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

We determined relative nitrate‐nitrogen (NO 3 ‐N) loss rates in 100 north‐mid‐European lakes from late spring to summer by using the exponential function N 2 = N 1 e −k(t 2 −t 2 ), where N 1 and N 2 are NO 3 ‐N concentrations at the beginning (t 1 ) and the end (t 2 ) of the time interval, respectively, and k is the specific NO 3 ‐N loss rate. We found that k decreased with increasing lake depth. Adjusting k to the lake depth (k adj ), we observed that k adj was positively related to spring NO 3 ‐N concentrations, but this relationship became insignificant at mean lake depths exceeding 12.5 m. A relationship between k adj and spring NO 3 ‐N concentrations in lakes shallower than 12.5 m implies that changes in spring NO 3 ‐N concentrations influence the NO 3 ‐N loss rate and thereby summer NO 3 ‐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 NO 3 ‐N concentrations have decreased over time everywhere, and the number of time periods exhibiting a NO 3 ‐N depleted condition, i.e., NO 3 ‐N levels below 10 mg L −1 , in these lakes has tripled since 1988. We explained the decreasing NO 3 ‐N concentrations by a reduction in external nitrogen loading including atmospheric deposition, and by changes in climate. The observed prolongation of NO 3 ‐N depleted conditions might be one possible explanation for the increasing occurrence of nitrogen‐fixing cyanobacteria in a variety of lake ecosystems.