Predicting the impacts of climate change on New Zealand's seaweed-based ecosystems

The impacts of global climate change are threatening the health and integrity of New Zealand's seaweed ecosystems that provide crucial ecological, economic, and cultural benefits. Important species that comprise these ecosystems include canopy forming large brown algae (fucoids and kelp), and understorey species. Here we review current knowledge of the measured impacts of climate change stressors on New Zealand seaweeds. Ocean warming has driven increasing frequencies, durations, and intensities of marine heatwaves globally and in New Zealand. Significant negative impacts resulting from heatwaves have already been observed on New Zealand's canopy forming brown algae (giant kelp Macrocystis pyrifera and bull kelp Durvillaea spp.). We predict that ongoing ocean warming and associated marine heatwaves will alter the distributional range and basic physiology of many seaweed species, with poleward range shifts for many species. Increased extreme weather events causes accelerated erosion of sediments into the marine environment and re-suspension of these sediments, termed coastal darkening, which has reduced the growth rates and available vertical space on rocky reefs in New Zealand and is predicted to worsen in the future. Furthermore, ocean acidification will reduce the growth and recruitment of coralline algae, this may reduce the settlement success of many marine invertebrate larvae. Mechanistic underpinnings of the effects of multiple drivers occurring in combination is poorly described. Finally, local stressors, such as overfishing, will likely interact with global change in these ecosystems. Thus, we predict very different futures for New Zealand seaweed ecosystems depending on whether they are managed appropriately or not. Given recent increases in sea surface temperatures and the increasing frequency of extreme weather events in some regions of New Zealand, predicting the impacts of climate change on seaweeds and the important communities they support is becoming increasingly important for conserving resilient seaweed ecosystems in the future.