Shifts in Underwater Grasses Impact Humans as well as Wildlife
By Briana M. Yancy
Shifts in Underwater Grasses Impact Humans as well as Wildlife
Last year, I worked for the Chesapeake Conservation Corps, a team of young adults who gain environmental experience while working for a year at nonprofit organizations and government agencies across the Chesapeake Bay region.
During my time with the corps, I soared in my kayak over beds of underwater grasses and stepped carefully among the beds to collect seeds for restoration. I watched fish follow the channels I made as I walked, had clumsy blue crabs brush up against my legs, and strategically avoided sea nettles caught in the grass canopy.
The health of the Chesapeake Bay depends on maintaining high levels of biodiversity, and that requires protecting beds of underwater grasses, also called submerged aquatic vegetation. They are one of the Bay’s most valuable resources, supporting an abundance of aquatic life. They also provide many benefits for humans, called ecosystem services, which include healthy fisheries, improved water clarity and shoreline protection.
Twenty species of native and non-native underwater vegetation are found in the Chesapeake. Unfortunately, their acreage has been declining for several decades due to human activity, climate change and pollution.
Having a mixture of grass species is important for their resilience. But in the southern Chesapeake Bay, where salinity is the highest, only two species coexist: eelgrass and widgeon grass.
Eelgrass populations have declined and are expected to keep declining as temperatures, disease and pollution increase. When eelgrass dies off, widgeon grass takes its place, but it is not clear if the ecosystem services provided by widgeon grass will be as beneficial as those provided by eelgrass.
What changes in ecosystem services can we expect and what do these shifts mean for life in and around the Bay?
Loss of eelgrass
The loss of eelgrass impacts ecosystem services such as shoreline protection, water clarity and wildlife habitat. Dense and healthy beds can reduce the force of waves and currents, shielding our shores from sea level rise and erosion. A 2008 report found that, compared with eelgrass, widgeon grass has a shallow root system and has difficulty surviving under high wave action. This interferes with its ability to weaken the impact of waves and currents.
Underwater grasses also trap suspended sediments, which improves water clarity. A study conducted in 2018 by researchers Emily French and Kenneth Moore found that eelgrass beds are better at trapping sediments than widgeon grass and thus better at improving water clarity.
Eelgrass is so crucial to life in the southern portion of the Chesapeake Bay that species declines can be linked to its disappearance. Declines in eelgrass caused Brant, a species of geese, to lose an important food source and almost completely disappear from the region.
Likewise, blue crab population declines can be linked to the retreat of eelgrass. Not only is blue crab survival higher in beds of eelgrass compared with widgeon grass, but in 2017 researchers estimated that the decline of eelgrass equals a total loss of $28.6 million–$76.7 million for fisheries.
Benefits of widgeon grass
One of the most important benefits of widgeon grass is its overall tolerance. It can withstand changes in water quality, heat and salinity better than eelgrass. In the 1990s, a team of researchers studied the relationship between water quality and underwater grasses in the Bay. They found that after major declines in water quality, widgeon grass was the only species found in some portions of the Bay.
Nutrient pollution also causes grasses to decline. The nutrients nitrogen and phosphorus entering the Bay in excess is due to deforestation, lawn treatments, agriculture and other human activity. Widgeon grass can survive better than eelgrass because it is more tolerant of nitrate enrichment, according to a 1994 study.
Many species prefer eelgrass habitat over widgeon grass habitat, but dense plots of widgeon grass provide valuable habitat comparable to eelgrass. In 2011, a researcher from the Virginia Institute of Marine Science found that pipefish foraged better in widgeon grasses’ thin blades. And, when eelgrass is unable to survive changes in water quality, widgeon grass becomes more valuable as habitat and a food source.
Strive for coexistence
Widgeon grass and eelgrass are both beneficial to the southern portion of the Bay, where species richness is low compared with other areas.
These two species work well in harmony. When stress causes eelgrass to die off, widgeon grass can expand to some areas and replace a portion of the lost eelgrass. And widgeon grass will retreat rather than displace eelgrass when the latter is able to return.
Both species are native to the Chesapeake Bay and, therefore, can support an abundance of native species. A study conducted at the Virginia Institute of Marine Science found that a higher percent coverage of seagrass, regardless of the species, positively correlated with blue crab density.
Eelgrass and widgeon grass provide food for waterfowl, such as ducks, coots, geese, swans and other aquatic animals, either from feeding on the plant matter directly or eating invertebrates that depend on them for habitat. Other animals that eat widgeon grass, such as muskrat and green turtles, also consume eelgrass.
Why does it matter?
This topic has significant management implications, but it is not only up to leadership to make changes. If you eat seafood, enjoy aquatic recreation or depend on shoreline protection, then these changes impact you. Everyone can take a few steps to reduce pollution and protect Bay grasses. Here are a few suggestions:
When boating, follow posted speed limits and no-wake laws to avoid harming grass beds.
Reduce herbicide and fertilizer use to minimize chemicals and nutrient overloading.
Clean up trash on or near the water and storm drains because waterborne trash can block light or physically damage grass beds.
Become a community scientist and monitor Bay grasses by joining the Chesapeake Bay SAV Watchers.
Briana Yancy is a Chesapeake Research Consortium environmental management staffer for the Chesapeake Bay Program’s Diversity Workgroup. She is also a graduate student studying biology with Project Dragonfly at Miami University in Ohio in conjunction with The Bronx Zoo. The focus of her studies is coastal ecosystem conservation.