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). The occurrence of this Northeast Pacific marine heatwave is attributed to decreased surface cooling and lower equatorward Ekman transport in the Gulf of Alaska due to a persistent atmospheric high-pressure ridge (Wang et al., 2014; Bond et al., 2015; Hartmann, 2015). The heatwave persisted over multiple years, from the end of 2014 to the end of 2016, before dissipating in 2017, but remains a concern for potential impacts to estuarine ecosystems and possible recurrences. 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). Whitney (2015) pointed out that just prior to the onset of the heatwave, the winter of 2013/2014 was accompanied by anomalous winds from the south that weakened nutrient transport in the eastern North Pacific, resulting in substantial decreases in phytoplankton biomass. This was followed by a coastwide bloom of the toxigenic diatom Pseudo-nitzschia in spring 2015 that resulted in the largest recorded outbreak of the neurotoxin, domoic acid, along the North American west coast. McCabe et al. (2016) demonstrated that this was caused by the anomalously warm ocean conditions. Similarly, Peterson et al. (2017) concluded that warm ocean conditions observed from late September 2014 through 2016 resulted in an overall drop in populations of copepods and potential collapse of the usual food chain up to forage fish (smalt, herring, and lance). Du and Peterson (2018) noted a shift in phytoplankton abundance and diversity from diatoms that favor lower temperatures to dinoflagellate that favor higher temperatures for optimal growth. 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.

As may be expected, the inner waters of coastal estuaries were also affected. This includes the Salish Sea, an inland fjord-like estuary comprised of the Puget Sound, the Strait of Juan de Fuca, and the Strait of Georgia basins in the U.S. and Canadian waters (see Figure 1). The Salish Sea is one of the world’s largest and most biologically rich inland seas that supports numerous species of mammals, birds, fish, and invertebrates, which in turn are vital to the regional economy, culture, and quality of life. There is continuing urgency to improve our understanding of the vulnerability or resilience of these coastal estuarine waterbodies to stressors such as anthropogenic nutrient and carbon pollution, sea-level rise, climate change, and now marine heatwaves.

Impacts on Salish Sea temperatures were seen throughout 2015 and 2016 before mostly dissipating in 2017. During this period, depth-averaged temperature increases up to 1.5°C were seen at the Washington State Department of Ecology monthly monitoring stations relative to 2013. Unlike typical conditions in such fjord-like estuaries, where variations are largest in surface waters, these high temperatures influenced by the higher-than-normal temperature of incoming exchange flow affected the entire water column. The largest annual average increase in surface water temperature of 1.89°C was recorded at the Admiralty Inlet station, while the largest annual average increase in bottom water temperature of 1.98°C was seen in Hood Canal. A daily average sea surface temperature increase as high as +2.32°C, was seen in 2015 at the National Oceanic and Atmospheric Administration (NOAA) National Data Buoy Center Seattle station. The presence of warmer and lower density water in the Strait of Juan de Fuca likely lowered the density gradient in this exchange pathway. Consequently, density-driven exchange flow would also have been reduced to a certain extent and may have affected overall circulation. Occurrences of early blooms of Alexandrium spp. in Hood Canal in April, of 2015 followed by an increase in 2016, and numerous examples of increased bacterial growth effects such as Vibrio-contaminated oysters, and impacts on higher trophic levels including herring, seabirds (e.g., rhinoceros auklets), and some species of marine mammals, may have been due to these warmer conditions (Bond, 2021). However, understanding the biogeochemical and ecological response of the Salish Sea to this marine heatwave thus far has remained qualitative and somewhat inconclusive in terms of quantifying the causes, extent, severity, and magnitude of projected impacts to the nearshore estuarine ecosystems. This could be partly because ecological monitoring and modeling assessments that were conducted during the marine heatwave years had a different focus, and the results simply did not show a particularly anomalous hydrodynamic or biogeochemical response inside the Salish Sea domain. These include modeling of years 2015–2017 by Olson et al. (2020) who successfully reproduced key aspects of the nitrogen cycle in the Strait of Georgia, region of the Salish Sea including the seasonal cycle and regional differences and highlighted mechanisms through which tidal currents lead to nutrient supply to the surface layer. They acknowledged that 2015 and 2016 were somewhat anomalous years in the Strait of Georgia due to the influence of the incursion of warm water associated with the heatwave. Similarly, Moore-Maley and Allen (2021) analyzed a 5-year hindcast record (2015–2019) along with model-data validation to examine wind-driven upwelling effects on surface nutrient delivery in the Strait of Georgia. MacCready et al. (2021) conducted a 3-year hindcast (2017–2019) covering mostly a period post marine heatwave, focused on circulation mixing and residence times in the Salish Sea. A marine heatwave impact or signature was hard to detect from the data and analysis presented in the above assessments as the warmer water had already occupied the Salish Sea or that marine heatwave had already passed by the time simulations were initiated. But a study by Jarníková et al. (2021) to characterize physical drivers of productivity dynamics did span years 2013–2016, and noticed potential inhibition of water column mixing by higher thermal stratification of the system, in 2015 relative to prior years which they attributed to possible marine heatwave effects.

