MEPS 223:39-48 (2001)  -  doi:10.3354/meps223039

Annual cycle of microalgal biomass in a South African temporarily-open estuary: nutrient versus light limitation

Christian Nozais1,*, Renzo Perissinotto1,*, Sarasvathi Mundree2

1School of Life and Environmental Sciences, G. Campbell Building, University of Natal, King George V Avenue, Durban 4041, South Africa
2Marine Science Unit, University of Durban-Westville, Private Bag X54001, Durban 4000, South Africa
*Corresponding author. Present address: Institut des Sciences de la Mer de Rimouski (ISMER), Université du Québec à Rimouski, 310 Allée des Ursulines, Rimouski, Québec GSL 3AI, Canada. E-mail:

ABSTRACT: The vast majority (73%) of South African estuaries are temporarily-open systems. There is virtually no information available on their trophic functioning, particularly at the lower levels of the food web. A whole-year survey was carried out to investigate variations in the biomass of phytoplankton and microphytobenthos in relation to major factors potentially controlling their dynamics in a subtropical temporarily-open estuary. Microphytobenthic chlorophyll a (chl a) concentrations ranged from 75 to 480 mg m-2, while phytoplankton biomass varied between 0.09 and 8.6 mg chl a m-3. Areal chl a concentrations in the sediment (microphytobenthos) were typically 1 to 2 orders of magnitude higher than in the water column (phytoplankton). The vertical attenuation coefficient in the water, Kd, varied between 0.94 and 28.9 m-1. Dissolved inorganic nitrogen to phosphorus molar ratios (DIN:DIP) varied between 0.3 and 127. The results suggest that nutrients rather than light might have limited phytoplankton growth during the closed phase of the estuary, since Kd values were lowest during this period. On the other hand, light rather than nutrients might have limited phytoplankton growth during the open phase, since nutrient concentrations in the water column were very low prior to the opening of the estuary at the mouth. The marked increase in phytoplankton biomass at the onset of the open phase further supports the hypothesis that phytoplankton cells might have been nutrient-limited during the closed phase. Resuspension by freshwater inflow may have also contributed to this increase. Microphytobenthic standing stock exhibited strong seasonal fluctuations, with the lowest values coinciding with the open phase. However, unlike phytoplankton, benthic microalgal biomass did not appear to be inhibited by either low nutrients or light availability.

KEY WORDS: Temporarily-open estuary · Phytoplankton · Microphytobenthos · Chlorophyll a · Nutrients · Light

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