Is trophic state or regional location the strongest determinant for Chl-a/TP relationships in lakes?

Based on data from the four Nordic countries: Denmark, Sweden, Norway and Finland, regressions have been developed relating phytoplankton abundance as chlorophyll-a, chl-a, to nutrient concentrations (as total phosphorus, TP, and total nitrogen, TN). Regressions were developed for the full data set from each country and for three ranges of trophy (expressed as TP < 20 mg · m^sup -3^, 20 mg · m^sup -3^ > TP > 200 mg · m^sup -3^, and TP > 200 mg · m^sup -3^). Our results suggest that trophic level is a stronger determinant for characterising the chl-a = f(TP) and Chl-a = g(TN) relationships than the regional localisation of the lakes. Thus, in contrast to many other pollution criteria, (e.g., acidification) equal guideline- or criteria values for phosphorus- or nitrogen- loads across regions will give equal end-point effects in lakes. Total phosphorus seems to limit phytoplankton growth when TP < 20 mg · m^sup -3^. There is no clear indication which of the nutrients, TP or TN, that limit phytoplankton growth for 20 mg · m^sup -3^ > TP > 200 mg · m^sup -3^. There are signs that neither TP nor TN are limiting factors for phytoplankton growth at TP > 200 mg · m^sup -3^.[PUBLICATION ABSTRACT]