The role of sediments in epilithic algal communities on coral reefs

Sediments comprise a mobile, physical component of all reef habitats. Forereef sediments can become trapped within epilithic algal communities (EACs) of hard reef substrata, however, the mechanisms of this process and the role of sediments in modifying EACs is poorly known. Sediment loads on reef substrata can vary naturally and increase from human-induced sedimentation. It is important to understand effects of sediments on EACs, as the EAC plays a significant role in primary production for reef ecosystems. A pump-operated sampling device was developed to quantitatively collect the mobile sediments that accumulate within EACs of hard substrata. Using this method, sediments were collected from EAC-covered substrata at Lizard Island, in the northern Great Barrier Reef (GBR), Australia. Replicate sediment samples were collected from four forereef habitat zones; the reef base, crest, fore-flat and mid-flat. Sediment distribution followed a predicted pattern, from hydrodynamic forces, of low loads on the crest and higher loads at the reef base and with leeward distance from the crest. The grain-size composition of sediments was even on the crest, but became sorted with leeward distance. The areas covered by algal forms (crustose, globular, turfing and macroalgae) and the mean heights of EACs from which sediments were collected were measured. Sediment distribution was strongly, positively correlated with the heights of EACs among and within habitat zones. Sediment load was also correlated, but to a lesser extent, to the area covered by algal forms within the sampling units; turfing and crustose forms were related to higher and lower sediment accumulation respectively. Spatial variations in sediment loads were still significant when significant effects of EAC heights were accounted for in an ANCOVA, showing that other factors (e.g. hydrodynamic forces) significantly modify sediment distribution on these substrata. New procedures for analysis of total organic carbon, nitrogen and phosphorus were developed and tested to alleviate problems of decarbonation of carbonates mixed with reef material. Total organic carbon, nitrogen and phosphorus were determined for both detritus (removed from EACs with sediment) and EACs (collected after sediment and detritus were removed). The biomass of EACs and detritus was variable at all spatial scales examined and both were positively correlated to sediment load. Algal biomass was significantly higher than that of detritus closer to the crest, while detrital biomass tended to be higher at the reef base. Although crest detritus was consistently low in terms of biomass per unit of substratum, it contributes a much greater fraction of the particulate mass when compared with the other zones. A similar pattern for algae (with high biomass:sediment load on the crest) suggests that crests on this reef should be favoured by sediment-avoiding herbivores and detritivores. The C:N ratios of algal tissue and detritus, as a predictor of food quality, were variable within but not among habitats. Phosphorus content of algae and detritus tended to increase with leeward distance from the crest, which supports the possibility of transfer of phosphate between algae and detritus and/or reef sediments. A trend of higher nitrogen and phosphorus content of detritus compared to algae suggests that detritus is of higher nutritional value on forereefs than algal tissue. A month-long field experiment examined the effects of cleared sediments on the growth and nutritive quality of EACs on coral tiles while partitioning the effects of herbivory with exclusion cages. EACs in the open were grazed to low biomass in all three sediment treatments. The final biomass of EACs within cages (as a measure of net algal growth), however, was significantly lower with increased sediment load. The magnitude of this effect depended on the site. Experiments using different types of cage controls supported these findings. C:N ratios of algal tissue were lower with high sediment loads, however, this appeared to be due to lower rates of carbon fixation. Nocturnal invertebrates were more abundant within EACs with lower sediment loads in open and caged treatments, indicating that sediments influenced their foraging preferences more than algal biomass or quality. A model involving interactions among sediments, EACs and herbivores is proposed, and explains the contradictory information on the effects of sediments on algal growth from observational and experimental work. In view of the findings, sediment loading is expected to have deleterious effects on algal-based food chains, but sediments may provide a local refuge from herbivory which permits high algal standing stocks on reefs