Amid growing concern about fish kills, U.S. Geological Survey (USGS) scientists have been researching sources of nitrogen entering Hood Canal, a fiordlike inlet off Puget Sound in Washington State. Excess nitrogen causes algae blooms, and when the algae die and settle to the bottom, their decay uses up dissolved oxygen, causing lethally low oxygen levels that kill fish and other species. In a trio of publications released in May 2006, the USGS scientists report that seawater flowing from Puget Sound into Hood Canal contains at least 17 times more algae-feeding nitrogen than all the freshwater inputs combined. Exactly how the seawater circulates in Hood Canal is not yet well understood.
Three of the scientistsUSGS Hood Canal project leader Tony Paulson, research oceanographer Ed Josberger, and hydrologist Bill Simondscontributed to a radio segment called "A Science Mystery," the second in a three-part series on Hood Canal that aired in late June on KPLU-FM, a National Public Radio affiliate in the Seattle-Tacoma area.
Hood Canal isn't really a canal but a long, narrow body of water with a relatively shallow entrance on Puget Sound, where the water depth is about 45 m. Just south of the entrance, the canal deepens to 150 to 180 m. The "sill" at the entrance hinders the exchange of water between the canal and the sound during changing tides and seasons. The water of Hood Canal can be highly stratified, with an upper layer that is fresher and warmer than the saltier, colder deep layer. A low rate of exchange between the canal's upper and lower layers means that dissolved oxygen removed from the lower layer by decaying algae will not be quickly replaced, putting deep-dwelling animals, such as rockfish, at risk. When strong southerly summer winds blow the oxygenated upper layer to the north, oxygen-depleted water rises from the lower layer to the surface, and many fish die.
In September 2002, it was observed that fish on the west side of Hood Canal near Hoodsport were under stress from low concentrations of dissolved oxygen. This observation prompted the Washington Department of Fish and Wildlife to temporarily close parts of the canal to some types of fishing during the month of October. In 2003, low dissolved-oxygen conditions worsened, some fish kills were observed as early as June, and by October an estimated 30 percent of rockfish had perished. Although low concentrations of dissolved oxygen in Hood Canal have been observed during late summer and early fall as far back as the 1950s, available data suggest that concentrations vary from year to year but have been trending lower over time, with longer durations of low concentrations. The USGS was asked by Congress to collaborate with stakeholders in studying the causes of low dissolved-oxygen concentrations in Hood Canal (see the Hood Canal Project Web page.)
In late summer and fall of 2004, USGS scientists examined both saltwater and freshwater pathways of nitrogen into Hood Canal, including streams, rivers, ground water, and seawater flowing in from Puget Sound. Water samples were analyzed for nitrogen compounds to determine how much nitrogen comes into Lynch Cove at the farthest reach of Hood Canal and by what pathways. Scientists measured currents, temperature, salinity, dissolved oxygen, and nitrogen isotopes in the upper and lower layers of the canal.
From these studies, USGS scientists Chris Konrad, Lonna Frans, Raegan Huffman, and Theresa Olsen estimate that 92 percent of the total freshwater load of dissolved nitrogen comes to the upper layer of Hood Canal from surface- and ground-water sources; point-source and flow from shoreline septic systems contribute only 4 percent of the total freshwater load. In comparison, the nitrogen load in seawater flowing into Hood Canal from Puget Sound was at least 17 times the total freshwater input to the upper layer. Similarities in nitrogen isotopic data from suspended matter in the upper and lower layers of the canal measured by Carol Kendall's group, in combination with high chlorophyll concentrations and other data collected where the upper and lower layers meet, strongly suggest that nitrogen-rich water from the lower layer mixes into the upper layer. A group headed by Marlene Noble observed that the near-bottom currents in the narrow strait connecting Lynch Cove to the rest of Hood Canal tended to flow toward the cove, pushing water in the lower layer into it. There was a tendency for water in the upper layer to flow out of Lynch Cove when near-bottom currents flowed into it, and vice versa.
"Seawater flowing into Hood Canal is a double-edged sword," said Tony Paulson, USGS research hydrologist and lead author of the main report. "Seawater currents flush out the system, but they also bring enormous amounts of nitrate into Lynch Cove. However, we really can't assess the human impact on Hood Canal until we can accurately model the very complicated movements of water, including complex effects caused by winds and by inflows of seawater from Puget Sound."
The USGS is working on Hood Canal with local groups and governments, agencies, tribes, and the University of Washington under the Hood Canal Dissolved Oxygen Program, enabled by funding secured by Congressman Norm Dicks. Three reports by USGS scientists contributing to this effort were published in May 2006.
The report "Loads of Dissolved Inorganic Nitrogen to Hood Canal and Lynch Cove, Washington," by Anthony J. Paulson, Christopher P. Konrad, Lonna M. Frans, Marlene Noble, Carol Kendall, Edward G. Josberger, Raegan Huffman, and Theresa Olsen, was published as USGS Scientific Investigations Report 2006-5106. A limited number of printed copies are still available as of July 2006; to request one, send an e-mail to email@example.com.
The report "Surface-Water Quality in Rivers and Drainage Basins Discharging to the Southern Part of Hood Canal, Mason and Kitsap Counties, Washington, 2004," by L.M. Frans, A.J. Paulson, R.L. Huffman, and S.N. Osbourne, was published as USGS Scientific Investigations Report 2006-5073.
The report "Transport Pathways in the Lower Reaches of Hood Canal," by Marlene A. Noble, Anne L. Gartner, Anthony J. Paulson, Jingping Xu, Edward G. Josberger, and Christopher Curran, was published as USGS Open-File Report 2006-1001.
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