Climate change represents a threat to coastal marine ecosystems through variable effects on community structure and function due to increasing mean sea-surface temperatures (SST), marine heatwaves, variation in salinity, and ocean acidification. Among the most at risk species are California kelps, which have already experienced significant die-offs over the past several years as a result of elevated SST and urchin grazing. However, the effects of these stresses on the very sensitive microscopic kelp life stage (gametophyte) are much less understood. Gametophytes are generally less resilient to changes in abiotic conditions, so global environmental change could result in drastic changes in kelp forest community structure and composition via impacts on this life stage. My dissertation research used manipulative laboratory experiments to investigate the interacting role of abiotic stressors on kelp reproduction and community compositions, specifically, the growth and survival of early kelp life stages. My first chapter focused on the effects of climate-driven temperature increases and ocean acidification on bull kelp (Nereocystis luetkeana) gametophytes from Point Arena, CA (Korabik et al. 2023). From 2014 to 2016, the largest marine heatwave in history appeared off the coast of California resulting in large kelp die off events. In this chapter, I asked how increased temperature and lowered pH impact the survival of bull kelp gametophytes and the production of juvenile bull kelp sporophytes. My results showed that increased temperature resulted in a significant decrease in the survival of gametophytes and a lower number of juveniles produced, whereas lowered pH only had a significant effect on the production of juveniles, slowing their rate of development. These results indicate that the predicted increase of marine heatwaves could have devastating effects on the persistence of bull kelp forest ecosystems. My second chapter considered the interacting effects of climate driven changes in temperature and salinity and interactions with the invasive seaweed (Sargassum muticum) on the growth and survival of giant kelp (Macrocystis pyrifera) gametophytes from Tomales Bay, CA. In my experiments, I tested: 1) how different salinities and temperatures impact giant kelp early life stages from different sources within Tomales Bay, 2) how the presence of invasive Sargassum propagules affect giant kelp gametophyte development, and 3) how the combined effects of salinity, temperature, and Sargassum presence affect giant kelp early life stages. My results indicate that 1) the presence of Sargassum had little effect on the survival of giant kelp gametophytes, 2) Sargassum accelerated development of giant kelp juvenile sporophytes, and 3) high temperatures resulted in the greatest reduction of giant kelp gametophyte survival. These results imply that giant kelp reproduction and presence within estuaries is more influenced by temperature than salinity and microscopic-stage competition with invasive species. My third chapter examined the effects of increased temperature and lowered salinities on invasive Wakame (Undaria pinnatifida) gametophytes in the San Francisco Bay. Previous studies have shown that low salinity can limit the distribution of Undaria, but there is no information about these effects on gametophyte stages. Using a full factorial design, I exposed Undaria gametophytes to five salinity conditions ranging from low to ambient salinity and two temperatures representing pre-2013 temperature maxima in San Francisco and maximum increased temperatures experienced under the 2014-2016 marine heatwave. I found that Undaria microstages were unable to survive below 20 psu and generally survived better under warmer temperatures of 18°C. Climate change in California is predicted to result in higher temperatures and reduced annual rainfall in drought years, which may facilitate future northward expansion of Wakame populations. With this research, I can better predict the impacts of climate change on kelp ecosystems to help coastal managers prioritize future protection efforts. Early life stages are often the most vulnerable to stress, and in this era of rapid climate change, understanding early life stage responses to stress will allow scientists and managers to better work towards the protection of our planet.

Among the many invasions that have stressed coastal marine systems in recent years, none have had such broad-reaching biological and political impacts as the Caulerpa taxifolia invasions of the Mediterranean Sea, Australia, and the United States. From its initial invasion of the northwest Mediterranean in 1984, to its discovery in Agua Hedionda Lagoon and Huntington Harbour in the summer of 2000 in California, this invasive alga has produced dramatic changes both in the biological landscape of coastal bays and estuaries and the political landscape of invasive species management.

The project hypotheses were as follows: A. Oyster recruitment will be higher at inner bay sites than the outer bay sites and density of recruits will be positively associated with adult oyster density. B. Mortality due to predators will be greater in the inner bay than the outer bay sites. C. Introduced predators have a greater per capita impact on oysters than native predators. D. Growth rates of oysters will be higher in the inner bay than the outer bay. E. Mortality due to overgrowth by space competitors will be higher in the inner bay than the outer bay. F. Oyster growth and survival will be higher at inner bay sites and will show a positive linear increase with increasing temperature and phytoplankton abundance.

