Kristy J. Kroeker

Summary: Pockets of the formerly glaciated Pacific coastline of North America likely remained ice‐free throughout the Last Glacial Maximum (LGM). These areas may have served as refugia for terrestrial species, but less is known about their role in the persistence of marine plants and other coastal species. We examined genetic diversity from > 1000 newly and previously sequenced whole genomes of canopy‐forming kelps of the genera Nereocystis and Macrocystis and built simple ecological niche models. We then reconstructed ancestral recombination graphs and modeled the geographic locations of genetic ancestors through time. We detected high genetic diversity in both species in north‐central British Columbia, in a region where suitable LGM habitat is plausible. Ancestor locations spatially converged backward in time toward this region, with multiple refugia inferred between northern Vancouver Island and southern Haida Gwaii. An expanded set of global samples for Macrocystis confirmed pre‐LGM divergence with California but hinted at the possibility of subsequent gene flow. Nereocystis and Macrocystis survived glaciation in northern refugia. The northern persistence of these foundation species raises the possibility that biodiverse kelp forest ecosystems could have continuously occupied portions of the northern Pacific coastline since the LGM.

O Pockets of the formerly glaciated Pacific coastline of North America likely remained ice-free throughout the Last Glacial Maximum (LGM). These areas may have served as refugia for terrestrial species, but less is known about their role in the persistence of coastal marine species. o We examined genetic diversity from >1,000 newly and previously sequenced whole genomes of canopy-forming kelps of the genera Nereocystis and Macrocystis, and built simple ecological niche models. We then reconstructed ancestral recombination graphs and modelled the geographic locations of genetic ancestors through time. o We detected high genetic diversity in both species in north-central British Columbia, in a region where suitable LGM habitat is plausible. Ancestor locations spatially converged backward in time toward this region, with multiple refugia inferred between northern Vancouver Island and southern Haida Gwaii. An expanded set of global samples for Macrocystis confirmed pre-LGM divergence with California but hinted at the possibility of subsequent gene flow. o Kelp forests survived in northern refugia and have continuously occupied portions of the northern Pacific coastline since the LGM. One implication of these results is their support for the Kelp Highway Hypothesis of the peopling of the Americas by a coastal route.

Predator loss can cause shifts in ecosystem state, especially when accompanied by changes in the behaviour of its prey. The recent decline of predatory sunflower sea stars Pycnopodia helianthoides and a coincident a decline of kelp forests across the northeastern Pacific raises questions about their role in kelp forest ecosystem maintenance and recovery. While P. helianthoides may support kelp forests by consuming herbivorous sea urchins, less is known about their non-consumptive effects, especially in degraded habitats. Here, we ask: How do the non-consumptive effects of this predator vary among prey species, and what are the emergent effects on community-level grazing pressure in a degraded habitat when kelp is reintroduced? We conducted a field experiment, where we deployed kelp blades in urchin barrens at discrete distances from caged P. helianthoides and control cages for 24 h. We demonstrate a reduction in the density of red sea urchins Mesocentrotus franciscanus nearest the sunflower star and no effect on green sea urchins Strongylocentrotus droebachiensis . Our results suggest the chemical cue of P. helianthoides elicits a localized landscape of fear of approximately 15 m ² that suppresses grazing, and that the non-consumptive effects of P. helianthoides on sea urchin behaviour may be important for kelp restoration.

