Benthic infauna and organism-sediment relations in a shallow, tropical coastal area: influence of outwelled mangrove detritus and physical disturbance

Sediment infauna and sedimentary structures within the shallow ( S 15 m depth) inshore of the central Great Barrier Reef lagoon were examined seasonally for 1 yr to relate benthic fauna1 abundance, community composition and biogenic activity to outwelling of mangrove detritus and natural physical disturbance. Standing amounts of mangrove litter exceeded 4000 g dry weight (DW) m'2 in a semi-enclosed area and declined to 3.5 g DW m-' at the inshore-middle shelf boundary. Macroinfaunal densities (mean = 1452; range = 308 to 3950 ind.m-2) and biomass (ash-free dry weight = 0.17 to 6.31 g m-2) varied greatly among sites and seasons, with densities correlating negatively with detritus standing stocks; biomass did not relate to outwelling. Total meiofaunal densities (mean = 715; range = 295 to 1568 ind.10cm-*) varied significantly among sites and seasons, but only turbellarians correlated positively to standing amounts of mangrove litter. Classification and ordination of nematode communities separated stations mainly on the basis of detntal loading. Species diversity (H') and evenness (J') correlated negatively with detritus standing stocks and extractable tannins derived from the litter. X-radiographs of sediment cores revealed sedimentary facies characterized by low to moderate rates of sedimentation, large patches of buried litter and bioturbation mainly in the upper 5 to 6 cm at the muddiest sites. X-radiographs also showed that these inshore sediments are devoid of large, equilibrium species, but subjected to episodes of scouring and resuspension by large tides and climatic disturbances. The poor nutritional quality of mangrove detritus and intermittent physical disturbances appear to be the major factors preventing the establishment of equilibrium communities and perpetuating the dominance of pioneering infaunal assemblages in this shallow, tropical inshore region.