This potential for marine heatwave-related impacts is particularly important to address given region-wide concerns associated with climate change and the general understanding that an increase in extreme event frequency, including marine heatwaves, may be expected (Pearce and Feng, 2013; Bond et al., 2015; Chen et al., 2015; Di Lorenzo and Mantua, 2016; Hobday et al., 2016; Scannell et al., 2016). Although climate change is not considered to be a major cause of the heatwave, findings by Laufkötter et al. (2020) validate the concern that the frequency of marine heatwaves has increased nearly twentyfold due to anthropogenic climate change effects. Marine heatwaves, which typically occurred once in hundreds to thousands of years in preindustrial times, are projected to occur on a decadal to annual basis if the global average air temperature rises by 1.5–3.0°C. The elevated temperatures during this marine heatwave serve as a preview of the type of conditions and estuarine biogeochemical stress we may expect under future climate conditions. The sustained high temperatures experienced here during the marine heatwave are comparable to the projected average high temperatures (+2.53°C on the shelf and + 1.5°C in the Salish Sea; Khangaonkar et al., 2019) corresponding to the high-emissions future scenario (the Intergovernmental Panel on Climate Change’s representative concentration pathway scenario, RCP8.5). The conditions experienced provide an opportunity for indirect validation of projected future ecological impacts.

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 water-quality 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].

We used a modeling-based approach to isolate and distinguish the effects of the high-temperature pulse that propagated through the Salish Sea. A biophysical model of the Salish Sea was first validated over the 5-year marine heatwave window from 2013 to 2017. “Existing” conditions were compared with “reference” conditions—defined as existing conditions without the anomalous heatwave forcing. This companion “reference” simulation was created using climatological ocean boundary and average meteorological heat load from pre and post marine heatwave years.

Propagation of the marine heatwave into the Salish Sea/Puget Sound region was examined using depth-time profile contour plots of temperature difference (ΔT) at selected stations starting from (1) the ocean boundary location directly across from Neah Bay at the mouth of the Strait of Juan de Fuca, to (2) Neah Bay, through (3) Port Townsend at Admiralty Inlet, to (4) Elliot Bay in the main basin of Puget Sound, and on to (5) Gordon Point at the southern end of Puget Sound as shown in Figure 7. The maximum ΔT (≈3–4°C) occurred during the first impingement of the marine heatwave relative to reference conditions was noted at the sea surface and over the continental shelf boundary. The ΔT reduced mostly to < 2°C as the heatwave propagated into the Salish Sea but increased in South Puget Sound in the range of 2–3°C. Results show that the marine heatwave peaked over the continental shelf and simultaneously reached the Salish Sea entrance at the mouth of the Strait of Juan de Fuca in mid-May 2014. Within a span of about 3 months by mid-September 2014, the heatwave had propagated and dispersed over the entire Salish Sea, reaching the innermost regions such as South Puget Sound.

