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Light Beneath the Surface: Requirements for Seagrass Growth

January 26, 2012
Graduate students Malee Jinuntuya, Billur Celebi and Meredith McPherson prepare to count submerged seagrass abundance. ©Carly Rose/VASG

Graduate students Malee Jinuntuya, Billur Celebi and Meredith McPherson prepare to count submerged seagrass abundance. ©Carly Rose/VASG

Virginia Marine Resource Bulletin
Volume 44, Number 1, Winter 2012
By Communications Intern Kate Schimel
Photography by Photography Intern Carly Rose

On a quiet day at the peak of summer, Virginia Sea Grant (VASG)-funded researcher Dick Zimmerman directs the deployment of a fleet of scientific instruments. The research vessel Riptide is positioned near the Goodwin Islands, where the York River meets the Chesapeake Bay. On deck, three graduate students in colorful scuba gear make last minute equipment checks.

Instruments drift behind the boat, some spouting water, others threatening to tangle with nearby crab pots. Then Zimmerman, a professor at Old Dominion University (ODU), sends his students out to collect samples of thread-thin wigeongrass and ribbon-like eelgrass—the most common species among the dwindling populations of Chesapeake Bay seagrass.

Since a wasting disease decimated seagrass in the Bay in the 1930s, decreasing water clarity and increasing temperatures have made it difficult for grasses to return. Thus far, researchers and managers have focused on improving water quality before planting seagrass. But trying to determine where to address which water quality factors has complicated efforts.

Zimmerman and Victoria Hill (ODU) and Charles Gallegos (Smithsonian Institution) are developing a model that should help restoration managers answer these questions. Managers could use the model to determine which water quality factors should be addressed and to predict where seagrass has the potential to grow if water quality does improve. The first step is building and testing the model using data on the effects of water quality and rising temperatures.

Water Quality Challenge
Seagrass is an essential part of the Chesapeake Bay ecosystem. Blue crabs and several species of fish live among the shoots and feed off the decaying leaves. The roots stabilize the sediment, which improves water quality and reduces storm damage.

The list of seagrass needs starts out simply enough. Just like grasses on land, seagrass needs light and nutrients. It also need salinity—at least 50% seawater—to grow. Yet dozens of factors influence the levels of these variables in the Bay. Runoff of mud and sand from the land and re-suspension of sediments by wind and wave energy can affect how much light gets through the water. Then there are more complex problems in some areas, such as excessive nutrients leading to algal growth that can shade or poison seagrass.

When it comes to addressing water quality, it’s a problem of too many possibilities. “Could we reduce the chlorophyll [from microscopic algae in the water]? Reduce the nutrient loading? Improve the water clarity? And in bare patches, if we planted grass here, would it succeed?” Zimmerman muses.

In some ways, seagrass’ needs have made them especially vulnerable to environmental issues in the Bay. The need for light limits seagrass to shallow near-shore areas, which are directly in the path of runoff and pollution. A large portion of the water in the Bay comes from the Susquehanna River, which flows through heavily populated areas of New York, Maryland, and Pennsylvania. The runoff from agriculture and dense cities can carry tons of nutrients, chemicals, and sediment into the Bay each year. These dissolved molecules and suspended particles promote the growth of nuisance algae and cloud the water.

These effects aren’t limited to one local area. Says Zimmerman, “We are talking about the whole Chesapeake Bay watershed.” Activities throughout the Mid-Atlantic region affect water quality in the Bay. Even global-scale activities affect the Bay, as warming waters threaten seagrass recovery.

Climate Change
Seagrass begins to die at water temperatures warmer than about 77 degrees Fahrenheit. This may not have been a problem during typical Chesapeake Bay summers in the past, but researchers have observed increasing summer water temperatures in the Bay. The effects on seagrass are starting to show.

A June heat wave in 2010, for example, brought water temperatures as high as 86 degrees and caused massive seagrass die-offs. According to the Chesapeake Bay Program, the heat wave caused a seven percent decrease in seagrass abundance.

The overall effect of climate change on seagrass is not simple, according to Zimmerman. Although warming waters make it hard for seagrass to survive, the increased levels of carbon dioxide resulting from fossil fuel combustion can stimulate seagrass photosynthesis, which may help the plants tolerate higher temperatures.

“Rising temperature makes life worse for seagrass, but rising carbon dioxide makes life better,” Zimmerman explains, “so we will see if one offsets the other.” To test how the effects of rising water temperatures and carbon dioxide levels will interact, Zimmerman and his colleagues will bring seagrass samples back to the lab where students will run experiments to find the tipping point between the harm done by high temperatures and the benefit of additional carbon dioxide.

Pulling It Together
Eventually, Zimmerman and his colleagues will combine data on water quality, seagrass needs, and even the effects of climate change. They will convert this portrait of the seagrass ecosystem into a mathematical model for use by managers throughout the Bay.

Once the model is complete, Zimmerman’s team will host a series of workshops to train resource managers in how to use the tool in restoration efforts. The findings will also be incorporated into education and outreach projects at the Virginia Aquarium and Marine Science Center—helping members of the public to demystify a complex and vulnerable ecosystem.

Zimmerman is also comparing the model’s results to the real-life observations he gathers from his instruments. Verification requires in-depth mapping of his research sites at the Goodwin Islands on the York River and Hog Island Bay off the Virginia coast. Existing maps of the river bottom have proven to be insufficiently detailed, so Zimmerman must use sonar imaging to build new maps from scratch. These bathymetric maps require extensive data collection in the form of hours spent motoring slowly over the same area in a boat loaded with sonar and GPS. Data on seagrass distribution and water quality will be overlaid onto the maps. For managers at his research sites, these maps are valuable tools in and of themselves.

As Zimmerman conducts his work, managers throughout the Chesapeake Bay watershed are debating what policy measures to take to improve water quality, and each summer brings uncertain temperatures and water quality conditions. Although many areas that had seagrass in the past are barren, some researchers have recently found seagrass where, in previous years, there were only wide stretches of sand. As restoration efforts move forward, Zimmerman’s model will help identify areas with the potential to support such regrowth and restoration.