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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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December 2022

Table of Contents

Bay Beaches Erode Differently

A 20-year analysis of Apam Beach in Ghana

In Ghana, the average rate of beach erosion is two meters per year, though rates can be as high as 17 meters per year. Most of the coast is eroding except for bay beaches, which are situated within or surrounded by headlands. To better understand this anomaly and assess their long-term stability, researchers analyzed the evolution of a 1.9-kilometer-long bay beach near Apam on the country’s busy south-central coast. They evaluated shoreline and sediment volume changes using data from diverse sources: two decades of freely available satellite images and tide and wave data, as well as UAV mapping over five months for a portion of the beach.

Between 2000 and 2020, Apam Beach experienced an average accretion of 0.60 meter per year. But the beach is dynamic: Intermittent years of erosion as well as beach-wide spatial variability was observed, with some years showing erosion in one area and accretion in another. This can vary by month as well. In places where the team obtained high-resolution UAV imagery, the shoreline advanced seaward from January to May 2021 at an average distance of 6.5 meters. In one instance, the sediment volume change from one month to the next declined by several thousand cubic meters, a greater than tenfold reduction. Tides and waves explained 46% of variation in the rate of shoreline change.

This overall net accumulation of beach sediment—albeit under low to moderate wave conditions—can be attributed to fluvial discharges from the estuary and the topography of bay beaches. The latter acts to modify waves and keep sand on the beach, making them relatively more resistant to erosion. This study highlights the importance of multiyear studies as well as understanding the dynamics of bay beaches. Because this bay beach is in a dynamic equilibrium state, future coastal developments must be carefully considered or risk changing an accreting beach to an erosional one.

Source: Abdul‑Kareem, R. et al. 2022. Shoreline Variability of a Bay Beach: The Case of Apam Beach, Ghana. Estuaries and Coasts. DOI: 10.1007/s12237-022-01110-9

Fish Communities Don’t Always Change With Salinity

Long-term monitoring reveals little impact of freshwater inflow in Florida

Coastal managers have long focused on the importance of managing freshwater flow to estuaries, as well as the timing of its arrival. And although there are many examples of the influence of salinity on fish communities, this may be an oversimplification. 

The Ten Thousand Islands in southwest Florida provide an ecosystem-scale opportunity to investigate how differences in freshwater flow affect the community structure of small and juvenile prey fish communities. Researchers compared three similar estuarine bays (Faka Union, Fakahatchee, and Pumpkin) with different hydrologic alterations that resulted in enhanced, reduced, or more natural freshwater inflow. Using water quality and bottom trawl monitoring datasets spanning 20 years (and more than 1,600 trawls), researchers evaluated relationships between fish community structure, salinity, and temperature.

Temperatures averaged over the two to three months prior to fish sampling had the highest correlation to fish communities in all three bays, suggesting that the main influence on fish communities appears to be simple seasonal patterns in spawning—rather than differences in salinity. They also found that differences in community structure between the bays were the result of different abundances of a few common species that were tolerant of a wide range of salinity, and not due to different suites of more specialized species. They conclude that restoration efforts underway to reduce freshwater drainage in Faka Union (and thereby increase salinity) will not shift the community as long as the conditions remain within the range of those experienced during the study.

These results contrast with studies in systems where sustained salinity fluctuations between wet and dry seasons have resulted in shifts in community structure. This once again highlights the risks of overgeneralization and the need to understand each system in context. 

Source: Kendall, M.S. et al. 2022. Too Much Freshwater, Not Enough, or Just Right? Long‑Term Trawl Monitoring Demonstrates the Impact of Canals that Altered Freshwater Flow to Three Bays in SW Florida. Estuaries and Coasts. DOI: 10.1007/s12237-022-01107-4

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Daily Mean Discharge Data Doesn’t Capture Compound Flooding

A historic analysis of extreme events in the UK

In estuaries, compound flooding arises when extreme sea levels and high river discharges occur at the same time, or in close succession. In fact, heavy rainfall, storm surges, and astronomical tides are drivers of flooding that rarely occur independently. This threatens low-lying coastal regions everywhere, including many of the world’s largest cities. To help identify the likelihood and timing of compound events and susceptible estuaries in Britain, researchers analyzed 30 years of historical sea level and river discharge data (at 15-minute instantaneous output frequency) from 27 tide gages linked to 126 river gages. They evaluated five different methods for identifying extremes in river records, calculating the durations, relative timings, and co-occurrences of these events.

Different methods of data selection and identification of peak river discharge events generated different results. But notably, daily mean discharge data did not accurately capture the likelihood of compound flooding—underestimating the magnitude and timing of peak discharge events in 68% of estuaries. Extreme river flow durations in most catchments across Britain from 1984 to 2013 were shorter than 24 hours, and therefore data at higher resolutions than daily are needed, preferably at 15-minute frequencies. The team identified 46 river gages in north and northwest Britain where river discharge and sea level extremes­ coincided the most. Additionally, at all study sites, compound events were more likely in the winter.

Future studies should try to understand how changing river and sea level climates may influence the susceptibility of estuaries to compound events. In the UK, for example, coastal flooding is rated as the second-highest risk of civil emergency. Given the widespread potential for compound flooding, understanding the timing and magnitude of interacting drivers will help support forecasts and warnings, emergency response, and long-term management plans. 

Source: Lyddon, C. et al. 2022. Historic Spatial Patterns of Storm‑Driven Compound Events in UK Estuaries. Estuaries and Coasts. DOI: 10.1007/s12237-022-01115-4

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Where Does Citizen Science Fit? 

Hundreds of stakeholders weigh in on the management of Chesapeake Bay

Despite extensive and sustained monitoring efforts in Chesapeake Bay, the resulting data have not been sufficient in spatial and temporal scale for detecting the efficacy of management actions taken to meet nutrient reduction requirements. In response, there’s been a recent push to integrate data collected by volunteers into the watershed-wide monitoring program and to make citizen science data more broadly usable. Not only would collaborations between scientists and volunteers help answer locally relevant questions, but greater sampling intensity would also provide much-needed responsive feedback on management actions. To better understand environmental stakeholders’ perspectives on the role of science and citizen science in bay management, a University of Maryland team surveyed 372 Chesapeake Bay managers, scientists, educators, policymakers, waterkeepers, and citizen scientists representing at least 190 organizations across the watershed.

On average, survey respondents felt they should have more influence in environmental management decisions, though the degree of desired influence varied among stakeholder groups. Waterkeepers and scientists within academia reported feeling the highest degrees of disempowerment, whereas environmental managers and policymakers perceived the smallest difference between their ideal and current levels of influence. Stakeholders broadly agreed that while citizen scientists should influence public policy comparatively less than professional environmental scientists, citizen scientists should ideally have more influence than they have currently.

Although there were mixed perspectives on the utility of volunteer monitoring for research and management, it was recognized that citizen scientists can play an important role by serving as advocates for change, helping to fill data gaps, and engaging more (and in particular, underserved) community members. The study authors support expanding stakeholder engagement and believe that citizen science is a promising avenue to help scientists and managers as they continue to develop more inclusive decision-making processes.

Source: Webster, S.E. & W.C. Dennison. 2022. Stakeholder Perspectives on the Roles of Science and Citizen Science in Chesapeake Bay Environmental Management. Estuaries and Coasts. DOI: 10.1007/s12237-022-01106-5

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