Future Studies

Little is known about the circulation of the landward region of Hood Canal and the physical forces that mix nitrate-rich bottom waters upwards into the euphotic zone that seem to be so important in sustaining phytoplankton blooms. The reversals in the normal estuarine circulation also could help to retain nitrate-rich bottom water within Lynch Cove, and thus provide greater opportunity for the upward flux of DIN. Without a better understanding of the physical forces that drive the circulation of Hood Canal that can only be obtained from additional current measurements for longer periods, the relative importance of nitrogen in freshwater and of nitrate-rich saline lower layer in stimulating the phytoplankton growth that depletes bottom-water oxygen can not be assessed.

The increase in δ¹⁵N of nitrate with landward distance from Sisters Point (fig. 16) in the absence of significant differences in the δ¹⁵N of the POM must be explained in order to fully understand the sources and internal cycling of DIN in Lynch Cove. At least four explanations are possible: (1) the load of high δ¹⁵N nitrate from ground water seeping below the pycnocline, (2) denitrification within the water column, and (3) oxidation of ammonium diffusing out of the sediments, and (4) downward mixing of residual nitrate in the pycnocline following phytoplankton uptake as a result of flow reversals. Available data obtained from Lynch Cove do not support or eliminate any of these explanations. Collecting additional data and interpreting them in the context of a model that couples biogeochemical processes with a hydrodynamic model could suggest the more plausible explanation.

The increase of δ¹⁵N values in nitrate in the bottom waters of shallow sites could be a result of greater influence of a load of ground-water nitrate with higher values of δ¹⁵N, such as septic tank effluent. Because of a greater sediment surface-to-volume ratio, a specific ground-water load will result in higher concentrations and higher δ¹⁵N of the nitrate in the shallow regions of Lynch Cove because of less dilution than would occur if the same load were added to the deeper region of Lynch Cove with its much larger cross-sectional volume. However, the absence of high values of δ¹⁵N in the POM either from the upper layer or from shoreline sites does not indicate that a terrestrial source of N with high value of δ¹⁵N was added in substantial amounts to Lynch Cove. If nitrate from a terrestrial source were responsible for the nitrate with higher values of δ¹⁵N, then the load would have to occur at depths greater than the pycnocline. Nitrate with higher δ¹⁵N values added to the regional ground-water flow in the subbasins through natural or anthropogenic activity could have been discharged to Hood Canal deeper than 6 m (the depth of the pycnocline) and caused this enrichment in δ¹⁵N in the bottom-layer water at sites shallower than 20 m.

Biogeochemical processes in the water column and the sediments can affect the δ¹⁵N of nitrate in the water column. The relations between DO, orthophosphate, and nitrate in the water column indicate that a significant amount of nitrate was denitrified in the shallow sites of Lynch Cove. Denitrification taking place in the water column increases both the δ¹⁵N and δ¹⁸O of the residual nitrate (Brandes and others, 1998; Kendall and others, 2001). In contrast, the δ¹⁵N of nitrate^- in the overlying water did not change during a 12-hour period when significant denitrification occurred in the sediments during an in-situ incubation core experiment (Brandes and Devol, 1997). Therefore, the portion of the denitrification that occurred within the water column of Lynch Cove would have determined the effect that denitrification had on δ¹⁵N values of the nitrate in the water column. In the Baltic Sea, less than 20 percent of the denitrification took place in the water column at DO concentrations of about 0.3 mg/L (Shaffer and Rönner, 1984). In a laboratory experiment simulating the sediment-water interface, denitrification occurred when DO concentrations were less than 0.5 mg/L (Kerner, 1996). DO concentrations in bottom waters of the shallow site L16 in Lynch Cove within a month preceding the δ¹⁵N determinations (August and September 2004) were about 0.3 mg/L. This DO concentration was well within the range in which denitrification could be occurring within the water column, given the analytical uncertainty of DO measurements at concentrations less than 0.5 mg/L. Understanding the relative proportion of denitrification occurring within the sediments and the water column in Lynch Cove is essential to resolving the effect that denitrification might be having on the increases in δ¹⁵N and δ¹⁸O of nitrate in the lower layer of the shallow sites.

Oxidation of dissolved ammonium diffusing out of the sediments as a result of sedimentary diagenesis of organic matter also could affect the δ¹⁵N of nitrate. The δ¹⁵N of ammonia diffusing out of the sediments has been shown to be elevated relative to the δ¹⁵N of the nitrate and to the δ¹⁵N of the organic matter in the sediments (Brandes and Devol, 1997). In September 2004, elevated ammonia concentrations as high as 0.08 mg/L were observed in the bottom waters of the shallow sites of Lynch Cove. Paulson and others (1993) also found elevated concentrations of ammonia in bottom waters of the shallow region of Lynch Cove, which appeared to been have been oxidized during estuarine transport. The effect of the diffusion of ammonia from the sediments on the δ¹⁵N of nitrate in the water column will depend on the rates of ammonia diffusion from the sediments and its subsequent oxidation rate.

The increase in δ¹⁵N of nitrate with landward distance from Sisters Point can be explained as a complex interaction between physical and biogeochemical processes. The low concentration of nitrate in the 3-m sample at L14 indicated that biological productivity removed most of the nitrate, thus providing an indication of the selectivity of algal uptake. Compared to bottom-layer nitrate (mean δ¹⁵N of +8.9 per mil with a range between +8.2 and +10.2 per mil, mean δ¹⁸O of +5.0 per mil with a range between +2.8 and +6.7 per mil), the residual nitrate in the 3-m sample at L14 had higher values of both δ¹⁸O (+18.5 per mil) and δ¹⁵N (+15.6 per mil). The higher value of δ¹⁵N in the remaining nitrate is consistent with the slightly lower value of δ¹⁵N of POM in the lower layer (+7.6 per mil). The higher value of δ¹⁵N in nitrate in the lower layer of the shallow sites in Lynch Cove could have been caused by downward mixing of the residual nitrate with its high δ¹⁸O and δ¹⁵N values within the pycnocline during the regular reversals of the sub-tidal currents.

Back to Table of Contents

For more information about USGS activities in Washington, visit the USGS Washington Water Science Center home page.

Accessibility FOIA Privacy Policies and Notices

U.S. Department of the Interior | U.S. Geological Survey
Persistent URL: https://pubs.water.usgs.gov/sir20065106
Page Contact Information: Publications Team
Page Last Modified: Thursday, 01-Dec-2016 19:03:37 EST