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 2019

Swimming signals: Atlantic mummichog and urbanization
A birds-eye view of shoreline change
How to build “softer” seawalls
How can aquaculture curb eutrophication?

Swimming signals: Atlantic mummichog and urbanization
Undisturbed salt marshes provide best habitat

Anthropogenic activities like eutrophication, sea level rise, and habitat loss and fragmentation threaten southern US salt marshes, but the effects of urbanization on these habitats are understudied. A recent study investigated the both the survival and abundance of a sentinel indicator species, the Atlantic mummichog, in six North Carolina creeks, and used a model to relate patterns in these demographics to urbanization and habitat change.

Between 2011 and 2014, weekly loss of mummichogs was high. What’s more, tagged fish showed high site fidelity to each creek, suggesting that this loss was largely an effect of mortality rather than emigration. The highest relative abundance of mummichog was seen in creeks in areas that were characteristic of undisturbed marshes, such as high percentages of marsh edges and greater coverage by marsh within a tidal creek’s watershed than by impervious surfaces. The authors suggest that preserving or restoring salt marsh habitat would help to preserve the abundance of mummichogs.

This work also uncovered a seeming contradiction: apparent survival rates, modeled using data on the resight or recapture histories of marked individuals, were lowest in the creek with the fewest impacts associated with urbanization. The authors attribute this to the creek’s connection to a larger body of water downstream, suggesting enhanced access by predators or greater rates of permanent emigration. These lower rates could also have been due to lower detection probability. More significantly, these results underlined the importance of examining multiple demographics in evaluating impacts on estuarine fish, given the contrasting relationships that survival and abundance each had with the hallmarks of urbanization.

Source: Rudershausen, P. et al. 2019. Survival and density of a dominant fish species across a gradient of urbanization in North Carolina tidal creeks. DOI: 10.1007/s12237-019-00575-5

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A birds-eye view of shoreline change
Combining satellite images can reveal dynamic trends over short time periods

Remote sensing can provide a helpful means of detecting shoreline change caused by both natural and anthropogenic coastal processes. Few studies have used multi-temporal remote sensing data to study gradual changes over time frames of less than ten years; because satellite imagery offers coarse spatial resolution, it can be difficult to detect subtle changes over short periods of time. Yet work completed in Mexico, using ten years of data from a satellite imaging system known as SPOT-5, showed that image fusion on these images could be used to detect trends in shoreline change over this relatively short time.

This work used SPOT-5 images from the northwest Yucatan coast. Data were collected across multiple light spectra, which were then combined to enhance both the visual quality and the spatial resolution of the images. This technique is known as image fusion or pan-sharpening. This method was used to examine patterns in coastal change, such as shoreline retreat, on both pristine and human-impacted coasts. The results demonstrate the real-world usefulness of this method. For example, the researchers found that an area that was too remote to otherwise study had almost no shoreline stability, despite being entirely removed from human impacts — indicating that some other processes are controlling shoreline change.

Previous research has applied data fusion to shoreline mapping, but this study is the first known application of this method to assess dynamic shoreline trends. The authors make a point of noting that the technique is not the most precise satellite detection method, and that enhancements may be needed to identify specific study conditions. However, this technique offers an alternative for studying areas that are inaccessible and/or lack field data, as well as a way of assessing shoreline trends where funding may be limited.

Source: Ruiz-Beltran, A.P., A. Astorga-Moar, P. Salles, C. Appendini. 2019. Short-term shoreline trend detection patterns using SPOT-5 image fusion in the northwest of Yucatan, Mexico. Estuaries and Coasts. DOI: 10.1007/s12237-019-00573-7

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How to build “softer” seawalls
Enhancing hard infrastructure with green solutions

In recent years, coastal engineers have increasingly considered “green” solutions to protect coastlines, such as shore nourishment, natural structures like oyster reefs, or combining human infrastructure with coastal habitat like salt marshes. These solutions cause fewer environmental impacts than traditional “hard” solutions -- structures like groynes, breakwaters, seawalls, and dikes. Yet the use of a soft solution is not possible on every piece of coastline, due to local hydrodynamic forcing and space considerations. With the expectation that more hard infrastructure will be needed to combat sea level rise, a literature review examined where soft-solutions can be applied (be it in combination with hard structures vs. completely soft), under what circumstances hard structures are still preferable over soft, and how hard solutions can be made into more sustainable “green” infrastructure in cases where they are needed.

This review describes design considerations for the most common hard structures used both inshore and foreshore, and how each of these structures may alter coastal processes. It also discusses how ecosystem engineering can be complementary to traditional hard solutions, measures that can mitigate morphologic and hydrodynamic changes, and adaptations that ecologically enrich conventional grey structures with living communities. For example, seawalls can be constructed or altered to maximize surface roughness and incorporate a variety of microhabitats, providing the same level of coastal safety while enhancing habitat diversity and complexity.

The review includes guidelines and policies from around the world that encourage natural processes. However, the authors note that there is a need for detailed design guidelines that address effectiveness and performance of nature-based coastal solutions, with the goal of making it just as easy for coastal managers to choose green infrastructure as it is to construct a jetty.

Source: Schoonees, T., A. Gijón Mancheño, B. Scheres et al. 2019. Hard Structures for Coastal Protection, Towards Greener Designs. DOI: 10.1007/s12237-019-00551-z

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How can aquaculture curb eutrophication?
Conservatively managed operations can limit the effect of nutrients

The aquaculture industry is growing quickly all over the world, and is expected to continue expanding. Yet aquaculture has a significant environmental impact that will need to be addressed as it grows, as these operations contribute organic matter that can cause eutrophication and algal blooms. In a study conducted in coastal south Korea, researchers examined how different types of aquaculture — and different management strategies — influence organic matter reaching the seafloor.

These authors measured oxygen demand and nutrient flux in sediments below a large finfish aquaculture operation, a long-line oyster aquaculture site, and a smaller, experimental finish operation where fewer fish were stocked. The differences they found were stark. Both of the large aquaculture sites had much higher rates of sedimentation than their paired controls, and saw up to ten times greater rates of nitrogen and phosphorus flux. Even more telling was the difference in sediment oxygen demand (SOD), which measures the total oxidation of organic materials and serves as a proxy for total organic material enrichment. SOD was 3.6 times higher at the fish farm than at the controls. Although SOD at the oyster farm was lower, it was still 2.7 times higher than that seen in the control.

In contrast, the sediment oxygen demand at the smaller fish farm was not significantly higher than that of the control sites, even given findings that suggested a significant portion of the demand came from epiphytes growing on the farm’s structures. Sedimentation rates at the experimental fish farm were half those of the finfish farm, and comparable to the controls. These results were encouraging, and suggests that the eutrophication from finfish aquaculture could be controlled via conservative management practices, including taking into account the effect of biofouling organisms.

Source: Kim, S.H. H.C , Kim, S.H. Choi et al. 2019. Benthic Respiration and Nutrient Release Associated with Net-cage Fish and Long-line Oyster Aquaculture in the Geoje-Tongyeong Coastal Waters in Korea. DOI: 10.1007/s12237-019-00567-5

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