Lake‐type‐specific seasonal patterns of nutrient limitation in German lakes, with target nitrogen and phosphorus concentrations for good ecological status

Abstract

Summary 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.

Mechanisms preventing a decrease in phytoplankton biomass after phosphorus reductions in a German drinking water reservoir—results from more than 50 years of observation

“…To assess the relative importance of phosphorus and nitrogen as limiting nutrients, the N:P ratio was used. As an indicator for discriminating between N and P limitation, we used the DIN:TP ratio, which has been identified as the best predictor for phytoplankton nutrient limitation (Bergstr€ om, 2010;Dolman, Mischke, & Wiedner, 2016;Ptacnik, Andersen, & Tamminen, 2010). Above a ratio of 3.4, phytoplankton usually shifts from nitrogen to phosphorus limitation.…”

Quantification and isotherm modelling of competitive phosphate and silicate adsorption onto micro-sized granular ferric hydroxide

“…Depending on the lake type, even lower values of 0.02-0.06 mg L À1 have been suggested in order to limit algae growth and to reach a good status. 3 Conventional techniques for P removal include precipitation with di-or trivalent metal ions and biological removal. Low residual phosphate concentrations are achieved with high doses of occulants.…”

“…At the pH PZC of the adsorbent the building of two different surface complexes is proposed by Hiemstra. 13 A binuclear bidentate complex with exchange of two ligands resulting in^(FeO) 2 Si(OH) 2 and a mononuclear monodentate complex^(FeOHFeO)Si(OH) 3 . At low concentrations and low pH values silicate is mostly present as monomers, whereas the amount of oligomeric and polymeric silicates increases steeply at circum-neutral pH values.…”

The response of nitrogen fixing cyanobacteria to a reduction in nitrogen loading

“…Phosphorus (P) was at one time thought to be the main nutrient limiting phytoplankton biomass in the majority of lakes (Schindler, ); however, since the 1970s, empirical and experimental studies have demonstrated that nitrogen (N) is also important in many lakes (see Lewis & Wurtsbaugh, ; Sterner, for review). In cross‐lake analyses, phytoplankton biovolume is more closely related to N when N:P ratios are low (Dolman & Wiedner, ) and conditions indicative of N limitation seem to predominate in shallow polymictic and riverine lakes (Dolman, Mischke, & Wiedner, ). In lakes that are identified as N limited, N loading reduction could be an ecologically meaningful way to improve water quality; however, concerns exist that compensatory N 2 ‐fixation by cyanobacteria (Nostocales) could render N load reduction measures ineffective (Schindler, , ; Schindler et al, ).…”