Figure 7 panel (1) shows the presence of the warmer sea surface temperature over the continental shelf from 2014 through 2016. The effects of downwelling-induced mixing are noticeable, particularly during the winter months of 2015 and 2016, where the temperature anomaly is seen to extend throughout the water column. Figure 7 panels (2) and (3) show the warmer waters in the Strait of Juan de Fuca and Admiralty Inlet (Port Townsend), mixed over the entire water column, being transported into the Puget Sound region of Salish Sea [Figure 7 panel (4) Elliot Bay] and South Puget Sound [Figure 7 panel (5) Gordon Point]. This early heat transfer into the Salish Sea is attributed primarily to estuarine exchange flow, given minimal atmospheric heat flux during the winter months. The effect of the marine heatwave in the Salish Sea was compounded by atmospheric heating during the summer. Figure 7 panel (1) shows that by mid-2016, the marine heatwave had already dissipated from the continental shelf. However, the resident heat in the Salish Sea remained trapped until the end of 2016, before being flushed out and replaced by cooler waters in 2017. It is important to note that in the deeper waters and the innermost regions of the Salish Sea, the effects of the heatwave lingered almost up to 2018 (Psemp Marine Waters Workgroup, 2020). These results are consistent with findings by Jackson et al. (2018) that warm conditions (0.3–0.6°C warmer than the normal monthly average) persisted below the surface mixed layer through at least March 2018 in adjacent Canadian waters to the north.

Sustained warming of Salish Sea waters was due to the combined effect of higher atmospheric heat flux (+ 19% relative to reference 45.2 W/m²) and conveyance of warmer waters from the shelf via strong estuarine exchange flow into the Salish Sea. The magnitude of the estuarine flow is estimated in the range of 100–150 × 10⁵ m³/s (Sutherland et al., 2011; Khangaonkar et al., 2017, 2018; Olson et al., 2020), MacCready et al., 2021). It is a function of mean water depth (∝H³) and depth-averaged density (salinity) gradient (∝∂⁡S¯∂⁡X) as presented analytically by Hansen and Rattray (1965), Dyer (1973), and MacCready (2004), MacCready (2007) for partially mixed estuaries, by Rattray (1967) for fjords, and by Khangaonkar et al. (2011) for Salish Sea sub-basins. The possibility that warmer waters occupying the Strait of Juan de Fuca may have adversely impacted density gradients and therefore the strength of exchange flow was examined.

The definition of exchange flow used here is “tidally averaged volume flux” across the selected transect that enters the estuary below the depth of net zero motion. Selected transects include (A) Strait of Juan de Fuca (inflow to the Salish Sea), (B) Haro Strait (inflow to Georgia Basin), and (C) Admiralty Inlet (inflow to Puget Sound). Table 4 presents exchange flow magnitudes computed at these transects for existing conditions from 2013 to 2017 including the marine heatwave years. Also presented are exchange flow magnitudes for reference for the same period along with percent difference. Results as expected show that during the marine heatwave, likely due to the reduced density gradient from higher temperatures in the Strait of Juan de Fuca channel, the strength of the exchange flow was reduced relative to reference conditions. This reduction was small (<−1% change) and seen in the Strait of Juan de Fuca and Haro Strait transects. Exchange flow into Puget Sound showed a small increase (< 1% change). This leads to the conclusion that the circulation and exchange flows within the Salish Sea were not significantly affected by the marine heatwave and remained strong, resulting in efficient warming of the Salish Sea by transport of warmer continental shelf waters. What is striking though is the ≈ 8% increase in magnitude of exchange flow that occurred during the period 2015–2017 relative to 2013. This increase in exchange flow is attributed entirely to increase in salinity/density gradient from the ≈10% higher average freshwater inflow in years 2014–2017 relative to 2013 and 2014. Note that high freshwater loads in 2014 occurred late in the year and as shown in Table 4, did not influence 2014 exchange flows significantly but contributed to the 2015 higher values. By the same argument, Year 2013 exchange flows should have benefitted from the high flows in 2012. But a cold start of the simulation was performed using 2013 flows and as a result 2012 flow effects are not included and 2013 exchange flows are likely under predicted.

