Seaweeds of the Emirates

Abstract

Seaweeds, Cyanobacteria, seagrasses and mangroves are the principal inshore primary producers in the southern basin of the Arabian Gulf. Of these the seaweeds are by far the most diverse with about 120 species recorded from those Emirates bordering the region. Little is still known of the seaweed floras of the two Emirates within the Gulf of Oman (Fujairah, Sharjah). Briefly discussed are the very extensive cyanobacterial mats association with inshore sedimentary environments. Described are the bands of seaweeds, cyanobacteria and sessile animals that are a feature of the intertidal of rocky shores. Much consideration is given to the striking forest-like community that develops seasonally on shallow and often seaward sloping rocky platforms. Large foliose brown seaweeds are the canopy dominants of this community that develops rapidly over the months of lowest sea temperature (‘winter’). Many of these seaweeds decay and are lost during the early summer resulting in a striking transformation of the seascape when the understory of smaller mat/turf-forming and crustose coralline seaweeds becomes evident. Since the late 1990s there has been a ‘phase shift’ with seaweeds replacing stony corals as the spatial dominants on many rocky platforms. The ecological significance of the large biomass of dead and decaying seaweed produced in early summer is discussed. Briefly mentioned are seaweeds as providers of ecosystem services.

1 Seaweed Ecosystems

1.1 Introduction to Seaweeds

The relatively low diversity of macroalgae or ‘seaweeds’ in those emirates bordering the Arabian Gulf reflects the extremely inhospitable nature of the environment within its shallow southern basin where sea surface temperatures (SSTs) now regularly exceed 35 °C in August and salinities are consistently above 44 PSU (see Chap. 4; Foster et al. 2012; Vaughan et al. 2019). The inhospitable environmental conditions encountered within the Arabian Gulf undoubtedly accounts in large part for its low seaweed diversity (<400 spp) (John and Al-Thani 2014). Conditions are particularly harsh in its southern basin and accounts for the relatively low number of seaweeds (about 120 species) recorded so far from those Emirates that border it. Until the end of the twentieth century the seaweed flora of the UAE was virtually unknown, with a catalogue of the benthic marine algae of the Indian Ocean region (Silva et al. 1996) mentioning only a single species along with seven Cyanobacteria from the emirate of Abu Dhabi. Often the seaweeds of the Arabian Gulf are considered as representing a depauperate subset of those in the Indian Ocean due to the environmental extremes.

The most comprehensively studied seaweed flora in the UAE is that of the 700 km long coastline of the emirate of Abu Dhabi where there are many natural rocky areas and lithified ‘hardgrounds’ along its flanking coast and associated with many barrier islands, shoals and sea mounts. Still little is known of the seaweeds of those other emirates bordering the Arabian Gulf whose coastlines are considerably shorter and possess few hard-bottom seaweed habitats. Very few seaweeds grow on sand and fine sediments with information still lacking on the inconspicuous forms usually associated with many coastal developments, including rough stone or concrete block breakwaters, sea walls, piers and revetments surrounding new or enlarged islands. Still scarcely studied are the seaweeds of those eastern emirates lying within the Gulf of Oman, namely Fujairah and Sharjah, where environmental conditions are less extreme and marine biodiversity might be expected to be significantly higher than within the Arabian Gulf.

The seaweeds of Fujairah are mainly confined to the wave-beaten rocky shores lying to the north of the enclave of Khor Fakkan since to the south the wave-exposed coast consists mainly of sand beaches (Chap. 4). Just over 20 seaweeds have so far been reported from Fujairah (John 2023) with even fewer known from the internationally important wetland conservation reserve of Khor Kalba in Sharjah (D. John, unpubl. data). It consists of an extensive system of mangrove-fringed tidal lagoons, where the seaweeds are mostly confined to the rocky channel connecting them to the open sea (see Chap. 8). Also belonging to Sharjah is an island of considerable conservation importance, Sir Bu Nair, within the Arabian Gulf and lying some 65 km from the Dubai-Abu Dhabi border. The conservations importance of the island relates to the unique composition of its corals that until recently remained in a healthy state in contrast to the increasingly degraded inshore reefs (Bejarano et al. 2022). Unfortunately, there is no published information on its seaweed flora, although it is likely that seaweeds will be inconspicuous and few in number because of the complete dominance of table corals (Acropora) although the situation might change following a warm water event that took place in summer 2021.

Seaweeds are one of the principal primary producers in the southern basin of the Arabian Gulf along with seagrasses, mangroves, Cyanobacteria, vascular plants in salt marshes, and the symbiotic dinoflagellate associated with corals. Seaweeds are benthic (i.e. bottom-associated) macroalgae, and therefore largely confined to habitats where suitable hard and relatively stable attachment surfaces are present, including rocks, lithified hard grounds, coral skeletons, breathing roots of the grey mangrove Avicennia marina, large sponges, man-made surfaces, and the shells of living and dead molluscs. Some smaller seaweeds do commonly grow as so-called epiphytes on some of the larger species. One of the few seaweeds colonising the surface of the soft mud of intertidal flats are the dark-green, felted masses of the coarse, tubular, branching filaments of Vaucheria piloboloides, a member of the yellow-green group of seaweeds.

An important group of photosynthetic organisms frequently considered along with seaweeds are the Cyanobacteria, formerly referred to as the blue-green algae (Cyanophyta). It has long been recognised that these organisms are more closely related to bacteria, hence now termed blue-green bacteria or Cyanobacteria. The Cyanobacteria and seaweeds sometimes occur together in fully marine situations although the Cyanobacteria are usually most abundant and conspicuous in some of the most inhospitable environments. In the UAE the Cyanobacteria form very extensive intertidal crusts or mats on sandy or muddy shores, and even above the normal high-tide level within coastal salt marshes or sabkhas where the sediments are often a mixture of sand, silt or clay and are sometimes associated with a crust of salt and other minerals.

Seaweeds are a common feature of natural rocky shores throughout the UAE and often become most evident to the general public along parts of the Abu Dhabi coast in the spring and early summer when masses of dead and decaying seaweeds accumulate in bays and become deposited onto beaches. Otherwise for much of the year seaweeds are not usually very noticeable unless visiting a rocky shore at low tide or venturing below the waves with mask and snorkel.

1.2 What Are Seaweeds

Seaweeds or benthic macroalgae are oxygen-producing marine organisms that are usually attached to hard surfaces and all members have somewhat similar ecological requirements. Like seagrasses and other vascular plants, seaweeds all possess chlorophyll a and b, although the green colour is frequently masked by accessory pigments except in the group commonly known as the green seaweeds (Chlorophyta). As a character colour can be misleading, although it does link to a suite of other characters enabling the recognition of the following major seaweed group: Chlorophyta (green seaweeds), Ochrophyta, Phaeophyceae (brown seaweeds), Rhodophyta (red seaweeds) and Xanthophyta (yellow-green seaweeds) (Box 10.1). The red seaweeds contain the red pigment phycoerythrin and the blue pigment phycocyanin and commonly range in colour from bright red to dark purplish. Sometimes seaweeds belonging to the red group (Rhodophyta) become bleached by bright sunlight and then range from yellow-brown to orange when growing in the intertidal or in shallow water. The brown seaweeds contain the accessory brown pigment fucoxanthin and vary from dark brown to a yellow-brown or straw colour. Besides differences in pigmentation, seaweed groups are separated upon many structural and biochemical features. Some seaweeds exhibit the phenomenon of iridescence and are either completely iridescent, only iridescent at the branch tips or it is in the form of bands and spots.

Seaweeds are an unnatural grouping of organisms, since they fall within different kingdoms on the biological classification system. The green and red seaweeds are in the Kingdom Plantae, whereas the brown and yellow-green seaweeds belong to a separate kingdom, the Chromista. Frequently considered along with the seaweeds are the Cyanobacteria whose cellular and biochemical characteristics are more closely related to the bacteria, hence they are no longer referred to as the blue-green algae. The Cyanobacteria are all microscopic and so only visible when cells, colonies or filaments grow together to form blue-green, black, brownish or red tufts, mats or crusts. Differences in colour relate to the different proportions of a red phycoerythrin and a blue phycocyanin pigment in the cells as well as to a brown scytonemin pigment produced within the sheath of filamentous forms. The Cyanobacteria are placed in the Kingdom Eubacteria and are only briefly considered here since have been scarcely studied in the region, are very difficult to identify and there is much disagreement even amongst specialists concerning their taxonomy and naming of species.

