A SUMMARY OF PAST AND RECENT PLANKTON OF LAKE WINNIPEG, CANADA USING ALGAL FOSSIL REMAINS

Although the phytoplankton and protozoan assemblages of Lake Winnipeg are quite variable both, spatial and temporally, knowledge of their cu rrent distribution and ecology enables them to be used as indicators p roviding unique information concerning past and recent ecosystem condi tions. Many of the main taxa have been similar throughout history alth ough there have been dramatic changes in their abundance. Lake Winnipe g is currently very responsive to the climatic conditions in its drain age basin and it appears, historically, to have been a diatom-bluegree n algal lake with Tintinnids and thecate amoeba forming significant co mponents of the protozoan community. A wide variety of microfossil rem ains have been used to infer past conditions in this large prairie lak e. Microfossils reported from the long core 103 (8 m) taken from the n orth basin of LakeWinnipeg and short core Namao 7a taken from the sout h basin of the lake in August 1994 indicate that the lake has experien ced several changes over its history. Diatom assemblages, low througho ut the core 103 below 50 cm, except for a peak around 300-400 cm are v irtually absent below 690-800 cm (Lake Agassiz). Stephanodiscus and Au lacoseira are the two major pelagic diatom genera represented througho ut the lake's history. Species changes occur near the top of the core indicate increased anthropogenic eutrophication. Shallow water littora l taxa were never abundant at either coring sites. The presence of pla nktic cyanoprokaryote (cyanophyte, cyanobacteria, bluegreen algae) rem ains (akinetes) from 600 cm depth and the progressive increase in abun dance from 400 cm to through the top 100 cm of the core indicates incr easing phosphorus levels, warming summer temperatures and increasing s ummer nitrogen limitation in the lake. Nitrogen fixing bluegreens (esp ecially Anabaena and Aphanizomenon akinetes) and the diatoms (Aulacose ira ambigua, A. granulata, A. islandica, S. binderanus, and S. niagara e) were abundant in the upper sediments and changes after 100 cm can b e interpreted as the effects of human impact. These taxa indicative of increase eutrophication, in addition to S. agassizensis, Melosira var ians and Cyclostephanos dubius, are representative of present day plan kton. The short core Namao 7a was dated by (Pb)210 and the fossil rema ins also show a rise in the Aulacoseira (particularly A. granulata and A. ambigua) and bluegreen akinetes in the last 40 years as was seen i n the upper sediments of the 103 core. There was an interesting peak i n chrysophyte cysts during the 1930's which corresponds nicely with cl imatic conditions during this period. The fires accompaning the hot dr y period in the mid 70's can be seen in the dramatic rise in charcoal during this period. Multiple types of micro fossils remains provides s everal useful tools for interpreting past lacustrine conditions.