Keywords

1 Distribution

1.1 Global Distribution

Found in intertidal as well as subtidal zones, seagrass meadows are either fully submerged or occasionally exposed in intertidal areas of the coastline. These marine plants occur in tropical, subtropical, and temperate zones (Fig. 9.1), and typically occur to a depth of 10 m in most of its range (Short et al. 2001), although they can be found to depths of 50 m or more when the water quality and clarity are optimal (Den Hartog 1979). Seagrass occurs in sediment types that range from muddy organic soil to sand with gravel substrates, and are often adjacent to mangrove and coral reef ecosystems (Lamine et al. 2021). Seagrasses are the most common coastal habitat globally, forming underwater meadows that are critical for coastal and marine wildlife and contribute to the primary productivity of the coastal waters (Costanza et al. 1997).

Fig. 9.1
A globe exhibiting dark shaded continents with few light area indicating the sea grasses appearance.

Seagrasses are widely distributed and found along most of the coastlines around the world. Data source: UNEP-WCMC (2021), published per the ENEP-WCMS General Data License

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1.2 Taxonomy

Seagrasses are marine angiosperms (flowering plants) that represent a small group of species compared with most other marine organisms, yet they hold a vital role in providing habitats to other species. There are about 300,000 species of angiosperms, and only around 60 of them are considered seagrasses. They fall into two families: Potamogetonaceae and Hydrocharitaceae, taxonomic studies of seagrasses are still ongoing with some current knowledge gaps (Hartog and Kuo 2006). With low taxonomic diversity compared to the more than 250,000 terrestrial flowering plants, seagrasses diverged from a lineage of monocot flowering plants (the same group that contains corn, lilies and all of the terrestrial grasses) between 70 and 100 million years back (Les et al. 1997). Research on their taxonomy and distribution is still on-going and questioned despite the limited diversity of seagrass flora (Hemminga and Duarte 2000). Despite of their low species diversity and specialist physiological characteristics, seagrasses have successfully and widely colonized in most coastal regions except the polar regions (Orth et al. 2006a). Seagrasses are a vascular (veinous) plant that produced roots and seeds and has various other adaptations that emerged as this group evolved on land and that they still retain despite moving into the sea; seed dispersal through water currents is one of the abiotic dispersal adaptation that seagrasses utilize as a method of reproduction, in which floating seeds, reproductive fragments or fruits are transported long distances until seeds arrive at the sediment surface as ‘seed rain’ (Orth et al. 2006b). They should not be confused with the evolutionarily divergent marine algae (seaweeds), which are taxonomically and biologically very distinct and are discussed separately in Chap. 10.

1.3 Arabian Gulf Seagrasses

In the relatively young Arabian Gulf in the North-Western Indian Ocean (NWIO) region, seagrasses are representative of Indo-Pacific and subtropical biogeographic conditions. Largest extent of the seagrasses mapped in the Arabian Gulf are found in Abu Dhabi coastal waters (Das 2021), although seagrass meadows have also been mapped from other emirates as well (Table 9.1), namely, Dubai, Umm Al Quwain and Ras Al Khaimah in khors (lagoons), channels and backwaters (Fig. 9.2), with relatively less areal coverage. The seagrasses of the UAE usually inhabit shallow, low wave energy areas up to a depth of 14 m (EAD 2020) in a varied sediment type that ranges from sandy substratum to muddy soil with nutrients and organics. The Arabian Gulf contains 6% of the total area of seagrasses around the world, which covers an area of around 6790–7320 km2 in the Gulf, with ca. 80% of this (i.e. 4% of global total) occur in Abu Dhabi alone (Erftemeijer and Shuail 2012; Lamine et al. 2021), and the most dense meadow could be found in Marawah Marine Biosphere reserve, an MPA established in 2007 to protect the rich ecosystems within (Mateos-Molina et al. 2021) (Fig. 9.2) (see also Chap. 8).

Fig. 9.2
A map indicates the U A E coastline with marked areas for M P As, seagrass, oyster beds, emirates borders, coastline, and study area.

