Talking Eelgrass
Feb 2, 2012 by Robin Just | Bio | 1 Comment »
Robin Just is a volunteer for CLF with an educational and professional background in biology and water quality issues. This blog was originally published on TalkingFish.org.
When we talk about fish, it’s good to remember that they not only come from somewhere but that the somewhere makes the fish. Habitat is essential; without it even many migratory fish won’t have a place to call home.
An eelgrass bed beneath the waters of New Hampshire's Great Bay (Photo credit: Ben Kimball, courtesy of NH Division of Forests and Lands).
Many North Atlantic fish spend an important part of their life cycles in coastal eelgrass habitat, and eelgrass is declining. Eelgrass is a native submerged aquatic plant found in shallow waters from Nova Scotia to North Carolina’s Outer Banks. In the northern areas this hearty plant spends part of each year under sea ice. It is not a true grass, but a flowering plant that evolved from terrestrial flora. With thin, streamlined leaves, and an extensive root system, it is uniquely adapted to thrive in ocean tides and swell. What it isn’t adapted to deal with is nutrient pollution, dredging, and other anthropogenic stressors that have our productive eelgrass meadow areas on the decline.
Why does this matter to fish? Eelgrass is one of the most valuable habitats in the northeast. For example, in the early 1930s a “wasting disease” decimated 90% of the Atlantic eelgrass communities. This decline took a heavy toll on, among other things, bay scallops. Bay scallops are a commercially important shellfish that range from Cape Cod to Florida, and are very dependent on seagrass meadows. Not only do they attach to living eelgrass leaves after their larval stage, but they consume decaying leaves for a significant portion of their diet. Bay scallops declined dramatically throughout their range, coincident with the wasting disease, and populations didn’t begin to recover until the mid-1940s. Some populations, such as those in the Chesapeake Bay, have never come back. Lobsters, clams, and other invertebrates also declined.
Loss of eelgrass habitat has an effect on other commercial fishery species as well. Some of these animals, such as cod, winter flounder, and lobster use eelgrass meadows as a refuge in their early life stages. The eelgrass provides places to stay hidden, feeding opportunities, and shelter from wave energy. Some species, such as striped bass, bluefish, tautog, and fluke will use eelgrass habitat as adults, as a place to hunt and forage.
A tautog in an eelgrass bed (Photo credit: MA Divison of Marine Fisheries).
In addition to providing a place to eat and live, eelgrass is part of the foundation of our marine food web. Eelgrass is a primary producer – turning aquatic carbon dioxide into food and energy through photosynthesis; it is then eaten by many animals that are then consumed by our commercially important species. In short: eelgrass matters a lot to New England fishery resources, and its decline is not good news.
The impact of the loss of eelgrass on these fisheries is hard to tease out from the many drivers of the decline in fish populations, including fishing pressure, habitat destruction, nutrient pollution, climate change, and other stressors. There has been almost no research done to numerically link the decline in eelgrass with population-level changes in commercial fisheries species. However, an 11-year study in Buzzards and Waquoit Bays found that loss of eelgrass was accompanied by significant declines in fish biomass, species richness, and other measures of community integrity. Worryingly, a recent investigation of New Hampshire eelgrass populations found they are declining by about 9% a year, and eelgrass mapping efforts in Massachusetts show significant declines as well. This is an issue the Conservation Law Foundation’s new Great Bay-Piscatqua Waterkeeper will be addressing.
While the link has yet to be fully characterized between commercial fish populations and healthy eelgrass, it is vital that recovering species have functioning near-shore ecosystems to support their reproduction and growth.
Why is eelgrass declining? The decline is worldwide. Our local populations are suffering from a combination of coastal development and nutrient pollution, dredging activities, over-grazing by Canada geese, and climate change. Here’s a brief description of each of those stressors:
- Eelgrass is extremely sensitive to light levels. Urban build-up and construction activities in our coastal areas put sediment in the water, which decreases water clarity, and that takes a toll on eelgrass populations . Nutrient pollution from wastewater, stormwater, and other human activities can promote blooms of algae that block light to these photosensitive plants and prevent them from growing.
- Dredging activities uproot eelgrass and can completely decimate an eelgrass meadow. It can take ten years or more for eelgrass to recover from this kind of stress.
- Grazing pressure from Canada geese is on the rise as the warmer winters encourage more of them to stay local instead of heading south. More geese means competition for food, and the geese increasingly turn to eelgrass to get them through the winters.
- Climate change: as ocean temperatures rise, native plants feel the heat. There is evidence that northern populations of eelgrass will not be able to adapt to warmer waters as easily as the southern populations might.
Efforts to restore eelgrass are underway around New England, but it’s not a simple process. Areas that once supported thriving eelgrass meadows can be re-planted, but unless the factors involved in eelgrass decline are addressed, the efforts will probably fail. For example, if construction activities degrade nearby water quality, leading to eelgrass loss, and then the water quality recovers, restoration is possible. But if the water is still dirty, it’s not going to help, and the habitat is lost.
Waiting until we know exactly how eelgrass ecosystems and commercial fish populations are linked before we address eelgrass decline is a dangerous path to follow. Since we know for sure that many of our economically important species utilize this habitat, it makes sense to try and protect it. We need good science about the utilization of eelgrass habitat, and we need effective restoration efforts that address water quality and other physical stressors. This will give our recovering fish populations every opportunity to grow and thrive.
