Temporal variation in riverine nutrient concentrations and the impact of short term storm events on nutrient loading in Hood Canal
As part of the Hood Canal Dissolved Oxygen Program Integrated
Assessment and Modeling study (HCDOP-IAM), this study seeks to help constrain
riverine nutrient export estimates into the Lower
region in order to determine the factors contributing to low Dissolved Oxygen
levels in Hood Canal, Washington. The current model utilizes
nutrient data from monthly samplings of the 43 streams that drain into Hood Canal;
however, this study proposes that there is substantial temporal variability in
river nutrient concentrations that must be considered. Samples were collected
during several major autumn and winter storm events from the Skokomish, Union,
and Tahuya Rivers
in the Lower Hood Canal
region. Dissolved nutrient data suggests that large amounts of nutrients
accumulate in soils during dry summer months, and are mobilized during large
storm events in autumn and early winter; the result is a high correlation
between river discharge and dissolved nutrient concentrations, meaning nutrient
export can be significantly higher during a storm event than would be estimated
by using a monthly average nutrient concentration. By mid-winter this response
is less marked, suggesting that this reservoir of accumulated nutrients is
exhausted. This study reveals that storm events may have a large influence on
total nutrient export into the Lower
region, which may be underestimated in current models.
- Sampling: ISCO
autosamplers, capable of autonomously collecting 24 one liter samples upon
activation, were setup on the Union, Tahuya, and Skokomish Rivers.
The autosamplers were programmed to automatically collect samples at 3 or
4 hour intervals, depending on the predicted storm length. Samples were
collected during significant storm events from October 2008 through
- Analysis: Total
Suspended Sediment analyses are performed. Dissolved nutrient (nitrate,
nitrite, ammonium, phosphate, and silicate), Dissolved Organic Carbon
(DOC), and Total Dissolved Nitrogen (TDN) concentrations are measured by
Kathy Krogslund's marine chemistry lab in the UW Oceanography department.
Particulate Carbon and Nitrogen analyses are performed by the Stable
Isotope Facility at UC Davis. Also a pilot study to measure lignin
phenols-organic biomarkers-is in progress.
- River Discharge Data: Real-time river discharge data for the Skokomish and Tahuya Rivers are obtained from USGS
(http://waterdata.usgs.gov/nwis/rt). Union River
discharge data is provided by the Hood Canal Salmon Enhancement Group
(HCSEG). Under several circumstances Union
River discharge data was not
available, so Huge Creek data from USGS was normalized to Union River values for this time period.
Huge Creek is within close proximity to Union River.
Results from storm samplings show that nutrient concentrations are
closely coupled with river discharge in early autumn storms. The figures
on the right show the correlation between river discharge and dissolved organic carbon,
total dissolved nitrogen, and total suspended sediment concentrations on short
timescales (days). Furthermore Carbon to Nitrogen and Nitrogen to Phosphorus
(not shown) ratios increase with river discharge indicating that during rapid
shallow flow (i.e. storm runoff) C is more readily mobilized from soils than N
and both C and N are mobilized more readily than P.
Winter and spring samplings do not show the
same pronounced increase in nutrient concentrations with increased river
discharge, indicating that nutrients accumulate in the drainage basin soils
during dry periods (i.e. summer) and are mobilized and exhausted by early and
late autumn storms. During the sampled January 2009 storm, a ~150% increase in
river discharge only results in a ~7% increase in TDN concentration, whereas
during the sampled November 2008 storm, a ~167% increase in river discharge
results in a ~167% increase in TDN concentration. Furthermore, regression
analysis suggests that "accumulated nutrients" are exhausted from soils more
rapidly in the Skokomish River basin relative to the Union
River likely because the much larger Skokomish River represents an integration of
small, Union-like streams.
Rough calculations suggest that properly
accounting for storm-induced N surges can make a profound difference in total N
export estimations depending on when monthly samples were taken. Relating this
storm data to 2007 monthly stream data suggests that October 2007 export
estimates may be ~10% too low, and November estimates may be as much as ~100%
too low for the Skokomish River.
Furthermore, sampling during non-average conditions can greatly influence
export estimates (e.g. Oct. 2007 monthly samples were taken during a period of
higher than average river discharge, meaning the "average" value obtained was
likely far in excess of the actual monthly average). Sampling will continue
until Fall 2009.
Ward, N.D., J.E. Richey, and R.G. Keil. 2011.
Temporal variation in river nutrient and dissolved lignin phenol concentrations
and the impact of storm events on nutrient loading to Hood Canal, Washington,
USA. Biogeochemistry. DOI: 10.1007/s10533-012-9700-9
Steinberg, P.D., M.T. Brett, J.S. Bechtold, J.E. Richey,
L.M. Porensky, and S.N. Smith. 2010. The influence of watershed characteristics
on nitrogen export to and marine fate in Hood Canal, Washington, USA.
Biogeochemistry. DOI 10.1007/s10533-010-9521-7.