UAE coastline showing seagrass areas, the location of marine protected areas and the occurrence of oyster beds. Source: Fig. 4 in Mateos-Molina et al. (2021), reproduced under Elsevier license 5513570228876

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Table 9.1 Seagrass area (in km2) of each emirate and a general description of the seagrasses in each area, with notes on observed occurrences and depths
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1.4 Seagrass in the United Arab Emirates

The seagrass meadows in the UAE coastal waters of the Arabian Gulf are composed of three key habitat forming species of seagrasses: Halodule uninervis, the most dominant species that contributes the highest biomass growing in all sediment types, Halophila ovalis, the second most dominant species that has a wider distribution and can be found in sand/gravel substrata in deeper waters, and Halophila stipulacea, which is has limited abundance and distribution and prefers muddy intertidal habitats with high organic content (Campbell et al. 2015). These meadows can be found as monospecific beds, as well as mixed species assemblages. In several areas in Abu Dhabi, especially during summer, seagrass meadows are intermixed with marine algae such as Sargassum spp. and Harmophysa spp. that usually grow within and around the seagrass meadows where substratum is dominated by gravel rocks and dead coral (see also Chap. 10). Up to 80% of coverage is contributed by H. uninervis in most meadows (Fig. 9.3 and Table 9.2).

Fig. 9.3
6 photographs exhibit a variety of seaweed plants and their roots. The seaweed plants are of different shapes and sizes, but they all have long, thin leaves. The roots of the seaweed plants are attached to the surface, and they appear like small, branching roots.

The three seagrass species: (a) and (d) Halodule uninervis, (b) and (e) Halophila stipulacea and (c) and (f) Halophila ovalis. This figure showcases these species in the field how they appear naturally, and then a closer view in the laboratory. Photos: Noura Al-Mansoori (a, d, e, f) and Himansu Das (b, c)

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Table 9.2 Depth ranges and substratum types these species prefer
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Seagrass cover of Abu Dhabi coastal waters was estimated to be 5500 km2 in 2000 using aerial as well as boat-based field surveys (Phillips et al. 2002). In 2021, combining data from remote sensing satellite imageries (Worldview and Sentinel) and data from available online sources and field survey using underwater video rays/drop camera, the Environment Agency Abu Dhabi mapped approximately 3000 km2 of seagrass in Abu Dhabi. Considering the survey methods of both the estimates, Abu Dhabi seagrass area may range from 3000 to 5000 km2 (Fig. 9.2).

The western region of Abu Dhabi as part of south-western Arabian Gulf is a sheltered waterbody with wide areas of shallow depth (<15 m) and sediments rich in nutrients and organics. Water circulation in the area is limited in this embayment and turbidity is relatively slow (see Chap. 4). Water depth, circulation, current and sediment types in the central and western region of Abu Dhabi create favorable conditions for seagrass growth, resulting in relatively higher density and wide distribution of seagrass beds. The area has natural channels, sheltered waters, bays and backwaters that supports the seagrass growth.

Due to high sea surface temperatures (SST) during summer, the seasonality of seagrass is apparent in UAE waters. Disappearance of seagrass meadows in very shallow waters (up to 5 m) during peak summer temperatures (July–September, SSTs >34 °C) is common, and this is associated with movement of seagrass-feeding marine megafauna to deeper (>5 m) waters during this period. Seagrass leaves shed copiously in these shallow depths during summer, leaving roots and rhizomes, with leaves reappearing only after the cessation of summer extremes. At times, seagrass-specializing marine animals such as dugongs will switch to feeding on marine algae during these periods where seagrasses decrease in density (Marsh et al. 1982). By October, when SSTs return to 24–26 °C, seagrass leaves start reappearing (EAD 2020). Similar pattern not observed in waters deeper than 5 m, where the slight difference in depth allows them to remain below physiological thresholds that cause stress, and seagrass beds in these slightly deeper depths can remain dense throughout the summer (Das 2021).

2 Diversity

Seagrasses have a wide distribution globally but with a relatively low number of species (~60 species worldwide), and because of their submerged nature they are often overlooked. Yet in comparison to most marine and coastal plants they are widely distributed (Short et al. 2007). Seagrasses have been classified into their respective bioregions in order to better understand their taxonomic distribution and provide a framework for understanding seagrass habitats, their dynamics in a geographic context, and their trophic pathways, while also providing a structure for scientists to compare and contrast species and study them worldwide (Short et al. 2007).

