Spatio-temporal variation in zooplankton community composition in the southern Salish Sea: Changes during the 2015-2016 Pacific marine heatwave

We examined zooplankton collected across a latitudinal gradient in the southern Salish Sea for changes during the 2015-2016 Pacific marine heatwave (MHW). Zooplankton were collected biweekly in vertical net tows from three regions of Puget Sound and one in northern Washington waters near the Strait of Juan de Fuca, from 2014 to 2020. Water temperatures were cooler in the north and warmer in the south, with anomalously high tem-peratures occurring in 2015-2016 in all regions. Chlorophyll biomass showed no consistent relationship with temperature. All regions had anomalously high zooplankton biomass in 2015, which continued in the three Puget Sound regions through 2017, with the two central stations showing the most dramatic increases. Several large crustacean species and molluscs showed positive biomass anomalies during the MHW. Euphausiid furcilia showed differential response by species. Puget Sound resident copepod species showed primarily positive anomalies, while some boreal and subarctic copepod species were anomalously low during this period. Gelati-nous zooplankton had somewhat higher biomass through the first few years of the study, but patterns showed no clear relationship with the MHW. Differing zooplankton patterns compared to those observed in the California Current, as well as prolonged effects on zooplankton biomass in Puget Sound, suggest that local forcing played a dominant role, likely differing from the mechanisms affecting zooplankton in the California Current. Primary postulated factors include increased growth and metabolism due to higher temperatures, nutrient-driven in-creases in primary production, and changes in species composition advected from the ocean. Further research is needed to determine the ramifications of the changes and how they may affect the food web in the future.