Is nitrate a driver for pigments of phytoplankton (a case study from a small shallow European lake)?

The traditional view of biogenic limitation in freshwater ecosystems considers phosphorus as the primary limiting element, but this concept has recently been challenged. There are a number of studies that suggest that photosynthetic indices may depend on the levels of nitrogen (especially, nitrate-nitrogen). To test the hypothesis about the possibility of nitrogen limitation in the lake ecosystem, the present study analysed the water in the littoral zone of a shallow model lake for the relationship between the content of phytoplankton pigments (chlorophyll a and carotenoids) and nitrate. The nitrate-nitrogen was selected because it is the most chemically stable nitrogen species. The study was conducted in a small lake with a pronounced autochthonous regime (as indicated by its small drainage ratio) during a vegetation season with contrasting hydrological conditions. The relationship with the nitrate concentration was not confirmed for chlorophyll a, but the levels of nitrate anion and carotenoids were shown to be related. Chlorophyll a, which is regarded as a metabolically active component of phytoplankton cells, shows a close correlation with air temperature. The corresponding prognostic regression models were created. The data on nominal availability of nitrate-nitrogen for primary production show possible nitrogen limitation on some of the observation dates. If the production potential represented by certain levels of nitrate is not realised by phytoplankton, it can be used by filamentous algae. The results of the study provide first steps toward prediction of one of the indicators of phytoplankton growth, the content of two main types of pigment (chlorophyll a and carotenoids).We found that the phytoplankton's carotenoids in a small shallow lake correlate with nitrate-nitrogen. We also show that primary production and the nitrate content can be in stoichiometric relation. The results of the present study provide some materials on nitrogen limitation of freshwater aquatic systems