Microorganisms and Detritus in the Water Column of a Subtidal Reef of Natal

Abstract

The parameters measured in this study were light and dark bottle uptake of NaH14CO3, the chlorophyll content of seawater, direct counts of bacteria, microbial upake of 14C glucose and a labeled algal extract, organic C in the seawater and the composition of debris washed onto the reef. The phytoplankton consisted largely of nannoplankton and primary production (P) was low with an annual mean of P = 12.82 mg C m-3 h-1, while the mean chlorophyll a content of 2.13 mg m-3 was equivalent to a dry biomass of 170.1 mg m-3. Microbial heterotrophic activity was relatively high with an annual mean glucose assimilation of Vmax = 0.612 .mu.g l-1 h-1 and a total bacterial count of 2.02 .times. 106 ml-1, equivalent to a dry biomass of 20.3 mg m-3. The largest proportion of this activity, measured by differential filtration and using antibiotic inhibitors, is performed by free bacteria as opposed to the smaller proportion conducted by bacteria attached to particles, or by microflagellates. Uptake of the labeled algal extract (Vmax = 13.2-72.6 mg C m-3 h-1) indicates that microbial heterotrophic activity exceeds phytoplankton production. C analyses gave an annual mean of 18.3 g m-3 total organic C, of which 20% was particulate, the rest being dissolved organic C. Analysis of debris revealed that it consisted almost exclusively of seaweed, except when rivers were in flood, providing a large influx of terrestrial plant debris. These findings are compared with other published results, and their ecological significance is discussed in relation to the remainder of the environment. Primary production by phytoplankton is not as important in supporting the biomass of filter feeders on the reef as is heterotrophic activity associated with organic detritus.