Regional biogeography is an important part of understanding seagrasses, because is based on the geographic range and is used to relate species of temperate and tropical regions. Even though taxonomic and genetic studies are still ongoing, the classification of most species have been well described into six bioregions globally: (1) Temperate North Atlantic (2) Tropical Atlantic, (3) Mediterranean, (4) Temperate North Pacific, (5) Tropical Indo-Pacific (part of which includes the UAE) and, (6) the Temperate Southern Oceans. Of all the tropical and temperate regions, the tropical Indo-Pacific region has the highest diversity of seagrass species, and which includes the Arabian Gulf (Short et al. 2007). Out of 60 known species globally, 24 predominantly occur in this bioregion, with only three species known to occur in the Arabian Gulf, presumably because the extreme environmental conditions of this region limit the number of species that are able to survive (Erftemeijer and Shuail 2012). Arabian Gulf seagrass diversity is low compared to the adjacent Red Sea, Northwestern Indian Ocean (NWIO), Western Indian Ocean (WIO) and rest of the Indo-pacific region (Table 9.3). This makes the Arabian Gulf a critical place in terms of climate change effects on these tropical species and hotspot for research to better understand mechanisms that seagrasses use to survive extreme temperature fluctuations.

Table 9.3 Regional distribution of seagrass species; Arabian Gulf, Sea of Oman, Red Sea, Mediterranean. Data from: (El Shaffai 2016; El-Shaffai et al. 2011; Green et al. 2003; Lipkin et al. 2003)
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3 Broad Importance of the Ecosystem

3.1 Global-Scale Importance

On a global scale, the complex seagrass ecosystems provide physical and biological functions such as stabilizing sediments, reducing sediment resuspension and erosion during storms, in addition to acting as a buffer for wave action. They also serve as a shelter for resident and transient adult and juvenile animals many of which have commercial and recreational importance to traditional fishing communities. Seagrasses and their epiphytes area major marine source of carbon and provide a food source to complex food webs through direct grazing or detrital pathways. They also trap detritus, sediment and nutrients (derived from land runoff) (Coles et al. 2002). Seagrass meadows have an extensive root system that stabilize the sediment and protect against coastal erosion, while also acting as effective carbon sinks for almost 10% of oceanic carbon burial, or 27.4 Tg C in a year (Duarte et al. 2005).

3.2 Ecological and Biological Importance: Arabian Gulf/UAE

Along the UAE coastline, seagrass ecosystems hold vital ecological and biological importance. They play a major role in purifying coastal waters, cycling nutrients and contribute to the food web structure (Hemminga and Duarte 2000). Seagrass meadows are known for carbon and nutrient sequestration. Organic carbon in seagrass sediment, known as “blue carbon,” originates from carbon sequestration ability of seagrass species. A study in 2013 suggests 52 tonnes/ha of blue carbon is stored in seagrass beds of Abu Dhabi and due to its large extent the overall blue carbon stock is estimated to be highest in the area out of all other ecosystems (algal mats and mangroves) (Skaalvik et al. 2013) (Fig. 9.4).

Fig. 9.4
4 photographs exhibit colorful coral reefs with a variety of corals, fish, and other marine life. The corals are in different shapes and sizes, and they range in various colors.

(a and b) Exposed root system of a seagrass meadow intermixed with algae. (c) Halodule uninervis monospecific seagrass meadow with some dispersed algae near Marawah Marine Biosphere Reserve in Western Region Abu Dhabi. (d) Intermixed seagrass meadow. Photos: Noura Al-Mansoori (a–c), Himansu Das (d)

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Seagrasses of the UAE is biologically critical as it supports at least 3000 dugongs and over 4000 green sea turtles (Das et al. 2021). They serve as important breeding and foraging grounds for the endangered green turtle (Chelonia mydas), and to the world’s second largest population of the vulnerable dugong (Dugong dugon) (Fig. 9.5) (Sheppard et al. 2010). Since dugongs and adult green sea turtles are herbivorous, seagrass is the staple food for these species. In addition, seagrass meadows are important fish nursery sites. Knowing the resilience of seagrasses to climate change, studies to estimate carbon sequestration capacity of seagrass and other coastal communities such as mangroves and saltmarshes have been undertaken by various research agencies (Elkabbany 2019; Skaalvik et al. 2013).

Fig. 9.5
An aerial view photograph depicts fish floating in the water.

Aerial view of a herd of Dugongs ((Dugong dugon) in the Western Region grazing on a seagrass meadow near Marawah Marine Biosphere Reserve, Abu Dhabi. Photo: Shamsa Al-Hameli

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Seagrass beds are a direct food source for many herbivores while also providing and indirect energy source to the detrital food web. They also provide nursery habitats for a variety of commercially important fishes, pearl oyster (Pinctada radiata), shrimp (Penaeus semisulcatus) and various other species that utilize these beds for food, shelter, and growth (Fig. 9.6).

Fig. 9.6
6 photographs exhibit a diverse variety of marine life swimming in a clear ocean. The creatures here are a seahorse, a jellyfish, and a school of fish. They swim through the seaweed present on the floor.

