Assessing the impact of successive marine heatwaves on the diversity of rocky intertidal communities

In recent years, clarification of the impacts of marine heatwaves (MHWs) on marine ecosystems has become an important topic. However, most previous studies that have assessed impacts of MHWs have not adequately reduced stochasticity, which has led to insufficient assessment of the direct effects of MHWs. There are also few studies that evaluate the accumulative carryover effects; which occur when the effects of successive events accumulatively increase over time. In addition, many studies have examined the effects of MHWs on abundance, but relatively few on diversity. From 2010 to 2016, southeastern Hokkaido in northern Japan experienced successive MHWs. Here, we assessed the accumulative carryover effects and the impact of MHWs on diversity (species richness and abundance-weighted diversity indices) with reduced stochasticity for four major functional groups (macroalgae, sessile invertebrates, herbivorous mollusks, and carnivorous invertebrates) of rocky intertidal communities. Surveys were conducted before (2004–2009), during (2010–2016), and after (2017–2018) MHWs. We found that the accumulative carryover effects were not detected in diversity. The results demonstrate that MHWs are to lead to accumulative carryover effects on diversity than on abundance. We also found that the direct effects of MHWs on diversity (i.e., increase or decrease of diversity indices during and after MHWs) were different among the four functional groups. These results were due to differences in species composition within the functional groups, such as the proportion of warm water species and cold affinity species and the proportion of dominant species. Thus, multiple metrics should be quantified simultaneously when assessing impacts on MHWs. • Response to marine heatwaves differed among rocky intertidal functional groups. • Marine heatwave responses differ due to species composition within functional groups, especially between warm and cool species affinity. • Accumulative carryover effects due to marine heatwaves was not detected in diversities. • Species richness of sessile invertebrates increased during and after marine heatwaves. • Species richness of carnivorous invertebrates decreased during and after marine heatwaves.