Chlorophyll concentrations and suspended particulate loads in St Lucia, a turbid estuary on the east coast of South Africa

Abstract

Concentrations of chlorophyll a and suspended particulate concentrations were measured during three lake-wide surveys of St Lucia, a shallow, turbid estuary on the east coast of South Africa. There was no salinity gradient in the system during any of the surveys, but between the surveys there were considerable salinity differences. Summer turbidities were higher than those of winter and spring, and turbidity along the eastern edge of the system was lower than elsewhere. Chlorophyll a was present over a wide range of salinities and turbidities and was generally highest in summer. However, there was no relationship between salinity and concentrations of chlorophyll a, and the concentrations were not significantly higher along the less turbid eastern shore. Concentrations of total paniculate matter (TPM) and particulate organic matter (POM) in the < 100 μm fraction were significantly correlated with turbidity throughout the year, but chlorophyll a and POM (< 100 μm) were significantly correlated only in summer. Concentrations of TPM in the > 100 μm fraction were two orders of magnitude lower than those in the < 100 μm fraction, but the organic content of the former fraction was very much higher. There was no relationship between turbidity and TPM (> 100 μm), nor between chlorophyll a and POM (> 100 μm) concentrations. Under conditions of high TPM load and in the salinity range 2–25 × 10⁻³, phytoplankton would probably still occur in St Lucia. Estimates of phytoplankton production ranged between 218 and 252 mg C·m−2·day⁻¹. A comparison of estimates of the standing stocks of carbon from phytoplankton and suspended POM < 100 μm indicated that carbon input from sources other than phytoplankton may be important.