Seaweed show considerably variation in form, size and complexity as well as in the nature of their reproduction. Some consist of simple or branched filaments of one to several rows of cells whereas others have a much greater morphological complexity ranging from cylindrical, compressed or flattened branches through to membrane-like forms. Some seaweeds have a stem-like portion known as a ‘stipe’ and are attached by a disc or a root-like structure, the so-called ‘holdfast’ (none possess true roots or vascular tissues). Some of the membrane-like forms are produced by division of cells in three planes to form what is known as a ‘parenchyma’. The majority have filaments as the basic unit of construction and these sometimes fuse to produce what is known as ‘pseudoparenchyma’. As none of the seaweeds have vascular tissues or roots, all gas and nutrient exchange occurs by diffusion through their tissues from the surrounding water, and hence typically they have thin, laminar surfaces.

Some red seaweeds are impregnated with calcium carbonate, so they are hard, stone-like and are referred to as ‘coralline seaweeds’ or ‘coralline algae’ because of their superficial resemblance to corals. Those calcareous forms having branches of calcified segments separated by darkly coloured uncalcified portions are known as ‘articulated corallines’. Most of the more ecologically important corallines are known as ‘crustose coralline seaweeds’ or ‘crustose coralline algae’ (sometimes abbreviated to CCA). Occasionally the crusts bear surface projects such as nodules or non-articulated branches as is the case in Lithothophyllum kotschyanum, the most widely distributed and common crustose form within the Arabian Gulf. These crustose corallines play an important role as ecosystem engineers since involved in the growth and maintenance of coral reefs by stabilising coral rubble and dislodged coral colonies and are known to induce the settlement of the larvae of corals and other benthic animals.

Box 10.1: Seaweed Groups

Green Seaweeds (Phylum Chlorophyta): Characteristically a bright green since no secondary pigments mask the chlorophyll colour. Range from single cells, simple or branched filaments of cells aligned in a single series through to compact spongy forms to flattened membrane-like fronds or tubes. A few are lime-impregnated.

Brown Seaweeds (Ochrophyta, Phaeophyceae): Characteristically range from olive-brown to various shaded of brown due the pigment fucoxanthin masking the green chlorophyll. Varying from filaments of cells in a single series through to strap-like forms or those with leaf-like appendages (foliose forms) and sometimes having a distinct holdfast, stem (stipe) and frond(s).

Red Seaweeds (Rhodophyta): Characteristically red in colour since the green chlorophyll pigment is masked by a combination of the red pigment phycoerythrin and the blue pigment phycocyanin. Often under high light conditions individuals become bleached to a brownish or straw colour. Varying from filaments of a single series of cells through to compact tissues in the form of cylindrical or flattened branches, sometimes membrane-like. Some members are impregnated with lime and are referred to as ‘coralline’ seaweeds since have been commonly mistaken for hard corals. Reproduction and life history is often very complex and is not considered here in any detail.

Yellow-Green Seaweeds (Ochrophyta, Xanthophyceae): Cells usually yellow-green due to predominance of a secondary pigment diatoxanthin in the chloroplasts (two or more chloroplasts per cell); unicellular, filamentous, colonial or coenocytic and motile forms with two subapical flagella; walls often with overlapping parts; storage material oil, fat or leucosin (never starch)

Blue-Green Bacteria (Cyanobacteria): Single-celled, colonies of large numbers of cells embedded in mucilage or of single filaments or grouped together to sometimes form distinctive filamentous colonies. Filaments are termed ‘trichomes’ and consist of a single linear row of living cells and these are sometimes enclosed within a sheath, occasionally sheath is lamellated and coloured. Some filamentous forms contain larger colourless cells (known as heterocysts) having one or two refractive polar nodes and are sources of nitrogen fixation in some species. Reproduce by large, thick-walled cells known as akinetes and by fragmentation to produce short lengths of trichome (hormogonia).

Seaweeds have complex life histories, with some species having stages showing so little resemblance to one another as to have once been considered separate species or genera. Some seaweeds reproduce vegetatively by fragmentation, although the majority have asexual reproduction by spores produced by a spore-producing stage known as the sporophyte. The males and female gametes are produced by the sexual stage (gametophyte) and fuse to form a zygote. For a discussion of the details of reproduction and complexity of the different life cycles in the algae, see general works on seaweeds and relevant websites.

1.3 Intertidal Habitats

1.3.1 Open Rocky Shores

The terminology used to describe biologically defined bands or zones on rocky shores throughout the world by Lewis (1964) and Stephenson and Stephenson (1972) has been applied to shores in the UAE (John and George 2004). According to the universally accepted scheme, the uppermost part of the intertidal (or littoral zone) is the ‘littoral fringe’ which lies above the very highest tides (Spring high tides) so is normally only wetted by wave splash and sea spray (Fig. 10.1). This fringe or ‘splash-zone’ is most conspicuous on wave-beaten rocky shores such as the rocky cliffs and platforms that are especially common to the west of Jebel Dhannah in Abu Dhabi and along much of the northern part of the 70 km long coast of Fujairah. Sometimes the Cyanobacteria form a blackish zone in and just below the littoral fringe and this is especially evident on limestone shores (Fig. 10.2a). There is little information on the composition of the Cyanobacteria within this zone except for the often most common and conspicuous species Chroococcus varius (Fig. 10.3c, d), whose colonies swell and become brownish-green when wetted by the incoming tide.

Fig. 10.1

Shore profile showing position of the tides in relation to the terminology used to describe the different zones based upon dominant plants and animal groups. Image credit: Oliver Farrell

Fig. 10.2

(a) Platforms of ancient reef limestone at low tide in western Abu Dhabi with lower parts very irregular due to many surface projections, pits and crevices and, where more steeply sloping, showing an upper dark coloured band of Cyanobacteria, (b) view of the zonation pattern on a cliff area in western Abu Dhabi showing a mid-littoral band of barnacles and mussels associated with an upper band of Cyanobacteria, (c) cliff area in northern Fujairah showing the upper littoral zone having a light coloured band of barnacles, often associated with darker patches of Cyanobacteria and these extending upwards, below the barnacle the rocks are covered by crustose corallines and dark coloured turf-like seaweed mats (d) close view of lower littoral zone showing the continuous crust of corallines and dark-coloured patches of turf-like mats associated with the large barnacle Tetraclita, (e) Intertidal platform in northern Fujairah revealing tidepools at low water and upper shore rocks covered by a carpet of the green membrane-like Ulva lacinulata and below a turf-like mat of red seaweeds, (f) Wave-battered littoral zone platform in northern Fujairah covered in lower parts by a carpet-like covering of Ulva lacinulata. Photo credits: David George (a, e, f) and David John (b, c, d)

Fig. 10.3

(a) Dark crusts or cushions of the cyanobacterium Kyrtuthrix maculans (left of oyster) on steeply sloping wave-exposed rocks in Fujairah, (b) light brown coloured mat of Gardnerula fasciculata (arrowed) growing on intertidal stone block close to an artificial island in Abu Dhabi, (d) Upper littoral zone platform showing wetted brownish band of Cyanobacteria, (c) upper shore showing light brown colonies of the cyanobacterium Chroococcus varius wetted by wave splash, (d) close view of the platform showing brownish-yellow polymorphous colonies of Choococcus varius, about 5–15 mm across. Photo credits: David George (a, c) and David John (b, d)

The mid-shore area of the littoral zone is known as the ‘eulittoral zone’ and immediately below lies the ‘sublittoral fringe’ which is the zone of the shore only exposed at the very lowest tides (known as ‘spring low tides’). A greyish band of barnacles often defines the upper part of the eulittoral zone and these typically occur immediately below or are mixed with the zone dominated by Cyanobacteria. Discovered at this level on rocks and cement blocks associated with an artificial island on Hail Shoal in Abu Dhabi were low growing light brown mats of the filamentous cyanobacterium Gardnerula fasciculata (Fig. 10.3b). Below the barnacle zone there is often a noticeable increase in biological diversity with the dominant organisms present often dependent on such factors as wave-exposure, nature of shore, slope and aspect. Sometimes on more steeply sloping rocks in Abu Dhabi were low growing turfs or mats of seaweeds that were frequently accompanied by clusters of calcareous tubes belonging to the serpulid polychaete worm Pomatoleios kraussii (Fig. 10.4c). Low growing mats were often particularly noticeable growing over crustose corallines on steeply-sloping, wave-battered rocks (Fig. 10.2c) and on the seaward side of rough stone breakwaters subject to an almost constant pounding by the waves.

