ECOLOGY OF PHYTOPLANKTON FROM 2 SHALLOW, ACIDIFIED LAKES, DENMARK, WITH SPECIAL REFERENCE TO THE GREEN-ALGAE

Lake Holm and Lake Kvie, two acidified, soft-water, shallow lakes in W estern Jutland, Denmark, have developed completely different phytoplan kton communities. In Lake Holm, the acidification process is more adva nced than in Lake Kvie.pH in Lake Holm decreased to 3.9 in 1992, ammon ia reached high levels (max. 4 mg 1(-1), total phosphorus concentratio n was mostly between 0.01 and 0.04 mg 1(-1) and inorganic phosphorus w as below the detection limit most of the year. The phytoplankton biovo lume was low (0.16-0.90 mm3 1(-1), max. 0.30 - 4.3 mm3 1(-1)) due to p hosphorus limitation. During the investigated period 1989-92, the dino flagellate Peridinium willei and unicellular desmid species were gradu ally replaced by tychoplanktic/epiphytic, filamentous green algae (Zyg nematales and Oedogoniales) and picoplanktic cyanophytes. In the summe r of 1992, Lake Holm partly dried-up. In this environment, tychoplankt ic/epiphytic, filamentous forms competed well against true plankti for ms. In spite of acidification in Lake Kvie (pH min. 4.5), elevated con centrations of nitrate and total phosphorus in this lake (max. 1.21 an d 0.15 mg 1(-1) respectively) resulted in irregular mass occurrences o f a species-poor, fast growing phytoplankton community of small Chloro coccales and Desmidiaceae (2.2-4.2 mm3 1(-1), max. 16-35 mm3 1(-1). La ke Kvie is deeper than Lake Holm and the phytoplankton of Lake Kvie is therefore less exposed to dessication than that of Lake Holm during h ot and dry periods and thus has more euplanktic species than that of L ake Holm.