Abstract

Effects and impacts of the Northeast Pacific marine heatwave of 2014–2016 on the inner coastal estuarine waters of the Salish Sea were examined using a combination of monitoring data and an established three-dimensional hydrodynamic and biogeochemical model of the region. The anomalous high temperatures reached the U.S. Pacific Northwest continental shelf toward the end of 2014 and primarily entered the Salish Sea waters through an existing strong estuarine exchange. Elevated temperatures up to + 2.3°C were observed at the monitoring stations throughout 2015 and 2016 relative to 2013 before dissipating in 2017. The hydrodynamic and biogeochemical responses to this circulating high-temperature event were examined using the Salish Sea Model over a 5-year window from 2013 to 2017. Responses of conventional water-quality indicator variables, such as temperature and salinity, nutrients and phytoplankton, zooplankton, dissolved oxygen, and pH, were evaluated relative to a baseline without the marine heatwave forcing. The simulation results relative to 2014 show an increase in biological activity (+14%, and 6% Δ phytoplankton biomass, respectively) during the peak heatwave year 2015 and 2016 propagating toward higher zooplankton biomass (+14%, +18% Δ mesozooplankton biomass). However, sensitivity tests show that this increase was a direct result of higher freshwater and associated nutrient loads accompanied by stronger estuarine exchange with the Pacific Ocean rather than warming due to the heatwave. Strong vertical circulation and mixing provided mitigation with only ≈+0.6°C domain-wide annual average temperature increase within Salish Sea, and served as a physical buffer to keep waters cooler relative to the continental shelf during the marine heatwave.

Highlights

  • Anomalous high sea surface temperatures occupied a large region off the coast of North America during the winter of 2013

  • In this paper, supported by synoptic field data, we present an assessment of the effects of the marine heatwave on the Salish Sea as a whole, including impacts on density-driven fjord-like estuarine circulation, followed by conventional waterquality indicator variables and parameters of concern such as temperature (T) and salinity (S) [nearshore habitat], dissolved oxygen (DO) [hypoxia], pH [ocean acidification], and nutrients and phytoplankton biomass [eutrophication], and zooplankton biomass [food web]

  • The data and simulation results relative to 2014 showed an increase in biological activity (+ 14%, and + 6% phytoplankton biomass, respectively) during the peak heatwave year 2015 and 2016 propagating toward higher zooplankton biomass (+14%, +18% mesozooplankton biomass). This was accompanied by lower DO levels and an increase in the volume of hypoxic water. This behavior was consistent with expectation that sustained warmer waters in the Salish Sea would result in higher biological activity

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Summary

Introduction

Anomalous high sea surface temperatures occupied a large region off the coast of North America during the winter of 2013 This mass of warm water, referred to as the “the blob,” was ≈3◦C warmer than normal in February 2014 and expanded its spatial extent to reach the U.S Pacific Northwest continental shelf toward the end of 2014 (Bond, 2014). Nearshore sea surface temperature anomalies along the Washington, Oregon, and California coasts reached a maximum of 6.2◦C off Southern California and only abated seasonally during spring upwelling-favorable wind stress (Gentemann et al, 2017) This event has been linked to numerous incidences of complex ecological impacts over the continental shelf waters directly exposed to the heatwave (Kintisch, 2015). More frequent and intense ocean warming events may have complex impacts on the food webs as concluded by Jones et al (2018) based on an assessment of massive mortality of planktivorous seabirds following the heatwave

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