Media Contact: Lisa Cugini, (401) 874-6642
URI Biological Oceanographers Test
How Shallow Marine Systems
Respond to Increased Nutrients
Narragansett, R.I. -- January 8, 2003 -- Most of the efforts to determine how estuaries respond to nutrient enrichment have been confined to relatively deeper and/or muddier river mouth estuaries. However, much of the Atlantic coast from Cape Cod to Cape Fear, as well as parts of the Florida coast and almost all of the Gulf Coast is characterized by a different type of coastal system, called by various names, including lagoon, inland bay, and salt pond. These more complex shallow systems are now facing increasing nutrient enrichment from agriculture and suburban housing development with associated on-site sewage disposal systems and ground water nitrogen enrichment.
A team of scientists at the URI Graduate School of Oceanography (GSO) have focused their research on these very shallow lagoon type estuaries to determine if there are predictable patterns of response to nutrient enrichment in these more complex systems. The research team includes biological oceanographer Dr. Scott Nixon, research associates Betty Buckley and Steven Granger, and recent Ph.D. graduate Joanne Bintz.
According to a recent article in the journal Human and Ecological Risk Assessment, the scientific team summarized data from 30 systems with mean depths ranging from 1-12 feet and water residence times from .3-100 days. In addition the team designed and built a coastal lagoon mesocosm facility where they replicated and controlled nutrient inputs, mixing rates, and water resident time. Fed by water from Narragansett Bay, the mesocosms had a variety of typical coastal lagoon organisms added to them, in addition to the plankton that enter with the bay water and the animals contained in the sediment.
While the scientists observed some changes with regard to phytoplankton blooms and the increase of epiphytic algae growing on seagrass leaves, they were most concerned with the impact of nutrient enrichment on the survival and health of eelgrass. The mesocosm experiments showed that the epiphytes appeared to have little or no role in eelgrass survival.
When early and prolonged algal blooms occur due to nutrient enrichment, eelgrass leaves elongate rapidly, but there is little energy put below ground in the development of roots and rhizomes. This may occur, in part, because of the natural shading provided by the algal bloom. But even when the researchers significantly increased the presence of filter feeding mollusks in nitrogen-enriched tanks, and the water of the tanks remained clear, the eelgrass still responded by developing longer leaves and little below-ground material.
Nixon and his team concluded that the development of long leaves at the expense of root and rhizome growth, regardless of whether caused by nutrient enrichment or by shading, means that there will be little or no lateral branching of rhizomes and little or no production of new shoots. As a result, the density of eelgrass beds exposed to nutrient enrichment and/or shading declines over the summer.
"Our experience suggests that the best indicators of health in eelgrass beds will include measures of the rate of leaf elongation, plant density, and the shoot:root ratio," said Nixon. Very rapid leaf elongation, low plant density at the end of the growing season, and high shoot:root ratios at mid-summer, taken together or individually, may indicate a poor prognosis for bed survival. Our experiments suggest that these indicators begin to appear with inorganic nutrient enrichment exceeding about 50 pounds of nitrogen per acre of lagoon per year."
The lagoon mesocosms were constructed using funds from the RI Sea Grant Program and the NOAA Coastal Ocean Program. Some of the shallow water indicator experiments were supported by the U.S. Environmental Protection Agency and the RI Sea Grant Program.