Marine eutrophication is defined as the reaction of a marine ecosystem to an excessive availability of a limiting nutrient. The ‘limiting nutrient’ concept implies that it is the availability of the limiting nutrient that determines the extent of the primary production in the ecosystem. Here, we assume nitrogen (N) as the limiting nutrient in marine waters.
The modelled impact pathway is limited to waterborne (as total-N) loadings from human activities into coastal marine waters increasing its N-concentrations there (Vitousek et al 1997; Galloway et al. 2004). The emission routes can be direct discharge of N into rivers or coastal areas, as well as nitrogen applications to the soil. Airborne emissions are excluded. The N input to marine coastal waters is assimilated by primary producers (mainly phytoplankton), promoting the increase in planktonic biomass (Nixon et al. 1996; Rabalais 2002). The organic matter (OM) thus synthesized is eventually exported to bottom waters (Ducklow et al. 2001) where its aerobic respiration by heterotrophic bacteria results in consumption of dissolved oxygen (DO) (Cole et al. 1988; Diaz and Rosenberg 2008). If excessive amounts of organic carbon reach the benthic (bottom) layer, DO may drop to hypoxic or anoxic levels (Gray et al. 2002), which may then lead to loss of species diversity (NRC 1993; Socolow 1999; Vaquer-Sunyer and Duarte 2008; Levin et al. 2009; Vitousek et al. 2012). The overall model builds on the environmental fate of N-forms, the biological processes in the entire water column of coastal areas, and on the species response to the depletion of DO, assuming linearity of cause-effect relationships along the adopted impact pathway.Cause-effect pathway for marine eutrophication from anthropogenic emission of waterborne nitrogen