GROWTH AND PHOSPHORUS UPTAKE OF SUMMER PHYTOPLANKTON IN LAKE ERKEN (SWEDEN)

The thermal stratification in Lake Erken was short and relatively unst able in 1989. Changes in the species composition of the phytoplankton between early May and August followed the general succession pattern o utlined for other temperate lakes. Fast-growing, r-strategist cryptoph ytes, dominant in the early phase of succession, could be separated su fficiently by 12 mu m membrane filters from larger K-strategists like Ceratium hirundinella and Gleootrichia echinulata which dominated in J uly. Under more turbulent conditions, the biomass of diatoms increased , and these species were also >12 mu m. Growth rates of the phytoplank ton and those of the two size groups were sensitive to the species com position, but fitted reasonably to the Droop model. Long turnover time s of orthophosphate in the water, the Phosphorus Deficiency Indicator defined here as the ratio of the light-saturated rate of photosynthesi s and the conductivity coefficient of phosphate uptake, and relative g rowth rates generally indicated low P-deficiency. Moderate deficiency was observed in late July, towards the end of the stratification perio d. Steady-state net P-uptake rates were calculated from the Droop mode l and compared with instantaneous net P-uptake rates estimated from P- 32 uptake kinetics by the linear force-flow relationship of Falkner et al. (Arch. Microbiol., 152, 353-361, 1989). The two data sets showed surprisingly similar seasonal trends. Depletion of epilimnetic soluble reactive phosphorus (SRP) resulted in enhanced utilization of intrace llularly stored P. Such periods were, however, interrupted by elevated SRP inputs to the epilimnion that led to luxury P uptake and a low in cidence of P deficiency.