Differential Responses of Eelgrass and Macroalgae in Pacific Northwest Estuaries Following an Unprecedented NE Pacific Ocean Marine Heatwave

Abstract

The 2013-2016 northeast Pacific Ocean marine heatwave (MHW) had myriad impacts on marine communities, but little is known about how this event affected coastal estuaries. We examined the extent to which elevated temperatures associated with the offshore MHW were observed in four estuaries (Willapa Bay, Washington and Netarts Bay, Yaquina Bay, and Coos Bay, Oregon) and the responses of macrophytes at marine dominated sites within these estuaries. Records of eelgrass (Zostera marina), ulvoid macroalgae (Ulva and Enteromorpha spp.), and environmental characteristics, including water temperature, were analyzed over three periods: before (2006-2010), during (2015-2016), and after (2017-2019) the MHW. During the MHW, all four estuaries experienced “estuarine heatwaves” (anomalous warm water events) at monitoring stations and there was an associated decline in macroalgae biomass. In northern estuaries (Willapa and Netarts bays) where water temperatures are normally higher and estuaries are shallower, aboveground eelgrass biomass declined and did not recover for at least three years after the MHW. In southern estuaries (Yaquina and Coos bays), where water temperatures are normally colder and estuaries are deeper, the MHW was associated with a neutral, or temporarily positive, change in aboveground eelgrass biomass. Our analysis supports the hypothesis that upwelling intensity and estuarine depth determined the severity of MHW impacts on macrophytes at marine dominated sites, and that the colder and deeper estuaries in the south may serve as refuges for eelgrass but not macroalgae. We suggest that estuary-specific responses were predictable given the drivers of macrophyte abundance at marine dominated sites, and that baseline ocean upwelling and estuarine temperature can be used to anticipate macrophyte responses at marine dominated sites to MHWs in the future.

Water temperature variability in the Coos Estuary and its potential link to eelgrass loss

“…Previously, Thom et al (2003) showed that stations in the Coos Estuary closer to the mouth had higher values of eelgrass density (100-200 shoots per m 2 ) related to the influence of oceanic, low-turbidity waters, while stations farther away from the mouth of the estuary had smaller density, related to increased turbidity due to the input of freshwater. A similar result was found across multiple PNW estuaries by Magel et al (2022). Thom et al (2003) also suggested that eelgrass decline was correlated with anomalously warm waters during the 1997-1998 El Niño; the degree to which this happens again between 2014-2016 is the subject of this study (Figure 1).…”

“…Our results suggest that the MHWs and increased air temperature contributed to the eelgrass density decline in 2015 (Figures 3, 10, Supplementary Figure 4). In contrast, Magel et al (2022) show an increase in eelgrass at one annually-surveyed station (Barview, see Figure 9) closer to the main channel than Valino Island. Their results show an increase in eelgrass density and biomass during the summer of 2015 and summer of 2016, followed by a decline to longer term average values.…”

“…Eelgrass, which plays a key role in the coastal zone worldwide (Phillips, 1984;Hosack et al, 2006;Lee and Brown, 2009), has decreased in abundance in PNW estuaries (Magel et al, 2022), including the Coos at one annually-sampled site. In the Coos, eelgrass is spatially variable, and estuary-wide eelgrass presence has been obtained through aerial photography and high density lidar intensity images from 2005 and 2016 with the aid of the US Environmental Protection Agency (EPA) and the Pacific Marine and Estuarine Fish Habitat Partnership (PMEP), respectively (Clinton et al, 2007;Sherman and DeBruyckere, 2018).…”

“…In-situ observations as well as remotely sensed surveys reveal that eelgrass has decreased in abundance in the up-estuary portions of South Slough and the greater Coos Estuary (Figure 2; Supplementary Figure 1). Remotely sensed data from 2005 and 2016 show a decrease of eelgrass-covered area in the Coos Estuary, with higher survival in the main channel than in South Slough (Supplementary Figure 2B), despite the proximity of the sub-estuary to the mouth of the estuary and influence of coastal waters (Raimonet and Cloern, 2017;Magel et al, 2022). Notably, stations in South Slough that were only 3-10 km from the mouth are less marine-influenced than the main channel stations that reside 5-15 km from the mouth, a result that is consistent with the structure of the salinity intrusion in the Coos Estuary (Conroy et al, 2020).…”

“…Many environmental parameters, outside a specific speciesdependent range, can cause stress on the fauna and flora of estuaries, including salinity, water temperature, turbidity, light availability, air temperature, water velocity, and nutrient levels (Thom et al, 2003;Echavarria-Heras et al, 2006;Lee et al, 2007;Nejrup and Pedersen, 2008;Kaldy, 2012;Kaldy, 2014;Salo and Pedersen, 2014;Basilio et al, 2017;Daly et al, 2017;Magel et al, 2022). For example, declines in eelgrass populations have been observed in the PNW and were related to increased water temperatures after the 1997-1998 El Niño event (Thom et al, 2003).…”