Community structure, biomass and productivity of epilithic algal communities on the GBR

Community structure, biomass and productivity of epilithic algal communities on the Great
Barrier Reef: Dynamics at Different Spatial Scales. Marine Ecology Progress Series, Sept 1992.
http://www.int-res.com/articles/meps/86/m086p077.pdf


Notes for this study:

Epilithic = Algae that is attached to rocks; also known at "benthic".
Productivity = Primary Production of food = "Primary Reduction" of nutrients.
Carbon = Biomass that is alive; is not GAC.
Turnover = How fast algae is eaten by grazers, followed by new growth.



"This study provides the first quantification of variation in the structure, biomass and productivity of the Epilithic Algal Community on a number of reefs over a wide area of the Great Barrier Reef. In addition, it describes within-reef and seasonal variability of these parameters.

The Epilithic Algal Community (EAC) of coral reefs is a diverse assemblage of crustose coralline and [fleshy] turf algae growing upon coral rock. In this case, the term 'turf' refers to the multi-specific and inconspicuous association of unicellular, and short (usually less than 1 cm high), simple filamentous algae [...].

The main goal of this study was to examine the natural (in situ) variations in community structure, biomass, photosynthesis-irradiance relationship, and primary productivity of the Epilithic Algal Communities on the Great Barrier Reef (GBR). Comparisons are made between seasons, successive years, and different habitats and reefs situated across and along the continental shelf. This is an extension of a study which focused on the differences between habitats and seasons within a single reef (Davies Reef, central GBR). Data on primary production of the EAC are compared with the [roughness] and nature of the reef substratum to estimate the contribution made by the EAC to whole reef productivity.

This study was carried out along 2 major [areas] designed to give a wide geographical coverage of the Great Barrier Reef. The first of these was a central GBR cross-shelf transect comprising a reef on the inner- (Pandora), mid- (Davies) and outer-shelf (Myrmidon). General environmental and structural features of these reefs are described elsewhere. The second [area] comprised a pair of reefs on the mid- and outer-shelf at 3 latitudes; McGillivray and Yonge, Davies and Myrmidon, and Heron and One Tree Islands.

Epilithic Algal Communities covered a high proportion of the reef flats (50 to 80 percent) and reef slopes (30 to 70 percent) on the coral reefs of the north, central and southern regions of the Great Barrier Reef (GBR). Crustose coralline algae and turf algae (fine and damselfish territory types) dominated the Epilithic Algal Communities in reef flat habitats, except in the near-shore region where turf algae predominated. Turfs also dominated the EAC on reef slopes. Patches of crustose coralline algae had a higher biomass [because of the calcium], but a lower photosynthetic rate [thus less nutrient reduction] than the equivalent area of fine turf algae. The net result was that these 2 main forms of epilithic algae had comparable rates of [per unit area] productivity. The [primary] productivity of turf-dominated communities was inversely correlated with algal biomass. Epilithic Algal Communities from various reef habitats at the same depth had equal [per unit area] and biomass-specific productivity, regardless of their location on [areas] extending both across and also along the Great Barrier Reef. EAC productivity changed in a predictable manner with season (maximum in summer, minimum in winter) and depth (decreasing with depth). [...] The EAC at 10 meters on reef slopes had approximately half the [per unit area] productivity of the community on the adjacent reef flat, but the EAC from these habitats had a similar biomass-specific productivity. Productivity of the EAC per unit area of reef, which takes into account the [roughness] and coverage of reefs by the EAC in particular habitats, varied between reefs, and ranged from 150 grams carbon (per square meter, per year) on the reef flat of the near-shore reef and on all reef slopes, to 500 grams carbon (per square meter, per year) on some mid- and outer-shelf reef flats. There was no apparent latitudinal pattern of change in EAC productivity per unit area of reef. Thus, availability of the EAC, the major food resource of grazers on coral reefs, appears to correlate well with known large-scale variations in grazing activity.