Figure 8 shows plan view contour plots of depth-averaged temperature difference ΔT, for the months of January, April, July, and October, representing winter, spring, summer, and fall seasons. Temperature response within the Salish Sea is complex as temperatures are affected by river flows and strong circulation effects modulated by reflux and mixing between numerous sills. Those effects are included in existing and reference simulations and likely cancel out as the effects of the heatwave on circulation were shown to be small (≈<1% relative change in magnitude). Therefore, the difference plots isolate direct effects of warming during the heatwave and the effect is seen clearly from fall of 2014 through the fall of 2016. The year 2013 temperatures prior to the marine heatwave were a bit cooler relative to reference, and year 2014 ΔT plots show warmer temperatures from April of 2014. In the summer and fall of 2014, as the marine heatwave propagated into the Salish Sea, inner sub-basins such as South Puget Sound, Lynch Cove, Sinclair Inlet, and Bellingham Bay and selected nearshore regions of the Strait of Georgia showed more pronounced warming effects. This is likely due to a combination higher net heat flux during the heatwave distributed over shallower depths in selected sub-basins. This anomalous local heating is also noticeable in Figure 7 Gordon Point station located in South Puget Sound. The noticeable warm waters over the continental shelf in January of 2015 and 2016 highlight the persistence of anomalous warmer waters at the Strait of Juan de Fuca entrance to the Salish Sea during winter months.

The impact of the marine heatwave on water quality/biogeochemistry due to sustained higher temperatures in the entire Salish Sea during the marine heatwave are of interest. Given the range of seasonal variation in temperature and concentrations of water-quality constituents, the incremental effect of the marine heatwave is not immediately noticeable in the time series plot of temperature (Figure 2 top panel) from the Elliot Bay region and likely varied in magnitude site to site. Of most interest and concern is the potential exacerbation of algal blooms, hypoxia, and ocean acidification due to the marine heatwave, although major toxic blooms were largely confined to the open coasts. Figure 9 presents the difference between modeled existing and reference conditions in annual average levels of major biogeochemical constituents such as algal biomass, zooplankton biomass, DO, and pH presented in the form of depth-averaged, annual average contour plots. The top row provides an annual summary of the difference in elevated temperatures relative to the reference, showing higher average temperatures in 2015 and 2016. The second and third rows show the difference in algal and zooplankton biomass relative to the reference condition without the temperature increase. Contrary to expectations based on the observation data, despite sustained higher temperatures during the heatwave, the modeled effect on algal biomass and zooplankton biomass relative to reference conditions was small. Some of the shallow embayments and nearshore regions in Puget Sound and around Discovery Islands in the north of Georgia Basin, show an increase in algae and zooplankton biomass. But in most of the larger basins, the predicted change does not show a dramatic increase in biomass relative to reference conditions, but contrary to expectations, shows a small reduction (gradients toward blue shade). The effect of warmer waters on DO as seen in the model was therefore primarily due to lower saturation at higher temperatures from 2014 onward. Most Salish Sea regions show a drop in DO relative to reference conditions except selected locations of noticeably improved DO levels such as the Strait of Juan de Fuca, and surprisingly, Hood Canal, a region known for its annual formation of the hypoxic volume of water. Observations and model results show noticeably lower DO levels in 2015 and 2016 relative to 2013 and 2014 (interannual variation see Figure 10). But small improvement is noted during the marine heatwave relative to reference for the same years. The simulation results for pH also show a similar change relative to the reference conditions in most of the Salish Sea. Many of these regions showed a drop in pH from 2014 to 2016 but small improvement in pH levels is noted in the Strait of Juan de Fuca and Hood Canal relative to reference for the same years.

Although the difference plots shown indicate that changes in biological activity and biomass due to the heatwave warming effects alone were relatively small, data and model results clearly show that biomass increased in 2015, 2016, and 2017. As summarized by Ummenhofer and Meehl (2017), the reported impacts of this Pacific Northwest heatwave have included the largest algal bloom on record that negatively impacted shellfish along the western coast of North America (NOAA, 2016), indicating increased biological response in the ecosystem. Available years of monthly monitoring of algal biomass by Ecology from 21 of 23 stations in the Salish Sea that was used in model validation also showed increased biomass: average measured algal biomass concentrations were higher in 2014, 2015, and 2016 by 4, 14, and 19% respectively relative to pre-marine heatwave year 2013. The data show that in 2017, measured concentration of phytoplankton biomass dropped back down to 2013 levels. Similarly, zooplankton monitoring data shown in Figure 3 from Puget Sound available from 2014 onward showed that average biomass concentrations in 2015, 2016, and 2017 were higher by nearly 30, 10, and 21% relative to 2014, matching an increased abundance of food sources associated with phytoplankton and particulate organic carbon. A review of monthly monitoring DO data from 20 Ecology stations in the Salish Sea showed that depth averaged concentrations increased in 2014 but were on average lower during the marine heatwave years 2015, 2016 by 2–3% relative to 2013 before recovering in 2017. The DO response also qualitatively reflects deterioration due to a combination of increased algal biomass in 2015 and 2016 and lower saturation due to higher temperatures before recovering in 2017.

