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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March 2021

Managed Ponds Offer Ample Zooplankton for Juvenile Salmon
Has Nutrient Loading to Chesapeake Bay Decreased?
Shorebirds Eat at Night, Too
Tracking Seagrass Survival in Virginia’s Long-Running Restoration Efforts

Managed Ponds Offer Ample Zooplankton for Juvenile Salmon
Chinook salmon grew quickly in San Francisco’s highly modified wetlands

The San Francisco Estuary has lost approximately 80 percent of its historical tidal marsh habitat. When restoration is not possible, taking advantage of managed ponds and other underutilized off-channel habitats could provide a unique opportunity for out-migrating fish. To evaluate the potential of heavily modified ponds as viable alternatives to traditional nursery habitats, researchers caged hatchery-raised juvenile Chinook salmon (Oncorhynchus tshawytscha) for seven weeks in four contrasting locations within Suisun Marsh: a preserved natural tidal slough, a leveed tidal slough, and the inlet and outlet of a tidally muted managed pond established in the 19th century for hunting waterfowl.

Salmon growth rates varied significantly across the four habitats, but surprisingly, the fastest growth didn’t occur in the natural tidal slough that best represents historic salmon habitat. Instead, fish grew fastest at the outlet of the managed pond. This area had the lowest concentration of dissolved oxygen, but that stress was offset by the abundance of zooplankton and cooler, less variable water temperatures. The team observed 100 percent fish survival at this managed pond outlet.

Tidal wetland habitats have decreased globally, and despite conservation efforts, it’s unlikely that highly altered estuaries will be restored to their historical conditions. This field experiment suggests that some potentially overlooked areas could help improve rearing conditions for declining fishes and increase habitat diversity along migratory corridors. Previous experiments in flooded rice fields, for example, also showed enhanced growth rates of juvenile salmon. In this case, managed wetlands could be used to supplement zooplankton during key intervals, such as peak salmon out-migration, and these typically inaccessible habitats could be outfitted with water control structures to enhance connectivity.

Source: Aha, N.M. et al. 2021. Managed Wetlands Can Benefit Juvenile Chinook Salmon in a Tidal Marsh. Estuaries and Coasts. DOI: 10.1007/s12237-020-00880-4

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Has Nutrient Loading to Chesapeake Bay Decreased?
N and P reductions from wastewater and the atmosphere were overwhelmed by increases in agricultural loads

The Chesapeake Bay has a long history of nutrient pollution, and there have been considerable efforts by the government and industry to monitor and reduce the inputs of N and P. These include public investments in clean air, upgrades to wastewater treatment plants, and the implementation of agricultural best management practices.

To see if efforts to reduce N and P loads have improved water quality, researchers turned to the Choptank estuary—a well-monitored basin on the eastern side of the Chesapeake that can serve as a microcosm of eutrophication in Chesapeake Bay as a whole. Both basins have low population densities, and they’re dominated by agricultural land. The team combined their own measurements with publicly available datasets to estimate inputs to the Choptank estuary in 2017, and these were then compared with inputs reported in 1998.

Remarkably, they found a 78 percent decrease in wastewater N and a 91 percent decrease in wastewater P over the study period. Atmospheric N deposition also dropped by 20 percent. In spite of these reductions, however, the overall N load increased by more than 50 percent and the P load rose by 20 percent—changes driven by increases in agricultural export of N and P, which was responsible for nearly all of the nutrient inputs estimated for 2017. Agricultural best management practices such as fertilizer management and drainage control structures have been increasingly implemented by farmers, but these are voluntary, are partially paid for by farmers themselves, and do not necessarily increase crop yields.

Although overall water quality in the Choptank estuary did not improve, the researchers did find evidence for local improvements at the outlets of some small agricultural watersheds and near the outfall of a large wastewater treatment plant—reinforcing the fact that reducing nutrient inputs can indeed help to improve water quality.

