Ecological stoichiometry of TN : TP : SRSi in freshwaters: nutrient ratios and seasonal shifts in phytoplankton assemblages

Our concept of the ‘balance of TN : TP : SRSi-ratios’ attempts to determine whether the seasonal succession of phytoplankton communities is influenced by or responds to nutrient stoichiometry. Because the molar average of TN : TP : SRSi = 20 : 1 : 17 for 118 measurements (7 lakes, 3 inflows, all seasons) was close to the marine optimum stoichiometric ratio, we used the ratio TN : TP : SRSi = 16 : 1 : 17 as a reference point for ecological stoichiometry. Triangular diagrams of TN : TP : SRSi ratios scaled in physiological proportions of 16 : 1 : 17 were used to identify seasonal fluctuation patterns of major phytoplankton nutrients. In lakes with ‘balanced nutrient ratios’ TN : TP : SRSi ratios vary evenly around the stoichiometric optimum of 16 : 1 : 17 within a year. Lakes with balanced nutrient ratios, however, were more the exception than the rule and only occurred in 2 of 7 lakes studied. Lakes with constant concentrations of one nutrient element in relation to those of the other two were common. Unbalanced nutrient ratios were associated with relatively constant concentrations of either SRSi, or TN or TP. The relative change in availability of soluble reactive silica was more affected by differences in lake morphometry than that of TN or TP. Ratios of TN : TP : SRSi have the advantage of synoptically presenting the relative nutrient availability for both diatoms and non-siliceous algae in phytoplankton communities. Shifts within the tripartite ratio correspond to successive substitution of phytoplankton groups, e.g. from diatoms to cyanobacteria.