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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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March 2019

Table of Contents

Eelgrass habitat not a straightforward boon for oysters
Assessing the risks of harbor dredging on diamondback terrapins
Bridging the harbor: Do phytoplankton respond?
High temperatures and macroalgae make a deadly duo

Eelgrass habitat not a straightforward boon for oysters
Influence of eelgrass is offset by species identity, predators, and food availability

Oysters and the habitats they create offer many benefits for coastal ecosystems, as well as the potential for commercial harvesting. The presence of seagrasses, such as eelgrass, is thought to benefit oysters, and seagrass beds are often targeted for oyster seeding -- yet a recent study shows that the relationship between oysters and seagrass is far from a simple one.

The authors hypothesized that, regardless of species, seagrass would reduce both predation and environmental stressors for oysters, providing a beneficial effect. However, they also predicted that because seagrass would reduce water flow, oysters in these habitats would have less food available. The study’s results were far from straightforward. Overall, survival of both native (Ostrea lurida) and non-native (Crassostrea virginica) oysters -- planted in two estuaries, across six sites that represented a diverse range of local pH, salinity, and temperature -- was significantly lower within eelgrass in all but the most upstream location. Although food availability was indeed correlated with growth, it did not vary in the presence of eelgrass. The prediction that eelgrass would shield oysters from predation was also disproven in a rather dramatic fashion, as two sites were decimated by drills prior to data collection regardless of eelgrass cover. 

Though seagrass beds are not necessarily the “safe haven” initially hypothesized, there were additional insights to be gained from this experiment: for example, shell strength of native oysters increased in eelgrass habitat, whereas that of non-native oysters decreased. Overall, this research shows the presence of seagrass is only one factor that influences oysters, with factors like local food availability, species identity and the pressures of predators playing just as large a role in their success.

Source: Lowe, A.T., J. Kobelt, M. Horwith, J. Ruesink. 2018. Ability of Eelgrass to Alter Oyster Growth and Physiology Is Spatially Limited and Offset by Increasing Predation Risk. Estuaries and Coasts. DOI: 10.1007/s12237-018-00488-9

Assessing the risks of harbor dredging on diamondback terrapins
Cape Cod’s terrapins at low risk during winter dredging

Wellfleet Harbor, in Cape Cod, Massachusetts, is host to the northernmost population of diamondback terrapin turtles, which are listed as threatened under the state Endangered Species Act. During the winter this turtle enters a hibernation state known as brumation, during which they lie in immobile clusters on the seafloor. Terrapins in this state are at considerable risk of  mortality if they brumate within part of a harbor that is dredged to maintain navigation channels, which is an activity generally reserved for the winter in order to minimize disruption to the ecosystem.

A team of researchers assessed the winter dredging risk to Wellfleet’s terrapin population by fitting a total of 100 terrapins with acoustic transmitters and subsequently tracking them across three years, using receivers stationed around the harbor from April to December. Their work examined both whether terrapins were brumating within the dredge zone, and whether they displayed enough mobility to suggest they could avoid dredging if needed.

The highest mobility of terrapins occurred in May, which is during the breeding season, whereas the lowest mobility was, unsurprisingly, during winter brumation. The highest occupancy within the dredge zone occurred from May through early July. In contrast, only three brumating individuals were detected in the dredge zone across the three years of the study, with approximately 97% of turtles moving into neighboring creeks over this period. As such, the authors calculate that the combined risk to terrapins during dredging months appears to be small, with reduced mobility in the fall and winter offset by the animals’ low occupancy of the dredge zone. The authors also affirmed that avoiding dredging in the warmer months helped to protect terrapins: while terrapins demonstrated the greatest mobility during summer months, their occupancy of the harbor was so elevated that dredging during this period would likely pose the greatest risk to these endangered turtles.

Source: Castros-Santos, T., M. Bolus, A.J. Danylchuk. 2018. Assessing Risks from Harbor Dredging to the Northernmost Population of Diamondback Terrapins Using Acoustic Telemetry. DOI: 10.1007/s12237-018-0481-9

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Bridging the harbor: Do phytoplankton respond?
Shallow reefs could shelter low-lying marshes from erosion

In the late 1950s a bridge was built across Shippagan Harbor, a coastal strait in New Brunswick, Canada. The bridge divided the area into an inner and outer bay, and decreased fetch over the inner bay by 76 percent. In a recent paper, researchers evaluated whether such a dramatic change affected phytoplankton. They hypothesized that the bridge should have led to a higher abundance of phytoplankton in the inner bay, as reduced hydrological flow concentrated nutrients from the land and altered the geochemistry of the inner as compared to the outer bay. They evaluated this based on algal pigment concentration and sediment organic matter, measured in a series of sediment cores that approximately dated back to 1930.

Although the inner and outer bays did display distinct geochemical patterns once the bridge was in place, there was no long-term relationship with phytoplankton abundance. Phytoplankton biomass did rise between 1950 and 1960, shortly after bridge construction, but this increase was temporary -- even as the inner bay saw an increasing influx of industrial and municipal waste, which might have been expected to fuel these communities. In addition, variations in the geochemistry of the inner bay over time were more closely linked with changes in how wastewater was treated, the release of effluent from a nearby fish plant, and the area’s connectivity with the open ocean. Combined, this suggests that, at least in this case of a bridge over a strait, the presence of a bridge did not have a strong effect on either sediment geochemistry or the phytoplankton that responds to it in the inner harbor, even as the bridge altered the surrounding hydrology.

Source: Patoine, A., M.K. Karmakar, P.R. Leavitt. 2018. Effects of Bridge Construction and Wastewater Effluent on Phytoplankton Abundance and Sediment Geochemistry in an Atlantic Temperate Coastal Bay Since 1930. DOI: 10.1007/s12237-018-0483-7

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High temperatures and macroalgae make a deadly duo
Warm waters and algal blooms combine to cause coastal hypoxia

Macroalgal blooms and rising water temperatures are increasingly frequent and co-occurring threats to marine life. Both of these stressors have long been linked to low oxygen conditions in coastal areas -- yet their combined effect on hypoxia occurrence and intensity remains poorly understood, especially given that drift algae is difficult to study. Recent research in the shallow, eutrophic Nakaumi Lagoon in western Japan used drift algae within enclosures to simulate the synergistic influence that these factors have on oxygen concentrations.

Over the course of ten 7-day experiments, the researchers found that daily variations in minimum dissolved oxygen were closely linked with the combination of high temperatures and the presence of macroalgae. When macroalgae was present, temperatures above 25°C rapidly increased the occurrence of hypoxia (defined as dissolved oxygen levels less than 2.0 mg/L), with daily minimum oxygen decreasing more steeply over time than seen at the same temperatures without algae. Anoxia (when dissolved oxygen levels equal zero) was reached when algae was present at temperatures above 29°C. The authors noted that these strong effects may be more likely in stagnant, poorly-flushed waters like those of the Nakumi lagoon.

As the authors emphasize, no variable of ecological importance to coastal ecosystems has changed so drastically over recent years as dissolved oxygen levels, which are decreasing worldwide. Moreover, oxygen levels are only predicted to continue decreasing with future warming. Research like this demonstrates the importance of managing nutrients in coastal waters; while worldwide management will be necessary to reduce global temperatures from rising, actions that prevent algal growth might be enough to help stave off hypoxia.

Source: Miyamoto, Y., T. Nakano, K. Yamada, K. Hatakeyama, & M. Hamaguchi. 2018. Combined effects of drift macroalgal bloom and warming on occurrence and intensity of diel-cycling hypoxia in a eutrophic coastal lagoon. DOI: 10.1007/s12237-018-0484-6

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