Effect of river flow on estuarine microalgal biomass and distribution

Impoundments and abstraction from rivers have reduced the amount of riverwater flowing into estuaries. This has necessitated research on the riverwater requirements of these important ecosystems. A study was initiated on the structure and biomass of microalgae in the Gamtoos estuary (Eastern Cape Province, South Africa) in order to improve our capability to predict changes and determine the acceptability of reduced riverwater input. The mean annual. runoff of the Gamtoos River has been halved by major impoundments. Agriculture in the Gamtoos estuary valley contributes high concentrations of nutrients through groundwater seepage and an extensive sub-surface drainage system. This study determined the responses of microalgae to the volume of river discharge. A strong longitudinal salinity gradient was present at water flow rates ranging from 0.3 to 30.5 m(3) s(-1). There was strong vertical stratification at flows above 1 m(3) s(-1). Nitrate concentration increased from 1.9 +/- 0.2 mu M (at 0.3 m(3) s(-1)) to 83.6 +/- 13.8 mu M (at 30.5 m(3) s(-1)). Phytoplankton biomass was highest in the oligo- and mesohaline reaches of the estuary and was strongly correlated to turbidity. Phytoplankton chlorophyll a was highest at flow rates of 0.8 (47.5 +/- 4.5 mu g 1(-1)) and 1 16m(3) s(-1) (49.9 +/- 7.3 mu g 1(-1)). Subtidal and intertidal benthic chlorophyll a were highest at flow rates of 1.0 (57.7 +/- 0.4 kg) and 1.16 m(3) s(-1) (8.7 +/- 0.1 kg) respectively. A riverwater input rate of similar to 1.0 m(3) s(-1) appears optimal for phytoplankton and benthic microalgal growth. At this flow rate, the estuary acts effectively at removing excess nutrients from the wafer before it discharges to the sea. (C) 2000 Academic Press.