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

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Spreading Dead Zones and Consequences for Marine Ecosystems

Seagrasses, marine flowering plants, have a long evolutionary history but are now challenged with rapid environmental changes as a result of coastal human population pressures. Seagrasses provide key ecological services, including organic carbon production and export, nutrient cycling, sediment stabilization, enhanced biodiversity, and trophic transfers to adjacent habitats in tropical and temperate regions. They also serve as “coastal canaries,” global biological sentinels of increasing anthropogenic influences in coastal ecosystems, with large-scale losses reported worldwide. Multiple stressors, including sediment and nutrient runoff, physical disturbance, invasive species, disease, commercial fishing practices, aquaculture, overgrazing, algal blooms, and global warming, cause seagrass declines at scales of square meters to hundreds of square kilometers. Reported seagrass losses have led to increased awareness of the need for seagrass protection, monitoring, management, and restoration. However, seagrass science, which has rapidly grown, is disconnected from public awareness of seagrasses, which has lagged behind awareness of other coastal ecosystems. There is a critical need for a targeted global conservation effort that includes a reduction of watershed nutrient and sediment inputs to seagrass habitats and a targeted educational program informing regulators and the public of the value of seagrass meadows.

https://www.jstor.org/stable/10.1641/0006-3568(2006)56%5B987:AGCFSE%5D2.0.CO;2

A Global Crisis for Seagrass Ecosystems

Ocean acidification represents a threat to marine species worldwide, and forecasting the ecological impacts of acidification is a high priority for science, management, and policy. As research on the topic expands at an exponential rate, a comprehensive understanding of the variability in organisms' responses and corresponding levels of certainty is necessary to forecast the ecological effects. Here, we perform the most comprehensive meta-analysis to date by synthesizing the results of 228 studies examining biological responses to ocean acidification. The results reveal decreased survival, calcification, growth, development and abundance in response to acidification when the broad range of marine organisms is pooled together. However, the magnitude of these responses varies among taxonomic groups, suggesting there is some predictable trait-based variation in sensitivity, despite the investigation of approximately 100 new species in recent research. The results also reveal an enhanced sensitivity of mollusk larvae, but suggest that an enhanced sensitivity of early life history stages is not universal across all taxonomic groups. In addition, the variability in species' responses is enhanced when they are exposed to acidification in multi-species assemblages, suggesting that it is important to consider indirect effects and exercise caution when forecasting abundance patterns from single-species laboratory experiments. Furthermore, the results suggest that other factors, such as nutritional status or source population, could cause substantial variation in organisms' responses. Last, the results highlight a trend towards enhanced sensitivity to acidification when taxa are concurrently exposed to elevated seawater temperature.

/pdf/impacts-of-ocean-acidification-on-marine-organisms-j52whpzuru.pdf

Impacts of ocean acidification on marine organisms: quantifying sensitivities and interaction with warming

http://www.jlakes.org/news/hal-200812-sdarticle.pdf

Eutrophication and harmful algal blooms: A scientific consensus

This review provides an integrated synthesis with timelines and evaluations of ecological responses to eutrophi- cation in Chesapeake Bay, the largest estuary in the USA. Analyses of dated sediment cores reveal initial evidence of organic enrichment in ~200 yr old strata, while signs of increased phytoplankton and decreased water clarity first appeared ~100 yr ago. Severe, recurring deep-water hypoxia and loss of diverse submersed vascular plants were first evident in the 1950s and 1960s, respectively. The degradation of these benthic habitats has contributed to declines in benthic macro- infauna in deep mesohaline regions of the Bay and blue crabs in shallow polyhaline areas. In contrast, copepods, which are heavily consumed in pelagic food chains, are relatively un- affected by nutrient-induced changes in phytoplankton. Intense mortality associated with fisheries and disease have caused a dramatic decline in eastern oyster stocks and associated Bay water filtration, which may have exacerbated eutrophication effects on phytoplankton and water clarity. Extensive tidal marshes, which have served as effective nutrient buffers along the Bay margins, are now being lost with rising sea level. Although the Bay's overall fisheries production has probably not been affected by eutrophication, decreases in the relative contribution of demersal fish and in the efficiency with which primary production is transferred to harvest suggest funda- mental shifts in trophic and habitat structures. Bay ecosystem responses to changes in nutrient loading are complicated by non-linear feedback mechanisms, including particle trapping and binding by benthic plants that increase water clarity, and by oxygen effects on benthic nutrient recycling efficiency. Observations in Bay tributaries undergoing recent reductions in nutrient input indicate relatively rapid recovery of some ecosystem functions but lags in the response of others.

/pdf/eutrophication-of-chesapeake-bay-historical-trends-and-1k0ro6bolk.pdf

Eutrophication of Chesapeake Bay: historical trends and ecological interactions

The eastern oyster (Crassostrea virginica) is a non-moving, filter feeding shellfish occurring naturally along the western Atlantic Ocean from the Gulf of St. Lawrence off Quebec, Canada to the Gulf of Mexico and the Caribbean Islands. The eastern oyster has been called the ultimate estuarine animal. It can tolerate a wide range of salinity, temperature, turbidity and dissolved oxygen levels, making it well adapted to the ever-changing conditions of the estuary. The distribution and survival of eastern oysters within habitat types is influenced by abiotic (non-living, physical) factors such as salinity, tide, oxygen levels and flow, as well as biotic (living, biological) factors such as disease, shell erosion caused by other species and predation. North Carolina's oyster stocks are composed of both intertidal (oysters growing between the mean high and low tide levels) and subtidal (oysters growing below the mean low water level) populations.

