Variations in nutrient ratios and aquatic food webs.
Turner, R. Eugene *,1, Rabalais, Nancy2, 1 Louisiana State University, Baton Rouge, LA, USA2 LUMCON, Chauvin, LA, USA
ABSTRACT- The dissolved Si : dissolved inorganic nitrogen (DSi : DIN) atomic ratios of riverine and coastal waters have been declining in many areas of the world to near a 1 : 1 ratio, primarily because of land use practices affecting nitrate concentrations. These changes will profoundly affect coastal food webs. Diatoms, for example, that are the a most important constituent of coastal phytoplankton, have an intracellualar DSi : DIN ratio of 1 : 1. Also, the regeneration of DSi and DIN in the worlds ocean is approximately 1:1. This led Redfield and others (e.g., Redfield 1934, 1958) to propose the existence of stoichiometric and physiological limits to phytoplankton growth. Results from field and laboratory studies have suggested that the lack of silica relative to nitrogen, can control phytoplankton community composition, and Elser et al. (1996) have shown how nutrient ratios (commonly discussed in terms of the nitrogen : phosphorus ratios) constrain organism organization at the cellular, organismal and community level. If the minimal DSi:DIN proportion of 1:1 for diatoms is not met, then an alternative phytoplankton community composed of non-diatoms may be competitively enabled. Officer and Ryther (1980) argued that as the DSi:DIN ratio fell below 1:1, the fisheries web would re-form and be composed of less desirable species. It turns out to be a correct prediction for the Louisiana shelf near the Mississippi River delta. Thus important fisheries could be affected by the relative quantities of nutrients being loaded into the receiving waters.
Key words: Redfield, ratios, nutrients, coastal
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