Scientific Investigations Report 2006–5106
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5106
In September 2002, fish on the west side of Hood Canal near Potlatch, Washington (fig. 1) were observed to be under stress from low concentrations of dissolved oxygen (DO). This observation prompted the Washington State Department of Fish and Wildlife to close parts of Hood Canal temporarily to some types of fishing during October. In 2003, DO conditions worsened, some fish kills were observed as early as June, and about 30 percent of rockfish were killed by October (http://www.hoodcanal.washington.edu/documents/document.jsp?id=1453, accessed Nov. 15, 2005). Low concentrations of DO in Hood Canal during late summer and early autumn have been observed as far back as the 1950s (Collias and others, 1974). Available data suggest that even though DO concentrations vary from year to year, low DO conditions recently have been occurring more frequently, and the duration of low concentrations is more persistent (http://www.hoodcanal.washington.edu/observations/historicalcomparison.jsp, accessed January 29, 2006).
The sluggish circulation and stratified water column of Hood Canal produce characteristics that are similar to classic fjords (Paulson and others, 1993; Warner and others, 2001). In fjords with sluggish circulation, the water column becomes stratified and leads to the natural condition of low DO concentrations in bottom waters. Unlike many fjords with shallow sills, the bottom waters of Hood Canal are not as anoxic because cold, saline, oxygenated ocean water sinks to the bottom as it spills over the sills and enters the fjord. In late summer/early autumn, the dense water displaces the existing bottom water. However, in the winter and spring, the flow of colder, less dense water into Hood Canal occurs at mid-depth (Warner and others, 2001). Because freshwater inflows from rivers and streams tend to stay on the surface of the canal, replenishment of DO consumed by decay of settling organic matter is limited.
The rates of nitrogen load to the upper layer of the water column typically limit algal growth in marine waters. Problems associated with sluggish circulation and a stratified water column are exacerbated when terrestrial sources of nitrate and other nutrients enhance the growth of algae and contribute to eutrophication. The biomass of algae produced by the added nutrients settles below the stratified layer and increases oxygen consumption through algal decomposition. Poor circulation and seasonal stratification of the water column set up conditions so that the re-oxygenation rate is not sufficient to meet the demand for oxygen caused by decomposition of algal biomass produced in the upper layer. Seasonal low oxygen conditions stress aquatic life and even force some species out of their normal habitat zones. The extent to which eutrophication is detrimental to ecosystems of coastal estuaries and bays has been recognized and studied on the Atlantic seaboard of North America (Dillow and Greene, 1999; Barlow, 2003). Similar studies have not been conducted in Hood Canal.
In 2003, the U.S. Geological Survey (USGS) was asked by Congress to study the causes of low DO concentrations in Hood Canal in collaboration with the Hood Canal Dissolved Oxygen Program (HCDOP). The HCDOP is a partnership of organizations that monitor and study Hood Canal, and evaluate potential corrective actions to address the low DO problem. There are more than 30 HCDOP partners, among them Puget Sound Action Team; Washington State Departments of Ecology, Health, Fish and Wildlife, and Natural Resources; Jefferson, Kitsap and Mason Counties; Skokomish and Port Gamble S’Klallam Tribes; Hood Canal Coordinating Council; Hood Canal Salmon Enhancement Group; University of Washington; and the USGS. After consultation with the HCDOP partners, the USGS began to assess the loadings of nitrogen-based compounds to Hood Canal. A study focusing on dissolved inorganic nitrogen (DIN) loads from surface water landward of The Great Bend was conducted to collect new data in 2004 because the most severe instances of low DO concentrations occur in this region. DIN refers to nitrate, nitrite and ammonia. Data collected for the Lynch Cove part of the study are presented and discussed in reports by Noble and others (2006) and Frans and others (2006). As part of the Lynch Cove study, the USGS also investigated the processes that affect DIN in the area by analyzing the water column from July to October 2004 for various constituents, including nitrogen isotopes.
The purpose of this report is to present the results of the assessment of freshwater and saline sources of DIN loads to Hood Canal through five pathways: (1) direct rainfall onto the surface of Hood Canal, (2) surface-water flow, (3) regional ground-water flow, (4) shallow ground flow from shoreline septic systems, and (5) landward transport in the lower layer of the water column by estuarine currents. This report presents (1) estimates of mean annual DIN loads to Hood Canal for each pathway, as well as from other sources, and compares the loads; (2) calculations of DIN loads to Lynch Cove for September and October 2004 for each pathway, and compares the loads; (3) discussion of internal cycling of DIN in Lynch Cove and the process that affects DIN in Lynch Cove; and (4) discussion of possibilities for future studies of DIN in Hood Canal.
Mean annual DIN loads for Hood Canal from each pathway were estimated using available data from 1971 to 2002. DIN loads to Lynch Cove were estimated using available data (including Frans and others, 2006) and current-velocity data (Noble and others, 2006) and constituent concentrations collected during September and October 2004. Water from Lynch Cove was analyzed for various constituents, including nitrogen and oxygen isotopes of nitrate during July through October 2004 to investigate internal cycling of DIN.
The Puget Sound Action Team (PSAT) and the Hood Canal Coordinating Committee (HCCC) recently developed an assessment of nitrogen loads into Hood Canal from only human activities (Fagergren and others, 2004). Most of the loads from human activities were included in the assessments of DIN loads from one or more of the five pathways into Hood Canal described in this report. The human activities associated with residential, forest, and agricultural use in the subbasins of Hood Canal are reflected in the measured DIN concentrations in the surface and ground water of the subbasins. For instance, mean DIN concentrations were highest in the highly populated Union River subbasin. In addition, the smaller lowland subbasins, which are more likely to be populated, contribute more of their fresh water to Hood Canal as ground-water discharges than do the larger subbasins. Because DIN concentrations were estimated to be higher in ground water, partially reflecting human activities, these populated lowland subbasins contributed more DIN for their size than the larger subbasins. DIN loads from human waste from the residential septic systems along the shores of Hood Canal are specifically addressed in this report. Loads from the direct non-point sources and commercial fishing practices are addressed separately in the section “Other Sources.”
Hood Canal is a 110-km long fjord that divides the Olympic Peninsula from the Kitsap Peninsula (fig. 1). The canal is between 2 and 4 km wide over most of its length. Sills rising to within about 50 and 75 m of the water surface separate Hood Canal from Admiralty Inlet, the entrance to Puget Sound. Landward of the inner sill, the depth of Hood Canal increases to about 175 m. The canal bends sharply at The Great Bend and ends in Lynch Cove at its landward end. Landward of The Great Bend, the water column is shallower and the water depths in this segment of Hood Canal are 55 m or less.
For more information about USGS activities in Washington, visit the USGS Washington Water Science Center home page.
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