The effects of summer and winter marine heatwaves on growth is determined by latitude and species traitsExtreme Events

Rhian Evans¹, Ben P. Harvey², Bayden D. Russell¹

¹The University of Hong Kong, China - ²University of Tsukuba, Japan

The incidence of marine heatwaves (MHWs) occurring multiple times over an annual cycle is increasing. While much research has gone into categorizing the impacts of singular events, the cumulative impacts of multiple events occurring in different seasons for organism physiology and ultimately population health and persistence is still little known. Here we use the growth performance index (GPI) to quantify the effects of MHWs occurring in different seasons on a cumulative physiological process. We first estimate the annual GPI of 40 economically important fish and invertebrate species from the western Pacific Ocean using thermal performance models across their current production range. We then predict annual changes in growth performance across the latitudinal range of each species incorporating daily variability in temperature under different MHW scenarios; an annual cycle with a summer MHW, an annual cycle with a winter MHW, an annual cycle with a summer and winter MHW, and finally a year-long MHW scenario. We find that MHWs in different seasons have very different predicted effects on growth performance, which is linked to the species’ geographic distribution, thermal tolerance and climate affiliation. Winter MHWs were beneficial for growth, particularly at mid-latitudes. In contrast, the effects of summer MHWs were generally negative, particularly at higher latitudes where most species only exhibit a summer growth period. Combined winter and summer MHWs produced a cumulative effect resulting in an overall negative impact, with the negative effect of summer heatwaves cancelling the positive effect of winter heatwaves for most species. Species were most vulnerable to the effects of multi-season MHWs at their range edges, e.g. subtropical and temperate species at their cold-range edge and subtropical species living at their warm-edge boundary in the tropics. Therefore, for cumulative processes such as growth, the occurrence of multi-season MHW events may present a significant challenge for mariculture operations looking to optimize growth performance, as well as cause significant coastal ecosystem restructuring for natural populations.