Abstract
Coastal eutrophication caused by anthropogenic nutrient inputs is one of the greatest threats to the health of coastal estuarine and marine ecosystems worldwide. To better understand and manage this threat, we compared six contrasting coastal ecosystems that are subjected to a range of riverine inputs of freshwater (buoyancy) and nutrients to address (i) impacts of anthropogenic nutrient inputs on ecosystem services; (ii) how ecosystem traits minimize or amplify these impacts; (iii) synergies among pressures (nutrient enrichment, over fishing, coastal development, and climate-driven pressures in particular); and (iv) management of nutrient inputs to coastal ecosystems. Globally, ~ 24% of the anthropogenic N released in coastal watersheds is estimated to reach coastal ecosystems. Our comparative assessment revealed that (i) in terms of the spatial extent of habitat degradation, Chesapeake Bay ranks number one followed in rank order by the northern Gulf of Mexico, the Baltic Sea, Great Barrier Reef, East China Sea and the northern Adriatic Sea; (ii) impacts of increases in anthropogenic nutrient loading are, and will continue to be, exacerbated by synergies with other pressures including over fishing, coastal development and climate-driven increases in sea surface temperature, acidification and rainfall; and (iii) when defined in terms of quantitative ranges of primary production, trophic status is not useful for relating anthropogenic nutrient loading to impacts. While managed reductions in point source inputs from sewage treatment plants are increasingly successful, controlling inputs from diffuse sources remains a challenging problem. Thus, it is likely that the severity of coastal eutrophication will continue to increase in the absence of effectively enforced, ecosystem-based management of both point and diffuse sources of nitrogen and phosphorus. This requires sustained, integrated research and monitoring, as well as repeated assessments of nutrient loading and impacts. These must be informed and guided by ongoing collaborations among scientists, politicians, managers and the public.
Highlights
During the course of the Twentieth century, increases in anthropogenic inputs of nitrogen (N) and phosphorus (P) to coastal ecosystems via river discharge to coastal ecosystems became the primary cause of eutrophication and consequent ecosystem degradation in coastal ecosystems worldwide (Rabalais et al, 2009, 2010; Paerl et al, 2014), a trend that is arguably the most widespread anthropogenic threat to the health of coastal ecosystems (Rabalais et al, 2009, 2010; IPCC, 2014)
Sustainable development depends on healthy ecosystems that provide four categories of services valued by society (Millennium Ecosystem Assessment [MEA], 2005; United Nations Environment Programme [UNEP], 2006; Malone et al, 2014; Culhane et al, 2020):
We have selected a set of coastal ecosystems that have been impacted by anthropogenic nutrient loading: three open continental shelf ecosystems, three semi-enclosed ecosystems (Baltic Sea, Northern Adriatic Sea, and Chesapeake Bay), and one eastern boundary upwelling system (Santa Barbara Channel in the California Current system)
Summary
During the course of the Twentieth century, increases in anthropogenic inputs of nitrogen (N) and phosphorus (P) to coastal ecosystems via river discharge to coastal ecosystems became the primary cause of eutrophication and consequent ecosystem degradation in coastal ecosystems worldwide (Rabalais et al, 2009, 2010; Paerl et al, 2014), a trend that is arguably the most widespread anthropogenic threat to the health of coastal ecosystems (Rabalais et al, 2009, 2010; IPCC, 2014). Evidence suggests that the coastal ocean as a whole has become net autotrophic (primary production of organic carbon > respiratory metabolism of organic carbon) due to increases in anthropogenic inputs of inorganic nutrients (Deininger and Frigstad, 2019). This is consistent with the conclusion that increases in autochthonous phytoplankton production are the primary cause of cultural eutrophication in coastal ecosystems (Rabalais et al, 2009; Bauer et al, 2013). Impacts of anthropogenic N enrichment on coastal ecosystems; (ii) how ecosystem-specific characteristics minimize or amplify these impacts; (iii) synergies between nutrient enrichment and other anthropogenic pressures; and (iv) the management of anthropogenic nutrient inputs and their impacts
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