Fig. 10.4

(a) View from edge of a wide intertidal platform revealing seaweed covered rocks and extensive shallow tidepools left as the tide drops at Shuweihat in Abu Dhabi, (b) shallow tidepool in ‘winter’ showing Polycladia myrica with its somewhat triangular outline and covered by the pinkish crusts of the coralline seaweed Hydrolithon, (c) lower littoral zone covered by colonies of green seaweed and brownish clumps of the common shore seaweed Palisada perforata, (d) close view of clumps of the green seaweed Dictyosphaeria cavernosa with its distinctive polygonal surface cells, (e) close view of the brownish branches of a clump of Palisada perforata, like many members of the red seaweed group it is brownish or straw-coloured when growing on the shore under stressful conditions. Photo credits: David George (a, b) and David John (c, d, e)

The lower littoral zone along much of the more sheltered parts of the coastline of Abu Dhabi and emirates to the east is normally characterised by low clumps of Palisada perforata (Figs. 10.4c, e and 10.5b), the bottle brush-like Digenea simplex (Fig. 10.5b) and the brittle green colonies of Dictyosphaeria cavernosa (Fig. 10.4d) whose distinctive polygonal surface cells are visible to the naked eye. These seaweeds persist throughout the year, whereas some of the mat-forming seaweeds are only evident during the winter months; often these seaweeds are most conspicuous along the seaward margin of those rocky platforms constantly wave splashed at low water. During the spring and early summer seasonally present seaweeds disappear, leaving some of the perennial forms mentioned above and these are often accompanied by pinkish or bleached crusts of crustose corallines. Sometimes there develop in early summer dense spongy, cushion-like growths of the green seaweed Cladophoropsis fasciculata on rocks in the lower littoral and shallow sublittoral zones (Fig. 10.5a). These have been observed covering extensive areas in the west of Abu Dhabi and sometimes become detached to accumulate in vast quantities in sheltered bays early in summer. This is unusual since nearly all seaweeds are much more conspicuous during the winter months (October-April) when the sea is normally below 25 °C.

Fig. 10.5

(a) Green clumps and mats of Cladophoropsis fasciculata growing in a sheltered rocky area, (b) close view of lower shore community in Abu Dhabi including Digenea simplex (Dg) and Palisada perforata (Pa), (c) the crustose coralline Lithophyllum kotschyanum showing the crust bearing nodules and cylindrical or flattened erect branches, (d) clump of the peacock tail brown seaweed Padina boergesenii showing the distinctive concentric zones of hairs on the fronds, (e) shallow water population of the inflated bladder-like brown seaweed Colpomenia sinuosa, (f) bleached clump of the fine red seaweed Spyridia filamentosa growing in the shallow sublittoral zone. Photo credits: David John (a–c), Chris Teasdale (d, e), and David George (f)

The uppermost limit of the infrequently exposed sublittoral fringe is usually defined by organisms normally growing in the shallow sublittoral zone. The black sea-urchin Echinometra and the oyster Pinctada radiata frequently define the fringe since are commonly present on the seaward margin of intertidal platforms along with mats of filamentous red seaweeds. During the winter months a few larger seaweeds sometimes grow within the fringe along with Dictyosphaeria cavernosa (Fig. 10.4d), sometimes this distinctive green seaweed persists throughout the year in the shallow subtidal. Some brown seaweeds similarly occur within the sublittoral fringe but are also more common in the subtidal zone, including the peacock tail weed (Padina boergesenii) (Fig. 10.5d), the foliose Polycladia myrica and Sirophysalis trinoidis and, often in March and April, the distinctive bladder-like Colpomenia sinuosa (Fig. 10.5e). The attractive but often bleached filamentous red seaweed Spyridia filamentosa (Fig. 10.5f), also occurs in the fringe but is more common in lower shore pools and in the shallow sublittoral zone.

1.3.2 Tidepools

Upper Shore Pools

Conditions for marine life are extremely harsh in these rocky pools, with some mostly supplied with water by wave splash and spray. Often Cyanobacteria are the only photosynthesising organisms surviving in such pools that are subject to exceptionally high salinities and water temperatures. These are usually filamentous Cyanobacteria forming brownish mats and are most common in nutrient-enriched pools associated with bird colonies.

Lower Shore Pools

During low water some shore platforms are partly covered by wide expanses of shallow water, as is the case at Shuweihat in Abu Dhabi (see Fig. 10.4a), rather than having many small pools. The environmental conditions are much more hospitable within lower shore pools compared to smaller ones at higher shore levels. Sometimes subtidal seaweeds are to be discovered in larger and deeper lower shore pools and frequently grow on the sometimes steeply sloping sides as well as on stones, shells and other debris lying on the bottom of those that are sandy floored. Some of those growing on the steep sides of pools are perennial forms that also occur on adjacent rock. Many seaweeds become bleached straw-coloured or brownish and this applies to Polyides myrica, a rhodiophyte that is sometimes heavily epiphytised by the small crustose coralline Hydrolithon (Fig. 10.4b). If sand is absent then lower shore rock pools might become lined by the pinkish crusts of coralline seaweeds, most commonly Lithophyllum kotschyanum (Fig. 10.5c). In Fujairah very small pools or depressions in the sublittoral fringe are typically lined by such crustose corallines and these are often occupied by the black sea urchin Echinometra. Easily overlooked is the beautiful umbrella-shaped green seaweed Acetabularia calyculus (Fig. 10.6b) that grows on partially sand or sediment-covered rocks and shells in lower shore pools and in the shallow subtidal.

Fig. 10.6

(a) Many umbrella-like individuals of Acetabularia calcyculus growing through sand in the fore and middle ground and a community in background dominated by clumps of brown and red seaweeds, (b) Close view of colonies of Acetabularia calcyculus bearing one or a series of the characteristic whorled fertile branchlets, often terminate in a cap, (c) soft, bushy clumps of Cladophora nitellopsis consisting of fine, clusters of densely branched filaments, (d) membrane-like fronds of Ulva lacinulata, (e) clumps of Ulva flexuosa with its tubular branches. Photo credits: Chris Teasdale (a, b) and David John (c–e)

1.3.3 Sedimentary Shores

1.3.3.1 Sandy Beaches

Sandy beaches range from steep, wave-exposed beaches of coarse sand through to those that are low to moderately wave-exposed, gently sloping and of fine-grained sand. Seaweeds are normally absent from such beaches unless sufficiently sheltered to enable hard sand-embedded sediments to be stable enough to allow colonisation by often fast-growing seaweeds. If partially sand-buried rocks, coral fragments, shells, cables or other artificial surfaces are present then these may be colonised by various seaweeds over the winter months. Most common are green seaweeds such as the much branched filamentous Cladophora nitellopsis (Fig. 10.6c), the fine tubular branches of Ulva flexuosa (Fig. 10.6e) and the membrane-like fronds of Ulva lacinulata (Fig. 10.6d). The latter commonly forms a low carpet-like mat on wave-exposed rocky platforms along the northern coast of Fujairah (Fig. 10.2e, f).

1.3.3.2 Mangrove Areas

The grey mangrove Avicennia marina is the only true mangrove tree to survive the temperature and salinity extremes encountered within the Arabian Gulf (see Chap. 7). The trees often form well-developed stands on the compacted muddy shores of creeks, channels, lagoon systems (Fig. 10.7a, b) and also along more wave-sheltered sandy shores on the mainland coast and offshore islands. Dark-coloured cyanobacterial mats commonly cover the frequently compacted mud associated with the trees. Also associated with the mangroves are some of the same seaweeds growing on partially buried hard surfaces on sandy shores. Occasionally the unbranched threads of the green seaweed Chaetomorpha linum become entangled with the breathing roots (pneumatophores) of the mangroves to form a curtain-like shroud (Fig. 10.7b, c). Sometimes associated with the mangroves are bushy clumps of the filamentous green seaweed Cladophora nitellopsis (Fig. 10.6c) and dense mats of the inflated, tubular branches of Ulva intestinalis (Farzanah et al. 2022). Sometimes such green seaweeds as Ulva are commonly association with dredged navigation channels close to industrial areas. Unsurprisingly the seaweeds on the uppermost margins of the calcareous limestone platforms through which these channels are cut are similar to those growing around and below the sublittoral fringe.