Commonly found associated species that utilize seagrass beds as a habitat: (a) Sea Pony (Hippocampus fuscus) (b) Juvenile fish grazing around seagarsses (c) Jellyfish (d) Goby fish (Cryptocentrus lutheri) burrowing between seagrass (e) Seagrass filefish (Acreichthys tomentosus) (f) Orange spotted trevally (Carangoides bajad). Photos: Shamsa Al-Hameli (a), Noura Al-Mansoori (b–f)

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3.3 Socio-cultural Importance

In terms of socio-cultural importance, seagrass beds have indirectly supported pre-modern communities around this region by providing food and refuge to dugongs. Historically, dugongs served as an important food source for coastal populations, as evidenced by the presence of their bones in UAE archaeological middens extending back as far as 4000 years ago (Lidour and Beech 2020). These coastal communities of the UAE consider seagrass meadows as sites for fish, dugong, and green turtles. In the past, prior to 1970 when there was no regulation for fishing marine megafauna, dugongs and green turtles were captured and meat consumed. The bones and skeletal remains of dugongs and green turtles in several coastal and offshore islands are a testimony of the activities in the past (Beech 2010).

4 Threats

Seagrasses occur in nearshore coastal waters, which are often close to human settlements. Therefore, most of the threats originate from human activities. Loss and degradation of seagrass meadows on global scales are often common to causes of seagrass impacts in the UAE and include factors such as eutrophication (Bulthuis 1983; Cambridge and McComb 1984; Neverauskas 1987) and coastal land-use modification through developmental activities such as dredging of navigational channels as well as operation of ports, harbours and industries (Kemp et al. 1983; Short and Wyllie-Echeverria 1996). Dredging and landfilling for coastal and offshore developments causes physical removal and smothering of seagrass meadows as well as deterioration of water clarity which is crucial of growth and recruitment of seagrasses (Erftemeijer and Shuail 2012; Short and Wyllie-Echeverria 1996). Mechanical damage by anchoring and propellor action of boats though common along the coast of offshore island, the threat has not been quantified. These actions not only damage seagrass physically but disturbs sediment to degrade water quality.

As the coastal marine environment of the UAE has experiened extremely rapid urbanization and industrialization since the 1970s (see Chap. 23), with seagrass beds occurring in some of the most heavily modified areas (e.g. lagoons and in shallow environments around oil concessions), pressures on these important ecosystems have been acute. Unfortunately, seagrass surveys only began in the late 1990s, and it is unknown to what extent seagrass beds were impacted by earlier development, particularly the extensive dredging of navigation channels throughout the central and western region of Abu Dhabi from the late 1960s onward, which were constructed to support the nascent oil and gas industry at Ruwais and its associated offshore platforms. Furthermore, after the discovery of oil in 1970, UAE escalated its development mostly along the coast and offshore areas for exploration of oil and gas. This increased developmental pressures resulted in low water clarity in terms of obstruction to penetration of light that may be considered as one of the most significant threat to seagrass growth and recruitment (Short and Wyllie-Echeverria 1996). Eutrophicated water supports growth of phytoplankton and marine algae in and around the seagrass meadows (Neverauskas 1987). This also increased nutrient content in the water column which accelerated macro algae growth and allowed it to dominate over seagrasses by covering the morphologically smaller seagrasses of the Arabian Gulf. Such invasion of macro algae may cause lower rate of photosynthesis (Larkum and West 1990; Walker et al. 1988). The epiphytes that grow in seagrass decreases diffusion of nutrients and gasses to seagrass thus affecting its physiology and growth which in large quantities may cause suffocation of meadows in a large scale (Fig. 9.7) (Borowitzka and Lethbridge 1989).

Fig. 9.7
2 underwater photography exhibits a floor filled with seagrass.

High nutrients and sedimentation may cause overgrowth by opportunistic epiphytic algae on seagrass beds, which may induce light stress and reduced seagrass growth. Figure (a) and (b) shows a seagrass bed near Mirfa, Abu Dhabi during June 2022, with sediment deposits and epiphytic growth on seagrasses. Photos: Noura Al-Mansoori (a), Himansu Das (b)

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5 Management and Conservation

5.1 Federal and Government Laws and Regulation

The conservation and management of seagrass meadows are in place in UAE through national and local regulations and laws, species conservation plans, and international and regional conventions and MoUs. The actions to protect seagrass meadows include research and monitoring, identification of threatening processes, development of policies and laws, and declaration of natural parks and protected areas, in addition to restoration of degraded habitats.

