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Coastal & Estuarine Science News (CESN)

Coastal and Estuarine Science News (CESN) is an electronic publication providing brief summaries of select articles from the journal Estuaries and Coasts that emphasize management applications of scientific findings. It is a free electronic newsletter delivered to subscribers on a bi-monthly basis.

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February 2022

Table of Contents

What Factors Boost Oyster Filtration Services? 

Eastern oysters can filter 60% of an estuary’s volume

One of the most valuable ecosystem services provided by oysters is their ability to improve water quality and clarity by removing suspended material as they feed. Healthy oyster populations exerting top-down control of phytoplankton can drastically enhance water quality, but what factors affect where these filtration services will be realized?

A team of researchers quantified the filtration services of Eastern oysters in the Guana-Tolomato-Matanzas (GTM) system, a well-flushed estuary in northeastern Florida with an extraordinary abundance of oysters believed to resemble the dense, pristine populations described centuries ago. They used a numerical model that included multiple spatial scales and teased apart the influence of reef size, residence time, particle concentration, and other physical factors to enhance ecological realism.

According to their analysis, oyster reefs in GTM can filter approximately 60% of the estuary’s volume within the residence time of the system. However, it was the refiltration of water by downstream populations—rather than residence time—that best explained contributions of individual oyster reefs to estuarine water quality.

They concluded that predictions of oyster filtration services should account for more than residence time and incorporate estimates of reef distribution and localized hydrodynamics. Additionally, accounting for downstream refiltration needs to be explicitly factored into restoration and management efforts where water quality is an important consideration. Prioritizing restoration efforts in areas with long residence times and high particle encounter rates can help optimize resource investment and maximize the effectiveness of restoration and management goals.

Source: Gray, M.W. et al. 2021. Beyond Residence Time: Quantifying Factors that Drive the Spatially Explicit Filtration Services of an Abundant Native Oyster Population. Estuaries and Coasts. DOI: 10.1007/s12237-021-01017-x

Modeling Transport in the Gulf of Mexico

Hindcasting the spread of oil from the Deepwater Horizon spill

Transport processes associated with river inputs, winds, waves, and tides play important roles in moderating coastal geomorphology, biogeochemistry, water quality, and food webs. Formed in response to the 2010 Deepwater Horizon oil spill, the Gulf of Mexico Research Initiative provided opportunities to study transport processes in the region. To more accurately forecast the spread of oil, nearly two dozen researchers reviewed numerical modeling efforts in the Gulf of Mexico, focusing on transport interactions along the river to ocean continuum: wetland, estuary, and shelf exchanges; river-estuary coupling; nearshore and inlet processes; open ocean transport processes; and river-induced fronts and cross-basin transport.

These five interconnected systems act at different scales and provide differing insights into the movement of surface oil patches, and an ensemble of coupled models is necessary to understand the full picture. Open ocean models are needed to determine the supply and movement of oil to nearshore areas, but river-induced fronts also play a role in channeling the oil at a distance from the discharge. Nearshore models help predict which inlets will receive oil. Models that couple rivers, estuaries, and the shelf are useful for determining where discharge from the Mississippi River and its many diversions will travel. However, manipulating water diversion outlets did not effectively prevent oil from reaching beaches and wetlands.

The efficacy of a suite of oil spill models was evaluated by particle tracking and correlation to fisheries and other biological resource data—which found weaknesses in the accuracy of the models’ simulations. Fully coupled models across the ocean continuum, as well as more baseline data (both physical and biological) and better coupling to atmospheric models and data, are needed to accurately determine where oil would end up. Without high quality coupled numerical models in place before a disaster happens, responses will be severely hampered and resources may be spent on the wrong efforts.

Source: Justić, D. et al. 2021. Transport Processes in the Gulf of Mexico Along the River‑Estuary‑Shelf‑Ocean Continuum: a Review of Research from the Gulf of Mexico Research Initiative. Estuaries and Coasts. DOI: 10.1007/s12237-021-01005-1

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What’s Up, Dock?

