Some effects of Eutrophication on pelagic and demersal marine food webs

This chapter reviews some of the possible consequences of increased nutrient loading and changing nutrient ratios to marine food webs, especially for larger organisms, and including the northern Gulf of Mexico. Selected results from various ecosystem models, knowledge of biomass size spectrum, literature and field studies, comparative ecosystem analyses, and implications from site-specific studies are summarized. Early models and subsequent improvements revealed the sensitivity of the pelagic and demersal consumers to the meager transfer of energy and carbon between trophic levels (upper limit about five or six), and are supported by empirical results documented for many systems. Less than 1% of the phytoplankton production will become fish biomass, which is most often as pelagic, not demersal, consumers. Eutrophication, by definition, increases the amount of primary production available, but the increase is not transferred throughout the food web with linearity or proportionality. Instead, higher nutrient loading may shift carbon flow within and through the 'microbial loop', diminish the percentage flowing into fecal pellet production, and increase flows toward phytoplankton cell aggregations that sink to lower layers. Eutrophication will be accompanied by greater carbon burial rates and export away from hypoxic/anoxic zones which become larger and longer-lasting; higher fisheries production should not be anticipated with eutrophication. Indeed, if the Si:DIN atomic ratios in water loading into continental shelves falls below 1:1, then there is likely to be a severe disruption in the diatom-copepod-fish food web, and more frequent toxic and noxious phytoplankton blooms.