Phytoplankton production in Agulhas bank waters (South Africa)

Master Thesis


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University of Cape Town

Carbon-14 measurements of community and size- fractionated production in continental shelf waters of the Agulhas Bank (South Africa) are presented. Auxiliary measurements included chlorophyll α, 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 - March) showed a thermally stratified euphotic zone (ZeuO.5%/Zm = 1.1 - 2.2), with a well deformed nitracline within the thermocline. A subsurface chlorophyll maximum closely associated with the region of maximum nitrate gradient (3 - 10 % surface irradiance) was typical of summer measurements. Subsurface chlorophyll α concentrations were lower in western Agulhas Bank waters (0.5- 6 μg.l⁻¹) than in eastern Agulhas Bank waters (l- 15 μg.l⁻¹). Highest subsurface chlorophyll α concentrations (15- 40 μg.l⁻¹) were measured during the late summer survey (March 1986) at the Alhard Banks station in a strongly stratified water column. For profiles showing a very well developed subsurface chlorophyll maximum, the production maximum occurred at or slightly shallower than the depth of the chlorophyll maximum. In western Agulhas Bank waters, during surveys when a shallow (<30m) thermocline persisted, production maxima at 90 and 100% surface irradiance intensities were frequently observed. Integrated production values ranged between 70 and 1390 mgC.m⁻².h⁻¹. Minimum areal production rates were observed during winter (July), whilst maximum rates were measured during late summer (March). High euphotic zone production was also measured during the spring survey in western Agulhas Bank waters (347 mgC.m⁻².h⁻¹), associated with the stabilising of the water column after winter mixing. The <15 μm size-fraction was observed to account for much of total production (62-97%) during conditions of deep mixing in winter and a deep upper mixed layer during summer. During the spring survey and those surveys where a shallow (<35m) 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.