Fig. 10.7

(a) The mangrove Avicennia marina fringing a creek, with breathing roots (pneumatophores) in the foreground and cyanobacterial mats on the muddy foreshore, (b) pneumatophores of Avicennia at low water and draped by the green filaments of Chaetomorpha linum, (c) close view of the unbranched thread-like filaments of Chaetomorpha linum. Photo credits: David George (a, b) and David John (c)

1.3.3.3 Mud Flats

Fine sediments and decomposing organic material tend to accumulate in inner harbour basins, sheltered embayments, quiet areas of lagoons and on the lee side of islands. These areas of very soft, muddy deposits are high in organic matter and often black and anoxic just a few millimetres beneath the surface. Sometimes the mud surface on the lower shore is covered by a dark green, felty mat consisting of the finely branched filaments of Vaucheria piloboloides, the only yellow-green seaweed so far reported from the UAE (Fig. 10.8a, b).

Fig. 10.8

(a) Sheltered lee side of an artificial island on Hail Shoal, Abu Dhabi showing the darker coloured zone of Vaucheria piloboloides close to the level of the low tide, (b) close view of mat showing the somewhat pyramidal clumps of Vaucheria filaments. Photo credits: David John (a) and David George (b)

1.3.3.4 Lagoons and Coastal Sabkha

Where lagoons have no through-flow or only flood during the highest tides (spring tides) conditions are hypersaline, especially in their innermost reaches (see Chap. 8). Commonly on the sand or muddy shores of such lagoons there are cyanobacterial crusts or mats (Box 10.2), often referred to as ‘algal mats’. These are usually most extensive along the seaward margins of low-lying areas of the coastal plain that only flood during very high spring tides and periods of strong onshore wind. The seaward margins of these coastal plains are subjected to very high rates of evaporation and are often referred to as evaporative coastal salt flats, called sabkha (pl. sabkhat) in Arabic. The sabkha plain extends well beyond the cyanobacterial mats and is influenced by changes in the groundwater level and the sediments are associated with surface deposits of gypsum and anhydrite (see Chap. 2).

Box 10.2: Cyanobacterial Mat Types and Composition

Cyanobacterial mats have distinctive surface features relating to a number of factors, including frequency of immersion by the tides, seawater salinity, drainage, water flow and what species of cyanobacteria are present (Golubic and Abed 2010). Two of the main mat-forming cyanobacteria, Microcoleus chthonoplastes and species of Schizothrix (principally S. splendida), consist of microscopically twisted bundles of very narrow filaments (known as trichomes) confined within simple or branched sheaths. Schizothriz splendida is common along the banks of creeks and the sandy floors of well-drained lagoons where it forms pinnacle-shaped mounds (Fig. 10.9b). The Microcoleus frequently forms flat mats in ponds and variously sized waterlogged depressions and is sometimes accompanied in the middle and upper shore by Lyngbya aestuarii, another filamentous cyanobacterium. In the mid-tidal zone the Cyanobacteria are rearranged at a finer scale in response to changes in the length of time surfaces are exposure to air and therefore to water loss.

Fig. 10.9

(a) Lagoon fringed by the grey mangrove Avicennia marina with mud in foreground covered by a cyanobacterial mat, (b) wave-sheltered sandy lagoon showing a well-drained tidal area with the lower littoral zone covered by the pinnacle-shaped mounds of the cyanobacterium Schizothrix splendida. Photo credits: David John (a) and David George (b)

According to Golubic and Abed (2010), the non-filamentous cyanobacterium Entophysalis major assists in stabilizing sediments by producing copious amounts of hydrated extracellular polymers and this accounts for much of the appearance of what they term the ‘cinder’ zone. This zone extends from about the lower to mid-shore level and is so-called because the surface is covered by minute blisters (pustular) or nipple-like projections (see Abed et al. 2010). Immediately above this zone lies a so-called polygonal zone where desiccation results in the surface mat cracking into polygons whose upturned edges enclose dark green, leathery, layers of cyanobacteria. These polygons tend to form in shallow pools and channels, especially those frequently flooded by the sea, with cracking of the surface resulting in the formation of different microhabitats. Different Cyanobacteria are dominant in the water-retaining depressed centre of the polygons compared to the upturned margins and cracks. In the littoral fringe there is often a thin, crinkled, crenulated or convoluted, leathery, blackened cyanobacterial mat that often dries out and crumbles. Usually very flat is the landscape in areas dominated by cyanobacterial mats and crusts due to sediment accumulation and poor drainage, with the mats developing initially on what becomes soft, swampy ground when flooded by the tide. The sabkha plain extends well beyond these mats and is influenced by changes in the groundwater level and associated with the sediments are surface deposits of gypsum and anhydrite.

Golubic and Abed (2010, p. 241, Fig. 1) include a montage of photomicrographs of the more common Cyanobacteria forming Abu Dhabi’s microbial mats.

1.4 Subtidal Habitats

1.4.1 Rock Surfaces of Low Relief

Many shallow water platforms have low physical relief and are partially sand-buried, often much fissured and have creviced hard surfaces. Periodic sand-burial of hard surfaces is especially common in shoal areas where there are strong tidal streams as well as considerable wave action. There is a diverse faunal assemblage associated with these surfaces, including the pearl oyster Pinctida radiata, the mussel Brachydontes variabilis, and the snail Cerithium scabridium to name a few. Also present are many annual and perennial seaweeds that are frequently attached to surfaces that are sometimes buried beneath a thin layer of sediment or sand. Often in winter this shallow water community is spatially dominated by foliose brown seaweeds and these extend into deeper water where growing on loose lying rocks and dead sand-embedded coral fragments that are sometimes on the bed of dredged channels. The most conspicuous brown seaweeds present are the distinctively flattened branches of Sargassopsis decurrens (Figs. 10.10c and 10.13a), the much-branched Sirophysalis trinodis (Fig. 10.12e), Hormophysa cuneiformis (Fig. 10.10b) and the rather rigid Polycladia myrica (Fig. 10.4b).

Fig. 10.10

(a) General view of a shallow water community dominated by various foliose brown seaweeds, (b) The wedge-shaped chainweed Hormophya cuneiformis growing on sand-buried hard surfaces, (c) a clump of Sargassopsis decurrens on a sand-embedded hard surface and consisting of compressed, distinctively alternate and pinnately divided secondary branches, (d) clump of the flattened and regularly forked branches of the brown seaweed Canistrocarpus cervicornis on relatively stable rock fragments, (e) brownish clump of the bleached red seaweed Acanthophora spicifera growing on a large detached sponge and showing its somewhat spiny lateral branches. Photo credits: David George (a–d) and Chris Teasdale (e)

On occasion orange-coloured clumps of Chondria dasyphyllum, with its irregularly divided and distinctively club-shaped laterals, form extensive beds on low sand-embedded rocks and lithified hard-grounds (Fig. 10.11a, b). It is often accompanied by other bleached red seaweeds such as Laurencia obtusa (Fig. 10.15d), Acanthophora spicifera (Fig. 10.10e) and Spyridia filamentosa (Fig. 10.5f) that are also attached to partially sand-buried hard surfaces including the shells of living or dead bivalve molluscs.

Fig. 10.11

(a) Shallow seabed with foreground showing clumps of Chondria dasyphyllum growing close to the natural island on Hail Shoal, Abu Dhabi, (b) large clump of Chondria dasyphyllum growing on a partially sand-covered area of lithified ‘hard ground’ and on small stones. Photo credits: David John (a) and David George (b)

Sand movement is a significant factor influencing the composition, abundance, distribution and biodiversity of organisms on low platforms or lithified ‘hard-grounds’ (Fig. 10.11). The mosaic of seaweed communities reflects the instability of these hard surfaces that are available for colonisation for varying lengths of time and at different times during the year.

