¹Plankton Canada, Burlington, ON, Canada L7L 2L8
²Fisheries and Oceans Canada, Canada Centre for Inland Waters, 867 Lakeshore Road, Burlington, ON, Canada L7R 4A6

Summary

         Spring and summer surveys of Lake Superior were conducted by Fisheries and Oceans Canada and Environment Canada aboard the research vessel CCGS Limnos. During 2001, the mean concentration of phytoplankton biomass was low in the spring (0.9 g m^-3) and summer (1.0 g m^-3) seasons. Spring was dominated by Diatomeae, Chrysophyceae, Chlorophyta and Dinophyceae. The phytoflagellates together contributed significantly in both seasons. Diatomeae were more prevalent in the summer compared to the spring. Very high species diversity across the lake was observed in both seasons. Most of the species identified were rare individually contributing less than 1% of the total biomass but collectively contributed 23 to 26% in spring and summer. Similarly, less common species (≥1 and <5% of total biomass) comprised 50-54%. Only a limited number of species belonged to the common category (≥ 5% of total biomass). The species composition and high biodiversity reflect a characteristic ultra-oligotrophic environment. The size composition of the phytoplankton in both seasons showed an overall dominance of nanoplankton. The size fractionated primary productivity also support nanoplankton dominance with high rates of production comprising a mixture of phytoflagellates and diatoms. Minute picoplankton, though contributing little to the total biomass, was found to possess high rates of photosynthesis (31 – 55% of the total primary productivity) resulting in high P/B quotients. The picoplankton fraction was composed mainly of Chroococcus dispersus var. minor. A comparison of ecosystemic changes in Lake Superior between the summers of 1973, 1983 and 2001 was carried out at comparable stations. Surface temperature showed a dramatic rise of 5^oC during the period of 1973 to 1983. The comparison further indicated that phytoplankton and its community structure changed considerably from 1973 to 2001 with a ten-fold increase in the mean biomass. The average size of algae increased from 9.9 µm in 1973 to 13.6 µm in 2001. The experimental data also shows important alterations in the physiological ecology of Lake Superior phytoplankton. Primary productivity in 2001 was ½ of its rate in 1973 with a corresponding decline in net and nanoplankton production rates. Cumulatively, P/B quotients showed a severe drop from 25.7 in 1973 to 1.2 in 2001. This is supported by an alarming drop in chlorophyll a to biomass ratios which are considered to be a barometer of algal health suggesting that the phytoplankton community is under stress. The structural and functional results indicate that Lake Superior phytoplankton reflect pristine oligotrophic conditions, in which the lower trophic levels are undergoing extensive changes. It is suggested that these alterations may be related to climate change and the observed warming of the lake from 13 – 18^oC.