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

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 5 N1e2 2 t2), 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 decreased with increasing lake depth. Adjusting k to the lake depth (kadj), we observed that kadj was positively related to spring NO3-N concentrations, but this relationship became insignificant at mean lake depths exceeding 12.5 m. A relationship between kadj 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. Nitrogen plays an important role for the primary productivity of aquatic systems and the microbial recycling of organic matter (Kalff 2002). In its dissolved form it is subject to a series of loss processes as it travels through the landscape. Within aquatic systems and their catchments these include denitrification, releasing gaseous nitrogen into the atmosphere, and biological uptake, followed by immobilization and accumulation of organic nitrogen in soils and sediments (e.g., van Breemen et al. 2002). High loss rates of nitrate–nitrogen (NO3-N), the main form of dissolved nitrogen in most lakes, might result in very low NO3-N concentrations in aquatic systems, a condition that has been associated with blooms of nitrogen-fixing cyanobacteria (Smith 1983; Ferber et al. 2004; Köhler et al. 2005). Nitrogen-fixing cyanobacteria can increase their toxicity when NO3-N concentrations become depleted (Velzeboer et al. 2001), explaining why an understanding of NO3-N loss rates is important for water management. In this study we examined NO3-N loss rates within and among 100 North–mid-European lakes. Additionally we analyzed temporal changes in NO3-N concentrations in a subset of 16 northern shallow lakes with complete time series between 1988 and 2003 and related them to changes in climate and atmospheric deposition. The results might help to gain further insight into changes in algal blooms in lakes, including recently observed increases in harmful algal blooms as reported by, e.g., Charmichael (2001), Huisman et al. (2004), and Peperzak (2005).