Phytoplankton primary production in a shallow, well-mixed, hypertrophic South African lake

This paper reports on a two-year analysis of the wind climate and its effect on phytoplankton primary production in a shallow (mean depth = 1.9 m), hypertrophic South African coastal lake, Zeekoevlei. The lake is subject to continuous mixing of the euphotic zone (Z(eu) = 0.8 m), and complete mixing of the water column to the mean depth on a daily basis. Median annual wind speeds, prevailing from either the north or the south, were 6.4 m s(-1). There was an almost total absence of calms, measured as hourly mean wind speeds of <1 m s(-1). Notwithstanding the high frequency of mixing, the lake supports a dense population of phytoplankton, dominated by Cyanophyte and Chlorophyte species. Mean concentrations of chlorophyll-a were 240 mu g 1(-1). The attenuation of photosynthetically available radiation, PAR, was high, with mean K-d values of 6.4 m(-1) and water transparencies of <0.5 m. Levels of primary productivity, determined using the light and dark bottle oxygen method, were very high, comparable to or exceeding that of the most productive systems yet studied. Maximum volumetric productivity ranged from 525 to 1524 mg C m(-3) h(-1), and was confined to the upper 0.5 m of the water column. Daily areal productivity, Sigma P-d, varied between 1.2 and 4.3 g C m(-2) d(-1), and that of the maximum chlorophyll-a specific photosynthetic rate, p(max)(B), between 1.6 and 7.9 mg C (mg Chl-a)(-1) h(-1). Primary production was limited by water temperature and the attenuation of PAR. The high frequency of wind-induced mixing resulted in regular mixing of the phytoplankton through the euphotic zone, and reduced the overall importance of P-max at a single layer in the depth profile. Similarly, the regularity of mixing was recognized as a limitation of the incubation of bottle chains to determine primary production levels.