The dynamics of phytoplankton populations, including a red-tide bloom, during a quiescent period in St Helena Bay, South Africa

Abstract

Phytoplankton species distribution and production in relation to hydrology were examined in St Helena Bay in the southern Benguela system during December 1987. The presence of a red-tide bloom of the dinoflagellate Ceratium furca provided the opportunity to examine the conditions required for the development of such an event. Hydrographic parameters indicated the intrusion of warm oceanic water adjacent to St Helena Bay whereas, within the Bay, aged upwelled water was evident following relaxation of upwelling and stabilization of the water column. C. furca was recorded in surface waters at a concentration of 1,3 × 106 cells·ℓ−1 and productivity estimates exceeded 500 mgC·m−3·h−1. Productivity indices were lowest within the red-tide bloom, indicating that the dominance of C. furca cannot be explained by rapid growth. The bloom appears rather to represent the accumulation of slow-growing ungrazed populations subject to concentration by physical entrainment and vertical migration. Phytoplankton succession is less likely to be disrupted in St Helena Bay because water masses are retained there for longer periods than outside the Bay. The prevalence of late successional forms such as dinoflagellates is therefore more likely within the Bay.