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.