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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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January 2021

Table of Contents

Invasive Seaweed May Benefit Juvenile Salmon
How Many Herbivores Can a Turtlegrass Meadow Support? 
The Resilience of Texas Estuaries to Hurricane Harvey
Predicting the Future of Seagrass Ecosystems in a Warmer World

Invasive Seaweed May Benefit Juvenile Salmon
Habitat diversity enhances nursery grounds

Over the past century, the invasive brown alga Sargassum muticum has become successfully established along the west coast of North America in areas that also support the native eelgrass Zostera marina. These same areas are home to endangered Chinook salmon, which spend months living in estuaries in the Pacific Northwest during their early (or smolt) life history stages before migrating to the open ocean.

A team of researchers set out to evaluate whether the presence of invasive Sargassum affected the nursery role of nearshore habitats in Washington state. They conducted a small-scale field experiment off San Juan Island where they set up fiberglass mesh cages containing native eelgrass, nonnative Sargassum, mixed eelgrass and Sargassum, or unvegetated bare sand as a control. Hatchery-raised Chinook smolts were introduced and their growth and survival, as well as the availability of invertebrate prey, were measured over three weeks. Salmon smolt growth and survival varied across treatments, as did prey availability. But when these factors were all considered together in a principal components analysis, the researchers concluded that the mixed treatment offered the most positive effects on nursery performance due to the high availability of amphipod prey. They suggest that the complexity created by mixing multiple habitats likely provides more refuge for both juvenile fish and the invertebrates they prey upon.

These findings point to the importance of habitat diversity in providing a particular ecosystem function, and they demonstrate that invasive species can help support coastal nursery functions—but only in the presence of the native eelgrass. Because invasive species are so widespread, future research that builds on this work could lead to more effective management of essential habitat for juvenile fish.

Source: Hughes, B.B. et al. 2020. Native and Invasive Macrophytes Differ in Their Effectiveness as Nurseries for Juvenile Endangered Salmon. Estuaries and Coasts. DOI: 10.1007/s12237-020-00845-7

How Many Herbivores Can a Turtlegrass Meadow Support?
Calculating the carrying capacity of a Florida Bay

After decades of conservation efforts, green turtle populations are bouncing back in many areas, including the northern Gulf of Mexico. In Florida’s St. Joseph Bay, green turtle populations were likely nonexistent three decades ago, but estimates based on aerial footage suggest there may now be as many as 82,000 turtles. Green turtles, as well as variegated sea urchins, feed on the bay’s turtlegrass meadows. But how many of these herbivores can the bay support? A group of researchers set out to estimate the herbivore carrying capacity of the bay by quantifying turtlegrass production, urchin and turtle densities, and their consumption rates.

The study estimated that sea urchins currently consume about 66 percent of the annual turtlegrass produced, and the amount left over can sustain approximately 278 juvenile green turtles per hectare—or about 556,010 turtles in the entire bay. That puts the current juvenile green turtle abundance (an average of 26 per hectare) far below carrying capacity. However, this estimate comes with a caveat: Previous simulations in St. Joseph Bay found that turtle herbivory can reduce turtlegrass productivity by 40 percent. If productivity were reduced, the study suggests that the bay would sustain fewer turtles at current urchin densities. Green turtles in Bermuda, for example, have recently expanded beyond carrying capacity and overgrazed turtlegrass meadows.

Because green turtles are major seagrass consumers, their past overexploitation had freed globally declining seagrass meadows from grazing pressure. With the relatively rapid global green turtle increase, evaluating current densities and future carrying capacities is critical for updating and achieving recovery goals for both green turtles and seagrass meadows. Where populations are approaching carrying capacity, top-down control will likely eventually be necessary to seek a balance between herbivore- and seagrass-dominated systems.

