On the Road
To look for differences in the sediment possibly caused by Bay grasses, Hengst puts peepers inside, outside and at the boundary of multiple Ruppia beds. After the water inside the peeper is the same as its surroundings, which occurs in roughly 7-10 days time, Hengst will retrieve the peeper and analyze the water for ammonia, nitrate, phosphate, and sulfide.
Like oysters, underwater grasses provide certain critical services to the environment — they are used as food and habitat for waterfowl, fish, invertebrates, and shellfish; serve as nursery habitat for young fish and crabs; filter and trap sediment that can cloud the water and bury bottom-dwellers; and oxygenate the water through photosynthesis. Together, the decline of grass abundance in the past few decades, along with the loss of oysters, has dramatically diminished water clarity and oxygen content, helping to maintain a state in the Bay that is turbid and dominated by phytoplankton primary production.
Kemp, a professor at Horn Point Laboratory, has identified specific cycles of positive feedback — where one change serves as a catalyst for other changes — in the interaction between grasses and the sediment, and he believes that these interrelated processes might help to jump-start recovery. Microorganisms in the sediment use oxygen to turn ammonia, a waste product, into nitrate through a process called nitrification. Other sediment microbes then remove fixed nitrogen (i.e., nitrogen available for algal growth) out of the environment by reducing it to nitrogen gas — through a process called denitrification. Since the roots of underwater grasses help bring oxygen to regions within the oxygen-starved sediment, they can accelerate the coupled nitrification-denitrification cycle, explains Kemp. In the Bay, as grasses died off in the 1970s, nitrification (which requires oxygen) and its coupled denitrification slowed down. This led to an accumulation of nitrogen, which supports more growth of algae. Since algae are better competitors for light than grasses, their presence further accelerated the demise of grasses by shading them.
Kemp believes that the positive feedback between nitrification-denitrification and underwater grasses could help to jump-start the Bay's restoration trajectory. Once water quality is good enough to sustain some healthy underwater grass beds, he says, the amplification process that accelerated their decline (positive feedback) should happen in reverse, helping to further improve water quality and decrease nutrient loading. But in order for the Bay to help catalyze its own recovery, water quality may need to be even better than it was before the decline of these grasses — the washed out Bay Bridge problem (hysteresis) again.
There are other ways by which the Bay might be able to accelerate its own recovery. For example, as oxygen penetrates bottom sediments, the release of phosphorus to the water column will decrease dramatically, explains ecologist Walter Boynton. In the absence of oxygen, phosphorus is less likely to separate from the iron-rich sediments, leach into the water and promote algal growth. Other good things will happen as bottom sediments become oxygenated again. For example, animals that live in the sediment, such as tubeworms, will become more abundant and will help to further stabilize the physical and chemical composition of the Bay bottom, he explains.
But first, says Boynton, we need to significantly reduce the nitrogen and phosphorus inputs to the Bay that cause sediments to become anoxic in the first place — rather than "nibbling at the fringes" of nutrient reduction. "That will start the process of rebuilding this resilience."
And what about oysters? The verdict is still out as to how much we can bring back the native oyster (Crassostrea virginica) and a whole host of issues must be resolved before we can introduce the non-native oyster (Crassostrea ariakensis) (see article, "A New Bay for the Oyster?" ). But restoring water quality would certainly be easier with a prolific filter feeder in the mix.
"We might be able to improve water quality through nutrient reduction alone, without restoring oysters, but it would likely be a less resilient Bay, one that is forever sensitive to perturbations like storms and other random events," says Breitburg. "Still, it is a better alternative than an un-restored Bay," she says.
A test of Breitburg's intuition on the Bay's continued susceptibility came in the third week of September, just after Hurricane Ivan pummeled the coasts of Florida and Alabama. Major flooding in Pennsylvania and New York rivers caused uprooted trees and pieces of debris to race through the Conowingo Dam into the Susquehanna River and, from there, into the Bay. A few days later, watermen noted large swaths of sediment-laden, deep brown water the color of dark coffee, as far south as one mile above the Bay Bridge.
Ivan's timing couldn't have been worse. The storm blew through just when the Susquehanna had been showing some signs of recovery — likely due to efforts in phosphorus reduction, says UMCES researcher Cornwell. Just a month earlier, scientists and managers found healthy stands of at least a dozen species of underwater grasses and clear water in the Susquehanna flats for the first time in years. Now there is concern that the huge sediment load that surged down the river might severely harm these newly grown grasses. The Susquehanna is clearly not resistant to a perturbation like Hurricane Ivan — no system would be. But if the Susquehanna can recover from the storm, if healthy underwater grasses and clear waters reappear next year, that will be a sign that some resilience has been restored to the river. If the new grasses are gone again and do not come back for several years, then the improvements seen last summer were ephemeral.
So the restoration of the Chesapeake Bay is back to the chicken and egg conundrum. Which comes first? Restoration of grasses and oysters? Or improved water quality? Can we help the Bay jump-start its own recovery by replanting oysters and underwater grasses? Or do we have to restore water quality first to help them survive and withstand perturbations like storms that flood the bottom with sediment? Can we upgrade water quality through nutrient reduction alone, without the filtering power of oysters and grasses — both of which create positive feedback for improving water clarity?
As the boat turns back towards SERC's pier, Breitburg finishes filling out her data sheets and sits down on the edge of the boat. The day has confirmed her suspicions — the plankton nets towed by the skiff were not too small. There were simply no nettles in the Rhode to be caught.
It's too soon to tell whether or not the Rhode is following a path similar to that of the Patuxent. Scientists do not have a parallel historical record of nettle abundance for comparison. In the Patuxent, however, Breitburg is confident that the declining oyster population is strongly implicated in the disappearance of nettles. She has plans to do follow-up studies to nail down the specifics of this broken link in the Bay's food web.
And, if Breitburg is right about a decline in oysters leading to a decline in sea nettles, then the opposite may also prove true. Although the return of oysters should bring improved filtration and cleaner water, it could also bring a big rise in sea nettle populations.
So one of the results of a restored Bay could be a return to the era of jellyfish-exclusion nets at swimming beaches, a state of the Bay that first sparked Maryland Congressman Edward Garmatz, the chair of the former U.S. House of Representatives Merchant Marine and Fisheries Committee, to author the "Jellyfish Control Act" in the 1960s. But now that we are beginning to understand the crucial role that these enigmatic creatures play in the Chesapeake's intricate food web, now that we see that a robust population of sea nettles comes with oysters, underwater grasses, clear waters, recreation, and healthy fisheries — it would be worth it, wouldn't it?
For More Information
Boesch, Donald F. and Jack Greer. 2003. Chesapeake Futures: Choices for the 21st Century. Edgewater, MD: Chesapeake Research Consortium. www.chesapeake.org/stac/futreport.html
Gunderson, Lance H. 2000. Ecological resilience-in theory and application.
Holling, C.S. 1973. Resilience and stability of ecological systems.
Horton, Tom. 2003. Turning the Tide: Saving the Chesapeake Bay. Washington, DC: Island Press.
Palmer, Margaret, et al. 2004. Ecology for a crowded planet.
Resilience Alliance: www.resalliance.org
Resilience Alliance, threshold database:
Scheffer, Marten, et al. 2001. Catastrophic shifts in ecosystems.
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This page was last modified September 15, 2018