How to minimize the cumulative impacts of docks

Small docks provide access to estuaries and coastal waterways, but they also impact shoreline ecological function by altering environmental conditions, which in turn can affect habitat and aquatic communities. A team of researchers reviewed the potential impacts of the main structural components of docks (the piles, decking, and floats) on estuarine and coastal flora and fauna—focusing on New England salt marshes and submerged aquatic vegetation (SAV), which are vulnerable to dock-induced habitat alteration.

Environmental impacts of docks depend on the structure size, design, and location, and can include both short- and long-term effects. Because piles displace existing habitats and create new ones, they represent a direct source of habitat alteration. Floats can also cause direct impacts such as crushing vegetation when the floats are grounded or lifting sediments as they rise and fall with the tide. Decking and piles can both cause indirect impacts through leaching, and all three structural components lead to chronic shading.

While the impacts of individual docks are generally minor, dense build-outs that overlap with sensitive coastal areas can result in greater overall fragmentation, alteration, and habitat loss. These effects can be avoided using alternative access approaches like public boat ramps and community docks. Where this is not possible, best management practices (BMPs) involving the siting, timing, installation methods, materials, and designs employed in dock construction can help to reduce impacts. However, adopting BMPs without considering the cumulative impacts of increasing dock proliferation may instill a false sense of environmental preservation.

The authors recommend mapping out important resources and quantifying cumulative impacts at the ecosystem level to place individual docks in context, using BMPs to minimize impacts to sensitive and cultural resources, and requiring mitigation strategies to avoid net habitat loss within a system.

Source: Logan, J.M. et al. 2021. A Review of Habitat Impacts from Residential Docks and Recommended Best Management Practices with an Emphasis on the Northeastern United States. Estuaries and Coasts. DOI: 10.1007/s12237-021-01006-0

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Getting to the Bottom of Hypoxia

Managing low dissolved oxygen in China’s Pearl River Estuary

The dissolved oxygen (DO) content in seawater is important for maintaining the growth and reproduction of marine life, but hypoxia is a worsening problem in estuaries worldwide. DO is an integrated measure of water quality that can be affected by both physical and biochemical processes. So, to better understand the causes of low dissolved oxygen observed in China’s Pearl River Estuary, researchers used a coupled physical-biogeochemical model to explore patterns of hypoxia over an intra-annual cycle. Located in the South China Sea, this estuary is surrounded by several megacities and has experienced increasing incidents of hypoxia over the past 30 years—concentrated in the bottom waters near the Humen outlet and the subestuary outside Modaomen and Jitimen.

Large freshwater inputs from the Pearl River during the wet summer season create strong water stratification, significantly hindering the exchange of DO between the upper and lower waters. Hypoxia begins to emerge in May, peaks in August, and mostly disappears by November. There were differences in the proximate cause for hypoxia in the two areas: Near Humen, hypoxia likely results from the input of low DO water from the river upstream, while outside Modaomen and Jitimen, low DO is likely the result of high sediment oxygen demand (SOD). Riverine-borne particulate organic carbon from terrestrial sources (which settles to the bottom of the estuary) is likely the ultimate cause of much of the SOD in the system. Because it takes two months to process, there’s a lag between the input of the terrestrial organic pollutant and low bottom water DO. As SOD decreases in fall and winter, DO levels gradually recover.

Although hypoxia can be difficult to address, models like the one used here can provide insight into how and why it develops in a given area. For the Pearl River Estuary, actions that increase oxygen concentrations of riverine water, along with those that reduce the input of terrestrial material, could help reduce the occurrence of hypoxia.

Source: Zhang, Z. et al. 2021. On the Intra‑annual Variation of Dissolved Oxygen Dynamics and Hypoxia Development in the Pearl River Estuary. Estuaries and Coasts. DOI: 10.1007/s12237-021-01022-0

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