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

The nutrient budget, phytoplankton primary production and sedimentation rate were studied weekly in the large (270 km2) and shallow (mean depth 2.8 m) eutrophic Lake Vortsjary in 1995. The annual external loading was 7.7 g m−2 y−1 of total nitrogen (TN) and 0.2 g m−2 y−1 of total phosphorus (TP), including 7% of both inputs as direct atmospheric precipitation. The external budget revealed a retention of 53% of TN and 28% of TP annual input. About 80% of the total loss of nitrogen was 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 major role in the upward nutrient flux formation and, thus, in the formation of the temporal pattern of nutrient concentration during the ice-free season. Other fluxes as the external loading or new sedimentation of autochthonous production were overcome and masked by the powerful resuspension — sedimentation cycle, exceeding the former by one or two orders of magnitude. The intensity of upward flux of nutrients (mainly caused by resuspension) increased in accordance to decreasing water level in autumn and correlated with the weekly average wind speed. The summer population of filamentous blue-greens dominated by Limnothrix redekei was light-limited until the minima of TN:TP ratio (< 10) and DIN:TN ratio (~ 0) in July initiated a clear peak of N2-fixing algae (Aphanizomenon gracile, Anabaena spp.) which lasted until September.