Carbon and nitrogen resources available to kelp bed filter feeders in an upwelling environment

Abstract

Environmental variables, and summer and winter particulate concentrations in the water column of a kelp bed in an upwelling region on the western Cape coast of South Africa, were measured daily to assess short-term fluctuations. There were significant differences between the summer and winter upwelling index, water temperature and wave height. Summer particulate carbon and nitrogen cancentrations of 533 .mu.g l-1 and 66 .mu.g l-1 respectively were lower than winter levels of 764 .mu.g C l-1 and 79 .mu.g N l-1. However, summer upwelling events did not reduce particulate carbon and nitrogen to the very low levels that might be expected from previous studies of chlorophyll concentrations in the kelp bed. Large waves resulted in C:N ratios that were significantly higher in winter than in summer (p < 0.01), and during continuous winter downwelling, a large proportion of the particulate material in the water column was of macrophyte rather than phytoplankton origin. In summer the carbon and nitrogen load was most highly correlated with temperature, while wind and wave size were the most important factors affecting particulate concentrations in winter. Mean particular organic concentrations were 380 .mu.g C l-1 and 43 .mu.g N l-1 and these compare well with concentrations of 301 .mu.g C l-1 and 41 .mu.g N l-1 estimated from previous primary production studies. The estimated carbon demands of Choromytilus meridionalis and Aulacomya ater, 2 mussels that commonly occur in kelp beds on the west coast, could be met in winter and summer by the particulate organic component in the kelp bed water column. Particulate nitrogen resources probably meet most of the nitrogen requirements of the 2 mussels.