Long-term trends and regional variability in extreme temperature and salinity conditions experienced by coastal marine organisms on Vancouver Island, Canada

Studies of ocean temperature and salinity typically focus on global or large-scale trends that may not accurately reflect the conditions experienced locally by marine organisms. When assessing impacts of climate change, it is important to document local conditions, and also to focus on the most physiologically stressful time of year. Our goals were therefore to (1) characterize long-term trends in sea surface temperature (SST) and salinity (SSS) experienced by coastal marine organisms during the most stressful time of year around Vancouver Island; and (2) document variation between east and west coasts of Vancouver Island in terms of SST, SSS, and daytime rock surface temperatures in the intertidal zone. Over an 82-yr period, from 1935 to 2016, summertime SST on both coasts increased by 0.67-0.78 degrees C (i.e., 0.82-0.97 degrees C per century). Trends in salinity differed between coasts: east coast salinity increased by 3.9 while west coast salinity decreased by 0.64. Although long-term SST trends are the same on both coasts, during the most stressful time of year, east coast waters are on average 4.3 degrees C warmer and salinity is 7.8 lower than on the west coast. Rock temperature in the mid and upper intertidal zone during daytime low tides is 3.9-4.2 degrees C warmer on the east coast. Populations of marine organisms inhabiting the coasts of Vancouver Island have therefore been experiencing long-term changes in abiotic stress, as well as persistent spatial variation in climate-related conditions during the most stressful months of the year.