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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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July 2021

Table of Contents

Measuring Vulnerability Along the East African Coastline
Can Sediment Supply Meet Demand in West Coast Marshes?
Capturing Change on Constantly Changing Barrier Islands
Managing Nitrogen With Oysters

Measuring Vulnerability Along the East African Coastline
Mangroves, coral reefs, and seagrass beds protect millions of people

East Africa is one of the most sensitive and vulnerable regions in the world to climate change. And while the area is also rich with mangroves, coral reefs, and seagrass beds that can provide protection against storms, these marine ecosystems and the human communities living nearby are threatened by a combination of coastal hazards, development pressures, and poor planning.

To help identify where ecosystem-based management should be prioritized, researchers created an Index of Vulnerability to Coastal Change that integrates information on social vulnerability with exposure to coastal hazards. In effect, they combined two indices. One index is formed by biophysical variables: topography and bathymetry, wind and wave exposure, surge potential, shoreline change rates, and presence of coastal habitats. The other index comprises socioeconomic metrics including population growth and density, housing standards, and sanitation. They also assessed the role of coastal habitats in reducing exposure through scenarios involving the presence or absence of mangroves, coral reefs, and seagrasses.

According to the analysis, 22% of East Africa’s coastline—and 3.5 million people—are at high levels of exposure to coastal hazards. These two figures would jump to 39% and nearly 6.9 million people if mangroves, coral reefs, and seagrasses are lost. Kenya and Tanzania benefit the most from natural coastal protection, leaving Madagascar and Mozambique with the largest proportion of their coastlines at higher exposure. In particular, coral reefs protect 2.5 million people from high exposure levels, mostly in large urban centers. Currently, 17 out of 86 coastal districts in Mozambique, Kenya, and Tanzania are considered areas of priority concern; habitat loss would raise this number to 24.

Habitat conservation and restoration must be prioritized to bolster social and environmental resilience in areas with high vulnerability to coastal change. An integrated assessment such as the one produced here can help inform risk-reduction strategies and decision making, including the allocation of international aid.

Source: Ballesteros, C. & L.S. Esteves. 2021. Integrated Assessment of Coastal Exposure and Social Vulnerability to Coastal Hazards in East Africa. Estuaries and Coasts. DOI: 10.1007/s12237-021-00930-5

Can Sediment Supply Meet Demand in West Coast Marshes?
More extreme precipitation events may deliver more sediment

In northern California’s Humboldt Bay, the impacts of climate change are exacerbated by local tectonics, and maintaining the surface elevations of salt marshes to keep pace with sea-level rise requires an adequate sediment supply. Modeling and field studies agree that sediment-rich marshes are more resilient to sea-level rise than sediment-limited ones. A team of researchers hypothesized that increases in the magnitude and frequency of storms will produce an increase in the amount of fine sediment coming downstream from rivers—which may help balance the area’s sediment demand with supply.

They tested their hypothesis by combining streamflow simulations with a sediment-transport model and applying climate change scenarios to investigate the potential impacts on sediment delivery from fluvial sources. These include local basins that discharge directly into Humboldt Bay, as well as the Eel River, which discharges sediment-laden plumes to the coastal margin.

The majority of fine-sediment transport occurs during peak flows in the mountainous basins that drain the study region. According to the analysis, sediment demand caused by the combined effects of tectonic subsidence and sea-level rise may be at least partially (if not wholly) mitigated under future climate conditions. Increases in the magnitude and frequency of extreme precipitation and streamflow events are expected to produce amplified increases in the regional sediment supply. The researchers projected sediment supply increases of 27% for local basins and 53% for Eel River by the middle of the century, and 58% and 99%, respectively, by the end of the century.

This study shows that an increasing sediment supply from fluvial sources may make some coastal areas more resilient to sea-level rise. Local projections of sediment supply under future climate conditions can be used to guide adaptation strategies and to assess the availability of sediment for implementing sediment-based solutions such as natural marsh accretion.

Source: Curtis, J.A. et al. 2021. Amplified Impact of Climate Change on Fine-Sediment Delivery to a Subsiding Coast, Humboldt Bay, California. Estuaries and Coasts. DOI: 10.1007/s12237-021-00938-x

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Capturing Change on Constantly Changing Barrier Islands
It’s important to study habitat oscillations and not just losses and gains

Barrier islands are dynamic environments that evolve with currents, waves, and tides. While these systems help with storm surge reduction and wave attenuation, accelerated sea-level rise and more frequent intense storms are expected to change habitats on these islands drastically over the coming century. Many studies have evaluated habitat shifts by examining net change over time. However, spatial dynamics can also be important.

To fully capture the dynamic nature of barrier islands, a team of researchers utilized an existing approach called change component analysis that can quantify habitat oscillation, which does not contribute to overall net gain or loss of habitat. They applied this tool to predicted habitat maps generated from a previously published modeling effort that examined climate change effects with and without beach and dune restoration on Alabama’s Dauphin Island. The previous study found more extensive changes without the proposed restoration, and this new work extended those results by explicitly quantifying how the study area changed under the modeled scenarios. They found that much of the modeled change that occurred was spatially complex—with habitat oscillations and shifts in habitat classifications that were masked when only the total gains and losses of individual habitat classes were considered. For example, when the habitat category of one area changes (such as from beach to dune) while the reverse change occurs in another area, there would be no net change in quantity of the two categories.

The additional information provided by change component analysis—whether applied to historical and contemporary habitat maps or to predicted habitat maps generated by models—will help better evaluate the effectiveness of restoration activities and help make more nuanced observations of habitat change to assist with complex natural resource management decisions on dynamic barrier islands.

Source: Enwright, N.M. et al. 2021. Assessing Habitat Change and Migration of Barrier Islands. Estuaries and Coasts. DOI: 10.1007/s12237-021-00971-w

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Managing Nitrogen With Oysters
Should bivalve-mediated denitrification be included in nitrogen removal strategies?

Nitrogen pollution, which can cause eutrophication symptoms like harmful algal blooms and low oxygen conditions, is one of the main threats to coastal ecosystems around the world. However, traditional mitigation strategies aimed at source reduction are difficult to implement, and they aren’t always enough to meet reduction goals. An in situ approach involving valuable shellfish like oysters and clams may complement and even enhance the cost efficiency of traditional source control efforts.

Bivalves such as oysters remove nitrogen directly from the water column by ingesting and assimilating suspended particulates containing nitrogen. The sequestered nitrogen can then be removed from the system by harvesting the organisms. However, bivalves also release biodeposits that create an environment that’s favorable for denitrification, the microbial-driven process of converting reactive nitrogen to non-reactive di-nitrogen gas. This provides another pathway for nitrogen removal via bivalves. Microbes in shellfish guts and on their shells may enhance denitrification too.

According to a new summary examining the opportunities and challenges of oyster-mediated denitrification, nitrogen removed via denitrification may be an important complement to biomass harvest. Despite that, bivalve-enhanced denitrification has rarely been incorporated into water quality policy, and the authors argue that incentivizing these in situ practices could help integrate them into existing nitrogen management programs. Additionally, government agencies responsible for implementing total maximum daily loads often consider ancillary benefits in addition to nitrogen reduction, and oyster reefs help clarify water, stabilize shorelines, and provide habitat for invertebrates and fish.

Source: Rose, J.M. et al. 2021. Opportunities and Challenges for Including Oyster-Mediated Denitrification in Nitrogen Management Plans. Estuaries and Coasts. DOI: 10.1007/s12237-021-00936-z

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