Changes in the seasonal environment substantially influence organisms. Understanding how species respond to such changes, such as in their feeding interactions and physiology, is key to predicting their resilience to both seasonal and longer term climate changes. The Sitka Sound in Southeast Alaska, USA, is an attractive natural laboratory for studying change, as the marine environment experiences substantial seasonal fluctuations in parameters such as temperature, pH, and productivity between summer and winter. By sampling a suite of dominant macrophyte algae and their benthic herbivores in winter (January) and summer (July) 2019, we investigated how producers and their primary consumers may respond to seasonal change inferred through fatty acid trophic markers. We used a fatty acid logratio selection and analysis approach to ask the following: (1) Do fatty acid biomarkers for algae differ between seasons? (2) Do the same fatty acid biomarkers differ between seasons when applied to herbivores, with herbivore fatty acids tracking the presumed trophic resources? (3) Do fatty acid biomarkers for herbivores differ between seasons, when considered independently from algae? Comparing logratio sets selected each for the algae and the herbivore fatty acids, we found that algae fatty acids were different between our sampling seasons, but the algae‐selected fatty acid logratios did not clearly separate most herbivores by season. In contrast, the herbivore‐selected logratios strongly distinguished herbivore species between the January and June samplings. Further, dispersions of fatty acid logratios were greater within herbivore species in the winter than in the summer, potentially due to the utilization of more diverse resources when preferred algae are less abundant. In total, results suggest that herbivores are not simply tracking the seasonal fatty acid changes in the dominant algae as trophic resources; instead, herbivore fatty acid seasonal changes may occur largely through some other nonexclusive mechanisms, such as omnivory, reductions or shifts in dietary composition, and/or endogenous physiological responses. Thus, the benthic herbivores in our study appear not to be locked into their diets when faced with seasonal change and may use a range of strategies, including diet diversification, to cope with environmental variation.

Global trends of ocean warming, deoxygenation, and acidification are not easily extrapolated to coastal environments. Local factors, including intricate hydrodynamics, high primary productivity, freshwater inputs, and pollution, can exacerbate or attenuate global trends and produce complex mosaics of physiologically stressful or favorable conditions for organisms. In the California Current System (CCS), coastal oceanographic monitoring programs document some of this complexity; however, data fragmentation and limited data availability constrain our understanding of when and where intersecting stressful temperatures, carbonate system conditions, and reduced oxygen availability manifest. Here, we undertake a large data synthesis to compile, format, and quality-control publicly available oceanographic data from the US West Coast to create an accessible database for coastal CCS climate risk mapping, available from the National Centers for Environmental Information (accession 0277984) at https://doi.org/10.25921/2vve-fh39 (Kennedy et al., 2023). With this synthesis, we combine publicly available observations and data contributed by the author team from synoptic oceanographic cruises, autonomous sensors, and shore samples with relevance to coastal ocean acidification and hypoxia (OAH) risk. This large-scale compilation includes 13.7 million observations from 66 sources and spans 1949 to 2020. Here, we discuss the quality and composition of the synthesized dataset, the spatial and temporal distribution of available data, and examples of potential analyses. This dataset will provide a valuable tool for scientists supporting policy- and management-relevant investigations including assessing regional and local climate risk, evaluating the efficacy and completeness of CCS monitoring efforts, and elucidating spatiotemporal scales of coastal oceanographic variability.

Global environmental change drives diversity loss and shifts in community structure. A key challenge is to better understand the impacts on ecosystem function and to connect species and trait diversity of assemblages with ecosystem properties that are in turn linked to ecosystem functioning. Here we quantify shifts in species composition and trait diversity associated with ocean acidification (OA) by using field measurements at marine CO₂ vent systems spanning four reef habitats across different depths in a temperate coastal ecosystem. We find that both species and trait diversity decreased, and that ecosystem properties (understood as the interplay between species, traits, and ecosystem function) shifted with acidification. Furthermore, shifts in trait categories such as autotrophs, filter feeders, herbivores, and habitat‐forming species were habitat‐specific, indicating that OA may produce divergent responses across habitats and depths. Combined, these findings reveal the importance of connecting species and trait diversity of marine benthic habitats with key ecosystem properties to anticipate the impacts of global environmental change. Our results also generate new insights on the predicted general and habitat‐specific ecological consequences of OA.

