Detritus in the epilithic algal matrix and its use by coral reef fishes

Abstract The epilithic algal matrix (EAM) is a ubiquitous component of coral reefs and is the primary grazing surface for many reef fishes. Detritus accounts for at least 10% to 78% of all the organic matter present in the EAM, variation being attributed to hydrodynamic forces such as wave energy and biological elements such as algal morphology. When compared with filamentous algae, the other major source of organic matter in the EAM, protein : energy ratios, C :N ratios and total hydrolysable amino acids all suggest that detritus is of higher nutritional value than the algae. Lipid biomarkers indicate that more than 70% of the detritus is derived from the filamentous algae but the addition of bacteria and microalgae add essential nutrients and improve the nutritional value of the detritus. The detritus is typically of an amorphic form with protein : energy ratios which indicate that it is capable of sustaining fish growth. Detritus within the EAM may be derived from dissolved organic matter, which reduces refractory material, enhancing the palatability and digestibility of detritus relative to filamentous algae. Detritus in the EAM may also come from settling material and fish faeces.