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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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2020 September

Table of Contents

Prior Land Use Affects Wetland Recovery
Fine-Scale Maps Offer Clearer View of Seagrass Dynamics
What Happens When Coastal Farmlands are Flooded for Restoration?
Oil Platforms Can Serve as Artificial Reefs

Prior Land Use Affects Wetland Recovery
Agricultural history leaves imprint on Tillamook Bay restoration

Removing dikes and tide gates can restore tidal flow, connecting fragmented habitats, reducing floods, and enhancing coastal ecosystem services such as fish production. But how the land was used before restoration may influence the recovery process, especially in its early stages.

At 169 hectares, the Southern Flow Corridor project in Oregon’s Tillamook Bay is one of the largest tidal wetland restoration projects implemented in the Pacific Northwest. Prior to restoration in 2016, land use and land cover varied considerably across the area and included non-tidal freshwater wetlands and pastures that were grazed by livestock, regularly cropped, or abandoned. Researchers sampled from these different zones during the two years before and after restoration and compared them to reference wetlands elsewhere in the bay.

Prior to restoration, cropped areas had lower elevation than reference high marsh sites due to soil compaction and were strongly dominated by nonnative plant cover; less intensively managed areas had intermediate elevations and more native plant cover. Restoring tidal inundation rapidly initiated changes toward reference conditions. However, the magnitude of change in soil and vegetation during this early post-restoration phase varied with pre-restoration conditions. Lower elevations had greater increases in soil salinity and pH and a greater loss of total and nonnative plant cover compared to the less frequently flooded higher-elevation areas within the restored site. Lower elevations also recruited native wetland plants, while freshwater invasive species tended to persist in the higher, and thus fresher, zones.

Understanding the disturbance history and spatial variability of a site can help inform restoration planning, such as the need to manage invasive species or to plant natives. Vegetation at higher elevations may transition more slowly from non-tidal species to diverse tidal wetland assemblages under passive restoration strategies such as removing a dike without other interventions.

Source: Janousek, C.N. et al. 2020. Early Ecosystem Development Varies With Elevation and Pre-Restoration Land Use/Land Cover in a Pacific Northwest Tidal Wetland Restoration Project. Estuaries and Coasts. DOI: 10.1007/s12237-020-00782-5

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Fine-Scale Maps Offer Clearer View of Seagrass Dynamics
Uncovering hidden changes in fragmented landscapes

Seagrasses in many nearshore waters around the world create distinct underwater features that are visible in remotely sensed imagery. And although broad-scale aerial surveys repeated over time can detect changes in seagrass distribution, fine-resolution mapping techniques are needed to better understand seagrass dynamics, especially in fragmented shallow water habitats.

By conducting new fine-scale measurements using existing imagery, researchers hoped to uncover previously undocumented patterns of seagrass dynamics in Florida’s Tampa Bay—where nearly half of all seagrass habitat is patchy despite recent system-wide seagrass recovery. Using GIS software, the team manually digitized and reinterpreted 30 randomly selected estuarine habitats in aerial photographs from 2004, 2006, and 2008. The resulting one-square-meter-resolution maps were used to quantify seagrass change to compare with broad-scale estimates.  

The study found that broad-scale mapping overestimated seagrass coverage in patchy areas and underestimated it in tidal flats. Higher-resolution maps helped to differentiate seagrass from bare sediment and revealed that seagrass coverage in patchy areas was about half of what was previously estimated using broad-scale mapping. On the other hand, fine-scale mapping detected the presence of seagrass on tidal flats that were previously considered “unvegetated.” Although both methods revealed an increase in seagrass cover over four years, the fine-scale maps were more dynamic and showed a larger net change.

As remote sensing classification techniques continue to advance, fine-scale mapping of complex or sparsely covered habitats can improve the accuracy of resource assessments, help validate semi-automated analyses, and assist managers in designating essential habitat and evaluating fragmentation as a habitat indicator.