Variability in primary producers' responses to environmental change may buffer higher trophic levels against shifts in basal resource composition. Then again, in instances where there is a lack of functional redundancy because consumers rely on a few species to meet their energetic requirements at specific times of the year, altered community production dynamics may significantly impact food web resilience. In high‐latitude kelp forests, a complementary annual phenology of seaweed production supports coastal marine consumers' metabolic needs across large seasonal variations in their environment. Yet, marine consumers in these systems may face significant metabolic stress under the pronounced low pH conditions expected in future winters, particularly if they lack the resources to support their increased energetic demands. In this study, we investigate how the growth and nutritional value of three dominant, coexisting macroalgal species found in subpolar kelp forests will respond to ocean acidification and warming in future winter and summer seasons. We find that the three kelps Macrocystis pyrifera, Hedophyllum nigripes, and Neoagarum fimbriatum differ in their vulnerability to future environmental conditions, and that the seasonal environmental context of nutrient and light availability shapes these responses. Our results suggest that poleward fringe populations of M. pyrifera may be relatively resilient to anticipated ocean warming and acidification. In contrast, ocean warming conditions caused a decrease in the biomass and nutritional quality of both understory kelps. Considering the unique production phenology of H. nigripes, we emphasize that negative impacts on this species in future winters may be of consequence to consumer energetics in this system. This work highlights how interspecific variation in autotrophs' responses to global change can disrupt the diversity and phenological structure of energy supply available to higher trophic levels.

This chapter reflects on how I came to understand that connecting scholarship in the natural sciences to solutions requires inclusive engagement and authentic relationships with community counterparts developed over time - relationships built upon understanding, empathy, and trust. These are not skills I was explicitly taught in college or graduate school, nor were they themes central to my curriculum or pedagogical approaches as an instructor. The chapter explains how I now endeavor to address these themes by teaching about engagement using activities centered around listening in my courses in the biophysical sciences. By doing so, I aim to promote equity, inclusivity, and belonging in the classroom and beyond. I further discuss how community-engaged research can provide a sense of ownership and belonging among all participants.

Biological processes play important roles in determining how global changes manifest at local scales. Primary producers can absorb increased CO₂ via daytime photosynthesis, modifying pH in aquatic ecosystems. Yet producers and consumers also increase CO₂ via respiration. It is unclear whether biological modification of pH differs across the year, and, if so, what biotic and abiotic drivers underlie temporal differences. We addressed these questions using the intensive study of tide pool ecosystems in Alaska, USA, including quarterly surveys of 34 pools over 1 year and monthly surveys of five pools from spring to fall in a second year. We measured physical conditions, community composition, and changes in pH and dissolved oxygen during the day and night. We detected strong temporal patterns in pH dynamics. Our measurements indicate that pH modification varies spatially (between tide pools) and temporally (across months). This variation in pH dynamics mirrored changes in dissolved oxygen and was associated with community composition, including both relative abundance and diversity of benthic producers and consumers, whose role differed across the year, particularly at night. These results highlight the importance of the time of year when considering the ways that community composition influences pH conditions in aquatic ecosystems.

The global trends of ocean warming, deoxygenation, and acidification are not easily extrapolated to coastal environments. Local factors, including intricate hydrodynamics, high primary productivity, freshwater inputs, and pollution, can exacerbate or attenuate global trends and produce complex mosaics of physiologically stressful conditions for organisms. In the California Current System (CCS), oceanographic monitoring programs document some of this complexity; however, data fragmentation and limited data availability constrain our understanding of when and where stressful coastal conditions manifest. Here, we undertake a large data synthesis to compile, format, and quality-control publicly available oceanographic data to create an accessible database for coastal CCS climate risk mapping, available at the National Centers for Environmental Information (Accession 0277984) under the DOI 10.25921/2vve-fh39 (Kennedy et al., 2023). With this synthesis, we combine publicly available observations and data contributed by the author team from synoptic oceanographic cruises, autonomous sensors, and shore samples with relevance to coastal ocean acidification and hypoxia (OAH) risk. This large-scale compilation includes 13.7 million observations from 67 sources. Here, we discuss the quality and composition of the synthesized dataset, the spatial and temporal distribution of available data, and examples of potential analyses. This dataset will provide a valuable tool for assessing regional and local climate risk, evaluating the efficacy and completeness of CCS monitoring efforts, and investigating spatiotemporal scales of coastal oceanographic variability.