1.4.2 Rock Surfaces of High Physical Relief

The diversity and abundance of UAE’s seaweed flora goes largely unnoticed since hidden beneath the waves. Of the submarine seaweed communities, the most diverse and complex is the forest-like one in which brown foliose seaweeds are the canopy dominants (Fig. 10.12a–c). These submarine forest usually occur on gently seaward sloping rocky platforms that fringe the mainland coast of Abu Dhabi as well as some of its offshore barrier islands and shoals. Many of these fringing slopes in the central and eastern region of this Emirate, somewhat sheltered from the north-westerly shamal winds, were probably once spatially dominated from about 1 to 4 m depth by table corals (Acropora) along with an understorey of mound and brain corals such as Porites, Platygyra and many other smaller merulinids (George and John 2004).

Fig. 10.12

Sublittoral zone seaweed-dominated communities: (a) dense community (‘forest’) spatially dominated by foliose brown seaweeds in March (‘winter’) and growing on a rocky platform fringing an offshore shoal, (b) closer view showing some of the brown seaweeds forming the canopy of the submarine forest, (c) diverse brown seaweed dominated community growing where a rocky platform abuts sand, (d) bushy clump of the very common Sargassum latifolium showing its conspicuous hair pits on the leaf-like appendages and subspherical to spindle-shaped bladders, (e) habit of the common brown seaweed Sirophysalis trinoidis with its branches terminating in three bladder like nodes, (f) the almost spherical, pom-pom like, often bleached clumps of the articulated coralline Jania rubens growing on a Sargassum. Photo credits: David George (a–e) and Chris Teasdale (f)

Over the past two decades there appears to have been what has been termed a ‘phase’ or ‘regime’ shift as seaweeds replace corals in some shallow rocky areas following bleaching and death of many stony corals as a consequence of episodes of warmer sea temperature. The first of such warm water episode was in the summer of 1996 and its impact was particularly severe on shallow water stands of Acropora table corals. A second and even more severe episode took place 2 years later and this now impacted many of other shallow water corals, especially the frame-building mound and pillar coral (Porites) colonies together with various corals that historically flourished in the understorey of the shallower Acropora (George and John 1999, 2000, 2001; Riegl 1999). Since the 1990s there have been further warm water episodes, with these becoming more frequent and sometimes more intense. It has been estimated by Burt et al. (2021) that coral cover has decline by 20% per decade and Riegl et al. (2018) mentions that Acropora table corals have all but disappeared from the UAE’s Gulf coast reefs (Sir Bu Nair island the one exception), now leaving only more temperature-tolerant species. The recent 2017 bleaching event was one of the most severe on record, with nearly three-quarters (73%) of corals lost across Abu Dhabi within a year of that event (Burt et al. 2019); as such events increase in frequency, the shift from coral to algal dominance is expected to continue.

The dense ‘forest-like’ seaweed community finds its maximum expression during the ‘winter’ months (about November-April) and during that time the principal canopy forming brown seaweeds are Sargassum’s (principally S. latifolium, Fig. 10.12d) with their leaf-like fronds, Sirophysalis trinodis (Fig. 10.12e) and the wedge-shaped chainweed Hormophysa cuneiformis (Fig. 10.10b), the latter so named because of its characteristically shaped branches. Smaller brown seaweeds commonly form an understorey and include the somewhat rigid and distinctively shaped Polycladia myrica (Fig. 10.4b), the peacock tail weed (Padina boergesenii) (Fig. 10.5d) and Canistrocarpus cervicornis with its regularly forked, flattened branches (Fig. 10.10d). Often seen between about March and April are the distinctive, spherical or somewhat convoluted bladders of the brown seaweed Colpomenia sinuosa (Fig. 10.5c), especially when washed up on beaches.

Many of the canopy-dominant brown seaweeds begin to die-off and decay as the temperature begins to rapidly rise in April/May and these often become heavily overgrown by other seaweeds (Fig. 10.13a), including the minute filamentous brown seaweed Sphacelaria rigidula (Fig. 10.13b), the pinkish spots of the crustose coralline Hydrolithon and the pinkish pom-pom like tufts of the articulate coralline Jania rubens (Fig. 10.12f). The on-going decay of the canopy dominant seaweeds leads to the eventual loss of the Jania which sometimes washes ashore in vast numbers along parts of the coast of Abu Dhabi as whitish balls. Occasionally large accumulations of these balls have been observed lying on the seabed in the shallows and remain pinkish in colour and seem still to be growing.

Fig. 10.13

(a) A moribund Sargassopsis decurrens heavily epiphytised by smaller seaweeds, (b) somewhat rigid tufts of the filamentous brown seaweed Sphacelaria rigidula growing as an epiphyte, (c) clump of a form of Lithophyllum kotschyanum with flattened fan-shaped branches and accompanied by mats of filamentous red seaweeds, (d) the semi-erect or prostrate flattened fronds of the brown seaweed Lobophora variegata, (f) this distinctive uncalcified crustose red seaweed Peyssonnelia simulans varies from red to purplish-red, (e) showing the creeping and prostrate branches of Gelidium pusillum, one of the more common turf-forming red seaweeds. Photo credits: David George (a, c, d) and David John (b, e, f)

Many of the canopy-dominant brown seaweeds begin to die-off and decay as the temperature begins to rapidly rise in April/May and these often become heavily overgrown by other seaweeds (Fig. 10.13a), including the minute filamentous brown seaweeds Sphacelaria rigidula (Fig. 10.13b), the pinkish spots of the crustose coralline Hydrolithon and the pinkish pom-pom like tufts of the articulate coralline Jania rubens (Fig. 10.12f). The on-going decay of the canopy dominants seaweeds leads to the eventual loss of the Jania which sometimes washes ashore in vast numbers along parts of the coast of Abu Dhabi and appear as whitish balls. Occasionally large accumulations of these balls have been observed lying on the seabed in the shallows and remain pinkish in colour and seem still to be growing.

Some of the canopy dominants survive into the following growing season despite losing many of their side branches and become reduced in summer to sometimes a perennial holdfast bearing a few almost naked main axes. The greatly reduced cover of these canopy-dominants during summer results in the submarine seascape becoming transformed, since now the most conspicuous seaweeds present are those forming turf-like mats, crustose corallines and various sessile animals (Fig. 10.13c). One of the more common of these low growing forms is Gelidium pusillum (Fig. 10.13e) whose creeping prostrate branches give rise to flattened, simple or divided erect branches. Also evident during summer are semi-erect fronds of the brown seaweed Lobophora variegata (Fig. 10.13d), reddish-purple crusts of Peyssonnelia simulans (Fig. 10.13f) and crustose corallines including Lithophyllum kotschyanum (Fig. 10.13c); the latter varies morphologically from crusts bearing simple, cylindrical branches through to clumps of somewhat flattened, fan-shaped and interweaving branches.

1.4.3 Coral Carpets

It has been suggested that the coral reefs of the southern basin of the Arabian Gulf are best described as ‘coral carpets’ since the colonies form but a single layer growing directly on rock surfaces (Sheppard et al. 1992). Seaweeds were always an inconspicuous component of healthy coral carpets, but are now becoming increasingly significant due to the demise of the corals caused by recurrent episodes of their bleaching and death. The first such incidence in the UAE took place in 1996 (George and John 1999, 2000, 2001; Riegl 1999) and only became apparent when the coral

skeletons were covered by a layer of fine silt and were colonised by the minute filaments of fast-growing seaweeds, principally the brown seaweed Feldmannia mitchelliae (Fig. 10.17e, f). Subsequently Cyanobacteria have been reported in Chap. 11 to be another group quickly colonising lesions on dead corals. By the spring of 1997 there was a succession of seaweeds on the still intact skeletons of the tabular coral Acropora with the crustose coralline the spatially dominant group (Fig. 10.14b). There was a large reduction in physical complexity as the framework of Acropora skeletons began to collapse, leaving many shallow rocky platforms in Abu Dhabi covered by mounds of coral rubble (Fig. 10.14a).

