NUTRIENT BALANCES AND PHYTOPLANKTON DYNAMICS IN 2 AGRICULTURALLY LOADED SHALLOW LAKES

The impact of agriculture was estimated on two shallow, eutrophic lakes, Lake Kotojarvi and Lake Villikkalanjarvi in southern Finland. The main emphasis was on phosphorus and nitrogen budgets and on the phytoplankton dynamics. Special attention was paid to internal P loading and blue-green algal blooms. The mean Tot-P load from agricultural land was 1.2 kg ha-1 a-1 in both basins and Tot-N loads were 19 kg ha-1 a-1 in L. Villikkalanjarvi and 12 kg ha-1 a-1 in L. Kotojarvi. The Tot-P input 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 annual 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. Villikkalanjarvi the retention was only 24-degrees for Tot-P and 19% for Tot-N. The difference was very probably due to a longer theoretical retention time in L. Kotojarvi. In L. Villikkalanjarvi the mean concentration of Tot-P was 120,ug l-1 and that of Tot-N 1700,ug l-1 and the corresponding 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 probably due to an internal P load in L. Kotojarvi: in 1988 the internal load 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 load coupled with a low DIN:DIP ratio resulted in a strong blue-green algal bloom in the summer of 1988. In L. Villikkalanjarvi blue-green algae were observed only in small amounts. Even in August 1990, when the DIN:DIP ratio was low enough to favor the occurrence of blue-green algae, they contributed only up to 10- 15 % of the total phytoplankton biomass.