The seasonal time course of phytoplankton primary productivity was studied weekly in a hypertrophic, gravel-pit lake close to Madrid, Spain. Chlorophyll a ranged 22-445 mg m-2. Gross primary productivity attained 0.28 +/- 0.14 g C m-2 h-1 (range: 0.06-0.60), its yearly value being approximately 900 g C m-2, but the shallow euphotic depths and the high plankton respiration ensured that net productivity was generally low. Respiration losses amounted to 0.31 +/- 0.24 g O2 m-2 h-1, with phytoplankton respiration roughly attaining one-half of overall plankton respiration. Areal phytoplankton productivity and plankton respiration followed a seasonal trend but this was not the case for photosynthetic capacity. Surface photoinhibition was evenly distributed throughout the study. Quantum yields showed an increasing depth trend, but no seasonal trend. Both P(max) and I(k) were both temperature- and irradiance-dependent. As compared with lakes of lesser trophic degree, phytoplankton primary production in hypertrophic lakes might be increased not only by higher nutrient contents but also by low chlorophyll-specific attenuation coefficients and low background, non-algal attenuation, thereby allowing for higher areal chlorophyll contents and hence higher areal productivity. Our study suggests that physical (irradiance and water column stability) as well as chemical features (dissolved inorganic carbon and soluble reactive phosphorus) may control seasonality of phytoplankton primary production in this lake despite recent claims that only physical factors are of significance in hypertrophic lakes. However, this does not explain all the variability observed and so a food web control is also likely to be operating.
