"Water quality management efforts aimed at protecting and preserving water quality and fisheries resources and habitat must be highly adaptive, taking both human nutrient enrichment and changes in freshwater input into consideration," said Dr. Hans Paerl, Kenan professor of marine and environmental sciences at the University of North Carolina at Chapel Hill's Institute of Marine Sciences.
"Estuaries are among the most productive and resource-rich waters on earth," Paerl said. "The recent hurricanes we have experienced have taught us that the growth and composition of phytoplankton are controlled and affected by both the freshwater inputs and nutrients contained in floodwaters accompanying hurricanes and other large storms."
Phytoplankton are key indicators of an estuary's ecological condition and health, he said. They are microscopic algae that form the base of the food chain, sustaining fisheries and overall fertility of large estuaries and other bodies of water, he said.
"This appears to be the major water quality management challenge over the next several decades, since climatologists have predicted that estuarine and coastal systems will face at least another several decades of elevated hurricane activity in the Atlantic," the scientist said.
The new study, scheduled to appear in late January in the journal Limnology and Oceanography, involved analyzing changes in water quality in the two largest estuarine systems in the United States before and after recent hurricanes such as Dennis, Floyd and Isabel.
Besides Paerl, authors of the report are postdoctoral researcher Lexia M. Valdes and graduate student Benjamin L. Peierls of the UNC College of Arts and Sciences' marine sciences institute and Drs. Jason E. Adolph, research scientist at the Maryland Center for Biotechnology, and Lawrence W. Harding Jr., professor at the University of Maryland Center for Environmental Science in Cambridge, Md.
"Both the Neuse River Estuary/Pamlico Sound and Chesapeake Bay systems have experienced large post-World War II increases in nitrogen and phosphorus loading from agriculture, industry and other sources, which often negatively affect water quality and marine life," Paerl said. "We've taken steps to reduce nutrients to alleviate such harmful symptoms as algal blooms and oxygen depletion, but this is a highly complex situation affected by draughts, hurricanes, floods, farming, development and other human activities."
In their study, the researchers uncovered similarities between the two largest U.S. estuaries, but also differences, he said.
In water systems, variability in the length of time the water remained in approximately the same place strongly affected seasonal and longer-term patterns of the amount and kinds of phytoplankton present, Paerl said. During and following hurricane years, when more freshwater drained into the estuaries, conditions favored fast-growing diatoms in the Chesapeake Bay but not in the Neuse River and Pamlico Sound. That probably was because the bay is more open to the sea than the N.C. sound is.
In the latter, spring growths of green algae (chlorophytes) and summer growth of blue-green algae (cyanobacteria) became more common, the scientist said. Such storm-driven changes have important ramifications for the animal plankton, or zooplankton, feeding on algae and, ultimately, on fish and shellfish that rely on specific types of phytoplankton and zooplankton. How an estuary's food web responds and its effects on fisheries are strongly dependent on the composition and abundance of phytoplankton sustaining the food web.
"What happens to phytoplankton dynamics ultimately cascades up the food web," Paerl said.
He and colleagues currently are investigating the longer-term ramifications of storm-induced changes for fisheries.
"Seasonally, droughts, floods and storm-related deep-water mixing overwhelmed attempts to control nutrients flowing off the land and into the sound and the bay," Paerl said. "This underscores the difficulty in predicting seasonal and longer-term phytoplankton production, composition and water quality responses to just nutrient management alone. The interactive effects of large, episodic freshwater discharges accompanying increased hurricane activity need to be incorporated in water quality management, at least for the foreseeable future."
Along with UNC and Duke University colleagues, Paerl began monitoring surface water quality in the Neuse River and Pamlico Sound in 2000 with the help of state-run ferries. This program, FerryMon, has helped track and document long-term, ecosystem-scale efforts of large storms and droughts on the ecological condition and overall health of the estuarine system, which is the mid-Atlantic region's largest fisheries nursery. That ongoing data collection and associated research has become a model for continuous ferry-based water assays throughout the nation.
Help and support from the N.C. Department of Transportation's Division of Ferries and Department of Environment and Natural Resources (DENR) deserves much of the credit, he said.
Since then, marine scientists in Florida, Massachusetts, Maine and New York and as far away as San Francisco Bay and Washington's Puget Sound have begun designing, and in some cases already using, comparable systems.