Conversion of Kelp Debris and Faecal Material from the Mussel Aulacomya ater by Marine Micro-Organisms

Abstract

Freshly powdered kelp debris from Laminaria pallida is characterised by a high C/N ratio of approximately 8.0, whereas material which has passed through the gut of the dominant kelp bed suspension-feeding bivalve Aulacornya ater has a reduced carbon content and a C/N ratio of 4.8. This suggests that Aulacomya ater may derive a substantial proportion, possibly amounting to as much as 40-50 % of its carbon requirements by absorption of the more labile carbon components of its diet whereas the more refractory components are returned to the environment as faeces. Both freshly- fragmented kelp debris and faecal material are colonised by bacteria which are subsequently replaced by flagellates and ciliates whose biomass commonly reaches approximately 10 % of that of the bacterial prey density. Calculation of the conversion efficiency of carbon into the carbon equivalent of bacterial biomass suggests that an efficiency of 10-15 % is achieved by the bacteria which colonise particulate debris, irrespective of the fact that the faecal material may be different in composition from that of freshly-fragmented kelp. Approximately 85-90 % of the fragmented kelp which is not consumed by the filter-feeders and the processed faecal material in the water column is mineralised within 33 d at 10 "C. The residual 10-15 % assimilated into bacteria could represent a potential food resource for the consumer community. Because of the relatively low conversion of particulate carbon into bacteria, however, it is suggested that their trophic significance as a carbon resource is likely to be small in coastal waters