Physiological effects of climate change and estimations of carbon standing stock of southern Salish Sea kelps

Abstract

Kelp forests are among the most diverse and productive ecosystems in the world, providing critical habitat for numerous ecologically and economically important species. However, kelps are at risk from climate change, particularly along the North American West Coast, demonstrating the need to characterize and quantify the effects of climate stress on kelp in this region. While the negative effects of ocean warming on kelps are well documented, the effects of ocean acidification are less well understood; increased pCO2 may help ameliorate some of the negative effects of warming and influence kelp resilience in future oceans. The first chapter of this dissertation presents a meta-analysis of responses of true kelps (order Laminariales) to ocean warming and acidification, based on a global literature synthesis. Our results showed that ocean warming has a strong negative impact on kelps at all life stages and across various physiological levels, including growth, reproduction, and survival. In contrast, ocean acidification generally has no effect, except for its negative impact on reproduction. Studies conducted in the temperate northern Pacific showed extreme negative effects of warming. We also identified key gaps in our understanding of kelp responses to climate change, such as the impacts on microscopic spores and the combined effects of warming and acidification. This analysis synthesized trends in a rapidly expanding field of literature and provided a deeper understanding of how kelps will respond to a changing ocean. The second and third chapters of this dissertation examined the effects of warming and acidification on bull kelp Nereocystis luetkeana, the primary canopy-forming kelp species in the Salish Sea, which has declined substantially in recent decades. First, we examined the short-term effects of climate change on sporophyte photophysiology. Adult sporophyte blades from two geographically proximate but genetically distinct and environmentally contrasting populations in Puget Sound, Washington, were collected and exposed for 1-3 hours to a range of temperatures (8, 10, 15, 20, 25, 30 C) and pH treatments (8, 7.7, 7.4). Following exposure, photosynthetic and fluorescence parameters were measured. In both populations, photosynthesis peaked between 10 and 25 C and decreased under high temperatures (25-30 C). Photosynthesis increased under low pH (7.4) but still declined sharply at high temperatures regardless of pH, indicating limited potential for ocean acidification to counteract warming stress. In our second experiment, we exposed microscopic gametophytes from the same two populations to a fully crossed experiment manipulating temperature (12 and 18 C) and pH (8.0 and 7.5) for one month. Because the gametophyte stage governs reproduction, recruitment, and long-term population resilience, understanding how this stage responds to climate change is essential. We measured survival, germination rate, sex ratio, oogonia production, sporophyte production, gametophyte and juvenile sporophyte growth, and photophysiology. As with the sporophytes, the effects of acidification were subtle and strongly temperature-dependent. Unlike in sporophytes, we observed positive effects of warming: high temperatures stimulated metabolic rates, leading to more rapid germination, higher growth rates, and increased photosynthesis and respiration. Sex ratio and oogonia production were not negatively impacted by warming or acidification. However, warming led to a severe decline in reproductive success, marked by reductions in sporophyte production. Our results indicate a temperature-sensitive reproductive bottleneck during early development, likely occurring between oogonia fertilization and sporophyte production, during which elevated temperatures shift energy allocation away from reproduction and towards growth. These findings highlight the vulnerability of bull kelp’s microscopic stages to warming and underscore the need to incorporate early life stages into predictions of kelp resilience under future climate scenarios. The population-level differences in photophysiological responses in sporophytes and in reproductive performance in gametophytes indicate local adaptation or acclimatization to different environmental regimes and inherent variation in thermal sensitivity. These results highlight the importance of accounting for fine-scale genetic and environmental variation when predicting kelp forest responses to concurrent ocean warming and acidification. Finally, we quantified kelp carbon standing stock across four bull kelp beds in the Salish Sea. The degree to which kelp sequester carbon remains highly debated; before kelp can be included in blue carbon accounting, accurate estimates of carbon standing stock are needed. The Salish Sea is a dynamic inland waterway home to 22 native kelp species. By incorporating both canopy-forming and understory kelps, we found that understory species account for a greater share of carbon standing stock than the canopy-forming N. luetkeana. We collected morphometric and tissue composition data for dominant understory and canopy species at each kelp bed and constructed allometric scaling relationships. We paired these data with bed area and species density information from routine government and NGO monitoring to estimate carbon standing stock estimates for each site. This research provided an initial baseline for future evaluations of kelp carbon sequestration potential in the Salish Sea and offers a framework for integrating morphological measurements into existing monitoring programs to generate carbon standing stock estimates.