The development and decline of phytoplankton blooms in the southern benguela upwelling system. 1. drogue movements, hydrography and bloom development

Abstract

A drogue was placed in a patch of newly upwelled water on five different occasions between 1979 and 1981 in order to follow the temporal sequence of events after upwelling. The drogue moved over distances of 67 – 200 km within 4–7 days at mean speeds of 18–39 cm·s−1. Drogue movement was variable and generally in the direction of the prevailing wind. However, significant “up wind” movement occurred when the drogue was entrained in counter-currents. Variations in hydrographic parameters at the start of each sequence, and in the bottom mixed layer along the drogue tracks, indicate that upwelling source waters are not always uniform. However, nutrient concentrations in source water were high (mean concentrations for nitrate, silicate and phosphate were 20, 8, 16, 6 and 1, 88 mmoln·m−3 respectively), whereas oxygen (4, 0 dm3·m−3) and chlorophylls a (0, 6 mg·m−3) concentrations were low. Once conditions stabilized, phytoplankton blooms developed rapidly in the upper layers, the cycle of bloom development and decline being completed in about 6–8 days. Nutrients decreased rapidly (and oxygen and chlorophyll a increased) with nitrates sometimes reaching concentrations of < 1 mmol·m−3, and oxygen and chlorophyll a peaking at concentrations of up to 9, 8 dm3·m−3 and 21, 2 mg·−3 respectively. Only once did light-limitation (due to deep mixing) influence bloom development. In this case, only moderately high chlorophyll a concentrations (∼ 10 mg·m−3) were attained. Bloom decline was usually associated with low nutrient concentrations and can be attributed mainly to sinking and dispersion of the phytoplankton, because zooplankton grazing appeared to have had an insignificant impact on the phytoplankton blooms. © 1987 Taylor & Francis Group, LLC.