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

The thermal stratification in Lake Erken was short and relatively unstable in 1989. Changes in the species composition of the phytoplankton between early May and August followed the general succession pattern outlined for other temperate lakes. Fast-growing, r-strategist cryptophytes, dominant in the early phase of succession, could be separated sufficiently by 12 mu m membrane filters from larger K-strategists like Ceratium hirundinella and Gleootrichia echinulata which dominated in July. Under more turbulent conditions, the biomass of diatoms increased, and these species were also >12 mu m. Growth rates of the phytoplankton and those of the two size groups were sensitive to the species composition, but fitted reasonably to the Droop model. Long turnover times of orthophosphate in the water, the Phosphorus Deficiency Indicator defined here as the ratio of the light-saturated rate of photosynthesis and the conductivity coefficient of phosphate uptake, and relative growth rates generally indicated low P-deficiency. Moderate deficiency was observed in late July, towards the end of the stratification period. Steady-state net P-uptake rates were calculated from the Droop model 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 intracellularly stored P. Such periods were, however, interrupted by elevated SRP inputs to the epilimnion that led to luxury P uptake and a low incidence of P deficiency.