The Eastern Oyster: Crassostrea Virginica

In many areas of the North American mid-Atlantic coast, seagrass beds are either in decline or have disappeared due, in part, to high turbidity that reduces the light reaching the plant surface. Because of this reduction in the areal extent of seagrass beds there has been a concomitant diminishment in dampening of water movement (waves and currents) and sediment stabilization. Due to ongoing declines in stocks of suspension-feeding eastern oysters (Crassostrea virginica) in the same region, their feeding activity, which normally serves to improve water clarity, has been sharply reduced. We developed and parameterized a simple model to calculate how changes in the balance between sediment sources (wave-induced resuspension) and sinks (bivalve filtration, sedimentation within seagrass beds) regulate turbidity. Changes in turbidity were used to predict the light available for seagrass photosynthesis and the amount of carbon available for shoot growth. We parameterized this model using published observations and data collected specifically for this purpose. The model predicted that when sediments were resuspended, the presence of even quite modest levels of eastern oysters (25 g dry tissue weight m−2) distributed uniformly throughout the modeled domain, reduced suspended sediment concentrations by nearly an order of magnitude. This increased water clarity, the depth to which seagrasses were predicted to grow. Because hard clams (Mercenaria mercenaria) had a much lower weight-specific filtration rate than eastern oysters; their influence on reducing turbidity was much less than oysters. Seagrasses, once established with sufficiently high densities (>1,000 shoots m−2), damped waves, thereby reducing sediment resuspension and improving light conditions. This stabilizing effect was minor compared to the influence of uniformly distributed eastern oysters on water clarity. Our model predicted that restoration of eastern oysters has the potential to reduce turbidity in shallow estuaries, such as Chesapeake Bay, and facilitate ongoing efforts to restore seagrasses. This model included several simplifiying assumptions, including that oysters were uniformly distributed rather than aggregated into offshore reefs and that oyster feces were not resuspended.

https://www.jstor.org/stable/info/3526891

Modeling Seagrass Density and Distribution in Response to Changes in Turbidity Stemming from Bivalve Filtration and Seagrass Sediment Stabilization

Suspension-feeding bivalves couple pelagic and benthic processes because they consume seston from the water column, and their biodeposits (feces and pseudofeces) settle on the sediment surface. Abundant stocks of bivalves can exert grazer control on phytoplankton, and this results in some nitrogen and phosphorus being regenerated to the water column as excreta and via microbial decomposition of biodeposits. Bivalve biodeposition, however, enhances net ecosystem losses of N and P via sediment burial and bacterially mediated, coupled nitrification-denitrification. Bivalve feeding also reduces turbidity and thereby increases light available for microphytobenthos. Although microphytobenthos may compete with nitrifying bacteria for N, potentially reducing coupled nitrification-denitrification, they retain N and P within sediments, further reducing net regeneration to the water column.

Influence of Eastern Oysters on Nitrogen and Phosphorus Regeneration in Chesapeake Bay, USA

Suspension-feeding eastern oysters, Crassostrea virginica, were once abundant in Chesapeake Bay and may then have exerted top-down control on phytoplankton and also reduced turbidities, thereby increasing light available to benthic plants. Alternatively, oysters may have simply recycled inorganic nutrients rapidly back to the water column, with no long-lasting reduction in phytoplankton biomass resulting from oyster feeding activity. To help distinguish between these scenarios, we explored changes in nitrogen fluxes and denitrification in laboratory incubations of sediment cores held under oxic and anoxic conditions in response to loading by pelletized phytoplankton cells, an experimental analog for oyster feces and pseudofeces. When organics were regenerated under aerobic conditions, typical of those associated with oyster habitat, coupled nitrification‐denitrification was promoted, resulting in denitrification of;20% of the total added nitrogen. In contrast, under anoxic conditions, typical of current summertime conditions in main-stem Chesapeake Bay where phytoplankton is microbially degraded beneath the pycnocline, nitrogen was released solely as ammonium from the added organics. We postulate that denitrification of particulate nitrogen remaining in oyster feces and pseudofeces may enhance nitrogen removal from estuaries. In aerobic incubations with sufficient light (70 mmol m 22 s 21 ), a benthic microalgal/cyanobacterial community grew that not only absorbed the inorganic nitrogen released from the added organics but also fixed N 2. This result suggests that an ecosystem dominated by benthic primary production may develop in shallow waters when reduced turbidity associated with bivalve feeding increases light penetration to a level that can sustain benthic microalgal production.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.2002.47.5.1367

Roger I. E. Newell

Papers

Eutrophication of Chesapeake Bay: historical trends and ecological interactions

The Eastern Oyster: Crassostrea Virginica

Modeling Seagrass Density and Distribution in Response to Changes in Turbidity Stemming from Bivalve Filtration and Seagrass Sediment Stabilization

Influence of Eastern Oysters on Nitrogen and Phosphorus Regeneration in Chesapeake Bay, USA

Influence of simulated bivalve biodeposition and microphytobenthos on sediment nitrogen dynamics: A laboratory study