Climate change impacts on kelp: Physiological responses across habitats, species, and populations

Abstract

Climate change is impacting organisms globally and increasing stressors like warming in both the atmosphere and ocean. Foundation species have disproportionate effects on their local environments through the provision of habitat therefore, declines can have major ecological impacts. Kelps are important foundation species in coastal marine ecosystems that support diverse understory algae, invertebrate, and fish assemblages and serve as a food source in natural systems and aquaculture. The environmental requirements of kelp, cold and nutrient rich waters, make them especially at risk from ocean warming. The Salish Sea is an extensive fjord estuary complex with a natural gradient of temperatures and nutrients with warm water temperatures in inlets like southern Puget Sound. This region is also a hotspot for intertidal thermal risk due to warm air temperatures in spring and summer coinciding with low tides. The combination of a gradient in water temperature and warm air temperatures during low tide makes this region ideal for studying multiple types of warming and stressors. In order to support conservation and management of ecologically important kelp species, we tested the effects of warming and multiple stressors across habitats (intertidal and subtidal), species (Hedophyllum sessile, Nereocystsis luetkeana, and Saccharina latissima), and populations in laboratory and field studies by measuring a comprehensive suite of physiological responses including blade growth, bleaching, photosynthesis, respiration, photosynthetic yield, nutrient uptake, and tissue C:N. We leveraged an extreme heatwave (2021 Western North American Heatwave) to understand mechanisms of survival through self-shading and protection of the meristem in H. sessile (Chapter 1). We built thermal performance curves for 8 N. luetkeana populations and found a trend of higher optimal temperatures in kelp from warmer sites but, surprisingly, the same critical thermal maximum of 22°C across all sites (Chapter 2). Finally, we tested the interactive effects of short-term exposure to warming and nitrogen limitation on N. luetkeana and S. latissima and found high temperatures but not low nitrogen availability was stressful for kelps (Chapter 3). Together, these studies indicate that inland seas can act as harbingers of climate change, some species are more resilient to warming more than others, management actions should protect sites that can serve as thermal refugia, and mitigation and restoration actions should be considered for sites nearing thermal tipping points.