NUTRIENT BALANCES AND PHYTOPLANKTON DYNAMICS IN 2 AGRICULTURALLY LOADED SHALLOW LAKES

The impact of agriculture was estimated on two shallow, eutrophic lake s, Lake Kotojarvi and Lake Villikkalanjarvi in southern Finland. The m ain emphasis was on phosphorus and nitrogen budgets and on the phytopl ankton dynamics. Special attention was paid to internal P loading and blue-green algal blooms. The mean Tot-P load from agricultural land wa s 1.2 kg ha-1 a-1 in both basins and Tot-N loads were 19 kg ha-1 a-1 i n L. Villikkalanjarvi and 12 kg ha-1 a-1 in L. Kotojarvi. The Tot-P in put to L. Kotojarvi was on an average 0.62 g m-2 a-1 (per lake surface area), and the Tot-N input 9.1 g m-2 a-1. The corresponding inputs to L. Villikkalanjarvi were 3.1 and 57 g m-2 a-1, respectively. The annu al variation followed the runoff volumes. About half of the Tot-P and one third of the Tot-N load was retained in L. Kotojarvi. In L. Villik kalanjarvi the retention was only 24-degrees for Tot-P and 19% for Tot -N. The difference was very probably due to a longer theoretical reten tion time in L. Kotojarvi. In L. Villikkalanjarvi the mean concentrati on of Tot-P was 120,ug l-1 and that of Tot-N 1700,ug l-1 and the corre sponding figures in L. Kotojarvi 67 and 990 mug l-1, respectively. The mean chlorophyll a concentration was, however, higher in L. Kotojarvi (26 mug l-1) than in L. Villikkalanjarvi (20 mug l-1). This was proba bly due to an internal P load in L. Kotojarvi: in 1988 the internal lo ad of dissolved P was estimated to be as much as twofold the external load. In L. Villikkalanjarvi the internal dissolved P load was only up to 50% of the external input. In L. Kotojarvi the high internal P loa d coupled with a low DIN:DIP ratio resulted in a strong blue-green alg al bloom in the summer of 1988. In L. Villikkalanjarvi blue-green alga e were observed only in small amounts. Even in August 1990, when the D IN:DIP ratio was low enough to favor the occurrence of blue-green alga e, they contributed only up to 10- 15 % of the total phytoplankton bio mass.