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|Title:||Nitrate-depleted Conditions on the Increase in Shallow Northern European Lakes|
|Authors:||WEYHENMEYER GESA; JEPPESEN Erik; ADRIAN Rita; ARVOLA Lauri; BLENCKNER Thorsten; JANKOWSKI Thomas; JENNINGS Eleanor; NOGES PEETER; NOGES Tiina; STRAILE Dietmar|
|Citation:||LIMNOLOGY AND OCEANOGRAPHY vol. 52 no. 4 p. 1346-1353|
|Publisher:||AMER SOC LIMNOLOGY OCEANOGRAPHY|
|Type:||Articles in periodicals and books|
|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 = N1 e-k(t2-t1), 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 was positively related to spring NO3-N concentrations but that this relationship decreased with increasing mean lake depth and diminished at mean lake depths exceeding 12.5 m. A relationship between k 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. Evaluating time series from 14 Swedish, 1 Estonian and 1 German lakes shallower than 12.5 m since 1988 we found that May to August NO3-N concentrations have decreased over time in all lakes, and that the number of time periods exhibiting NO3-N depletion, i.e. NO3-N levels below 10 µg 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 periods might be one possible explanation for the increasing occurrence of nitrogen-fixing cyanobacteria found in a variety of lake ecosystems.|
|JRC Institute:||Institute for Environment and Sustainability|
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