Fig. 10.14

(a) The piles of rubble left following the collapse of the skeletons of the tabular coral Acropora at Al Dhabiya (now Al Nouf) in Abu Dhabi at about 6 months after the bleaching event of summer 1996, (b) the inflated bladder-like brown seaweed Colpomenia sinuosa growing on the upper dead branches of the coral Acropora and below are overgrown mainly by crustose coralline seaweeds; photographed near Marawah Island, Abu Dhabi in March 1997. Photo credit: David George

The island of Sir Bu Nair remains the only place of those emirates within the Arabian Gulf where relatively healthy stands of the tabular coral Acropora still exist, although even these have been damaged by recent severe warm water episodes (Bejarano et al. 2022; Burt et al. 2021). Seaweeds are more commonly associated with rocky areas where the corals have become degraded and coral communities are becoming dominated by more stress-tolerant brain and mound corals (Burt and Bauman 2019). One seaweed particularly associated with dead and living corals is Lobophora variegata (Fig. 10.13d) whose flattened, brown fronds are most frequent towards the base of living corals where sometimes also present are the reddish crusts of Peyssonnelia simulans (Fig. 10.13f). Otherwise, turf-like mats are typically confined to dead or damaged areas on coral where they are occasionally accompanied by the green filamentous seaweeds Cladophora and Cladophoropsis. Most commonly associated with corals are crustose coralline seaweeds, with Lithophyllum kotschyanum the most conspicuous and varies from a crust bearing nodules and simple or divided, cylindrical branches (Fig. 10.5e) through to one having flattened, often somewhat convoluted, interweaving, fan-shaped branches (Fig. 10.13c). Often crustose corallines develop on the steep sides of rock ledges and coral skeletons (Fig. 10.14b) and are occasionally accompanied by reddish or purplish tufts or mats of filamentous Cyanobacteria.

1.4.4 Seagrass Beds

There are vast seagrass beds or meadows covering comparatively shallow (<10 m) sand-covered inshore areas in those emirates bordering the southern basin of the Arabian Gulf, particularly Abu Dhabi (see Chap. 9). By far the most abundant seagrass is the narrow leaved Halodule uninervis and is accompanied in places by the two broader leaved species belonging to the genus Halopila, namely H. ovalis and H. stipulacea (Fig. 10.15a, b). Sometimes there is a mosaic of seagrass ( vascular flowering plants) and seaweed communities where the sea bed is of sand and also contains surfaces suitable for seaweed attachment. The only seaweeds growing directly on sand in the UAE are Caulerpa sertulariodes, Caulerpa racemosa and Avrainvillea amadelpha. The most common Caulerpa is Caulerpa sertularioides form farlowii (Fig. 10.16a, b) and this occurs within seagrass beds as well as on the sides of floating pontoons (Fig. 10.17a). Avrainvillea amadelpha is mostly encountered in very shallow sandy areas where its large holdfasts produce solitary or clusters of branches that often terminate in flattened blades (Fig. 10.16c, d).

Fig. 10.15

(a) Shallow seagrass meadow dominated by the broader leaved Halophila stipulacea and occasionally with small seaweed on sand-embedded hard surfaces, (b) close view of narrow leaves of Halodule uninervis, oval leaves of the seagrass Halophila ovalis and Halophila stipulacea, (c) clump of the bleached, filamentous red seaweed Polysiphonia kampsaxii, about 8 cm high, (d) bleached clump of the red seaweed Laurencia obtusa growing attached to a bivalve shell. Photo credits: David George (a–c) and Chris Teasdale (d)

Fig. 10.16

(a) Caulerpa sertularioides form farlowii growing in a shallow sandy area of Hail Shoal in Abu Dhabi, (b) close view of the form showing its dense, bottle-brush, erect branches of compactly arranged pinnules arising at intervals from creeping stolon-like branches, (c) clumps of the dark green seaweed Avrainvillea amadelpha showing its long stipes bearing anvil- or fan-shaped terminal blades (about 20 cm high), (d) closer view of flattened fan-shaped terminal blades. Photo credits: Chris Teasdale (a–c) and David John (d)

Fig. 10.17

(a) Floating pontoon with the submerged sides colonized by a diverse assemblage of organisms, including branched bryozoan colonies and the green seaweed Caulerpa sertularioides form farlowii, (b) insert of floating buoy and closer view showing dense cover of fouling organisms around and just below the water surface, (c) the bushy, somewhat plumose branches of the sexual life-history stage of Asparagopsis taxifolia (asexual stage forms delicate pinkish red tufts), (d) the bushy, straw coloured Hypnea cornuta with its upwardly curved branchlets, often these bear anvil-shaped propagules, (e) brown tuft of the filamentous seaweed Feldmannia mitchelliae, (f) line drawing of microscopic filaments of Feldmannia mitchelliae showing its characteristically shaped organs of reproduction (plurilocular gametangia). Photo credits: David George (a) and David John (b–e); image (f) from Borgesen (1939)

The seaweeds accompanying seagrasses usually grow on a variety of partially sand-buried surfaces, including low lying rocky areas, lithified ‘hard grounds’, large sponges, coral fragments, stones and living or dead bivalve mollusc shells. Common on these hard surfaces are a mix of filamentous green seaweeds, such as Cladophora nitellopsis (Fig. 10.6c) and Chaetomorpha linum (Fig. 10.7c), accompanied by various red seaweeds that include Spyridia filamentosa (Fig. 10.5f), Acanthophora spicifera (Fig. 10.10e), and clumps of the fine filaments of Polysiphonia kampsaxii (Fig. 10.15c). Some seaweeds and the surfaces to which they are attached occasionally become dislodged by strong tidal streams and sometimes leave a distinctive trail when dragged across the sandy seabed (Fig. 10.15d). Smaller seaweeds grow epiphytically on the blades of the seagrass Halodule uninervis, including the crustose coralline Hydrolithon that often appears as small pink spots.

1.5 Man-Made Habitats

1.5.1 Fixed Structures

Suitable seaweed habitats continue to be created as the coastal zone of the UAE is increasingly modified by the construction of new breakwaters, stone jetties, metal piers, floating marinas, harbours, channels, landfill, rock armoured islands and peninsulas (e.g., Dubai’s ‘Palm Islands’). Many islands and breakwaters have revetments of natural stone blocks (usually of limestone) or, less commonly, of concrete tetrapods or metal piles. Most of these surfaces become quickly colonised by fouling animals along with fast-growing seaweeds and sometimes are in areas where no such hard-bottom had existed previously. Suspension-feeding invertebrates frequently foul pier piles and floating pontoons whereas hard corals are recorded on the sediment-free seaward side of rough stone breakwaters and are accompanied by red seaweeds, such as the bushy seaweed Asparagopsis taxifolia (Fig. 10.17c), various brown seaweeds and the ubiquitous crustose corallines. Breakwaters and other shoreline developments might be thought of as large-scale artificial reefs capable of supporting abundant and diverse marine communities. Consideration has therefore been given to applying ecological engineering principles to such shoreline developments in order to enhance biodiversity, with the first step been to determine those best suited to enhance recruitment of desirable organisms (Burt and Bartholomew 2019; Burt et al. 2009).

1.5.2 Floating Structures

Channel marker buoys and pontoons are frequently festooned by a diverse assemblage of seaweeds during the winter months, together with many invertebrates. Immediately at or just below the water surface (Fig. 10.17a, b) there is often a dense seaweed mat that sometimes includes the articulated coralline Jania rubens (Fig. 10.12f), Acanthophora musciformis (Fig. 10.10e), the creeping green Caulerpa sertularioides form farlowii (Figs. 10.16b and 10.17a), the peacock/turkey tail brown seaweed (Padina boergesenii) (Fig. 10.5d), the often straw-coloured Hypnea cornuta (Fig. 10.17d) and the inflated colonies of Colpomenia sinuosa (Fig. 10.5c) that are usually present in the spring. Occasionally present are clumps of the form of the crustose coralline Lithophyllum kostchyanum whose branches are flattened (Fig. 10.13c). These man-made structures are periodically cleaned to remove fouling organisms so a stable animal-dominated assemblage is probably never attained. Small buoys in sheltered marinas and the hulls of small boats frequently have a covering of the green filaments of Cladophora nitellopsis (Fig. 10.6c) and sometimes mats or tufts of the filamentous brown seaweed Feldmannia mitchelliae (Fig. 10.17e, f); these fast-growing seaweeds are usually the first conspicuous colonizers of newly available surfaces.

