Scientific Investigations Report 2006–5106
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5106
The discovery that fish populations in Hood Canal, Washington, were stressed and dying because of low concentrations of dissolved oxygen (DO) in the water prompted studies to determine the causes of the low DO concentrations. The U.S. Geological Survey contributed to the studies by assessing the quantities and sources of loads of dissolved inorganic nitrogen (DIN) to Hood Canal and to Lynch Cove, the landward-most part of the canal and the area where DO concentrations were lowest. DO concentrations in the bottom waters of parts of Hood Canal, especially Lynch Cove, were low because of oxygen consumption from the decay of settling labile organic matter (for example, algae) produced in the fresh, upper layer. The presence of phosphate in the upper layer of Lynch Cove in the absence of nitrate supports the hypothesis that the production of this labile organic matter was limited by the availability of nitrogen. At the entrance to Hood Canal, about 1,000 to 3,000 cubic meters per second (m3/s) of saline Admiralty Inlet water flowing over the sills into the lower layer of the water column carries substantial amounts (0.29 milligram per liter of N) of dissolved inorganic nitrogen (DIN). An estimated 10,100 to 34,000 metric tons (MT) of DIN as N are transported annually by estuarine circulation. As the saline water in the lower layer travels landward, it increasingly becomes entrained in the seaward-flowing upper layer of the fresh, less dense water, and the net transport of water within the lower layer decreases.
River and shoreline basins of Hood Canal contributed most of the 689 MT of DIN added annually by freshwater loads to the upper layer of Hood Canal during the last 30 years (92 percent of total freshwater load to the upper layer). A portion of the DIN load from these basins ultimately originated as DIN in precipitation falling on the terrestrial landscapes. Although the estimates of DIN load from surface‑water discharges to the upper layer of Hood Canal were based on numerous measurements of flow and DIN concentration from various creeks and rivers in the Hood Canal and Lynch Cove drainage basins, very little is known about the load of DIN associated with direct discharges of ground water to Hood Canal. Rain falling directly on the surface of Hood Canal is estimated to have contributed 4 percent of the annual load to the upper layer of Hood Canal, based on data from a nearby atmospheric deposition station. Along-shore septic systems were estimated to contribute 4 percent of the annual load of DIN to the upper layer of Hood Canal.
The water column of Hood Canal shallows to a depth of less than 50 meters as Hood Canal turns sharply at The Great Bend. Both tidal currents and the net estuarine circulation decrease landward of The Great Bend. In autumn 2004, the net transport of landward-flowing water in the lower layer of the water column decreased to 119 m3/s at Sisters Point, compared to about 1,000 to 3,000 m3/s transported over the sill at the entrance of Hood Canal. This net circulation transported 24 times the amount of DIN into Lynch Cove (132 MT per month) than freshwater loads of nitrogen added to the upper layer of Lynch Cove (3.6 MT per month). In fact, the mean concentration of DIN in the bottom saline waters (0.42 mg/L of N) was higher than that of the Union River (0.20 mg/L of N), which is surrounded by the most populous residential area and included loads from septic systems. During September and October 2004, 72 percent of the total freshwater loads to the upper layer of Lynch Cove originated from surface and regional ground-water sources from Mission Creek and Union River subbasins, as well as three shoreline subbasins. About 23 percent of the DIN load to the upper layer of Lynch Cove from freshwater sources was estimated to have originated from shallow shoreline septic systems in Lynch Cove during September and October 2004. Rain contributed about 4 percent of freshwater DIN load to the upper layer of Lynch Cove during September and October 2004
The loss of turbulent energy from the tides allows thermal heating and the supply of fresh water to maintain a stratified upper layer in the summer in Lynch Cove. Any DIN that was added to the upper layer of Lynch Cove from atmospheric or terrestrial freshwater sources (about 3.6 MT per month) was quickly taken up by the abundant phytoplankton population, which consumed oxygen while settling and decaying. In order for DIN in the lower layer that was transported by estuarine circulation to be taken up by phytoplankton, the DIN in the saline water must mix upward into the stratified euphotic upper layer. The similarity of the δ15N of the upper-layer POM and bottom-layer nitrate, and the maximums in turbidity, chlorophyll a, particulate organic carbon, and particulate organic nitrogen at the base of the pycnocline where saline bottom water mixed upward into the upper layer, strongly suggest that nutrient-rich, saline bottom water was largely responsible for sustaining the productivity of the euphotic upper layer. However, not all DIN in the lower layer mixed upward and was taken up by phytoplankton. The transport of nitrate from Lynch Cove to The Great Bend due to horizontal gradients within the pycnocline also limits the upward diffusion of nitrate from bottom waters, as does denitrification in the shallow regions of Lynch Cove.
Under some environmental conditions, isotopic composition of nitrogen can provide insights into the sources of nitrate and the transformations that the nitrogen may have undergone. For instance, δ15N of residual nitrate usually is higher than the δ15N of the material formed by biological processes such as uptake and denitrification. A concerted effort was made to differentiate terrestrial sources of nitrogen from sources of nitrate in the saline water by locating one-half of the water-quality sites near the shoreline. If terrestrial sources of nitrogen were significant and they had a distinct isotopic composition, the most likely location of finding an isotopic signal from terrestrial sources would be within the water column of these shoreline sites. In September, nitrate in the upper layer was almost completely taken up by phytoplankton. Therefore, the isotopic signature of nitrate from terrestrial sources generally would be transferred to the particulate matter. No difference in the composition of particles between shoreline and open water sites was found (carbon:nitrogen ratio, δ13C, and δ15N). The interpretation of isotopic data is subject to significant uncertainty, and the inability to detect differences in the source of particulate nitrogen does not necessary mean that the nitrogen taken up by the phytoplankton all originated from the same source. In contrast, there were significant and consistent changes in the isotopic composition of nitrate of the bottom waters that could be either related to the source of nitrate or, more likely, reflect the nature of geochemical reactions occurring in Lynch Cove. Relations between DO, orthophosphate, and nitrate in the water column in shallow region of Lynch Cove suggest some denitrification of nitrate after decomposition of particulate organic matter. Denitrification decreases the upward flux of nitrate that stimulates phytoplankton growth, and could possibly affect the δ15N of nitrate.
The preliminary assessment of the annual loads of DIN to Hood Canal and the loads of DIN to Lynch Cove during 2 months of the critical autumn season provide a framework to develop a more comprehensive understanding of the effects of DIN loads on DO concentrations. In particular, the findings in this report indicate that horizontal transport and upward mixing of DIN in the lower layer are driven by a complex set of physical forces that are not fully understood. In addition, validation of any future coupled biogeochemical-hydrodynamic model will require examining the causes of the isotopic composition of nitrate in the lower layer of shallower regions by further study.
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