DETRITAL PATHWAYS IN A CORAL-REEF LAGOON .2. DETRITUS DEPOSITION, BENTHIC MICROBIAL BIOMASS AND PRODUCTION

Coral reef lagoons have generally been regarded as sinks for organic matter exported from more productive reef front and reef flat zones. The object of this study was to examine the importance of detritus as a carbon source for benthic communities in the lagoon at Davies Reef, central Great Barrier Reef. We report the results of seasonal measurements, taken in 1986, of bacterial numbers and production, protozoan numbers, community primary production and respiration in the sediments of Davies Reef lagoon. Deposition rates of organic matter in the lagoon were also measured. Deposition rates (+/- 1 SE) of carbon ranged from 9.2 (+/- 1.5) to 140.7 (+/- 10.3) mg Cm-2 d-1. Deposition rates were highest in winter and spring, lowest in summer. Rates of bacterial production ranged from 4.7 (+/- 0.2) pmol thymidine in-corporated g-1 dry wt (DW) h-1 in winter to 23.5 (+/- 1.0) pmol thymidine incorporated g-1 DW h-1 in spring. The number of ciliates ranged from 65 (+/- 10) to 356 (+/- 50) cm-3 through the year and the number of large (greater-than-or-equal-20-mu-m) flagellates from 38 (+/- 7) to 108 (+/- 16) There were no clear relationships between the sediment organic content, detrital input or temperature and the rates of bacterial processes, community metabolism or the standing stocks of microbes in the lagoon. The relative significance of detritus and in situ primary production as sources of carbon in the lagoon varied with season. In summer and autumn, detritus was less important than primary production as a source of carbon (4 to 27% of total carbon input). In winter and spring, detritus input became more significant in supply of carbon to the sediments (32 to 67% of the total carbon input). The lagoon does not simply act as a sink for carbon exported from the reef flat. We calculate that only 5% of the net reef flat primary production reached lagoon sediments in summer, but nearly 40% in winter.