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

Abstract

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 atthe 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 (k adj), we observed that k adj waspositively related to spring NO3-N concentrations, but this relationship became insignificant at mean lake depths exceeding 12.5 m. A relationship between k adj 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 L21, 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.