For a number of reasons, the Epilithic Algal Community is thought to play an extremely important role in the [feeding] dynamics of coral reefs. Firstly, a large proportion of the net primary production within specific habitats of coral reefs is provided by the EAC. This is most clearly evident for the shallow habitats, where it has been relatively easy to measure the metabolic rates of benthic [sea floor] reef communities. However, relatively little is known about the productivity of benthic plant communities on reef slopes and in deep lagoons. Epilithic [attached to rocks] algae are intensively grazed [by fish], and in the process are maintained in a state of low biomass, but rapid turnover. In the few cases where whole reef systems have been examined, the biomass of the Epilithic Algal Community is found to be considerable due to its extensive coverage of reef surfaces (up to 80 percent in some habitats), and the high [roughness] of the reef substratum. Hence, the EAC almost certainly makes a substantial contribution to the [per unit area] productivity of most reefs. Moreover, a high proportion of the carbon [food] produced by the coral reef EAC is thought to be directly available to the reef food web via herbivorous grazers. Indeed, recent studies [in 1992] of reef flats have demonstrated that grazers consume around half of the annual net production of the EAC, with the balance presumably channeled into detrital pathways in the form of [DOC] exudates and particulate matter.

Experimental plates (8 X 8 X 2 cm) cut from the coral Porites spp. were bolted directly to the reef substratum in a haphazard manner within different reef zones. [...] Plates were left in the field for 6 to 12 months to establish a 'natural' Epilithic Algal Community.

The type and irregularity of the reef surface, at the sites where algal production was measured, were surveyed at Pandora, Myrmidon, Heron and One Tree Reefs in October 1989, and at MacGillivray and Yonge Reefs in December 1989. Reef surface type, expressed as proportional coverage by sand, and 7 functional groups of biota: fine turf, damselfish-territory turf, crustose coralline algae, encrusting brown algae, macroalgae (e.g. Halimeda spp.), hard corals and other fauna (e.g. soft corals, sponges), was quantified using line transects.

The percent cover of coral plates by the 4 major functional components of the Epilithic Algal Community (fine turfs, damselfish territory turf, crustose coralline algae, and encrusting brown algae), averaged over all seasons, was similar for the 2 reef-flat zones (4 & 7) on Davies and Myrmidon Reefs. Both turfs and coralline algae were important in these habitats, but coralline algae were more abundant on reef crests (Zone 7). Fine turf dominated (57 to 67 percent cover) and coralline algae were much less abundant (20 percent cover) in the reef-slope algal community. The reef flat on Pandora on the inner-shelf differed markedly from similar habitats on the mid- and outer-shelf reefs, in that coralline algae were rare, and 87 percent of algae was damselfish territory turf. Epilithic Algal Community structure did not vary significantly with season.

The maximum net photosynthetic rate [and reduction of nutrients] of the Epilithic Algal Communities, in both [per unit area] and biomass-specific terms, varied seasonally, with the maxima in summer and the minima in winter in all zones, on all reefs.

[Primary] production of the EAC based both on area and biomass varied strongly with season for all zones and reefs of the cross-shelf and latitudinal [areas]. Productivity was highest in summer and lowest in winter.

Thus, annual production of the EAC (without taking EAC coverage, or irregularity of the reef surface into account) is 400 grams carbon (per square meter, per year) on reef flats, and 220 on reef slopes.

The EAC (turfs plus coralline algae) covered a high proportion of the surface on all mid- and outer-shelf reefs examined. [...] In general, hard corals were the other major occupant of space on these reefs (11 to 44 percent on flats; 14 to 35 percent on slopes). Although there were some significant differences between reefs in terms of the cover of particular
types of EAC, these did not suggest any latitudinal trends, nor any consistent differences between mid- and outer-shelf reefs. The one inner-shelf reef examined in this study (Pandora Reef) was distinctive in being dominated by zooanthids (61 percent), as well as its comparatively low EAC coverage. Moreover, damselfish-territory turf dominated the EAC (75 percent) on the natural surfaces of Pandora Reef, as was observed with [the experimental] coral plates from this reef, whereas in comparable habitats on mid- and outer-shelf reefs, fine turfs and coralline algae were equally important components of the EAC. Reef flats had consistently higher cover of EAC (range: 51 to 81 percent) than the adjacent reef slopes (range: 33 to 73 percent), which had extensive areas of sand. In addition, coralline algae were important on reef flats, and filamentous algae dominated the reef slopes.

The EAC covers a high proportion of reef flats (up to 80 percent) and reef slopes (up to 50 percent) throughout the GBR. However, the extent of this cover, as well as the community structure of the EAC, varies with location, such that cover of substrata by algae is greatest on reef flats in the mid- and outer-shelf region, where the community is usually dominated by a mixture of well grazed turfs and crustose coralline algae.

