Nitrogen cycling in eutrophic freshwaters

Agriculture and urbanization are important sources of N input to aquatic ecosystems, and this input is tending to increase as agricultural practice is intensified and as urban populations increase. Although a great deal of information is readily available on the concentrations of nitrate entering lakes, lochs and rivers, less is known about the precise fate of the added N, how it is cycled within ecosystems, and the ways in which it is lost from ecosystems. This paper presents information obtained from selected habitats in the northern U.K. on the quantitative significance of N inputs from agriculture and urbanization, and on the ways in which the added N is cycled. Particular attention is paid to key microbiological processes involved in such cycling and to the environmental factors that affect these.