Size-fractionated phytoplankton production in western Agulhas Bank continental shelf waters

Abstract

Carbon-14 measurements of community and size-fractionated production during a mid-winter, spring and three summer research cruises in stratified continental shelf waters of the western Agulhas Bank (South Africa) are presented. Auxilliary measurements included chlorophyll a, nitrate concentrations and thermal profiling of the water column. During the winter and spring surveys, the mixed layer extended deeper than the euphotic zone, with nitrate and phytoplankton biomass being evenly distributed through the euphotic zone. Production maxima occurred at the 40–90% surface irradiance light depths, with some evidence of surface photoinhibition. All surveys during summer months (December–January) showed a thermally stratified euphotic zone ( Z eu0.5%/ Z m= 1.1–2.2), with a well defined nitracline within the thermocline. A subsurface chlorophyll maximum (0.5–6 μg l −1), situated at the 3–10% surface irradiance light depth and closely associated with the region of maximum nitrate gradient, was typical of summer measurements. The primary production maximum was located at the depth of the subsurface chlorophyll maximum only on limited occasions, primarily when conditions of a deep thermocline (>35 m) prevailed. During surveys when a shallow (<30 m) thermocline was observed, the primary production maximum occurred frequently at the 90 and 100% surface irradiance intensities. Average integrated production values ranged between 95 and 289 mg C m −2 h −1. Minimum areal production rates were observed during winter (July). Maximum euphotic zone production was measured during the spring survey in western Agulhas Bank waters (347 mg C m −2 h −1). The <15 μm size-fraction accounted for 62–97% of total primary production during mid-winter and one summer survey. During the spring survey with stabilization of a winter mixed euphotic zone, and two summer surveys where a shallow (<35 m) thermocline persisted, production was predominantly by the netplankton >15 μm size-fraction (60–87%). Periods of enhanced netplankton growth may be crucial for the sustaining of the large biomass of spawning anchovy present in western Agulhas Bank waters. Vertical profiles, the subsurface chlorophyll and production maximum and size-fractionated production measurements are discussed with relevance to the control of phytoplankton production and potential trophic fluxes in Agulhas Bank waters.