The filtration rate of Perna perna (L.) at the approximate annual mean local water temperature (20 "C) is high, being 2.7 X I O -~ (shell length in mm)'-86 1 h-' or 8.85 (dry flesh weight in 1 h-'. It was found to be able to filter latex particles down to at least 0.46 pm in diameter, these being roughly the mean size of free coccoid bacteria in the study area. Its mean assimilation efficiency, determined by the Conover method, was 61 % on a natural diet of particles < 100 p m in diameter which had a mean organic content of 3.16 mg 1-'. The faeces production rate was also established and an energy budget for P. perna is discussed in the light of available data.

The parameters measured in this study were light and dark-bottle uptake of NaHI4CO3, the chlorophyll content of seawater, direct counts of bacteria, microbial uptake of 14C glucose and a labelled algal extract, organic carbon in the seawater and the composition of debris washed onto the reef. The phytoplankton consisted largely of nannoplankton and primary production was low with an annual mean of P = 12.82 mg C n1r3h-', 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 V,,, = 0.612 @g l-'h-' and a total bacterial count of 2.02 X 106 ml-', equivalent to a dry biomass of 20.3 rng 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 labelled algal extract (V,,, -13.2-72.6 mg C m-3h-') indicates that microbial heterotrophic activity exceeds phytoplankton production. Carbon analyses gave an annual mean of 18.3 g m-3 total organic carbon, of which 20 % was particulate, the rest being dissolved organic carbon. 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. It is concluded that 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.

An attempt was made to quantify microbial heterotrophy using a labelled algal organic extract (D014C) as a tracer. The heterotrophic potential technique was used and values obtained for V, , , T and K + S in parallel experiments were 13.2-72.6 pg C l-'h-', 14.5-82.3h and 604.0-4024.7 pg C 1-' respectively for D0I4C uptake, and 0.1-2.9 pg l-'h-', 3.241.8h and 1.348.2 pg 1-' for D-[U-l4 C] glucose uptake. Greater growth yields (i.e. a lower % label respired) were measured for DOi4 C than glucose. Production figures of 34.9-780.1 mg C m-3h-' were obtained by relating the proportional uptake of tracer quantities of D 0 I 4 C to the DOC in the seawater and these were much greater than increases in biomass established from acridine orange direct counts (0.25-2.81 mg C m-3h-' cf. related DOx4 C figures of 34.9-369.5 mg C m3h-l). However, it appears that direct counts do not adequately reflect growth, and microbial assimilation of the D014 C conversely overestimates consumption of environmental DOC since this contains an unknown proportion of organics resistant to microbial degradation (K+S derived from the heterotrophic potential technique < directly measured S as DOC). On evaluation of the results, the heterotrophic potential technique applied to D014 C uptake appears to provide a realistic estimate of microbial production.

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