Early spring development of phytoplankton in Lake Erken, Sweden, as monitored by delayed fluorescence

摘要

Lake Erken is a moderately eutrophic lake, situated 60 km NNE of Stockholm and 50 km E of Uppsala, Sweden. Its altitude is 11 m above sea level and it has an area of 24 km~2, with a maximum depth of 21 m an average depth of 9 m. The theoretical water residence is 7 years, and thus internal processes strongly affect water quality. The ice-cover period lasted an average of 123 days for the period 1961-1990, with an ice-breakup on 13 April (Persson et al. this vol.). Scenarios for the future indicate a much shorter ice-cover period (Persson et al. this vol.). Spring development of phytoplankton has been studied over several years in Lake Erken (Rodhe et al. 1958, Pechlaner 1970, Bostroem & Pettersson 1977, Pettersson 1980, 1985). Climatic factors influence species composition and biomass development of phytoplankton (Weyhenmeyer et al. 1999, Pettersson et al. 2003). Diatoms usually dominate the spring peak, but years with dominance of dinofla-gellates have been noted as well. A fascinating change from almost dark to good light conditions and from surplus nutrient conditions to limitation occurs during the period when snow and ice disappears. The water column is stable until ice-out, when total mixing appears.