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Master of Science (MS)
McPhee-Shaw, Erika E.
Crosby, Sean Christopher
Nutrient supply is a fundamental driver of primary productivity, and often a limiting factor for organism growth in both open-ocean and coastal systems. Continental shelves are locations of high primary productivity in the world’s oceans because of high nutrient supply. Understanding the dominant transport mechanisms of nutrients in these locations is paramount to understanding patterns in primary productivity. While there have been extensive studies of nutrient transport and productivity on the Pacific Northwestern coast of the US (Davis et al., 2014, Siedlecki et al., 2015, Ware and Thomson, 2005, Banas et al., 2009), there is renewed interest in understanding these transport processes as dissolved oxygen (DO) decreases with climate change, leading to persistent hypoxia (Siedlecki et al., 2015, Hickey and Banas, 2003). While the presence of energetic internal waves have been documented on the Washington Shelf (Alford et al. 2012, Zhang et al. 2015), the impact of this mechanism on coastal nutrient concentrations has not yet been evaluated in detail in this region.
The purpose of this study is to describe and quantify the across and along-shelf transport mechanisms of nitrate on the WA shelf and identify the dominant processes bringing deep, dense, nutrient rich, low DO waters to coastal surface waters, focusing on internal waves and upwelling. Data was sourced from the NANOOS (Northwest Association of Networked Ocean Observing Systems) NEMO (Northwest Enhanced Moored Observatory) array, consisting of two moorings off the coast of La Push, WA in 100-m water depth on the WA shelf. Observations of nitrate concentration, water velocity, and surface wind velocity during the summer of 2014 are used to estimate transport. Transport by canyon-enhanced upwelling and internal waves are estimated from spectral variance as well as a classification scheme I developed to parse out baroclinic events. While wind driven up and downwelling dominated summer variability (34%), internal-wave-driven variance was significant (28%±13%). Sets of internal waves inundated the shelf on approximately 12-hour timescales, increasing the nitrate concentration by approximately 7 umol/L during periods of internal wave influence (26.5% increase) with a mean nitrate flux increase of 3.7 μmol×m×L-1×s-1(18-fold increase). Wind driven upwelling increased surface nitrate concentration by approximately 8 umol/L (30.3% increase) with a mean nitrate flux increase of 4.5 μmol×m×L-1×s-1(22-fold increase). Confounding factors including Salish sea nutrient pumping, the Columbia river plume, and the Juan de Fuca eddy were not quantified, but the potential impact of their influence is discussed. Though during the study period (summer 2014), upwelling was weaker than usual, enhancing the relative impact by internal waves, internal waves are likely a significant contributor to nitrate transport on the WA shelf.
internal waves, upwelling, nutrient transport, nitrate flux, primary productivity, Washington Shelf, circulation, nutrient retention, canyon enhanced upwelling, dissolved oxygen, Juan de Fuca Eddy
Western Washington University
Subject – LCSH
Continental shelf--Washington (State)--La Push; Marine productivity--Washington (State)--La Push; Water--Dissolved oxygen--Washington (State)--La Push; Nitrates--Washington (State)--La Push; Upwelling (Oceanography)--Washington (State)--La Push; Internal waves
La Push (Wash.)
Copying of this document in whole or in part is allowable only for scholarly purposes. It is understood, however, that any copying or publication of this document for commercial purposes, or for financial gain, shall not be allowed without the author’s written permission.
Danyluk, Liesl G., "Transport Mechanisms of Nitrate on the Washington Shelf" (2022). WWU Graduate School Collection. 1133.