Depth limit for reef building corals in the Au’au Channel, S.E. Hawaii
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- Grigg, R.W. Coral Reefs (2006) 25: 77. doi:10.1007/s00338-005-0073-6
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In this paper, the relationship between reef building (accretion) and depth in an optimal inter-island channel environment in Hawaii is analyzed. For accretion, the growth rate of Porites lobata is used as a proxy for the reef community, because it is the most abundant and dominant species of reef building coral in Hawaii. Optimal growth of P. lobata occurs at a depth of 6 m, below which both growth rate and abundance decrease with increasing depth. A lower depth limit for this species is found at about 80–100 m, yet reef accretion ceases at ~50 m depth. Below 50 m, rates of bio-erosion of colony holdfasts equal or exceed the growth of basal attachments, causing colonies to detach from the bottom. Continued bio-erosion further erodes and dislodges colonies leading to their breakdown and ultimately to the formation of coralline rubble and sand. Thus, within this channel environment in Hawaii, a threshold for reef building exists at ~ 50 m depth, where coral accretion is interrupted by bio-erosion. Conceptually viewed, this depth horizon is analogous to a vertical Darwin Point, although quite narrow in space and time. More importantly, it explains the history of reef morphology in the Au’au Channel where a chronological hiatus exists at a depth near 50 m. This hiatus separates shallower modern growth (about 100 years or less) from the deeper reef which is all due to accretion during the early Holocene or Pleistocene epochs.
KeywordsHawaiiDepth limitCoral growthReef accretionDrowningDarwin Point
It is well known that the growth of coral reefs is light limited and rapidly attenuates with increasing depth (Buddemeier et al.1974; Baker and Weber 1975; Dustan 1975; Chalker 1983; Barnes and Chalker 1990; Kleypas et al. 1999). This, of course, is a result of the relationship between calcification and symbiotic zooxanthellae which can be several times faster in the light versus the dark (Barnes and Chalker 1990). In fact, corals were originally described as plant–animal associations; lithophytes or zoophytes (Quoy and Gaimard 1825). Clearly, any factor that potentially affects light on the reef, such as turbidity, would also be expected to affect the growth of reef building corals.
Temperature may also limit the growth of corals with increasing depth. Stratification in some tropical latitudes may produce a shallow thermocline that sets a depth limit for corals. For example, at the northwestern end of the Hawaiian Archipelago, a shallow thermocline develops during summer months and limits reef growth to depths of 20m or less (Grigg 1981). Upwelling in some settings may also inhibit reef development. In contrast to these examples, at the southeastern end of the Hawaiian chain, temperature does not appear to exert an effect on the profile of vertical growth of reef corals. This is because the mixed layer there normally exceeds 100 m which is below the depth limit set by light for most corals. The same argument would apply to the carbonate saturation state because a depth gradient for this factor does not exist within the mixed layer (Kleypas et al. 1999). In terms of reef growth, the Au’au Channel off Maui at the southeastern end of the Hawaiian chain exemplifies an environment where optimal conditions for coral growth exist (Grigg 1982).
The question addressed in this paper deals with the relationship between coral colony potential growth and reef accretion (Chave et al. 1972). Is the depth limit for coral reef accretion (reef building), the same as the depth limit for coral colony growth? In other words, do coral reefs accrete as deep as individual reef building corals can grow?
Materials and methods
In this study, Porites lobata was selected as representative of the total reef community as it is the most abundant and dominant species of reef building coral in Hawaii (Grigg 1982). P. lobata is also considered representative of other Hawaiian corals, because its growth approximates the growth in mean solid radius of corals of substantially different growth form, i.e., branching and encrusting species (Maragos 1972) and because it is intermediate in growth rate (Buddemeier et al. 1974).
Dominant taxa responsible for bio-erosion were identified and described in terms of their abundance and the degree to which they led to the destruction of holdfasts. Bio-erosion was measured by deduction. At depths below about 50 m, it was found that colonies failed to remain attached to the substratum. It was therefore reasoned that the rate of bio-erosion of colony holdfasts was equal to or exceeded the radial growth rate at this depth. At 50 m, vertical and horizontal (radial) growth at the colony base are nearly equivalent. In other words, the rate of bio-erosion on the undersurface of colonies was assumed to have exceeded the rate of linear accretion at a size and depth when the holdfast was no longer attached to the substratum. While qualitative and approximate, this method proved to be a direct measure of the depth limit of reef accretion. Colonies that do not remain fixed to the substratum do not contribute to the reef building process. In the Au’au Channel, colonies below about 50 m are unattached and gradually break down into sand due to further biological and physical erosion. The sand is ultimately washed out of the channel by strong bottom currents onto the deeper shelves that surround the islands.
