The budgets of nitrogen and phosphorus in shallow eutrophic Lake Võrtsjärv (Estonia)

The nutrient budget, phytoplankton primary productionand sedimentation rate were studied weekly in the large(270 km2) and shallow (mean depth 2.8 m)eutrophic Lake Võrtsjärv in 1995. The annualexternal loading was 7.7 g m™2 y™1 of totalnitrogen (TN) and 0.2 g m™2 y™1 of totalphosphorus (TP), including 7% of both inputs asdirect atmospheric precipitation. The external budgetrevealed a retention of 53% of TN and 28% of TPannual input. About 80% of the total loss of nitrogenwas accounted for by denitrification (3.3 g m™2 y™1) and only 20% was buried into the sediment.Wind-induced sediment resuspension played the majorrole in the upward nutrient flux formation and, thus,in the formation of the temporal pattern of nutrientconcentration during the ice-free season. Other fluxesas the external loading or new sedimentation ofautochthonous production were overcome and masked bythe powerful resuspension – sedimentation cycle,exceeding the former by one or two orders ofmagnitude. The intensity of upward flux of nutrients(mainly caused by resuspension) increased inaccordance to decreasing water level in autumn andcorrelated with the weekly average wind speed. Thesummer population of filamentous blue-greens dominatedby Limnothrix redekei was light-limited untilthe minima of TN:TP ratio (<10) and DIN:TN ratio(∼ 0) in July initiated a clear peak of N_2-fixingalgae (Aphanizomenon gracile, Anabaena spp.)which lasted until September.