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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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May 2020

Table of Contents

Modeling Coastal Marsh Restoration Benefits in the Gulf of Mexico
Marshes Continue Nitrogen Removal Services Despite Oil Spill
Satellite Images Provide Synoptic Pictures of Hurricane Damage and Recovery
Can Conservation Mitigate Flood Risks in Coastal Cities?

Modeling Coastal Marsh Restoration Benefits in the Gulf of Mexico
Using regional data to predict ecological benefits and set restoration targets

As a result of the Deepwater Horizon oil spill, billions of dollars will be available for habitat restoration in the northern Gulf of Mexico. One approach that’s often used to assess damage and determine the type and quantity of restoration needed is resource equivalency analysis (REA), which looks at both natural resource losses and the predicted ecological benefits of restoration projects. Despite the widespread use of REAs, few detailed applications have been published.

To help set realistic restoration targets, researchers devised a method for quantifying anticipated ecological benefits associated with marsh restoration. In order to demonstrate the utility of their REA model, they quantified the benefits gained from a hypothetical, 20-year salt marsh creation project using dredged material in Louisiana’s Barataria Bay, which was heavily impacted by the 2010 oil spill. The team input existing data on vegetation cover, above- and belowground plant biomass, and the abundance of indicator species including periwinkle snails and amphipods, as well as their rates of recovery following restoration in a typical Louisiana salt marsh.

They found that the recovery trajectories for each marsh component were important drivers of the modeled restoration benefits, with each component recovering at a different rate. For instance, aboveground biomass and amphipod abundance increased immediately after restoration with benefits generally steady over time, though amphipod abundance never fully recovered. In contrast, belowground biomass increased slowly throughout the project’s duration.

This approach can be adapted for marshes elsewhere and could also incorporate other site-specific parameters such as the recovery trajectories of soil, birds, or terrestrial arthropods. REA models can help managers synthesize existing information to make informed assumptions about the ecological benefits associated with their restoration projects.

Source: Fricano, G.F. et al. 2020. Modeling Coastal Marsh Restoration Benefits in the Northern Gulf of Mexico. Estuaries and Coasts. DOI: 10.1007/s12237-020-00706-3

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Marshes Continue Nitrogen Removal Services Despite Oil Spill
Nitrogen cycling recovered after Deepwater Horizon

Anchored between the land and sea, salt marshes are positioned to intercept nitrogen before it enters coastal waters where it can cause problems such as algal blooms and hypoxia. Through a process called denitrification, microorganisms in the soil remove anthropogenic nitrogen – from sources like fertilizer and fossil-fuel combustion – by converting nitrate into inert nitrogen gas.

To see if the 2010 Deepwater Horizon oil spill impacted the nitrogen removal ecosystem service provided by salt marshes, researchers studied four marshes in Louisiana’s Terrebonne Bay from 2013 to 2014. This study builds on work conducted in 2012 at the same sites – two marshes that were exposed to oil and two that remain unoiled. Over the course of one year, they measured soil properties, nitrification and denitrification potentials, and the abundances of archaea and bacteria that are involved in nitrogen cycling.

Although the study found seasonal patterns and variability across sampling locations, the analysis revealed no differences in either nitrification or denitrification potential between the oiled and unoiled marshes. This may be a sign that the marshes have recovered, as other studies have reported that nitrogen cycling is initially affected by the presence of oil. However, the researchers caution that their results may also reflect the fact that oiled sediment was lost to rapid erosion.

Regardless of the reason, the good news is that Deepwater Horizon did not have a lasting effect on the nitrogen removal service provided by salt marshes. However, the loss of coastal land along the Gulf of Mexico – accelerated substantially by the oil spill – is both rapid and ongoing. And when these marshes are lost, the services they provide will be lost as well.

Source: Schutte, C.A. et al. 2020. No Evidence for Long-term Impacts of Oil Spill Contamination on Salt Marsh Soil Nitrogen Cycling Processes. Estuaries and Coasts. DOI: 10.1007/s12237-020-00699-z

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Satellite Images Provide Synoptic Pictures of Hurricane Damage and Recovery
High-resolution time series offer insight into storm effects

Tropical cyclones are known for their vast potential to cause ecological damage, but even within their core path, the damage can vary wildly. High-resolution satellite images are not only useful for observing changes immediately after a hurricane, but importantly, they can also monitor the long-term survival of wetlands – and their possible recovery. 

To study vegetation changes in the Florida Keys after category 4 Hurricane Irma made landfall in September 2017, researchers compiled a time series of cloud-free images taken from the Sentinel-2 satellites: Three were taken in the year before the storm, and 13 were taken over the next 19 months. The team compared leaf density in these 10-meter-resolution images using a common vegetation index combined with available field data and drone imagery.

Significant physical damage to trees was seen in the first post-storm image taken 23 days after the hurricane. The severity of damage varied across the area, with the greatest destruction occurring on islands just east of the hurricane eye. The time series also revealed substantial regrowth of multiple inland vegetation classes (such as palms and hardwood species) in the first three months. However, the storm caused extensive die-offs of both black and red mangroves, which continued to decline for four to five months, with some areas still exhibiting 90% tree mortality after 19 months.

Imaging such a large study area using aerial surveys would be cost-prohibitive, and inaccessible areas are challenging for field surveys. While field sampling is necessary for adequate ground truthing, high-resolution satellite remote sensing makes it possible to track a study area at scales that would otherwise be impossible – an approach that could be broadly applied to monitor not only storms, but also restoration success or wetland response to sea level rise.

Source: Svejkovsky, J. et al. 2020. Satellite Image-Based Time Series Observations of Vegetation Response to Hurricane Irma in the Lower Florida Keys. Estuaries and Coasts. DOI: 10.1007/s12237-020-00701-8

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Can Conservation Mitigate Flood Risks in Coastal Cities?
The importance of nature-based protection in urban hot spots

Although engineered defenses like dams and levees can help coastal cities cope with rising flood and erosion risks, protecting and restoring coastal habitats is comparatively inexpensive and could potentially help the world’s most vulnerable flood-prone populations.  

Using GIS and previously published data on bathymetry and topography, a duo of researchers evaluated 136 coastal cities with more than one million inhabitants to assess the potential for nature-based mitigation strategies. In each case they measured the extent of mangrove forests, salt marshes, seagrass meadows, and coral reefs present along the most likely storm surge pathways from the open sea.

The vast majority of the cities they studied had existing coastal habitats that could provide some protection. In big cities located on large river deltas – such as Guangzhou in China, Guayaquil in Ecuador, Khulna in Bangladesh, New Orleans, and Ho Chi Minh City in Vietnam – the conservation of mangroves and marshes would be a valuable add-on to engineered defense structures. In contrast, 35 cities are located right along the coast or on former wetlands reclaimed for human use, with little or no space between themselves and the sea. These include Amsterdam, London, Montreal, New York, and Tokyo. These cities must rely on classic engineering solutions or, if space is available, some combination of artificial structures and the creation of coastal habitats.

More detailed local-scale assessments will be needed to evaluate the effectiveness of coastal restoration (or even creation) as a strategy for mitigating flood risk. However, in addition to long-term sustainability, ecosystem conservation could also offer beneficial services such as carbon sequestration, production of fisheries, and water quality regulation.

Source: Van Coppenolle, R. et al. 2020. Identifying Ecosystem Surface Areas Available for Nature-Based Flood Risk Mitigation in Coastal Cities Around the World. Estuaries and Coasts. DOI: 10.1007/s12237-020-00718-z

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