Long and short term variations in suspended particulate material: the influence on light available to the phytoplankton community

The magnitude and frequency of events leading to changes in turbidity have been studied in a large (61 km²), shallow (mean depth 3.4 m) wind-exposed lake basin at the western end of Lake Mälaren, Sweden. In this paper we couple changes in suspended particulate inorganic material (SPIM) resulting from wind driven sediment resuspension, and variations in the discharge and sediment load, to spectral variations in subsurface light and estimates of photosynthetically active radiation (PAR). To accomplish this we use a semi-analytical model which predicts the spectral variations in downwelling irradiance (E _d()) and the attenuation coefficient of downwelling irradiance (K _d()), as a function of the concentrations of chlorophyll, dissolved yellow substances, and suspended inorganic and organic particulate material. Unusually high river discharge, led to large inputs of yellow substances and large in lake yellow substance concentrations (a _ys(420) 20 m^–1), causing variations in yellow substance concentration to have the greatest role in influencing temporal trends in the attenuation of PAR and variations in the depth of the euphotic zone (Z _eup). In spite of this, variations in SPIM could account for approximately 60% of the variation in Z _eup attributed to changes in yellow substances alone. Our results show that changes in suspended sediment concentration leads to both long-term and short-term changes in the attenuation of PAR, even in the presence of high concentrations of dissolved yellow substances.