Nutrient Cycling

Abstract Nutrient concentrations within aquatic ecosystems have increased markedly during the last fifty years due to a variety of causes. Significant changes to the rates of nutrient cycling, particularly of nitrogen and phosphorus, have influenced their biological availability on a global scale. Three broad groupings of processes that directly influence nutrient cycling within freshwaters are described. Retention modifies the availability of nutrients for transport, while selectivity between individual chemical species results in preferential uptake and/or transport, and finally transformation processes alter the physical state or reactivity of particular nutrients. Differences in spatial and temporal dynamics of delivery and transport mechanisms among individual nutrients have caused variable relative changes in nitrogen and phosphorus concentrations and fluxes. These composite dynamic factors make it difficult to couple causal relationships between nutrient sources and their impacts.