Large fleshy algae (e.g. Sargassum spp.), which are usually relatively rare in all habitats on the mid- and outer-shelf reefs, dominate in some seasons and habitats on inner-shelf reefs. Although large fleshy algae were not common in the study site at Pandora Reef, the adjacent windward edge of this reef has an extensive Sargassum community. The transition from an algal community, comprising mainly coralline and low turfs on reefs of the mid- and outer-shelf, to one characterized by lush turfs and large fleshy algae, correlates with a significant decrease in grazing intensity.

[Maximum per unit area] productivity occurs in summer in shallow habitats, and minimum productivity occurs in winter on the reef slope.

In shallow reef areas, EAC biomass is lowest [i.e., highest grazing and primary production regrowth] in summer, and peaks around winter, whereas at 10 meters on the reef slope, biomass is lower than on the reef flat, and it is seasonally stable. The seasonal variation in biomass of EAC on the reef flat can be explained by the seasonal changes in grazing intensity relative to productivity. While variations in productivity and grazing intensity are positively correlated, the magnitude of seasonal change in rate of grazing on algae exceeds that produced, thus resulting in the observed seasonal fluctuations in biomass. On the reef slope, presumably EAC productivity and losses, such as those due to grazing, are in balance over the year. Similarly, [previous researchers] observed that epilithic algal biomass decreased towards summer on the reef flat at One Tree Island, and that this corresponded with an increase in grazing activity. The lower biomass of the EAC on the reef slope compared with the reef flat is in part due to the difference in [algal] community structure; reef slopes are dominated by turfs (80 percent cover), which have a lower biomass per unit area than coralline algae [due to the calcium].

It is now well established [in 1992] that the EAC represents an extremely important trophic [food] resource on coral reefs through the interaction with grazers. In turn, grazers exercise a strong influence on the biomass and community structure of reef algal communities. Grazing activity, within certain limits, is also thought to stimulate productivity of epilithic algae by selecting for fast growing forms of algae [and thus the most "primary reduction" of nutrients], the removal of [dying] material, and an enhanced availability of [food]. Over an entire reef flat, it has been estimated that grazers account for [consuming] 40 to 70 percent of EAC net production, but there is considerable spatial variability in grazing intensity. For example, grazing rates are highest on the outer shallow reef crests and slopes, and lowest in leeward edges of reef flats and in lagoons. On a larger scale, the abundance of herbivorous fishes (the major grazers on the GBR), and the rate of removal of the EAC by grazers (determined by grazer exclusion experiments) is much higher on mid- and outer-shelf reefs than on reefs near-shore. These differences in grazing pressure within reefs, and between reefs located across the GBR, do not influence the rate of turnover of the epilithic algae (i.e. that established on our [experimental] coral plates) being grazed. For example, algal communities from different habitats at the same depth on Davies Reef are equally productive. Furthermore, the EAC on Pandora Reef was as productive per unit area and per unit biomass as the communities on Myrmidon and Davies Reefs.

The rate of EAC [primary] production per unit area of reef surface, or actual algal food availability (i.e. the [per unit area] production of EAC on [our experimental] coral plates, corrected for the irregularity and algal coverage of reef surfaces), does however correlate with the rate at which algae are removed by grazers, as was demonstrated in the comparison between reef-flat habitats on Davies Reef. Similarly, in considering the cross-shelf gradient, the availability of algal food resources on Davies and Myrmidon Reefs (400 to 500 grams carbon per square meter, per year) is 3 times that of Pandora Reef (160 grams). This compares with a 3- to 5-fold differential in abundance of grazers on these 2 groups of reefs. These differences in algal food availability between sites are due to differences in algal coverage of reef surfaces. [Previous researchers] estimated grazing rate at 0.1 grams carbon (per square meter, per day) on Pandora Reef, and 0.6 on the mid- and outer-shelf reefs. Hence, grazing accounts for 23 percent of the EAC net production on Pandora Reef and approximately 50 percent of the production on the reefs offshore. This is in close agreement with independent estimates of the average annual grazing impact on the EAC at Davies Reef (57 percent) and on One Tree Island (50 percent). It remains unknown, as pointed out by [a previous researcher], whether grazer abundance controls algal food availability, or vice versa."