Due to its importance as foraging and nursery area for marine wildlife and fish, seagrass meadows have been considered as a critical habitat and protected by Law 23 and 24 (1999). Under International conventions for biological diversity, and the Abu Dhabi 2030 plan, the conservation target for seagrass habitats have been included among other key important marine habitats and associated species, which aids the protection and conservation of seagrass habitats (Mateos-Molina et al. 2021). All developmental project along the coast of Abu Dhabi goes through EIA (Environmental Impact Assessment) to limit impacts to these ecosystems. Five marine protected areas in Abu Dhabi encompass extensive seagrass meadows that are known to support diverse fish assemblages, as well as marine mammals and reptiles. Marawah Marine Biosphere Reserve protects 30% of the UAE seagrasses and contains the largest herd of dugongs after Al Yassat Marine Protected Area (Mateos-Molina et al. 2021).

As part of conservation, (a) regular assessment and mapping of seagrass meadows is crucial to take management actions, (b) mitigating threats, (c) taking actions to improve water clarity and quality, and (d) restoring seagrass in lost and degraded habitats are few of the initiatives that are ongoing or being planned. The value of seagrass for the UAE in terms of supporting marine wildlife and fisheries and rate of carbon sequestration has been realised. Blue carbon assessment program (AGEDI 2013, 2016) includes seagrass beds, along with related ecosystems such as mangroves, algal mat and salt marshes.

Human activities along the coast have sometimes resulted in loss and degradation of seagrass meadows. Keeping ecosystem in mind, and to achieve targets of Abu Dhabi 2030 for protection of coastal communities, seagrass restoration is the applied aspect of recovery of seagrass. However, seagrass restoration by sexual as well as asexual means are not cost-effective. Success of a restoration program depends on several factors (Orth and Moore 1988) such as, (a) site selection (depth, water quality, clarity, and circulation), (b) selection of species (species that has better chance of survival). (c) standard operating protocol for plantation (spacing, patch location). The biggest challenge to the success of the UAE’s seagrass restoration is its distribution. Since most of the seagrasses in the UAE grow in sub-tidal region—unlike Southeast Asian countries where seagrasses grow in an intertidal area—planting and monitoring a patch underwater is challenging.

5.2 Assessment of Seagrass Meadows of UAE

DPSIR (Driver – Pressure – State – Impact – Response) assessment is a useful tool to asses and investigate the cause-effect within an ecosystem to aid better frameworks for stakeholders, policy makers and governance managers to draft response measures. International Union for Conservation of Nature’s (IUCN) Red List of Threatened Species classifies the dugong as ‘Vulnerable’ and green turtles as “Endangered”, indicating that there is a high risk of extinction in the wild in the medium term. The dugong’s and green turtle’s life cycle and their reliance on seagrass for food make it highly vulnerable to threats. Seagrasses are restricted to shallow, coastal waters where the seabed receives enough light for photosynthesis to occur. These areas are also subject to high levels of human activities, which can have short- and long-term impacts on seagrass ecosystems. The DPSIR analysis indicates a summary and overall situation of seagrass meadows in UAE (Table 9.4).

Table 9.4 DPSIR (Driver – Pressure – State – Impact – Response)—summary assessment for Seagrass meadows of UAE
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6 Conclusions

Baseline information for seagrass ecosystems in terms of their extent, species composition, biomass and water and sediment characteristics is available for the UAE waters. The gap in research and knowledge includes phenology, seasonality of distribution, interaction of flora and fauna, impact of frugivory, ecosystem values and economic evaluation. To conserve seagrass meadows, we need to respond and manage several challenges such as (a) understanding the importance of value of seagrass, (b) improved knowledge on seagrass ecology and their resilience to climate change, (c) identifying and mitigating threatening processes, (d) developing and implementing conservation actions including restoration of lost and degraded areas. Many of these responses requires multidisciplinary approach, regional and international collaborations as well as stakeholder involvement. Seagrass research, conservation and communication in the UAE may continue to be a priority as part of coastal and marine conservation strategies.

7 Recommended Readings

For those interested in learning more about the seagrass systems, we recommend World Atlas of Seagrasses (2003) and for a more in depth scientific reference Global Seagrass Research Methods (2001) and Seagrasses: Biology, Ecology and Conservation (2006). We also recommend visiting Seagrasswatch.org to learn about the global seagrass monitoring efforts by scientists and citizens.