Environmental Control of Nitrate Uptake in Surface Oceanic Waters: an Overview

Abstract

Nitrate uptake by natural plankton populations are controlled by external environmental factors. These environmental factors include light intensity, temperature, and concentrations of nitrate, ammonium and other nutrients. Experimental observations on nitrate uptake in oceanic waters are reviewed here. Environmental factors affect nitrate uptake in a complex and indirect way, and the relationship between a certain environmental factor and uptake rates may not be readily identified in field observations. Several characteristic features, however, have been noted. In most cases, larger size species of phytoplankton are dominant in the bloom region where algal biomass is building up, and they contribute to the bulk of nitrate uptake in such waters. Smaller nanoplankton and picoplankton, however, also assimilate nitrate as well as ammonium and urea. In a relatively limited spatial and time scale where other factors are not limiting, a clear temperature dependency of nitrate uptake is demonstrated. Light intensity effectively controls nitrate uptake in most oceanic environments, and results in characteristic diel and vertical variation of the uptake. Low nitrate concentration strongly limits uptake in the surface layer of stratified water columns. The presence of ammonium and urea generally reduce nitrate uptake, although some blooming population shows preference for nitrate over ammonium and urea. The importance of the regenerated forms of nitrogen in the control of nitrate uptake in nitrate-replete waters is suggested, but limited availability of light, iron and silicate is also suggested to suppress nitrate utilization. These observations are yet somewhat qualitative, but will be essential for quantitative provision of nitrate utilization in the world ocean and, hence, new production flux.