Assessing marine heatwave impacts on the pelagic ecosystem of the San Juan Archipelago

Abstract

In a rapidly changing world, understanding the interactions between the biological and abiotic world is crucial to predict change within our ecosystems and more importantly manage them. With increasing occurrences of marine heatwaves due to climate change, we are provided the unique opportunity to assess how ecosystems respond to intense periods of warming, that produce signals in a pulse-like form. This allows us to gain insight on both how components of our natural marine systems are impacted by warming, as well as how this differs from the more gradual effects, we see with steady increases in temperature due to climate change. In this study, the pelagic ecosystem of the San Juan Archipelago is assessed in relation to its response to the recent 2014-2016 marine heatwave to investigate this. A simplified food web consisting of chlorophyll, pacific sand lance, alcids, gulls, cormorants and pinnipeds is used while chlorophyll, sea surface temperature and photosynthetically active radiation are used as environmental covariates that are tested against the biological components listed above through a multivariate auto-regressive model to assess this question. Time series analyses are also conducted to assess group trends through time, and in relation to pre, during and post heatwave periods. Overall, it was found that all three environmental components (sea surface temperature, photosynthetically active radiation, and chlorophyll) are increasing through time, however sea surface temperature additionally shifted baselines after the heatwave had subsided. When examining the biological components, alcids, gulls and cormorants all have recently made a recovery to pre-heatwave levels after a decline through the heatwave in numbers. In contrast, pinnipeds have been declining continuously throughout time. Several significant interactions were found to be apparent, with sea surface temperature, photosynthetically active radiation and chlorophyll influencing several of the biological groups. Ultimately this indicates that although groups are directly influenced by the environmental covariates tested, which may indicate increased vulnerability to environmental changes due to climate change, biological groups may have the capacity to return to pre-heatwave levels after heatwaves subside. Information as such is critical for further understanding how our ecosystems will change in relation to climate change and also informs that ecosystem responses to marine heatwaves may be different than the more gradual climate change effects, which is important for the management of our systems as a whole.