Sammanfattning

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 N-2 5 N-1e(-k)( (t)(2) - (t)(2)), where N-1 and N-2 are NO3- N concentrations at the beginning (t(1)) and the end (t(2)) 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) was positively 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 mu g 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

Publicerad i

Limnology and Oceanography
2007, Volym: 52, nummer: 4, sidor: 1346-1353
Utgivare: AMER SOC LIMNOLOGY OCEANOGRAPHY

SLU författare

• Weyhenmeyer, Gesa

  • Institutionen för miljöanalys, Sveriges lantbruksuniversitet
    

UKÄ forskningsämne

Miljö- och naturvårdsvetenskap
Fisk- och akvakulturforskning


Publikationens identifierare

DOI: https://doi.org/10.4319/lo.2007.52.4.1346

Permanent länk till denna sida (URI)

https://res.slu.se/id/publ/16681