『Abstract
Hood Canal, Washington, USA, is a poorly ventilated fjord-like
sub-basin of Puget Sound that commonly experiences hypoxia. This
study examined the influence of watershed soils, vegetation, physical
features, and population density on nitrogen (N) export to Hood
Canal from 43 tributaries. We also linked our watershed study
to the estuary using a salinity mass balance model that calculated
the relative magnitude of n loading to Hood Canal from watershed,
direct precipitation, and marine sources. The overall flow-weighted
total dissolved N (TDN) and particulate N input concentrations
to Hood Canal were 152 and 49μg l-1, respectively.
Nitrate and dissolved organic N comprised 64 and 29% of TDN, respectively.
The optimal regression modes for TDN concentration and areal yield
included a land cover term suggesting an effect of N-fixing red
alder (Alnus rubra) and a human population density term
(suggesting onsite septic system (OSS) discharges). There was
pronounced seasonality in stream water TDN concentrations, particularly
for catchments with a high prevalence of red alder, with the lowest
concentrations occurring in the summer and the highest occurring
in November-December. Due to strong seasonality in TDN concentrations
and in particular stream flow, over 60% of the TDN export from
this watershed occurred during the 3 month period of November-January.
Entrainment of marine water into the surface layer of Hood Canal
accounted for ≒98% of N loading to the euphotic zone of this estuary,
and in a worst case scenario OSS N inputs contribute ≒0.5% of
total N loading. Domestic wastewater discharges and red alders
appear to e a very important N source for many streams, but a
minor nutrient source for the estuary as a whole.
Keywords: Hood Canal; Puget sound; Stream chemistry; nitrogen;
Red alder; Onsite septic systems; Eutrophication; Hypoxia』
Introduction
Study site
Methods
Basin attributes
Field and laboratory procedures
Streamflow estimation
Loading rates
Principal component analyses
Multiple regression models
Salinity box model of N inputs to hood canal
Results
Seasonal and regional trends in concentrations and export
Statistical models of watershed effects on nutrients
Monte Carlo simulations
Box model of N inputs to Hood Canal and Lynch Cove
Discussion
Terrestrial controls on stream N export
Vegetation effects
Population
Soils
Biological versus physical drivers
A box model of watershed TDN contributions to Hood Canal
Conclusions
Acknowledgments
References