Source: Kaufman, K.A. et al. 2020. The Use of Imagery and GIS Techniques to Evaluate and Compare Seagrass Dynamics across Multiple Spatial and Temporal Scales. Estuaries and Coasts. DOI: 10.1007/s12237-020-00773-6

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What Happens When Coastal Farmlands are Flooded for Restoration?
Past fertilizer use may cause temporary eutrophication 

Because of the high cost of building defenses to protect coastal farmlands against sea level rise, managers in Denmark have opted instead to restore the least productive of these low-lying agricultural areas into tidal wetlands. Deliberately breaching older engineered defenses allows coastlines to migrate landward, helping to increase wave attenuation and reduce storm impacts. But due to fertilizer use, agricultural soils accumulate large inventories of nutrients, which can be rapidly released by biogeochemical processes when the area is flooded with seawater.

To study the ecological implications of these exported nutrients, researchers sampled from Gyldensteen Coastal Lagoon, which was formed after 214 hectares of farmland were deliberately flooded with seawater in 2014. Originally an old marine lagoon, the area was diked, drained, and then cultivated for agricultural purposes for more than a century. The team used multiple approaches to measure the loss of nitrogen and phosphorus from the soil during the first five years after restoration. According to their analysis, nitrogen loss was initially high but decreased rapidly after the first two years, while phosphorus loss declined after an excessive release during the first year but remained high and stable for at least five years thereafter.

Agricultural soils are a substantial source of nitrogen and phosphorus to the surrounding marine areas, and newly restored coastal lagoons may experience eutrophic conditions for several years until the nutrients are depleted. How quickly an area improves depends on the water residence time and volume of the recipient marine ecosystem. In the case of Gyldensteen Coastal Lagoon, where the excess nutrients are discharged into an open marine environment with rapid water exchange, the eutrophication impact is minimal. However, the authors suggest that it would be detrimental to flood agricultural areas adjacent to small, stagnant water bodies that are more vulnerable to eutrophication.

Source: Kristensen, E. et al. 2020. Nitrogen and Phosphorus Export After Flooding of Agricultural Land by Coastal Managed Realignment. Estuaries and Coasts. DOI: 10.1007/s12237-020-00785-2

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Oil Platforms Can Serve as Artificial Reefs
Platforms in the Gulf of Mexico offer shelter for young fishes

Thousands of oil and gas platforms can be found throughout the northern Gulf of Mexico. Because hard substrate is so rare in this region, these platforms attract a high diversity of fishes looking for food and shelter. To better understand this phenomenon, researchers conducted remote video surveys of the structure and seafloor around 322 small unmanned platforms in waters up to 18 meters deep for two years; surveys were also conducted at a subset of these platforms by a pair of divers.

Fifty-five fish species were documented around these platforms in shallow coastal waters, including the once critically endangered Atlantic goliath grouper. Notably, 29 species were represented at least partially by young-of-the-year or by juveniles less than two years old. The abundance and diversity of fishes differed based on environmental variables such as river discharge, sediment composition, dissolved oxygen, and the distribution of Sargassum macroalgae drifts. For example, the likelihood of red snapper young-of-the-year occurrence increased with bottom dissolved oxygen saturation. Given the seasonal hypoxic zone found in this area, the platforms may provide refuge for young fishes forced by low-oxygen patches to swim up the water column where they would otherwise be exposed to predators. Likewise, when Sargassum drifts wash up on beaches, the associated fishes turn to platforms as alternative habitat.

Currently, decommissioned platforms must be removed within a year, and this is often done using explosives—resulting in high fish mortality. However, this study suggests that nearshore platforms function as nurseries and suitable reef habitat for fishes, which can be important in areas with high nutrient inputs that are susceptible to eutrophication and widespread bottom water hypoxia.

Source: Munnelly, R.T. et al. 2020. Spatial and Temporal Influences of Nearshore Hydrography on Fish Assemblages Associated with Energy Platforms in the Northern Gulf of Mexico. Estuaries and Coasts. DOI: 10.1007/s12237-020-00772-7

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