Dead zones in the coastal oceans have spread exponentially since the 1960s and have serious consequences for ecosystem functioning. The formation of dead zones has been exacerbated by the increase in primary production and consequent worldwide coastal eutrophication fueled by riverine runoff of fertilizers and the burning of fossil fuels. Enhanced primary production results in an accumulation of particulate organic matter, which encourages microbial activity and the consumption of dissolved oxygen in bottom waters. Dead zones have now been reported from more than 400 systems, affecting a total area of more than 245,000 square kilometers, and are probably a key stressor on marine ecosystems.

Supporting Online Material for Spreading Dead Zones and Consequences for Marine Ecosystems

Chesapeake Bay has been the subject of intensive research on cultural eutrophication and extensive efforts to reduce nutrient inputs. In 1987 a commitment was made to reduce controllable sources of nitrogen (N) and phosphorous (P) by 40% by the year 2000, although the causes and effects of eutrophication were incompletely known. Subsequent research, modeling, and monitoring have shown that: (i) the estuarine ecosystem had been substantially altered by increased loadings of N and P of approximately 7- and 18-fold, respectively; (ii) hypoxia substantially increased since the 1950s; (iii) eutrophication was the major cause of reductions in submerged vegetation; and (iv) reducing nutrient sources by 40% would improve water quality, but less than originally thought. Strong public support and political commitment have allowed the Chesapeake Bay Program to reduce nutrient inputs, particularly from point sources, by 58% for P and 28% for N. However, reductions of nonpoint sources of P and N were projected by models to reach only 19% and 15%, respectively, of controllable loadings. The lack of reductions in nutrient concentrations in some streams and tidal waters and field research suggest that soil conservation-based management strategies are less effective than assumed. In 1997, isolated outbreaks of the toxic dinoflagellate Pfiesteria piscicida brought attention to the land application of poultry manure as a contributing factor to elevated soil P and ground water N concentrations. In addition to developing more effective agricultural practices, emerging issues include linking eutrophication and living resources, reducing atmospheric sources of N, enhancing nutrient sinks, controlling sprawling suburban development, and predicting and preventing harmful algal blooms.

/pdf/chesapeake-bay-eutrophication-scientific-understanding-57pxkb4mp1.pdf

Chesapeake Bay Eutrophication: Scientific Understanding, Ecosystem Restoration, and Challenges for Agriculture

A 52-yr record of dissolved oxygen in Chesapeake Bay (1950–2001) and a record of nitrate (NO3−) loading by the Susquehanna River spanning a longer period (1903, 1945–2001) were assembled to describe the long-term pattern of hypoxia and anoxia in Chesapeake Bay and its relationship to NO3− loading. The effect of freshwater inflow on NO3− loading and hypoxia was also examined to characterize its effect at internannual and longer time scales. Year to year variability in river flow accounted for some of the observed changes in hypoxic volume, but the long-term increase was not due to increased river flow. From 1950–2001, the volume of hypoxic water in mid summer increased substantially and at an accelerating rate. Predicted anoxic volume (DO<0.2 mg I−1) at average river flow increased from zero in 1950 to 3.6×109 m3 in 2001. Severe hypoxia (DO<1.0 mg I−1) increased from 1.6×109 to 6.5×109 m3 over the same period, while mild hypoxia (DO<2.0 mg I−1) increased from 3.4×109 to 9.2×109 m3. NO3− concentrations in the Susquehanna River at Harrisburg, Pennsylvania, increased up to 3-fold from 1945 to a 1989 maximum and declined through 2001. On a decadal average basis, the superposition of changes in river flow on the long-term increase in NO3− resulted in a 2-fold increase in NO3− loading from the Susquehanna River during the 1960s to 1970s. Decadal average loads were subsequently stable through the 1990s. Hypoxia was positively correlated with NO3− loading, but more extensive hypoxia was observed in recent years than would be expected from the observed relationship. The results suggested that the Bay may have become more susceptible to NO3− loading. To eliminate or greatly reduce anoxia will require reducing average annual total nitrogen loading to the Maryland mainstem Bay to 50×106 kg yr−1, a reduction of 40% from recent levels.

/pdf/hypoxia-in-chesapeake-bay-1950-2001-long-term-change-in-483naxog62.pdf

Hypoxia in Chesapeake Bay, 1950–2001: Long-term change in relation to nutrient loading and river flow

A reduction in dissolved oxygen concentration is one of the most important direct effects of nutrient over-enrichment of coastal waters on fishes. Because hypoxia can cause mortality, reduced growth rates, and altered distributions and behaviors of fishes, as well as changes in the relative importance of organisms and pathways of carbon flow within food webs, hypoxia and anoxia can lead to large reductions in the abundance, diversity, and harvest of fishes within affected waters. Nutrient enrichment, however, typically increases prey abundance in more highly oxygenated surface waters and beyond the boundaries of the hypoxic zone. Because of this mosaic of high and low oxygen areas within a system, not only the actual oxygen concentration of bottom waters, but the spatial arrangement, predictability, and persistence of highly oxygenated, high productivity habitats, and the ability of fishes to locate and use those favorable habitats, will determine the ultimate effect of low oxygen on fish populations. Negative effects of hypoxia on fish, habitat, and food webs potentially make both fish populations and entire systems more susceptible to additional anthropogenic and natural stressors.

https://moritz.botany.ut.ee/~olli/eutrsem/Breitburg02.pdf

Effects of hypoxia, and the balance between hypoxia and enrichment, on coastal fishes and fisheries

▪ Abstract Oysters have been introduced worldwide to 73 countries, but the ecological consequences of the introductions are not fully understood. Economically, introduced oysters compose a majority of oyster harvests in many areas. Oysters are ecosystem engineers that influence many ecological processes, such as maintenance of biodiversity, population and food web dynamics, and nutrient cycling. Consequently, both their loss, through interaction of overharvest, habitat degradation, disease, poor water quality, and detrimental species interactions, and their gain, through introductions, can cause complex changes in coastal ecosystems. Introductions can greatly enhance oyster population abundance and production, as well as populations of associated native species. However, introduced oysters are also vectors for non-native species, including disease-causing organisms. Thus, substantial population, community, and habitat changes have accompanied new oysters. In contrast, ecosystem-level consequences of oyste...

Introduction of Non-Native Oysters: Ecosystem Effects and Restoration Implications

Ecological studies, including those focusing on coastal eutrophication, vary in the emphasis they place on species-level vs. ecosystem-level processes. The degree of variation among interacting species in their response to perturbations to the physical environment is likely to be important in determining when species- or population-level processes will strongly affect attributes measured at higher levels of ecological organization. We conducted mesocosm and small-scale laboratory experiments to determine how low oxygen affects predation rates in a zooplankton–fish larvae–larval predator food web typical of mesohaline areas in the Chesapeake Bay. Dissolved oxygen concentrations in bottom waters of the Chesapeake Bay decline during summer to levels that can be physiologically stressful or lethal to animals dependent on aerobic respiration. Our results indicate that the effects of low oxygen on trophic interactions vary among interacting pairs of species in the food web studied. Low but nonlethal dissolved o...

Varying effects of low dissolved oxygen on trophic interactions in an estuarine food web

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Varying effects of low dissolved oxygen on trophic interactions in an estuarine food web