Nitrogen uptake and the importance of internal nitrogen loading in Lake Balaton

Abstract

SUMMARY1. The importance of various forms of nitrogen to the nitrogen supply of phytoplankton has been investigated in the mesotrophic eastern and eutrophic western basin of Lake Balaton. 2. Uptake rates of ammonium, urea, nitrate and carbon were measured simultaneously. The uptake rates were determined using 15 N and 14 C methodologies, and N 2 -fixation was measured using the acetylene-reduction method. The light dependence of uptake was described with an exponential saturation equation and used to calculate surface-related (areal) daily uptake. 3. The contribution of ammonium, urea and nitrate to the daily nitrogen supply of phytoplankton varied between 11 and 80%, 17 and 73% and 1 and 15%, respectively. N 2 -fixation was negligible in the eastern basin and varied between 5 and 30% in the western region of the lake. The annual external nitrogen load was only 10% of that utilized by algae. 4. The predominant process supplying nitrogen to the phytoplankton in the lake is the rapid recycling of ammonium and urea in the water column. The importance of the internal nutrient loading is emphasized.

Effects of Environmental Factors on the Uptake Rates of Dissolved Nitrogen by a Salt-water Green Alga (Oocystis borgei Snow)

“…Gu and Alexander (1993) indicated a linear relationship between light intensity and urea uptake rates by A. flos-aquae. A similar finding was observed in marine phytoplankton by Presing et al (2001). Results from this experiment showed increases in DN uptake rate with increases in light intensity, but the uptake rate for all forms of DN decreased when light intensity reached 153 lmol m -2 s -1 (Fig.…”

“…-N, NO 3 --N, urea-N and N 2 -N by Anabaena flos-aquae were closely related to algal biomass, water temperature and light level in a subarctic lake. Presing et al (2001) indicated a positive relationship between the uptake rates for NH 4 ? -N, urea-N and NO 3 -N, temperature and light intensity in Lake Balaton.…”

A five-year study of autotrophic winter picoplankton in Lake Balaton, Hungary

“…Water temperature, however, is not the only factor regulating picoplankton abundance. In summer, the filamentous N 2 -fixing Cyanobacteria seem to outcompete the other algal groups (including picocyanobacteria and picoeukaryotes) suggesting a possible nitrogen limitation of phytoplankton in Lake Balaton (Présing et al 2001).…”

“…This finding agrees with the hypothesis (Weisse 1993) that the factors controlling the abundance of picoeukaryotes differ from those that control picocyanobacteria. In Lake Balaton, during the coldwater period when both water temperature and light are appropriate for picoeukaryotes, the concentrations of nitrogen and phosphorus are also at their highest (Présing et al 2001), therefore the entire phytoplankton is temperature and light limited. During the warm period, filamentous nitrogen-fixing Cyanobacteria have competitive advantage over picocyanobacteria.…”

“…This supports the hypothesis (Weisse 1993) that in shallow lakes factors other than macronutrients (N, P) have more pronounced impact on picoplankton abundance. In Lake Balaton, the concentration of nitrogen and phosphorus increases during the winter months (Présing et al 2001), because phytoplankton-including picophytoplankton-have a limited ability to take up nutrients due to reduced light and temperature. Our results suggest that picoeukaryotes are better competitors than picocyanobacteria at conditions characterized by low light and temperature and are able to grow under the ice cover, although their growth is constrained and they cannot fully exploit the available nutrients.…”