Nutrient Inputs and Fish Production
Scott W. Nixon
It has been recognized for most of the past century that there is a causal link between nutrient supply, the level of primary production, and the yield of fish and shellfish from freshwater and marine ecosystems. Quantifying this relationship in the sea has proven difficult for many reasons, not the least of which is that a variety of factors in addition to nutrient supply also influence primary production and fisheries yields. Nevertheless, it has recently proven possible to use field and mesocosm experimental data to establish an empirical relationship between the input of dissolved inorganic nitrogen and primary production by the phytoplankton in a range of coastal, shelf, and open ocean areas.
Similarly, there is a good correlation between reported levels of primary production (14C uptake) and commercial fisheries yields across a wide range of marine systems. Both empirical relationships are non-linear and they can be combined into one computed relationship between nitrogen loading and fisheries yield that is roughly hyperbolic in form. Such a positive result does not, of course, imply that increased nitrogen input will necessarily result in increased fish production in all systems. For example, in areas with strong vertical stratification, nitrogen fertilization may lead to hypoxia or anoxia in the deeper waters with consequent loss of fishery habitat and yield.
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