Nitrate‐depleted conditions on the increase in shallow northern European lakes

We determined relative nitrate‐nitrogen (NO3‐N) loss rates in 100 north‐mid‐European lakes from late spring to summer by using the exponential function N2 = N1e−k(t2−t2), where N1 and N2 are NO3‐N concentrations at the beginning (t1) and the end (t2) of the time interval, respectively, and k is the specific NO3‐N loss rate. We found that k decreased with increasing lake depth. Adjusting k to the lake depth (kadj), we observed that kadj was positively related to spring NO3‐N concentrations, but this relationship became insignificant at mean lake depths exceeding 12.5 m. A relationship between kadj and spring NO3‐N concentrations in lakes shallower than 12.5 m implies that changes in spring NO3‐N concentrations influence the NO3‐N loss rate and thereby summer NO3‐N concentrations. Time series from one Estonian, one German, and 14 Swedish lakes shallower than 12.5 m since 1988 revealed that May to August NO3‐N concentrations have decreased over time everywhere, and the number of time periods exhibiting a NO3‐N depleted condition, i.e., NO3‐N levels below 10 mg L−1, in these lakes has tripled since 1988. We explained the decreasing NO3‐N concentrations by a reduction in external nitrogen loading including atmospheric deposition, and by changes in climate. The observed prolongation of NO3‐N depleted conditions might be one possible explanation for the increasing occurrence of nitrogen‐fixing cyanobacteria in a variety of lake ecosystems.