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To identify the seasonal pattern of nitrogen (N) and phosphorus (P) limitation of phytoplankton in four different lakes, biweekly experiments were conducted from the end of March to September 2011. Lake water samples were enriched with N, P or both nutrients and incubated under two different light intensities. Chlorophyll a fluorescence (Chla) was measured and a model selection procedure was used to assign bioassay outcomes to different limitation categories. N and P were both limiting at some point. For the shallow lakes there was a trend from P limitation in spring to N or light limitation later in the year, while the deep lake remained predominantly P limited. To determine the ability of in-lake N:P ratios to predict the relative strength of N vs. P limitation, three separate regression models were fit with the log-transformed ratio of Chla of the P and N treatments (Response ratio = RR) as the response variable and those of ambient total phosphorus:total nitrogen (TN:TP), dissolved inorganic nitrogen:soluble reactive phosphorus (DIN:SRP), TN:SRP and DIN:TP mass ratios as predictors. All four N:P ratios had significant positive relationships with RR, such that high N:P ratios were associated with P limitation and low N:P ratios with N limitation. The TN:TP and DIN:TP ratios performed better than the DIN:SRP and TN:SRP in terms of misclassification rate and the DIN:TP ratio had the highest R₂ value. Nitrogen limitation was predictable, frequent and persistent, suggesting that nitrogen reduction could play a role in water quality management. However, there is still uncertainty about the efficacy of N restriction to control populations of N₂ fixing cyanobacteria.

Due to their competitive advantage when inorganic nitrogen (N) sources are scarce, it is widely assumed that the abundance and N2‐fixation rate of N2‐fixing cyanobacteria (Nostocales) would increase in response to reduced N loading. If realized, this response would render efforts to improve water quality by N reduction ineffective. To assess this, we performed a microcosm experiment using water from an N limited lake, Langer See, in which phosphorus loading was held constant, while a gradient of decreasing N loading was simulated. Nostocales biovolume increased over time in all microcosms, regardless of the N addition rate, so that no difference in Nostocales biovolume developed between high and low N microcosms. In contrast, the biovolumes of other taxa (especially non‐fixing cyanobacteria) were lower at low N addition rates. N2‐fixation increased in low N microcosms, compensating for 36% of the difference in N addition by day 6. However, this compensation rate was achieved at Nostocales biovolumes far higher than those typical in the studied lake. At biovolumes typical for summer the compensation rate would only account for 7% of the omitted N. If these results were to hold over longer time scales, in shallow polymictic lakes like Langer See, reduced N loading may lower both in‐lake N concentrations and biovolumes of non‐fixing phytoplankton without significantly impacting Nostocales biovolume. [ABSTRACT FROM AUTHOR]

Eutrophication is a global environmental problem that leaves many lakes with impaired ecological status. Human activity has increased the total concentrations of both nitrogen and phosphorus in aquatic systems, but their relative influence on phytoplankton biomass is uncertain. Their action as alternative limiting resources complicates assessment of their relative influence and disagreement may be in part due to seasonal shifts and lake-type-specific differences in the prevalence of limitation by nitrogen versus phosphorus. Debate continues as to whether measures to reduce nitrogen would be beneficial in addition to controls placed on phosphorus., We used a piecewise model to test whether total nitrogen (TN) concentrations, in addition to total phosphorus (TP), influence phytoplankton biomass in 369 lowland German lakes. The piecewise model predicts biomass from TN for low N : P ratio lakes, and from TP for high N : P ratio lakes. We tested three N : P mass ratios to divide lakes: dissolved inorganic nitrogen to TP (DIN : TP), DIN to dissolved reactive phosphorus (DIN : DIP) and TN : TP. TN was a better predictor of biomass than TP when either the DIN : TP ratio was below 1.6, DIN : DIP was below 8.4, or TN : TP below 29; predictions were most accurate when using the DIN : TP ratio., To investigate seasonal and lake-type-specific patterns of N and P limitation, we used the DIN : TP ratio, together with absolute concentrations of DIN and DIP, to predict the limiting nutrient at each lake in each month of the vegetation period. N limitation was much more common in polymictic than stratified lakes. While a high proportion of both stratified and polymictic lakes were P limited in early spring (60-70%), for polymictic lakes, we found a strong shift from P limitation to N limitation in summer: more than 50% of polymictic lakes were N limited between June and September and only 15-30% were P limited., To obtain lake-type-specific nutrient targets we estimated the average TN and TP concentrations at which lakes of different types achieved good ecological status according to EU water framework directive criteria. Stratified lakes achieved good ecological status at concentrations of 400-500 μg L−1 TN or 20-35 μg L−1 TP, while for polymictic lakes values of 500-1000 μg L−1 TN, or 35-75 μg L−1 TP were required., We estimate that nitrogen has an important influence on phytoplankton biovolume, and thus ecological status, for many polymictic lakes in Germany. While there is some uncertainty in the nutrient targets required to achieve good ecological status, this uncertainty is small compared with the range of concentrations currently observed, and lakes with moderate or worse status have concentrations of both TN and TP that are far above these current target estimates. [ABSTRACT FROM AUTHOR]