SEASONAL VARIATION IN THE CONTRIBUTION OF PHYTOPLANKTON, BACTERIA, DETRITUS AND INORGANIC NUTRIENTS TO A ROCKY SHORE ECOSYSTEM

SUMMARY

The seasonal variation in stocks of detritus, bacteria and phytoplankton and concentrations of inorganic nutrients in solution have been monitored on an intertidal and subtidal rocky reef at Dalebrook in False Bay. The results are compared with those from other inshore environments and their significance is discussed in relation to the large filter-feeding community on the reef. Concentrations of nitrate and silicate were lowest in summer. Phytoplankton standing stocks showed a similar trend; between October and March chlorophyll α concentrations were generally lower than 2 µg.ℓ-⁻¹. Large numbers of bacteria were encountered, the majority of which were free-living, rod-shaped cells; only 10 per cent of the cells were found attached to particulate matter. Numbers of bacteria were highest in summer. Despite the calmer seas in winter, the amount of particulate matter in suspension was slightly higher than in summer. The organic fraction, with an annual mean of 1,5mg.ℓ-¹, comprised approximately 39 per cent of the total. Detritus was the largest source of organic material on the reef, averaging 1,2 mg.ℓ-¹. Much of this detritus is thought to have originated from the large seaweed community growing along the coast, although microscopic examination showed that most of this material was in an advanced state of decomposition and there was little fresh, easily identifiable plant material. In view of the small phytoplankton standing stocks in the vicinity of the reef it appears to be detritus and associated bacteria which support the large biomass of filter feeders.