Relative significance of phytoplankton, bacteria and plant detritus as carbon and nitrogen resources for the kelp bed filter-feeder Choromytilus mendionalis

Abstract

Standing stocks of phytoplankton on the West Coast of the Cape Peninsula, South Africa, showed marked short-term variations, reaching maximal values of 8.9 .mu.g chlorophyll a l-1 at the end of a downwelling phase when warm surface water of approximately 16.degree. C impinges on the coast. Following a period of south-east winds, cold water of .apprx. 11.degree. C or less upwells in the nearshore waters and chlorophyll a concentrations may fall within 24 h to 0.6 .mu.g l-1 or less. At the same time, the standing stocks of bacteria ranged from 0.4 .times. 106 cells ml-1 during strong upwelling pulses to 5.9 .times. 106 cells ml-1 during downwelling phases. Total resource availability in the water column is generally in the range 307-450 .mu.g C l-1 with an associated N pool of 46-71.1 .mu.g N l-1 during upwelling and downwelling phases. At the end of a downwelling phase the C represented by the phytoplankton component amounts to > 50% of the total particulate C in the water column and may dominate the resource as a whole. Bacteria, however, represent a C resource which is of comparable importance to the detrital component especially during periods of weak upwelling. Because of their relatively low C:N ratio of 3.5 compared with values as high as 18.9 for the detrital component, bacteria represent a N resource which may even exceed that of the phytoplankton under some conditions. Estimates of the C consumption requirements of the black mussel C. meridionalis Krauss in relation to the potential yield from phytoplankton, bacterial and detrital pools suggest that a major utilization of the detrital component would be required to meet the C demands even if phytoplankton were retained with a high efficiency. In contrast, the estimated N demands of 2.8 to 45.5 .mu.g N h-1 at 12.degree. C, depending on shell length, could be met from the phytoplankton alone which could supply 12.5-127.4 .mu.g N h-1 at 100% efficiency. C. meridionalis does not therefore appear to require utilization of bacteria to support its nutritional needs, a conclusion which is substantiated by the experimental work of others which suggests that particles in the bacterial size range are inefficiently retained by this mussel. Phytoplankton and detrital pools are thus of importance as a C resource for bivalve consumers such as C. meridionalis. Bacteria, however, represent a N pool of comparable importance to the phytoplankton and which is likely to be of major significance to those filter-feeders which are capable of exploiting particles in the bacterial size range.