Source: Fisher, T.R. et al. 2021. Localized Water Quality Improvement in the Choptank Estuary, a Tributary of Chesapeake Bay. Estuaries and Coasts. DOI: 10.1007/s12237-020-00872-4

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Shorebirds Eat at Night, Too
Godwits wintering in France forage whenever mudflats are available

Birds are typically active during daylight hours since they rely on their vision to forage, though they often change their behavior depending on predation risk, competition, food availability, and weather conditions. Shorebirds that feed on mudflats face an added complication: They’re subject to the tidal cycle, so they can feed only when mudflats are exposed.

The quality of shorebird wintering grounds directly affects their survival, subsequent migration, and breeding success, yet their nocturnal habits are rarely studied. To compare the differences in daytime and nighttime foraging behaviors, researchers fitted GPS loggers on bar-tailed godwits (Limosa lapponica) along the French Atlantic coast. The locations of six females on Ré Island were recorded every 30 minutes over the course of two winters.

The godwits used the entire window of time when mudflats were available—from two hours before low tide to three hours after—spending as much time in feeding areas at night as during the day. Their nocturnal foraging ranges were much smaller than their diurnal ones, and they distributed themselves to avoid competition with each other at night. When it’s harder to visually detect prey, they likely switch from sight-feeding to probing the sediment for polychaete worms. Additionally, individual godwits were extremely faithful to their feeding areas, and they showed strong habitat selection: They preferred seagrass beds in the daytime and oyster parks and beaches at night, potentially because oyster farmers and beachgoers were more likely to be out during the day.

Nocturnal foraging appears to be crucial in the winter for godwits to meet their daily energy requirements. Although their numbers are stable now, godwit populations have previously fluctuated due to degraded wintering habitats and increasing human disturbance. Understanding the nighttime distribution of vulnerable species can help define areas of importance for management and conservation efforts.

Source: Jourdan, C. et al. 2021. Nycthemeral Movements of Wintering Shorebirds Reveal Important Differences in Habitat Uses of Feeding Areas and Roosts. Estuaries and Coasts.

DOI: 10.1007/s12237-020-00871-5

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Tracking Seagrass Survival in Virginia’s Long-Running Restoration Efforts
Short water residence time is the best predictor of eelgrass success

As ecosystem engineers, seagrasses create dense, submerged meadows that provide habitat for fish and shellfish, stabilize coastal sediment, and sequester carbon. But seagrass restoration projects are expensive and time-consuming, and they often fail. Selecting an appropriate site remains a big challenge.

To better understand how seedlings survive and spread, researchers examined both the successes and failures of long-term Zostera marina eelgrass restoration in the Virginia Coast Reserve—the largest and most successful seagrass restoration effort on record, now in its third decade. More than 70 million seeds have been dispersed to reestablish eelgrass, creating a seed source for additional, natural meadow expansion.

For this study, the team tracked plant survival from 2001 to 2015 within restoration plots as well as natural recruitment sites and used models to identify whether success was associated with various environmental predictors. Low water residence time was the single best predictor of eelgrass survival and could be used to prioritize areas for additional seeding and restoration. However, for eelgrass to expand naturally, the seeds need to get there: The researchers found that eelgrass established persistent patches in low fetch areas that were shallow and sandy with low water temperatures and short residence times.

According to the study’s predictive models, the area where eelgrass could potentially survive is triple that of the area they currently occupy—corroborating historical records that show how widespread eelgrass meadows used to be in these bays before they were wiped out in the 1930s during an outbreak of wasting disease. Mapping the total habitable area can help managers plan for successful restoration projects. These results are also useful in the context of marine spatial planning since anticipating eelgrass spread can minimize potential conflict with other uses of water bottom areas, including shellfish aquaculture.

Source: Oreska, M.P.J. et al. 2021. Defining the Zostera marina (Eelgrass) Niche from Long-Term Success of Restored and Naturally Colonized Meadows: Implications for Seagrass Restoration. Estuaries and Coasts. DOI: 10.1007/s12237-020-00881-3

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