TEMPORAL VARIATION IN RIVER NUTRIENT AND DISSOLVED LIGNIN PHENOL CONCENTRATIONS AND THE IMPACT OF STORM EVENTS ON NUTRIENT LOADING TO HOOD CANAL, WA. Nicholas D. Ward, University of Washington School of Oceanography, Seattle, WA, nickward@uw.edu; Jeffrey E. Richey, University of Washington School of Oceanography, Seattle, WA, jrichey@uw.edu; Richard G Keil, University of Washington School of Oceanography, Seattle, WA, rickkeil@uw.edu Abstract A year-long data set measuring dissolved nutrient, particulate nutrient, and dissolved lignin phenol concentrations in several temperate Pacific Northwest rivers, shows a strong correlation between river discharge rates and nutrient concentrations during significant storm events following a long dry period. Furthermore, dissolved lignin concentrations increase with river discharge during autumn and winter storms; systematic changes to C/V, S/V, and Ad/Al (v) ratios indicate a mobilization of relatively more woody/gymnosperm-derived and degraded material during storm events. Results from this study suggest that a shallow nutrient-rich pool of particulate matter accumulates in watersheds during long dry periods; this nutrient-rich particulate matter degrades in surface soils, creating a pool of accumulated dissolved nutrients in surface soils during periods of soil-saturation deficiency. Surface runoff during autumn and winter storms mobilizes the nutrient-rich particulate pool, whereas dissolved nutrients are mobilized from soils once the soil saturation deficiency is alleviated. This pool of shallow soil nutrients, secondary to a deep soil pool sustaining base flow conditions, is exhausted with successive winter storms; by spring there is little to no response in river nutrient concentrations and a dilution of dissolved lignin concentrations with increased rainfall and river flow. INTRODUCTION The magnitude of nutrient export from a watershed is closely coupled with the rate of water discharge. On a seasonal scale, there is a logarithmic relationship between river discharge and concentrations of dissolved Carbon and Nitrogen constituents (e.g. Guillaud, et al. 2007). Furthermore, recent studies have shown a tight coupling between river discharge rates and dissolved nitrogen and carbon concentrations on shorter timescales (e.g. Sigleo and Frick, 2003; Boyer et al, 1997). Addition of nutrient-rich freshwater has been shown to cause eutrophication in nutrient-limited estuarine or coastal systems, which in certain cases results in hypoxic events (Howarth and Marino, 2006; Rabalais, et al., 2001); thus, understanding the timing and magnitude of river nutrient delivery is critical for predicting potential impacts of a watershed on the basin in which it drains. Observations in Hood Canal, WA—a branch of the fjord-like Puget Sound estuary—have shown that low dissolved Oxygen (DO) conditions in Hood Canal, especially in the southernmost Lynch Cove region (see figure 1), have become more persistent and widespread in recent decades compared to the 1930’s to 1960’s, resulting in increased frequency of hypoxic and fish kill events (Newton et al, 1995; Newton et al 2002). The Hood Canal Dissolved Oxygen Program Integrated Assessment and Modeling study (HCDOP-IAM) seeks to quantify the complex processes that contribute to low DO in Hood Canal, by utilizing marine models and a terrestrial Distributed Hydrology Soil-Vegetation Model (DHSVM). A subset of the HCDOP-IAM effort, this particular study seeks to better constrain river nutrient exports estimated by the DHSVM. The DHSVM, as it is, calculates total watershed nutrient export based on data gathered from monthly samplings from the 43 streams draining into Hood Canal; however, it is hypothesized that actual river-bound nutrient concentrations vary significantly on much shorter timescales than a Figure 1. Dissolved O 2 concentrations in the Hood Canal. Sample sites for this study are the Skokomish and Union Rivers, which drain into the southwest and southeast corners of Hood Canal, respectively. 2nd Joint Federal Interagency Conference, Las Vegas, NV, June 27 - July 1, 2010