Detrital pathways in a coral reef lagoon

Abstract

Coral reef lagoons have generally been regard- ed as sinks for organic matter exported from more pro- ductive reef front and reef fiat 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 bacte- rial numbers and production, protozoan numbers, com- munity primary production and respiration in the sedi- ments 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 C m- 2 d - ~. 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 -~ in winter to 23.5 (___1.0) pmol thymidine incorporated g-1 DWh-a in spring. The number of ciliates ranged from 65 (_ 10) to 356 ( + 50) cm- 3 through the year and the number of large (>20/~m) flagellates from 38 (__+7) to 108 (+ 16)cm -3. 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 rela- tive significance of detritus and in situ primary produc- tion as sources of carbon in the lagoon varied with sea- son. 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 in- put 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.