A comparison of temperature and salinity conditions on the east and west coasts of Vancouver Island: Implications for intertidal invertebrate population persistence in the face of climate change

Climate change is altering the physical and chemical conditions of ocean habitats, including changes to seawater temperature, pH and salinity. Such changes to ocean conditions have the potential to impact marine organisms by altering population abundance or by driving evolutionary change in adaptive traits. The rate at which these abiotic conditions change is important, as this may determine whether populations adapt or are extirpated. Given the significant effects of temperature and salinity on the physiology and performance of marine animals, knowledge of temporal trends and the extent of spatial variations in these conditions is essential to understand the selective pressures that have influenced the evolution of extant populations and to make predictions regarding their persistence in the face of climate change. Therefore, to improve our understanding of the regional climate conditions on the southern coast of Vancouver Island, I have (1) characterized the long-term trends in sea surface temperature (SST) and salinity (SSS) experienced by coastal marine animals during the most stressful time of year, and (2) documented variation between east and west coasts of Vancouver Island in terms of SST, SSS, and intertidal rock surface temperature during low-tide emersion. I then examined the effects of the distinct local climate conditions on east and west coasts on the tolerance thresholds of populations on each coast. Using a series of common garden experiments, the tolerance thresholds of populations of four benthic intertidal invertebrates (Littorina scutulata, Littorina sitkana, Balanus glandula and Nucella lamellosa) were determined for (1) elevated temperature during low tide emersion, (2) elevated water temperature, and (3) low salinity. This study found that over an 82 y period, from 1935 to 2016, summertime SST on both coasts increased by 0.67– 0.78 °C (i.e. 0.82 – 0.97 °C per century). Trends in salinity differed between coasts: east coast salinity increased by 3.9 PSU while west coast salinity decreased by 0.64 PSU. Although long-term SST trends are the same on both coasts, east coast waters are on average 4.3 °C warmer, and salinity is 7.8 PSU lower, than on the west coast. Rock temperature in the mid and upper intertidal zone during daytime low tides is 3.9 – 4.2 °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. Laboratory experiments revealed three important findings regarding population tolerance thresholds to SST, SSS and emersion temperature among marine invertebrate species. Firstly, substantial differences in tolerance to increased SST and emersion temperature conditions were discovered between species, secondly, similar tolerances to the abiotic parameters existed between east and west coast populations of species, and finally, acute exposure to increased SST and emersion temperature or decreased SSS conditions is not an immediate threat to the populations studied. Overall, it appears that populations are living well within their tolerance limits and their present-day tolerances are well-suited to withstand the predicted changes in ocean conditions.