Total nitrogen, total phosphorus, and nutrient limitation in lakes and oceans: Is there a common relationship?

Total nitrogen (TN) and total phosphorus (TP) measurements and contemporaneous measurements of chlorophyll a (Chl a) and phytoplankton nutrient deficiency have been made across a broad range of lakes and ocean sites using common methods. The ocean environment was nutrient rich in terms of TN and TP when compared with most lakes in the study, although Lake Victoria had the highest values of TN and TP. TN concentrations in lakes rose rapidly with TP concentrations, from low values to TN concentrations that are similar to those associated with the ocean sites. In contrast, the TN concentrations in the oceans were relatively homogeneous and independent of TP concentrations. The hyperbolic shape of the TN:TP relationship created a broad range of TN:TP values for both lakes and oceans. The TN:TP ratios of the surface ocean sites were usually well in excess of the Redfield ratio that is noted in the deep ocean. Phytoplankton biomass, as indicated by Chi a, was strongly dependent upon TP in the lakes, and there was a weaker relationship with TN. Oceanic Chi a values showed a positive relationship with TP, but at much higher TP values than were observed in the lakes; there was no relation with TN. P-deficient phytoplankton growth was inferred using independent indicators when TP was <0.5 mu mol L-1 at both freshwater and marine sites. N-deficiency indicators were highly variable and did not show any clear dependence on TN concentration. The TN:TP ratio was indicative of which nutrient would become limiting for growth in both lakes and oceans. When all sites are compared, N-deficient growth was apparent at TN:TP < 20 (molar), whereas P-deficient growth consistently occurred when TN:TP > 50 (molar). At intermediate TN:TP ratios, either N or P can become deficient. We conclude that N or P limitation of algal growth is a product of the TN and TP concentration and the TN:TP ratio rather than a product of whether the system of study is marine or freshwater.