2 Ecological Interactions

A feature of many rocky areas and lithified ‘hard grounds’ is the complexity of biological interactions taking place, including competition for space, avoidance of predation and herbivory through structural or chemical adaptations, and temporal escape through rapid growth and regeneration. The ability of some seaweeds to quickly colonise and become establish might impact on the successful colonization and re-establishment of corals following episodes of bleaching and death. It is known that turf-like mats very soon colonize coral skeletons following bleaching and yet Birrell et al. (2005) was able to show experimentally that turf-like seaweed mats only have a negative impact upon coral recruitments if containing trapped sediment.

Some seaweeds are adapted to survive the ravages of herbivorous fish and sea urchin grazing through morphological and chemical defence mechanisms (Duffy and Hay 1990; Hay and Fenical 1992). The heavy calcification of crustose corallines means they are not normally grazed by fish other than parrotfish and are well adapted to survive chronic grazing or scraping by sea urchins. These forms also survive in habitats where there is violent wave action hence are conspicuous on very wave-battered and often steeply sloping shores, including the seaward side of breakwaters. For example, the crustose corallines form a very distinctive band in the lowermost zone on rocky headlands along the northern coastline of Fujairah (see Fig. 10.2c, d). The success of crustose corallines is not only due to their hardness but also their ability to continuously replace those surface cell layers lost through grazing, heavy wave-action and sand abrasion.

Some of the larger seaweeds, such as the foliose browns, are known to be an important food source in the UAE for animals including green turtles and dugongs. It has been suggested by Fulton et al. (2020) that in diverse tropical seascapes, what they term macroalgal meadows (p. 3), ‘…could provide stepping-stones or refuge habitats for fishes occupying a diverse tropical seascape subject to disturbance events. Depending on the trophic diversity of these macroalgal-associated fishes, such overlaps in habitat occupation could help stabilise ecosystem structure and function in the face of disturbances affecting a particular habitat type (e.g., mass-bleaching of corals)’. Much of the large biomass of decaying seaweed often seen floating in the sea along parts of the western coast of Abu Dhabi in spring and early summer becomes deposited on beaches and probably contributes significantly to detrital food webs as well as to the microbial loop.

Small turf-like mats of seaweeds are typically formed of short, tightly packed, highly branched filaments and consist of erect branches arising from a system of creeping or prostrate branches. Such growth forms are common and survive loss or damage to the apices of the erect portion by fish browing, desiccation, wave action and sand abrasion by having basally positioned growing points or meristems. These mats are conspicuous in the lower littoral zone of rocky shores or breakwaters  including those subject to considerable heavy wave action. Such mats are also commonly associated with shallow rocky areas and ‘hard grounds’ (0.5–5 m), especially where there is much physical relief and are sometimes associated with large populations of herbivorous fish. Still to be investigated in the UAE are such plant-animal interactions, including the impact of herbivory on the distribution and composition of the seaweeds in the littoral and shallow subtidal zone.

3 Seaweed Seasonality

The maximum summer sea temperature and seasonal temperature range in the southern basin of the Arabian Gulf is the greatest worldwide. Temperatures range from 19 to 36 °C with the lowest ones are from February to April (winter-spring period) and the highest from about August to October (summer-autumn) (see Chap. 4). The annual temperature regime accounts for the pronounced seasonal changes in the distribution, abundance and composition of the UAE’s seaweeds flora (John and George 2003). The temperature changes are not as extreme outside the Arabian Gulf, and therefore seaweeds in the eastern emirates are unlikely to display anything like the same very pronounced seasonality, although such information is lacking since they have been little studied in Fujairah or Sharjah’s coastal waters.

Observations on seasonal changes to the seaweed flora have been confined to Abu Dhabi, where seaweeds undergo a dramatic transformation as the sea temperature rapidly rises in spring and early summer (April-May). Many seaweeds begin to undergo decay and are lost during this period, and large floating masses have been observed in the western region of Abu Dhabi which frequently accumulate in sheltered bays before washing ashore (Fig. 10.18a, b).

Fig. 10.18

(a, b) Rocky bay in the west of Abu Dhabi showing the mass of seaweeds washed ashore in spring and early summer following the seasonal die-back of offshore seaweed communities. Photo credits: David George

Most seaweeds appear to be moribund or dormant over the summer when sea conditions tend to be very calm and most submarine surfaces become coated in what is believed to be a film of sediment mixed with microalgae.

A further transformation of the seascape takes place from about November onwards and corresponds to the sea temperate dropping below about 25 °C, when many seaweeds begin to reappear and the larger brown seaweeds once more spatially dominate many shallow rocky areas. Recovery of these seaweeds often takes the form of new branches from often persistent basal portions and of small juveniles arising from resting spores and microscopic life history stage. The winter period is the most appropriate time for studying this important group, since this is at the time when seaweeds are most abundant and diverse.

4 Invasive Seaweeds

The only seaweed believed to have been introduction in recent times is Caulerpa racemosa form requienii (Fig. 10.19). This green seaweed was first discovered in September 2010 in Palm Jebel Ali (Dubai) where it formed dense and extensive beds over the sandy and muddy seabed at depths ranging from 3 to 5 m (Venneyre et al. 2014). It consists of creeping stoloniferous branches attached by clusters of rhizoids and giving rise at intervals to cylindrical or slightly compressed, regularly forked branches. The only other site from which it is reported is a sheltered muddy area lying about 700 m off Ras Ghantoot and close to a what at the time was a causeway connecting the mainland to the Dubai ‘Waterfront Islands Development’; a site just a few kilometers from the Abu Dhabi border.

Fig. 10.19

Dense bed showing the cylindrical or slightly compressed erect branches of Caulerpa racemosa form requienii growing on the sandy floor of Palm Jebel Ali in Dubai. Image credit: Laurence Vanneyre

One possible vector responsible for its introduction are the suction dredgers employed for the construction of Palm Jebel Ali and the Dubai waterfront. These dredgers use sand as ballast in their hoppers and, just prior to travelling to Dubai, had been deployed extracting and transporting sand from near Jakarta in Indonesia for the construction of a harbour in Singapore. To determine whether this is the source of the introduction it will be necessary to carry out a genomic investigation to compare the Dubai population with that from Indonesia.

This introduced species is a potential invasive species, since other members of the genus have become widely distributed and have brought about disastrous changes in shallow water environments in different parts of the world, especially in the Mediterranean. There is no recent information on these seaweeds within the Arabian Gulf so its current status is unknown.

5 Importance of Seaweeds

Seaweeds, along with seagrasses, are the key primary producers lying at the base of inshore food webs within the southern basin of the Arabian Gulf. The larger brown seaweeds are key primary producers involved in nutrient cycling and the seasonally developed, complex three-dimensional subtidal ‘forests’ in which they are the canopy dominants are important spawning and nursery habitats for many fish, as well as foraging areas for adult shrimps. Some seaweeds are a direct food source for fish, invertebrates, as well as turtles and dugongs, but much seaweed biomass enters the food web as detritus. They are therefore seasonally important to suspension and detrital feeders alike and, through decomposition, make a significant contribution to the microbial loop.

Detrital feeders are important general consumers of organic material and along with microalgae probably account for the importance of inshore mud flats and offshore shoals and islands as feeding grounds for millions of migratory and resident water fowl (see Chap. 15). The spring and the early summer are when a large quantity of decaying seaweeds and seagrasses are cast up on some beaches with brown seaweeds and contributing to much of this biomass (Fig. 10.18a, b), principally species of Sargassum, Hormophysa cuneiformis and Sarcothalia trinodis whose floatation is aided by air-filled vesicles or bladders. Decaying seaweeds deposited on beaches (i.e. beach wrack) are consumed by animals living in the strandline and, therefore, make an important contribution to terrestrial food webs. This justifies leaving them in situ rather than removing them simply because they are considered unsightly when on leisure beaches.