The concurrence of (1) warmer waters from the heatwave and higher atmospheric net heat flux and (2) evidence of increased phytoplankton and zooplankton biomass from monitoring programs in the Salish Sea led to the expectation/speculation that the two may be directly related. However, this expectation precludes the possibility that interannual variability in freshwater inflows, associated nutrient loads, and magnitude of exchange with the Pacific Ocean can also result in a similar response and may be a dominant contributor resulting in this observed increase in biological response. Past work in the Salish Sea in connection with nutrient pollution management has shown that the system is extremely sensitive to nutrient loads from rivers, wastewater discharges, and oceanic loads through exchange flow (Khangaonkar et al., 2018; Ahmed et al., 2019). Figure 10 shows simulated algal biomass in the photic zone (25 m) from the simulations conducted from 2013 through 2017. The annual algal growth and die-off pattern show similarities but also include subtle variations. For 2013, the total biomass (area under the curve) is larger than in 2014, but 2014 shows a higher peak. The peak drops off a bit in 2015 and 2016 before increasing in 2017. This pattern in Figure 10A appears to match the observed net freshwater inflow (and associated nutrient) loading to the system but is difficult to interpret in a stand-alone manner because a part of algal biomass is consumed by zooplankton. The transmitted effect is more visible in increasing zooplankton biomass in years 2014, 2015, and 2016 (Figure 10B).

The year 2014 was a higher-flow year immediately following 2013, which was a low-flow year. It is important to note that higher nutrient loads from 2014 also benefited 2015 algal growth as most of the 2014 high loads occurred during late fall and winter likely after the algal activity had ceased. The effect of sustained higher flow/nutrient loads in 2015, 2016, and 2017 is reflected in higher algal biomass in Figure 10A. This, in turn, results in higher zooplankton biomass, as shown in Figure 10B. The blue line in Figures 10A,B represents existing conditions with full marine heatwave effects included. The red line represents simulated reference conditions, cooler waters using climatological ocean boundary and reduced heating, and all other inputs remaining the same. Two features stand out in this result: (1) interannual variability and increased biological productivity have been reproduced in both simulations and appear to be dominated by nutrient loads and estuarine circulation/exchange and (2) heatwave effects in the Salish Sea appear to have caused a minor reduction in biological response from what may have otherwise occurred, in that the reference simulation biomass is a little higher (≈0–4% higher algal biomass in reference condition years 2014 to 2017) than the existing condition simulation with heatwave effects included. Relative to existing conditions, the zooplankton biomass in the reference simulation is also correspondingly lower (≈0–3% higher algal biomass in reference condition years 2014 to 2017).

Figure 11 presents time series plots of the volume of hypoxic water with DO < 2 mg/L, and volume of corrosive water with aragonite saturation state.

Figure 11A shows the time series of hypoxic volume in the Salish Sea for the existing and reference conditions without heatwave effects. Results show that the volume of hypoxic water increases steadily over the years 2013, 2014, and 2015 and maintains its high level through 2016 and 2017, consistent with the effects anticipated from increased primary production over these years. The peak of hypoxia in the Salish Sea occurs during the fall months of September and October at the culmination of the algal growth period and does not appear to be significantly affected by the heatwave perturbation. The relative impact of the heatwave on hypoxic volume is plotted as the difference between existing and reference conditions. The difference relative to the reference, plotted as a gray curve, indicates that the exposure to hypoxic water (volume-days) is generally reduced during 2015, 2016, and 2017 compared to conditions that might have occurred without the heatwave. This is also consistent with improved DO in Hood Canal presented in Figure 9 relative to conditions without the heatwave warming.

Figure 11B shows the time series of the volume of acidic water in the Salish Sea for the existing and reference conditions without heatwave effects. As shown in Figure 11B, peak acidic volume occurs during the winter months due to lower pH associated with freshwater inflows. Unlike DO, the time series of the volume of water with Ω_A < 1 does not show a noticeable trend over the years 2013 to 2017. The predicted difference relative to reference, plotted as a gray curve, indicates that the exposure to corrosive water (volume-days) is likely reduced during the 2015, 2016, and 2017 heatwave compared to conditions without heatwave effects.

