Seagrass swap could reshape Chesapeake Bay food web

Beneath the surface of the Chesapeake Bay, a subtle but dramatic shift is taking place as eelgrass gives way to its warmer-water relative, widgeon grass. A new study from researchers at William & Mary's Batten School & VIMS shows that this seagrass swap could have ecological impacts across the Bay’s food webs, fisheries and ecosystem functions.

Published in Marine Ecology Progress Series, the study reveals that while both seagrass species offer valuable habitat, they support marine life in very different ways. The researchers estimate that the continued shift eelgrass to widgeon grass could lead to a 63% reduction in the total quantity of invertebrate biomass living in seagrass meadows in the Bay by 2060.

“Several factors including water quality, rising temperatures and human development are threatening eelgrass in the Chesapeake Bay. In its place, particularly in the middle Bay, widgeon grass has expanded due to its ability to tolerate warmer, more variable conditions,” said Associate Professor Chris Patrick, who is also director of the Submerged Aquatic Vegetation (SAV) Monitoring & Restoration Program at the Batten School of Coastal & Marine Sciences & VIMS.  “However, the two grasses provide structurally distinct habitats that shape the animals living within.”

All grasses are not created equal

While working with Patrick and earning her master’s degree at the Batten School & VIMS, lead author Lauren Alvaro M.S. ’23 engaged in extensive fieldwork studying seagrass meadows in Mobjack Bay. Her team surveyed and compared habitats consisting of eelgrass, widgeon grass as well as mixed beds. They documented everything from burrowing clams and snails to crabs and fishes to get an idea of life living within the sediment and amongst the grasses.

The findings showed that while widgeon grass supports more individual invertebrates per gram of plant material, eelgrass meadows are home to larger animals and have more plant biomass per square meter. As a result, eelgrass supports a greater total animal biomass per square meter.

“Our findings suggest that we’re likely to see a fundamental shift in the structure of the food web that favors smaller creatures as eelgrass is replaced by widgeon grass,” said Alvaro. “The eelgrass meadows produced fewer animals, but they’re bigger and more valuable to predators like fish and blue crabs.”

Much of the difference is due to the physical characteristics of the two types of seagrasses. Widgeon grass beds have a greater surface-to-biomass ratio due to their narrower leaf structure, which provides more area for small invertebrates to cling to. However, eelgrass’ broader leaves provide a type of canopy favored by animals like pipefish, blue crabs, and larger isopods, which are small shrimp-like crustaceans.

The bigger picture

The researchers extrapolated their findings and estimated that current seagrass habitats in the Chesapeake Bay support approximately 66,139 tons of invertebrate biomass living in the sediment and amongst the grass beds and produce 35,274 tons of new animal biomass each growing season. Termed “secondary production,” this is the biomass the habitat makes available to higher levels of the food chain.  

If seagrasses continue to shift as expected, by 2060 secondary production could be reduced by more than 60% under a scenario where no further nutrient reductions occur.  Nutrient runoff into the Bay is the largest threat to submerged aquatic vegetation. Even in a best-case nutrient management scenario, the Bay could still lose approximately 15% of secondary production biomass.

“Within the limits of our study, it wasn’t possible to determine whether it was the meadow’s physical structure, the meadow area, or available food sources that contributed to greater numbers of fish in the eelgrass meadows,” said Alvaro. “This makes it difficult to accurately estimate fishery-level impacts of changes in meadow composition, but several lines of reasoning support an expectation of reduction in numerous commercial and recreational species.”

The study adds to a growing body of research documenting the effects of changes in foundational species influenced by a warming planet. The authors cite similar research involving Florida’s mangroves and a worldwide shift from coral to algae-dominated ecosystems.

As states within the Bay’s extensive watershed work to maintain and improve the health of the estuary, the team hopes their findings will help inform management decisions and restoration strategies. Protecting and restoring the remaining eelgrass and better understanding the role of widgeon grass may help preserve ecological resources for future generations and provide a buffer against future shocks.

Visit the website for the SAV Monitoring and Restoration Program for more information about seagrass research at the Batten School & VIMS.