Growth and phosphorus uptake of summer phytoplankton in Lake Erken (Sweden)

The thermal stratification in Lake Erken was short and relativelyunstable in 1989. Changes in the species composition of thephytoplankton between early May and August followed the generalsuccession pattern outlined for other temperate lakes. Fast-growing,r-strategist cryptophytes, dominant in the early phase of succession,could be separated sufficiently by 12 µm membrane filtersfrom larger K-strategists like Ceratium hirundinella and Gloeotrichiaechinulata which dominated in July. Under more turbulent conditions,the biomass of diatoms increased, and these species were also>12 µm. Growth rates of the phytoplankton and thoseof the two size groups were sensitive to the species composition,but fitted reasonably to the Droop model. Long turnover timesof orthophosphate in the water, the Phosphorus Deficiency Indicatordefined here as the ratio of the light-saturated rate of photosynthesisand the conductivity coefficient of phosphate uptake, and relativegrowth rates generally indicated low P-deficiency. Moderatedeficiency was observed in late July, towards the end of thestratification period. Steady-state net P-uptake rates werecalculated from the Droop model and compared with instantaneousnet P-uptake rates estimated from ³²P uptake kinetics by thelinear force-flow relationship of Falkner et al. (Arch. Microbiol.,152, 353–361, 1989). The two data sets showed surprisinglysimilar seasonal trends. Depletion of epilimnetic soluble reactivephosphorus (SRP) resulted in enhanced utilization of intracellularlystored P. Such periods were, however, interrupted by elevatedSRP inputs to the epilimnion that led to luxury P uptake anda low incidence of P deficiency.