Physical aspects of coral reef lagoon sediments in relation to detritus processing and primary production

Abstract

Physico-chemical analysis of sediments showed there to be 4 main sediment types in One Tree Lagoon (Southern Great Barrier Reef) ranging from coarse sand to very fine sand. These sediments showed characteristic depth profile of dissolved oxygen and Eh. Oxygen was depleted within the top 5 to 10 mm of the sediment and redox values were correspondingly low. These values showed seasonal differences. The sediments were generally oxidized in winter (Eh values .gtoreq. -38 mV) but reduced in summer (Eh .ltoreq. -100 mV). Sediment concentrations of total organic carbon (TOC) and total nitrogen (TN) were typically 0.28 and 0.06% respectively with no significant difference between seasons. A significant difference was observed, however, for TOC levels between sediment types with highest concentrations observed in the very fine sand (0.29 .+-. 0.005%). Mean values for community primary production (P) and net community production (Pn) over all seasons were higher in the coarse sand (P = 0.44 gC m-2 d-1; Pn = 0.05 gC m-2 d-1) than in the very fine sand (P = 0.26 gC m-2 d-1; Pn = -0.003 gC m-2 d-1). The coarse sand had a P/R ratio .gtoreq. 1 whilst the very fine sand had P/R ratios of .gtoreq. 1 summer but .ltoreq. 1 in winter. Community respiration values (R) were higher in winter than in summer (R = 0.29 .+-. 0.03 and 0.24 .+-. 0.02 gC m-2 d-1 respectively). Bacterial abundance in sediments was 2 to 3 times higher in winter in both the very fine and coarse sediment types. Sediment community production was low compared to detrital input and any net accumulation of C and N in sediments is likely to be low, this production is probably consumed by sediment grazers.