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.

WEYHENMEYER Gesa;  JEPPESEN Erik;  ADRIAN Rita;  ARVOLA Lauri;  BLENCKNER Thorsten;  JANKOWSKI Thomas;  JENNINGS Eleanor;  NOGES Peeter;  NOGES Tiina;  STRAILE Dietmar; 

2007-11-09

AMER SOC LIMNOLOGY OCEANOGRAPHY

JRC35017

0024-3590,   

https://publications.jrc.ec.europa.eu/repository/handle/JRC35017,