Effects of nitrate load on submerged plant biomass and species richness: results of a mesocosm experiment

Increased nitrogen loading may lead to changes in productivity or biodiversity in freshwater systems. Field surveys have shown reduced species richness of submerged and floating-leaved plant communities in shallow lakes as winter nitrate concentrations, a surrogate for nitrate loading, have risen above 1-2 mg NO(3)-N L(-1). Experimental tank mesocosms, containing about 3 m(3) of water and sediment from Hickling Broad, Norfolk, UK were initially planted with eleven submerged plant species from the lake and its connected waterway. Constant phosphorus loadings (designed to give added concentrations of 50 mu g P L(-1)) were given to all tanks. Four nitrate loadings were given in a randomised block design with twelve-fold replication. Loadings were designed to increase the concentration in the water by 1, 2, 5 adn 10 mg NO(3)-N L(-1) (N1, N2, N5 and N10, respectively) Nitrate loading increased phytoplankton and periphyton chlorophyll a in the N2, N5 and N10 treatments compared with N1. It complementarily decreased total plant volume and had varied effects on different species, with most species indifferent, a few (mostly charophytes) declining above the N1 treatment, and one (Elodea canadensis) performing best in N2 and N5 compared with N1 and N10. Species richness of submerged macrphytes declined with time in all treatments and with increasing nitrogen load in the first year. In the second year, species richness did not further decline in the N1 treatment but declined at increasing rates with increasing nitrogen load in others. The rate of decline in the second year, plotted against nitrate load, fitted an exponential relationship, allowing calculation in inflow water, or of an empirically determined equivalent TN concentration in the lake water of about 1.50 mg N L(-1). This value broadly corresponds with estimates from field data for concentrations associated with declining species richness and is much lower than values currently often found in lowland agricultural areas in Europe.