The increase in abundance in rocky subtidal areas of crustose coralline seaweed following the mass bleaching and death of corals was first observed in Abu Dhabi in the late 1990s (George and John 1999, 2000, 2002). The increased importance of crustose corallines has been confirmed by a study of colonisation of tile surfaces in which these seaweeds were a major component of the settlement community and this led Bento et al. (2017) to conclude that in the southern Gulf ‘the CCA [crustose coralline algae] appear to be a persistent, long-term characteristic of this highly disturbed extreme environment’. In the same study it was discovered that ‘turf algae’ were more important than crustose corallines in the Sea of Oman (including Fujairah) with sea squirts (ascidians) and moss animals (bryozoans) dominating the settlement community in what was considered to be a more benign region.

The crustose corallines play an important role in coral-dominated areas by reinforcing dead coral skeletons. They stabilise surfaces by ‘cementing’ coral debris and dislodged blocks of living coral and reduce erosion. Seaweeds are known to induce the settlement of coral larvae (Ritson-Williams et al. 2014; Tebben et al. 2015) so the presence of crustose corallines should favour coral colonisation following disturbance. This has important implications given the increasingly degraded nature of coral reefs around the UAE since such beneficial coral-algal interactions might aid their recovery.

Crustose corallines are able to survive where wave-exposure is extreme, hence are conspicuous on wave-beaten shores such as the rocky headlands along the northern coast of Fujairah (John 2023). These are predominant and sometimes the only conspicuous seaweeds to survive in submarine rocky areas where there is chronic grazing by large aggregations of the black sea urchin Echinometra (Fig. 10.20a, b). Crustose seaweeds persist because they have a subsurface growth or meristematic layer that continually replenishes the outer surface cells removed by the scraping or rasping of urchins. One of the only threats to crustose corallines comes from parrotfish that feed by removing entire portions of crust along with any branches or surface nodules, although parrotfishes are relatively rare along the southern Gulf coast of the UAE (Hoey et al. 2016).

Fig. 10.20

(a) Submarine rocky reef at Hail Shoal in Abu Dhabi chronically grazed by large populations of the sea urchin Echinometra and showing its much-eroded surface, (b) closer view of chronically sea urchin grazed rocky area now spatially dominated by crustose coralline algae (CCA). Photo credits: David George

So far 16 species of crustose coralline seaweeds have been recorded in the UAE (John and Al-Thani 2014) of which three are only known from Fujairah. All these corallines were originally described in three unpublished reports written in the late 1990s by Dr Yvonne Chamberlain, a specialist in crustose corallines, and reproduced in a report prepared for what was then the Abu Dhabi Company for Onshore Oil Operations (John and George 2001).

There is an urgent need for a taxonomic revision of this increasingly important, and yet much neglected group of seaweeds, ideally involving combining the more traditional approach based on morpho-anatomical characters with modern DNA sequence analysis.

6 Seaweeds as a Resource

There have been several studies in the southern Gulf focusing on chemical elements extracted from local seaweeds, and Al-Adilah et al. (2021) have reported on the nutritionally important polysaturated fatty acids and suggested local seaweeds to be a possible commercial source of such compounds. Less convincing is the suggestion by Farzanah et al. (2021) that UAE seaweeds might be a feasible source of biomass for refining and producing fuel for jet aircraft and as high value chemicals. All such studies have focused on a few of the more readily recognised and relatively common seaweeds, including the green seaweed Ulva. A species of Ulva, referred to as Ulva intestinalis, has been discovered to have high concentrations of essential minerals such as potassium, magnesium, iron and zinc, so making it a promising novel food source in the UAE according to Farzanah et al. (2022). Other seaweeds investigated elsewhere in the southern Gulf include the seasonally common inflated brown seaweed Colpomenia sinuosa, the so-called peacock tail weed Padina boergesenii, various species of Sargassum and Sarcothalium trinodis. These studies all fail to address the problem of having a constantly available and reliable source of supply since most species are only present in any quantity during the months of cooler sea temperature (‘winter’). That is not to say a small scale or ‘cottage’ industry approach might not be adopted if it were acceptable for harvesting to cover a very short period when seaweeds are locally available in quantity.

7 Conclusions

The submarine seascape of the shallow inshore environment of the emirates bordering the Arabian Gulf and Gulf of Oman has undergone a dramatic transformation or ‘phase’ or ‘regime’ shift over the past two decades. The degradation and decline in coral cover over the last two decades has been very significant and has resulted in new opportunities for seaweeds to colonise and replace them as the spatial dominants in some shallow rocky areas (Fig. 10.20). All indications are that environmental stressors such sea temperature will continue to favour seaweeds that are likely to continue to become an ever more significant component of benthic marine communities.

Differences in the composition and abundance of seaweeds between the emirates reflects the availability of suitable substrates for seaweeds, extent of these hard substrates and the amount of collecting effort. Abu Dhabi has received, by far, the greatest amount of attention, in part due to many nearshore and offshore habitats suitable for the development of seaweeds and the presence of extensive rocky areas once completely dominated stony corals (see Grizzle et al. 2016). Surveys are needed of the seaweeds of those emirates lying to the west of Abu Dhabi since few, if any, records exist for most of these areas (see John and Al-Thani 2014).

The seaweed flora has been scarcely studied in the emirates bordering the Gulf of Oman (Fujairah, Sharjah), unlike in the neighbouring sultanate of Oman where they have been well researched and for which over 400 species have been recorded (see Wynne 2018). The exceptional high seaweed diversity in Oman compared to Fujairah and Sharjah is not just a reflection of greater research effort but, rather, the exceptional development of seaweeds that takes place where there is upwelling of colder, nutrient-rich water along its southern coast during the seasonal monsoon (Savidge et al. 1990). The rockier parts of the coastline of Fujairah are more comparable to northernmost parts of the Omani coast where seaweed diversity is low due to high air temperatures, desiccation and low nutrient levels (B. Jupp, pers. comm.).

Since the 1990s new coastal and offshore developments in the UAE will have benefitted seaweeds by providing hard surfaces suitable for colonisation, sometimes where none existed before. Although there is no evidence that increasing summer sea temperatures has had an adverse impacted upon the seaweed flora, it is impossible to know whether this will continue to be the case as the environment becomes more stressful. There are a multitude of environmental factors that impact seaweeds, corals and other marine organisms besides sea temperature, including sea water acidity that is increasing in the Arabian Gulf (see Uddin et al. 2012) as well as seas and oceans worldwide. The decline in pH and carbonate levels combined with rising CO₂ and bicarbonate levels will be a threat to crustose coralline seaweeds, corals as well as marine invertebrates having shells of calcium carbonate. Acidification might not only affect the organisms themselves, but results in softening of the lithified sediments and debris (‘hardgrounds’) which are often crucially important surfaces for seaweeds and other benthic organisms (Purkis et al. 2011). Uncertain surrounds the threat from acidification because the seawater in the Arabian Gulf is supersaturated with aragonite as a result of regional environmental conditions.

One of the groups most likely to benefit from some of the predicted longer-term changes in the marine environment will probably be the Cyanobacteria. The most significant cyanobacterial communities, often referred to as ‘algal mats’, principally develop on low lying and flat or gently sloping sedimentary shores (e.g., coastal sabkha, intertidal mudflats). These mats will be at risk from storm surges predicted to increase linked in response to rising temperatures in the region. Other risks include inundation resulting from land subsidence linked to oil and groundwater extraction and sea-level rise due to thermal expansion and melting of land-based ice elsewhere on the globe (Melville-Rea et al. 2021).

Seaweeds and Cyanobacteria remain some of the least-researched primary producers in the UAE, with very few genomic studies yet undertaken using DNA sequence data to identify seaweed floras. There are some questionable new records from the southern region since these are not supported by informative illustrations/photographs and no attempt has been made to fully described the specimens upon which they are based or confirm by DNA sequence analysis. It is of crucial importance to ensure material is correctly identified and representative specimens suitably stored in national institutions (e.g., herbaria, museums, universities) for future reference and further study by taxonomic specialists. Representatives of all the species reported from the UAE are housed in the herbarium at the Natural History Museum in London (BM).

8 Recommended Readings

For further information on the seaweeds of the Emirates, the following sources are recommended as accessible readings: Al Abdessalaam (2007), John (2005, 2012), John and George (2004, 2005), George and John (2005).