Samples of carbonate substratum and 14C ages from Station 6 off Lahaina, Maui
Calcite age (years before present)
P. lobata skeleton
Clean coral skeleton
Clean coral skeleton
P. lobata skeleton
P. lobata skeleton
P. lobata skeleton
P. lobata skeleton
P. lobata skeleton
P. lobata skeleton
The growth of P. lobata in the Au’au Channel off Lahaina, Maui (Fig. 1) can be considered optimal relative to other locales in the Hawaiian Islands for the following reasons. Water quality is optimal, because the Au’au Channel is flushed by tidal currents that transport pure oceanic water between the islands of Maui and Lanai on a semi-diurnal basis. Run-off from the land is minimal and infrequent and rarely diminishes water quality in the channel habitat. Light transmission is therefore optimal. At 80–100 m where P. lobata drops out, percent surface irradiance is about 5% (Fig. 3), approximating the compensation point of this species.
Being near the southern-most latitude in the archipelago, temperature is also non-limiting (Fig. 4). From the surface to 90 m, the annual temperature range is approximately 28–24°C, well within optimal temperature limits for this species (Kleypas et al. 1999). The location of stations running offshore from Lahaina, Maui can also be considered optimal for coral growth because the entire area is well sheltered from both summer and winter long period swell.
Mean linear extension rates of P. lobata at six depths in the Au’au Channel, Maui, Hawaii
Number of colonies
Cumulative annual growth bands (years)
Mean linear extension (mm years-1)
Standard error (± mm years-1)
During the submersible dive series, no colonies of P. lobata larger than ~15 by 30 cm were observed fixed to the substratum at depths deeper than 50 m, e.g., all were unattached. In all cases, colonies of P. lobata at depths below 50 m were platy in growth form, being broader than tall (Dustan 1975) (Fig. 5). At 50 m, lateral or radial growth was approximately equal to vertical growth.
As mentioned above, the rate of bio-erosion at 50 m was measured indirectly. At 50 m in the Au’au Channel, the vertical growth rate of P. lobata was 3.02 (±0.05 SE) mm year-1(Table 2). Because permanent reef structures were not forming below this depth, the negative inverse of this rate was taken as the average rate of bio-erosion of colony holdfasts. In other words, losses due to bio-erosion at 50 m must equal or exceed gains due to the growth of P. lobata. It is reassuring that the bio-erosion rate calculated in this manner falls well within the range of estimates (0.5–4.0 mm year-1) that have been calculated elsewhere in the world, e.g., Bikini, Guam, Aldabra, Bermuda, Western Australia and the Great Barrier Reef (Davies 1983, Table I).
In summary, as colonies slowly accrete vertically at depths greater than 50 m, bio-erosion actively undermines their basal attachment sites (Fig. 5). The most active bio-eroders are clionid sponges which alone account for about 80% of the loss of calcium carbonate. The large boring bivalves, Lithophaga sp., Arca ventricosa and Rocellaria hawaiiensis also contribute significantly to weakening holdfasts of P. lobata.
Clionid sponges are endolithic borers that excavate a multitude of tiny chambers with even smaller galleries branching off the main chambers. Literally thousands of holes produced by these borers were found on the undersurface of all colonies of P.lobata collected below 50 m (Fig. 5). The thickness of clionid penetration was rarely more than 2–3 cm along the undersurface. Nevertheless, the sponges slowly etch inward eventually pinching off the holdfast. Apparently, the undersurfaces of platy colonies of P. lobata are ideal substrata for clionid sponge larvae to settle since they are not covered by living coral tissue (Wilkinson 1983). Boring clionid sponges are considered to be the most destructive bio-eroding organisms found on coral reefs globally (Wilkinson 1983; Wood 1999).
