CESN Main Page

Coastal & Estuarine Science News (CESN)

You can have future issues delivered to your email inbox on a quarterly basis. Sign up today!

2018 March

Contents

Do Seagrass Meadows Provide Refuge from Acidification?
Assessing Fish Responses to Drought
Living Shorelines’ Benthic Benefits
Blue Carbon in Tampa Bay

 Do Seagrass Meadows Provide Refuge from Acidification?

Seagrass buffering capacity may be limited

Ocean acidification makes it more difficult for many marine organisms to grow and maintain their shells and skeletons. Some have speculated that highly productive marine habitats such as seagrass meadows could offer a refuge from the effects of ocean acidification, thanks to their ability to mineralize excess carbon dioxide and locally increase seawater pH. To test this idea, the authors of a new study evaluated two very different seagrass habitats—one in Bailey’s Bay, Bermuda, and one in Mission Bay, California.

The researchers examined the seagrass beds’ potential to provide local buffering against acidification at each site by collecting water chemistry data across space and time at each site with a combination of autonomous data loggers and boat-based surveys. Dissolved oxygen, dissolved inorganic carbon, and pH varied according to daily cycles at both sites, rising and falling in patterns that suggested that seagrass metabolic processes such as photosynthesis and respiration were their main drivers. These factors also varied spatially within each site, as did total alkalinity, although alkalinity was affected by tidal cycles and changes in salinity rather than seagrass metabolism.

The pH response during the study shows that seagrass metabolic processes have detectable local effects. However, although small benefits may occur during peak photosynthesis, these effects may actually reverse overnight as pH drops during times of peak respiration. Ultimately, seagrass meadows may not be the magic bullet some had hoped for, and further studies are needed to assess how water chemistry within seagrass beds varies across longer time periods.

Source: Cyronak. T., A.J. Andersson, S. D’Angelo, P. Bresnahan, C. Davidson, A. Griffin, T. Kindeberg, J. Pennise, Y. Takeshita, and M. White. 2018. Short-Term Spatial and Temporal Carbonate Chemistry Variability in Two Contrasting Seagrass Meadows: Implications for pH Buffering Capacities. Estuaries and Coasts. DOI: 10.1007/s12237-017-0356-5

Return to Top

Assessing Fish Responses to Drought

Drought alters fish communities in the Mississippi Sound

Eighty percent of the commercially and recreationally important fisheries in the Gulf of Mexico depend on estuaries. But when the flow of freshwater into an estuary drops due to drought, its physical and biological characteristics change—and with them, the fish species it can support. As drought frequency rises due to climate change, understanding how they can alter fish communities is becoming more and more important.

Researchers working in the Mississippi Sound conducted monthly sampling across the region from 2006 to 2014, collecting fish and recording data on water temperature, salinity, clarity, and dissolved oxygen. Although the Palmer Drought Severity Index classified 2006, 2007, and 2011 as drought years for this area, the estuary was not strongly affected in 2011, perhaps because large amounts of freshwater were released via a spillway into Lake Pontchartrain during that year. In 2006 and 2007, however, the estuary experienced higher-than-average salinity, water temperature, and clarity, and was home to different species assemblages than in non-drought years. Of eight individual fish species that were strongly influenced by drought, seven were less abundant during drought conditions, while only one, the white mullet, appeared to be more prevalent.

Drought conditions clearly alter fish species communities, and anthropogenic activities that mimic drought conditions—water withdrawals from aquifers, impoundments of stream flow, etc.—are likely to have similar results. The researchers hope that tracking what low flow does to fish may help guide upstream management decisions during drought years.

Source: Mickle, P.F., J.L. Herbig, C.R. Somerset, B.T. Chudzik, K.L. Lucas, and M.E. Fleming. 2018. Effects of Annual Droughts on Fish Communities in Mississippi Sound Estuaries. Estuaries and Coasts. DOI: 10.1007/s12237-017-0364-5

Return to Top

Living Shorelines’ Benthic Benefits

Bivalves increase after living shoreline construction in Chesapeake Bay

Coastal armoring structures are often installed to ease the effects of sea level rise and coastal erosion. A demand for eco-friendly alternatives to traditional shoreline hardening has increased interest in “living shorelines,” which typically involve the use of natural materials. However, few studies have evaluated the ecosystem services provided by living shoreline projects. To learn more, a recent study tracked the effects of living shoreline construction on benthic communities in Chesapeake Bay.

Researchers took annual samples of sediment-dwelling fauna at two study sites—one where a living shoreline replaced an existing bulkhead, and one where work was done to stabilize an eroding saltmarsh—before construction and for two to three years afterward. They also collected samples at nearby control locations for comparison. The results were variable but suggested that the construction of these living shorelines altered local benthic communities to more closely resemble those in adjacent marshes. The density and biomass of clams generally increased, particularly in the second year after construction. This was especially encouraging, as the presence of bivalves can be a strong indicator of a healthy ecosystem. Although the density and biomass of polychaete worms declined, an upward tick toward the end of the study hinted they may recover eventually as well.

Seeing the full benefits of the construction of living shorelines takes time, and the authors recommend future longer-term studies. However, their results show that living shorelines can provide benefits to benthic communities and suggests that they should continue to be considered as an alternative to traditional shoreline hardening.

Source: Davenport, T.M., R.D. Seitz, K.E. Knick, and N. Jackson. 2018. Living shorelines support near-shore benthic communities in upper and lower Chesapeake Bay. Estuaries and Coasts. DOI: 10.1007/s12237-017-0361-8

Return to Top

Blue Carbon in Tampa Bay

Mangrove expansion has increased carbon sequestration

Carbon stored in the soil and plant biomass of coastal wetlands is often referred to as “blue” carbon. These habitats sequester disproportionately large amounts of carbon, especially in places such as mangrove forests that accumulate rich peat deposits. However, mangrove losses in many areas have spurred growing interest in preserving them and quantifying their value.

In the Tampa Bay area of Florida, researchers collected soil cores and recorded vegetation characteristics at sixteen sites that included mangrove forests, salt marshes, and salt barrens. The total organic carbon stock in mangroves around Tampa Bay averaged 134 metric tons per hectare, with salt marshes averaging 66 metric tons per hectare and salt barrens 27. Most of this carbon was sequestered underground in all three habitat types. Total organic carbon stocks did not differ significantly between natural and restored mangrove forests, probably because the area’s natural mangrove forests are relatively young and have experienced significant human disturbance.

Mangroves currently comprise 74% of the coastal wetlands in the Tampa Bay area, but this is a recent development, with mangroves replacing areas of salt marsh within the last century. Tampa Bay’s mangroves sequester less carbon than those elsewhere in the world, which the researchers attribute to their location at the northern limit of their range and the relatively dry climate. Despite this, however, mangrove encroachment has resulted in the sequestration of approximately an additional 141,000 metric tons of carbon in Tampa Bay’s coastal wetlands in the past 140 years. The future depends on how well these habitats adapt to rising sea levels—in order to migrate inland and continue locking away carbon, the study’s authors say, they’ll need a protected “buffer zone” waiting for them to move into.

Source: Radabaugh, K.R., R.P. Moyer, A.R. Chappel, C.E. Powell, I. Bociu, B.C. Clark, and J.M. Smoak. 2018. Coastal Blue Carbon Assessment of Mangroves, Salt Marshes, and Salt Barrens in Tampa Bay, Florida, USA. Estuaries and Coasts. DOI: 10.1007/s12237-017-0362-7

Return to Top