Marine heat waves' impact on zooplankton populations and implications for biogeochemical cycling

Abstract

Climate change is causing marine heat waves to become more common, leading to increased stratification of the water column. This in turn causes hypoxic events and reduced mixing between the surface layer and the deep ocean. Food webs rely on nutrients brought up by this mixing, and marine heat waves can disrupt this process, impacting zooplankton populations through bottom-up interactions with their main food source, phytoplankton. In the Salish Sea, this nutrient rich water comes from the North Pacific through the Strait of Juan de Fuca and up into the San Juan Channel. Zooplankton populations from 2007 – 2022 were examined in the channel during the fall to determine whether a marine heat wave known as The Blob affected zooplankton diversity and abundance. The Blob formed in the North Pacific in 2013 and lasted until 2016. It was followed by several years of abnormally high sea surface temperatures (SST) lasting until the present day (2022). Years before The Blob reached the Salish Sea (2007 – 2013) were compared with years after it arrived (2014 – 2022). After the heat wave, zooplankton densities were lower at the Pelagic Ecosystem Functions’ North (p = 0.0079) and South (p = 0.0026) stations while diversity was unchanged (p > 0.05 at both stations). This was driven mainly by patterns in copepod dominance. Zooplankton are vital to commercially important fisheries in the Salish Sea, as well as for biogeochemical cycling. Future marine heat waves could continue to affect zooplankton populations, potentially destabilizing the ecosystem in the Salish Sea.