While the biogeochemical response with respect to heatwave perturbation was counter to our expectations, the behavior when examined in its entirety correlates quite well with the interannual variation in conventional key variables that the system is sensitive to. Figure 12 summarizes the Salish Sea conditions for key nutrient loads, exchange flow magnitudes, algal biomass, and zooplankton biomass for the years 2013 through 2017. Annual average nutrient loads, shown in Figure 12A, increased during 2014 to 2017 relative to pre-marine heatwave conditions of 2013, mirroring the increase in freshwater inflow (see Figure 6). Most of the 2014 freshwater inflow increase occurred late in the year, the effects of which carried over to 2015. The increased freshwater-induced salinity gradients resulted in higher exchange flows to the Salish Sea in 2015, 2016, and 2017, relative to 2013 and 2014 as shown in Figure 12B and Table 4. During this period, the Salish Sea experienced an increase in nutrient flux from land-based freshwater sources as well as from the Pacific Ocean. The higher loads and stronger tidal exchange resulted in the availability of more nutrients in the photic zone that led to higher primary productivity, which was transmitted to zooplankton through predation on algae, as shown in Figures 12C,D. Simulation results show a Salish Sea wide average 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 zooplankton biomass) relative to 2014. Biomass calculations are volume weighted average values computed over the entire Salish Sea domain for each year. Figures 12B–D show a comparison with simulated results without the effect of heatwave-related warming which once again demonstrate that the heatwave effect on algal growth was small relative to interannual variability in nutrient loads and estuarine processes.

The results of the 5-year simulation spanning the heatwave validated against monitoring data from the same period showed clearly that there was a notable increase in biological activity during the years 2015–2017 relative to pre-heatwave conditions of 2013. The data and simulation results relative to 2014 showed 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). 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. However, sensitivity test simulations of reference conditions without the heatwave perturbation showed that the warming with all other conditions unchanged would potentially have caused an opposite effect on algal growth in the Salish Sea that is dominated by diatoms which favor cooler temperatures. The increase in biological activity experienced by the Salish Sea was most likely due to year-to-year variation in key parameters that Salish Sea’s biogeochemistry is most sensitive to: (a) nutrient loads from land-based sources and (b) magnitude of the estuarine exchange with the Pacific Ocean and associated oceanic supply of nutrients. Both quantities are directly influenced by freshwater inflow to the Salish Sea. Freshwater during the marine heatwave went through a sharp change from a low-flow condition in 2013 to a sustained period of higher than average flows from 2014 to 2017. Based on these results, it appears this increase in hydrologic loads was the primary driver of increased biological activity during the passage of the 2014–2016 Northeast Pacific marine heatwave through the Salish Sea. However, we also acknowledge that the question whether this sustained multi-year higher-than-normal freshwater inflow to the Salish Sea, was influenced by the marine heatwave due to factors other than precipitation such as higher snow melt, remains unresolved.

Relative to reference conditions each year, the sensitivity tests show a small reduction in primary production during the heatwave. This result is contrary to expectation of higher biological activity at higher temperatures and must be treated with caution. But, several factors have contributed to this result. Stronger stratification during the marine heatwave likely reduced the diffusive flux of nutrients to the photic zone. While some sub-basins experienced higher temperature increases (≈2.0°C), the average temperature increase in the Salish Sea was only ≈0.6°C thanks to the mitigating benefit provided by strong circulation between the sills present in this fjord like estuary. As expected, there was a small reduction in the growth of diatoms at higher marine heatwave temperatures, but a corresponding increase in higher temperature favoring dinoflagellates was not realized due to stronger control exerted by light availability later in the summer.

We note that most literature and reported impacts from the heatwave were based on studies from coastal regions directly exposed to the heatwave over the continental shelf. The results are consistent with our prior finding (Khangaonkar et al., 2019, 2021) that strong physical circulation and reflux mixing processes within the Salish Sea attenuates temperature impacts from the continental shelf.

Publicly available datasets were analyzed in this study. This data can be found here: https://ecology.wa.gov/Research-Data/Data-resources/Environmental-Information-Management-database.

All authors listed have made a substantial, direct, and intellectual contribution to the work, and approved it for publication.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.