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The fate of fish in a New England estuary could predict what’s ahead for Louisiana marsh life should the state activate a cornerstone of its coastal restoration plan.
That’s according to Dr. James A. Nelson, a University of Louisiana at Lafayette biologist.
Nelson and other researchers introduced nutrient-enriched seawater into three watersheds in Plum Island Estuary, an area of tidal marshes and interconnected creek channels in northeastern Massachusetts.
The team’s goal was to determine how nutrients such as phosphate and nitrate affected the estuary’s food web, the relationship between predator and prey in nature.
Researchers focused on mummichog, a fish about 1.5 inches long that’s common in salt marshes and brackish inlets along the Atlantic coast. Mummichog eat algae and detritus during times of low tide, but get energy by eating worms and insects when water levels permit access to creek banks.
For the first six years of the decade-long experiment, the artificially introduced nutrients stimulated algae growth. With an abundant food source to fuel reproduction, the number of mummichog increased. After the initial boom, however, researchers noted a sharp decrease to population levels lower than when the experiment began.
The discovery coincided with an increase in trophic decoupling. That’s when changes in the structure of waterways separate an organism from its food source.
Nitrate introduced into the water is a type of fertilizer. It set off a complex series of events that weakened creek banks, resulting in large fractures.
The fertilizer enabled plants along the water to receive nutrients artificially, so they no longer put down extensive roots. In addition, microbes began to eat peat that lined the water's edge. Both helped hold creek banks together. Without them, cracks eventually appeared.
Preliminary examinations of mummichog behavior indicated the fish avoided the hollowed out banks. Researchers concluded the fish feared fractures might conceal predators such as juvenile American eels.
That trepidation isolated fish from their principal source of food.
And that’s a cautionary tale for Louisiana as it considers how to counter coastal land loss, said Nelson, an assistant biology professor.
Plans to divert Mississippi River water – and the sand, silt, mud and sediment it carries – into south Louisiana marshes are a centerpiece of the state’s restoration initiatives.
Mississippi River levees, while designed to protect communities from flooding, also have contributed to coastal land loss. They blocked natural spring flooding, which carried sediment and nutrients that replenished marshes. Since the 1930s, the state has lost more than 2,000 square miles of wetlands.
Among other suggestions, Louisiana’s coastal master plan calls for countering land loss through river diversions. By removing levee sections, water and sediment would again enter the marshes and replicate the natural land building processes that created the delta.
The Plum Island experiment in Massachusetts could offer a preview of what might be in store for coastal Louisiana should the planned river diversions take place.
It’s likely the diversions of the nutrient-heavy river water would result, at least initially, in the creation of marshland and an increase in fish and shrimp populations, just as researchers observed in New England.
But the river water will carry higher nutrient concentrations than marshes are used to, Nelson warned. It’ll also contain agricultural pesticides and fertilizers, industrial byproducts, wastewater and other pollutants.
“Although we think we know how the marsh will respond to these diversions, our experiment showed there are tipping points that may not show up for six, eight, 10 or more years after the diversions begin.
“What we are going to do here in Louisiana is an exponentially larger version of our study. While the final results may not be exactly the same as we saw in Plum Island, our results demonstrate that the water, marshes, and animals are connected in ways we cannot fully anticipate.
“If things are not going as planned in a decade or so, simply turning off the water will not undo what we already set in motion” Nelson concluded.
Ecosystems, an international peer-reviewed journal, published the team’s findings in May.
In addition to UL Lafayette’s Nelson, the article’s authors are David S. Johnson, Virginia Institute of Marine Science; Amanda C. Spivak, Woods Hole Oceanographic Institution, Massachusetts; and Linda A. Deegan and Nathalie R. Sommer, both of the Marine Biological Laboratory’s Ecosystem Center, Massachusetts.
Photo caption: Plum Island Estuary, an area of tidal marshes and creek channels in northeastern Massachusetts, was the site of UL Lafayette biologist Dr. James Nelson and other researchers’ decade-long study on the effects of artificial nutrients on salt marshes. (Photo courtesy of the U.S. Geological Survey)