A transformation in the seagrass meadows hidden under the surface of the Chesapeake Bay may have huge implications for fish and crabs living in the estuary.
Eelgrass, once the dominant species of underwater grass in the Bay, has declined in recent decades — a casualty of climate change in a warming Bay — and is steadily being replaced by a more adaptable species known as widgeon grass.
New research finds that the transition to widgeon grass, or in some cases no grass at all, likely means much less food for fish in the future.
Scientists have long worried that stringy widgeon grass, now the Bay’s most abundant species of submerged aquatic vegetation, or SAV, does not provide the same ecological value as lush eelgrass meadows.
A new study backs that hunch with hard numbers: The shift from eelgrass to widgeon grass could mean a 60% reduction by 2060 in worms, clams and other fish food typically found in the grass beds.
The findings “have several serious implications for the Chesapeake Bay,” researchers from the Virginia Institute of Marine Science cautioned in a paper recently published in the Marine Ecology Progress Series.
Their research provides a case study of how climate change will fundamentally alter critical parts of the Bay ecosystem, resulting in a future system that is significantly different — and potentially less productive.
The bottom line, said Chris Patrick, a VIMS scientist specializing in seagrass research, is that each square meter covered by eelgrass, compared with widgeon grass, produces a greater abundance of the worms and clams that fish and crabs like to eat.
The research was conducted by Patrick and two former VIMS students, Lauren Alvaro and Marc Hensel, who set out in 2021 to study differences in organisms that were living in comparable beds of widgeon grass and eelgrass.
“We’ve got this change that’s happening, and we really need to know, ‘What does it mean?’” Patrick said. “What are the consequences of a species swap?”
As recently as the 1960s, eelgrass could be found as far north as Kent Island in Maryland. But worsening water quality and the devastating deluge from Tropical Storm Agnes in 1972 restricted it to lower portions of the Bay.
Eelgrass, though, has turned out to be especially sensitive to warm temperatures, suffering several severe heat-related die-offs in recent decades. And the plants are even more vulnerable when warm temperatures are combined with cloudy water conditions because they are less able to get sunlight energy needed to reduce heat-related stress.
But the transition to widgeon grass, the other species that tolerates high salinities, has not been a one-to-one trade when it comes to the Bay’s ecosystem. Eelgrass is present year-round with peak growth in the spring and fall, providing areas for juvenile fish and crabs to hide and producing food for waterfowl to eat most of the year.
Widgeon grass can also be a perennial plant, but its peak growth is limited to late spring and summer, meaning it provides much less shelter and food during most of the year.
The new study found that the differences in the two plants also results in a significant variation in the amount and type of fish food the beds produce.
Widgeon grass leaves are thin, stringy and only a few inches long. Eelgrass leaves are thick and strap-like and can reach lengths of nearly two feet, which forms sheltering canopies over bottom areas.
“There’s no real comparison,” Patrick said. “Widgeon grass has very short, clumpy, really thin leaves. It looks like a lawn, whereas the eelgrass is dense and thick, like a jungle.”
The study found that a square meter of ground covered with eelgrass produced a greater biomass of tiny invertebrates — the base of the food chain — and produced significantly more of the larger clams and worms that sustain predators like fish and blue crabs.
The study estimated that widgeon grass Baywide is producing 28.6% less biomass of small organisms and 22.2% less of the larger organisms — secondary production — that fish and other predators rely on.
That disparity is expected to grow as eelgrass continues to decline because of warming Bay temperatures.
Computer models used to predict future conditions anticipate that eelgrass will shrink from about 20,000 acres to about 6,200 acres in 2060 if Bay nutrient reduction goals are met or could fall as low as 5,200 acres if water quality remains mostly unchanged.
The difference is more extreme when it comes to available fish food. Under the scenario where water quality goals are met, the scientists estimated that secondary production would decline 15% from current levels. But if conditions remain unchanged, it could decrease by 60% —in part because widgeon grass too would suffer significant losses.
“We think that we’re going to have some loss of productivity and that, if we can continue to reduce nutrients, we can ameliorate that somewhat,” Patrick said.
One glimmer of good news is that those numbers were produced using 2022 estimates of underwater grass coverage. In the last two years, eelgrass has made a comeback in some areas as clearer water has allowed it to move into deeper waters, which may also be cooler.
If those areas prove to be what biologists call “thermal refugia” for eelgrass, and if water remains clear enough for it to persist, those future predictions might change.
“Water clarity has been improving,” Patrick said. “If it continues to improve, and the beds get deeper and deeper, and [there are] thermal refugia, maybe these massive collapses we’re expecting won’t come to pass, at least not on the time horizons that we’ve been talking about.”
But, he added, “Those are a bunch of ifs and maybes.”