Phosphate concentrations in lakes

Phosphate is an important nutrient that restricts microbial production in m any freshwater(1-3) and marine environments(4-6). The actual concentration of phosphate in phosphorus-limited waters is largely unknown because common ly used chemical and radiochemical techniques overestimate the concentratio n(7,8). Here, using a new steady-state radiobioassay to survey a diverse se t of lakes, we report phosphate concentrations in lakes that are orders of magnitude lower than estimates made spectrophotometrically or with the freq uently used Rigler radiobioassay. Our results, combined with those from the literature, indicate that microbes can achieve rapid turnover rates at pic omolar nutrient concentrations. This occurs even though these concentration s are about two orders of magnitude below the level where phosphate uptake is estimated to be half the saturation level for the picoplankton community . Also, while phosphate concentration increased with the concentration of t otal phosphorus and soluble reactive phosphorus in the lakes we sampled, th e proportion of phosphate in the total phosphorus pool decreased from oligo trophic to eutrophic lakes. Such information, as revealed by the phosphate assay that we use here, should allow us to address hypotheses concerning th e concentration of phosphate available to planktonic microorganisms in aqua tic systems.