Nutrient Limitation of New Production in the Sea

Abstract

Light and nutrients are the two well-known basic requirements for primary production in the sea and are usually supplied in opposing vertical gradients. When radiant energy of correct wavelengths is available, the vertical advection of nutrients from below the euphotic zone sets the maximum rate of absorption of these new nutrients and the ensuing primary production is considered to be new production (Dugdale and Goering, 1967) in contrast to the regenerated production based upon recirculating nutrients. When Sverdrup (1955) presented a map of world ocean primary production based upon his understanding of the vertical advective regimes of nutrients in various regions, he was actually providing the first global map of new production. Dinitrogen fixation also fuels new production, but this source of new nutrient (along with atmospheric and terrestrial inputs of nitrate and ammonium) is relatively minor compared to the advection of nitrate (e.g., Carpenter, 1983). The fractionation of nitrogen species from new into regenerated forms, through grazing and bacterial activity, and the existence of a practical tracer for nitrogen, the stable isotope 15N (e.g., Dugdale and Wilkerson, 1986), has made it possible to investigate the new and regenerated pathways in the marine ecosystem. The flow of nitrogen through the euphotic zone ecosystem has proved more complex than originally suggested (Dugdale and Goering, 1967) and is more realistically described by several possible schemes, including that of Michaels and Silver (1988), where the size fractions of the phytoplankton and bacteria are included explicitly along with other elements of the microbial loop. Their analysis showed that the microautotrophs provided the major source of new nitrogen for sinking particle formation, with the picoplankton participating primarily within the microbial grazing loop. However, both nanoplankton and picoplankton are capable of new production. For example, unicellular cyanobacteria (Glibert and Ray, 1990) and bacteria (Brown et al., 1975) may use nitrate and were shown to fix dinitrogen (e.g., Mitsui et al., 1986; Martinez et al., 1983).