Nitrogen dynamics in the steeply stratified, temperate Lake Verevi, Estonia

The dynamics of different nitrogen compounds and nitrification in diverse habitats of a stratified Lake Verevi (Estonia) was investigated in 2000-2001. Also planktonic N-2-fixation (N(2)fix) was measured in August of the observed years. The nitrogen that accumulated in the hypolimnion was trapped in the nonmixed layer during most of the vegetation period causing a concentration of an order of magnitude higher 3 than in the epilimmon. The ammonium level remained low in the epilimnion (maximum 577 mgN m(-3) average 115 mgN m(-3)) in spite of high concentrations in the hypolimnion (maximum 12223 mgN m(-3), average 4807 mgN m(-3)). The concentrations of NO2- and NO3- remained on a low level both in the 3 epilimnion (average 0.94 and 9.09 mgN m(-3), respectively) and hypolimnion (average 0.47 and 5.05 mgN m(-3), respectively). N2fix and nitrification ranged from 0.30 to 2.80 mgN m(-3) day(-1) and 6.0 to 107 mgN m(-3) day(-1), respectively; the most intensive processes occurred in 07.08.00 at depths of 2 and 5 in, accordingly. The role of N(2)fix in the total nitrogen budget of Lake Verevi (in August 2000 and 2001) was negligible while episodically in the nitrogen-depleted epilimnion the N(2)fix could substantially contribute to the pool of mineral nitrogen. Nitrification was unable to influence nitrogen dynamics in the epilimnion while some temporary coupling with ammonium dynamics in the hypolimnion was documented.