THE BUDGETS OF NITROGEN AND PHOSPHORUS IN SHALLOW EUTROPHIC LAKE VORTSJARV (ESTONIA)

The nutrient budget, phytoplankton primary production and sedimentatio n rate were studied weekly in the large (270 km(2)) and shallow (mean depth 2.8 m) eutrophic Lake Vortsjarv in 1995. The annual external loa ding was 7.7 gm(-2) y(-1) of total nitrogen (TN) and 0.2 gm(-2) y(-1) of total phosphorus (TP), including 7% of both inputs as direct atmosp heric precipitation. The external budget revealed a retention of 53% o f TN and 28% of TP annual input. About 80% of the total loss of nitrog en was accounted for by denitrification (3.3 gm(-2) y(-1)) and only 20 % was buried into the sediment. Wind-induced sediment resuspension pla yed the major role in the upward nutrient flux formation and, thus, in the formation of the temporal pattern of nutrient concentration durin g the ice-free season. Other fluxes as the external loading or new sed imentation 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 nutrie nts (mainly caused by resuspension) increased in accordance to decreas ing water level in autumn and correlated with the weekly average wind speed. The summer population of filamentous blue-greens dominated by L imnothrix redekei was light-limited until the minima of TN:TP ratio (< 10) and DIN:TN ratio (similar to 0) in July initiated a clear peak of N-2-fixing algae (Aphanizomenon gracile, Anabaena spp.) which lasted u ntil September.