Source: Rodriguez, A.R. and K.L. Heck Jr. 2020. Approaching a Tipping Point? Herbivore-Carrying Capacity Estimates in a Rapidly Changing, Seagrass-Dominated Florida Bay. Estuaries and Coasts. DOI: 10.1007/s12237-020-00866-2

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The Resilience of Texas Estuaries to Hurricane Harvey
Effects on water quality were short-lived

Despite the potentially severe impacts of tropical cyclones, many studies have found that the water quality of affected estuaries typically returns to pre-storm conditions within months, even days. However, longer-term effects have rarely been documented. To quantify both short- and long-term effects of Hurricane Harvey on water quality indicators in three Texas estuaries—Guadalupe, Lavaca-Colorado, and Nueces-Corpus—researchers collected both discrete and continuous water samples from 26 sites before and up to nine months after the Category 4 storm made landfall in August of 2017.

The researchers found estuarine water quality to be resilient, which was surprising given the severity of winds and magnitude of rainfall associated with Hurricane Harvey. A rapid increase in salinity due to storm surge was followed by a rapid decrease as floodwaters reached the estuary. During this period of low salinity, bottom water hypoxia developed and lasted nine days. The decrease in dissolved oxygen led to a decrease in pH, though this returned to pre-storm levels within two weeks. Salinity returned to pre-storm levels within a month. All three estuaries experienced a post-Harvey increase in inorganic nutrients, though these mostly returned to their baseline levels by wintertime.

It’s possible that the storm’s effects on water quality were muted because these are healthy watersheds without prior water quality degradation. Regardless, these short-lived effects did have a negative impact on marine life: Previous studies found a post-Harvey decline in benthic biomass, abundance, and diversity, and reduced catches and minor shifts in fish community structure were observed for a month. This work demonstrates the value of combining high-frequency, continuous monitoring with discrete manual sampling, especially since both the intensity of tropical cyclones and the frequency of high-rainfall events are projected to increase with climate change.  

Source: Walker, L.M. et al. 2020. Timescales and Magnitude of Water Quality Change in Three Texas Estuaries Induced by Passage of Hurricane Harvey. Estuaries and Coasts. DOI: 10.1007/s12237-020-00846-6

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Predicting the Future of Seagrass Ecosystems in a Warmer World
A mechanistic understanding of climate change effects on seagrass performance

The relationship between water clarity and seagrass success is well-established, and the management of seagrass meadows often focuses on nutrient reduction as a means of enhancing light availability. However, seagrasses are also facing threats due to climate change, such as increased temperatures and ocean acidification, both of which also affect their growth and survival. In this perspective, the author reviewed nearly five decades of seagrass studies—ranging from experiments with tissue segments and whole plants to satellite imagery of entire meadows—to help understand how seagrasses respond to shifts in both temperature and CO₂ concentration, and what this means for their productivity.

Studies have consistently found that increasing CO₂ enhances productivity in a wide range of seagrass species by stimulating photosynthesis and plant growth and decreasing light requirements. On the other hand, increasing temperature raises seagrass metabolism, leading to decreased production. Because of the way temperature, CO₂, and light affect plant physiology, the extra CO₂ associated with ocean acidification may allow seagrasses to offset the damaging effects of temperature stress. In fact, light can dampen the temperature response even at present-day CO₂ levels. Consistent with this, model predictions show that coastal areas with good water quality (and hence more light) are more likely to withstand increasing temperatures. Despite this potentially good news, the author points out that there may be other potentially damaging effects associated with this response such as reduced chemical defenses, and seagrasses still remain vulnerable to nuisance algal blooms and local heat waves.

Since the 1970s, seagrass science has transformed from a largely descriptive field to an increasingly quantitative and predictive undertaking that evaluates how the environment influences growth, reproduction, and the flow of energy. This type of mechanistic understanding is important for predicting the future trajectory of seagrass systems.

Source: Zimmerman, R.C. 2020. Scaling up: Predicting the Impacts of Climate Change on Seagrass Ecosystems. Estuaries and Coasts. DOI: 10.1007/s12237-020-00837-7

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