The boring bivalves Lithophaga and Arca are also commonly found within the holdfasts of P. lobata. They excavate bare oval areas up to 3 by 6 cm. The mantle glands of Lithophaga secrete acid that dissolves and weakens the surrounding limestone structure making holdfasts more prone to fracture. There appears to be no significant secondary infilling due to cementation or lithification of sediment which in this environment is swept away by vigorous bottom current. Like the clionid sponges, all of the species of boring bivalves found in P. lobata are filter feeders and are well adapted to high current environments.
Perhaps, the most unique result of this study was the discovery of a sharp hiatus in the age of samples of bare limestone substratum at a depth of about 50 m. Above 50 m, all samples carbon dated turned out to be modern, younger than about 100 years (Table 1). In sharp contrast, samples collected below 50 m, were all between 8,000 and 9,600 years old. Although these samples are few in number, they clearly demonstrate that reef accretion below 50 m during most of the Holocene has not occurred. In essence, the modern reef stops at 50 m even though the growth of individual corals may extend considerably deeper. Colonies of Leptoseris spp. which grow as deep as 110 m in the Au’au Channel are also unattached.
A decrease in the rate of growth of reef building corals as a function of increasing depth is not a new finding. Similar patterns have been described for massive corals in both the Pacific and Caribbean (Buddemeier et al. 1974; Dustan, 1975). What is new, is the limiting effect of bio-erosion which effectively decreases by about 50% the critical depth to which coral reefs can build permanent structures (accrete) at least in this Hawaiian inter-island channel environment. Reef building corals can and do grow at depths to over 100 m in this environment, yet reefs do not accrete deeper than 50 m under present day conditions. Limestone structures deeper than this represent fossil reef growth deposited during the early Holocene or the Pleistocene when the reefs were in shallower water (Grigg et al. 2002). The extent of structures below 50 m covers almost 100 sq km of bottom area extending across the entire Au’au Channel.
The depth limit to which reef building corals develop permanent reefs has long been considered a function of decreasing light (Wells 1957; Dustan 1975; Buddemeier et al 1974; Kleypas et al. 1999). This study shows reduced growth rates of P. lobata with increasing water depth which mirrors the reduced availability of PAR. Both the measured rate of growth (Fig. 2) and the calculated light availability (Fig. 4) suggest that there is still potential for coral growth down to about 100 m in the Au’au Channel. However, the results also demonstrate that bio-erosion, by many endolithic boring organisms, intervenes by setting a shallower depth limit of 50 m for reef building than would otherwise occur by light availability alone. At this threshold for reef building, net carbonate production approaches zero. This threshold is produced because growth rates are exceeded by rates of bio-erosion. In other environmental settings where bio-erosion is of less significance, the depth limit of reef building may be more a direct function of light, or even temperature in areas of upwelling or shallow stratification.
Conceptually viewed, the depth horizon at 50 m in the Au’au Channel is analogous to a vertical Darwin Point, although quite narrow in space and time. A broader zoogeographic Darwin Point exists at the northwestern end of the Hawaiian Archipelago, where islands drown because net carbonate production of the reefs there approaches zero due to reduced coral growth, bio-erosion, mechanical losses and subsidence (Grigg 1982). At the southeastern end of the Hawaiian chain, reefs drown if they fall below 50 m due to rising sea level. This threshold for drowning is set by reduced coral growth due to decreasing light, and the inability of coral colonies to permanently attach to the substratum due to bio-erosion. In less optimal environments for coral growth in Hawaii, the depth limit for reef building may even be shallower (Grigg and Epp 1989).
This research was supported by a grant from the National Sea Grant Program to the author. I would also like to thank Eric Grossman for helping to collect and analyze the samples of carbonate. Robin Lee provided vessel time at no cost to the project and provided diver support. Submersible time (five dives) in the Pisces V was awarded to the project by the Hawaii Undersea Research Laboratory. Terry Kirby and Chuck Hathoway were the pilots of the Pisces V. Eric Hochberg greatly assisted with the modeling of down-welling irradiance and the preparation of Fig. 4, Nancy Hulbirt prepared Figs. 1 and 5 of the manuscript, and Richard Dunne assisted with the preparation of Fig. 2. Eric Hochberg and Sam Kahng reviewed the manuscript and provided helpful suggestions for improvement.