Climate change is expected to increase heatwaves (MHWs) and disease transmission, potentially incurring losses of aquaculture stocks. Ostreid herpesvirus (OsHV-1) is a temperature-associated pathogen that predominantly affects the commercially important Pacific Oyster (Magallana gigas) and other species around the world, with some µVars causing > 95% mortality of oyster stocks. Over the course of three studies, we explored the utility of stress hardening (SH) in ameliorating the effects of MHWs (Chapter 1) and disease outbreaks (Chapter 2) by exposing multiple oyster species to a stressor before the onset of an event as well as a comprehensive overview of OsHV-1 detections in different locations and hosts (Chapter 3). In Chapter 1, we exposed juvenile Olympia oysters (Ostrea lurida), Kumamoto oysters (Crassostrea sikamea), and M. gigas to a two-week-long SH phase that involved exposure to a combination of temperature (15˚C v. 21˚C) and tide (immersion v. 6-hour tidal cycle) prior to a 72-hour simulated MHW at one of four temperatures (15˚C, 18˚C, 21˚C, 24˚C). Once the MHWs ended, a portion of O. lurida seed were grouped by their SH treatments, outplanted in Tomales Bay, CA, USA, and then assessed for mortality after nine months. Outplanted O. lurida that experienced tidal SH had perished by the time they were retrieved 279 days later. In contrast, 53.4% of O. lurida fully immersed during SH at 21˚C survived outplanting, while only 13.3% of those in the 15˚C SH treatment remained. Less than 10% of oysters across all species perished during the SH and MHW phases of the experiment, with C. sikamea experiencing the greatest losses. For Chapter 2, we investigated the effects of laboratory-based stress hardening (SH) via temperature (15˚C/16˚C v. 21˚C) and tide (tidal cycle vs. full submersion) on M. gigas and C. sikamea with the expectation that short-term exposure to warmer conditions and a simulated tidal regime would improve the performance of outplanted oysters. In 2021 and 2022, we exposed oysters to SH mechanisms before outplanting oysters at three sites spanning a thermal gradient in Tomales Bay, where they were exposed to OsHV-1 outbreaks in the field. We compared these outcomes to a baseline study completed in 2020, where M. gigas that had not been conditioned were exposed to OsHV-1 at two sites. M. gigas submerged in the warmer SH treatment had lower mortality in 2021, and those that experienced tidal fluctuations during SH had higher growth rates in 2022. In 2021, we saw no effect of SH on C. sikamea growth or mortality, but oysters were infected with OsHV-1. To our knowledge, this is the first study to provide OsHV-1 copy numbers (mg tissue-1) for C. sikamea. In Chapter 3, we used peer-reviewed literature, government reports, and unpublished data to describe the timeline of OsHV-1 variant detections, countries where M. gigas has tested positive, and other known hosts. We also summarized mean prevalence and peak infection intensity when that information was provided in the studies we assessed. Overall, SH has the potential to improve the resilience of aquaculture impacted by MHWs and disease, but it, along with the database of OsHV-1 detections we collated here, will likely be best used as part of an array of solutions addressing the multifaceted effects of climate change.

As global environmental challenges intensify, understanding the multifaceted dynamics of ecosystem adaptation, as well as what constitutes effective habitat restoration, becomes paramount. The first part of this dissertation aims to understand the ecological success of habitat restoration efforts and its correlation with public perception for enhanced project outcomes and public support. Focusing on tidal marsh ecosystems, we gathered qualitative and quantitative data on public perceptions and ecological responses to restoration in Oregon, USA. Our analysis revealed an increase in hydrology scores with the number of restoration actions, while no distinct relationship emerged between restoration actions and vegetation scores. To bridge social and ecological metrics, we developed a linking matrix and found that although restorationists and the public shared similar values, community priorities often went unaddressed in project assessments. Furthermore, human perceptions and values were rarely considered in restoration evaluations. These findings highlight the need to integrate social values into restoration projects and improve communication between stakeholders.An important environmental challenge for tidal marshes is sea-level rise (SLR) and the second part of this dissertation investigates the combined effects of SLR and biological invasions on a tidal marsh cordgrass (Spartina foliosa) ecosystem. Field experiments in San Francisco Bay, CA, USA, demonstrated reduced cordgrass survival in the presence of invasive crabs, along with varying responses of benthic microalgae and macrofaunal grazers to tidal inundation. Contrary to expectations, no interactive effects between increased inundation and invasive species were observed. This highlights the importance of considering sequential or latent stressor effects on ecosystems. In light of the impending changes brought about by SLR, the final part of this dissertation explores the intricate trophic interactions between aquatic and terrestrial habitats within tidal marsh ecosystems. We simulated sea-level rise using experimental structures that increased tidal inundation and assessed changes in various ecological indicators and species responses. Cordgrass exhibited negative responses to increased inundation, likely due to oxygen limitations resulting in elevated sulfide levels. Additionally, insect responses varied with some species showing positive reactions to inundation, while others exhibited negative responses. The presence and abundance of Song Sparrows (Melospiza melodia) were influenced by elevation and year, potentially linked to alterations in S. foliosa integrity and Chironomid abundance. This underscores the importance of integrating aquatic-terrestrial connections into predictive models for sea-level rise effects and conservation strategies, offering valuable insights for proactive management and sustainable coastal planning. Collectively, these results suggest that effective restoration and adaptation strategies for tidal marsh ecosystems require a holistic approach that bridges ecological and societal considerations. Recognizing the interplay between human values, ecosystem responses to restoration, and the complexity of trophic and stressor interactions is pivotal to craft strategies that safeguard these vital ecosystems in a changing world.

Fouling communities are diverse assemblages of sessile, filter feeding invertebrates found in marine and estuarine environments. They are an experimentally tractable study system that is commonly used to test ecological theories, and most research on fouling communities has taken place on artificial structures near docks and marinas. Fouling species are also found in natural habitats within estuaries, such as seagrass beds and cobbles, but few studies have tested ecological theories or processes influencing fouling communities in these natural habitats. Additionally, estuaries are dynamic ecosystems that are highly vulnerable to effects of climate change, and many estuaries contain a large number of non-native fouling species. Since climate change is likely to favor non-native over native species, processes structuring fouling communities may vary over space and time, resulting in biodiversity shifts and the proliferation of invasive species. In this dissertation, I explore how processes influencing fouling communities vary spatially across an estuarine gradient as well as inside and outside of seagrass beds, and vary temporally over three years in Tomales Bay, CA.

In my first chapter, I examined how fouling communities and effects of predators change across the estuarine stress gradient of Tomales Bay, CA. The Environmental Stress Model predicts that effects of predators decrease with increasing stress. In estuaries, this stress gradient occurs from the ocean to freshwater habitats, with increasing stress for marine organisms at greater distances from the ocean. However, this theory might not apply when stress-tolerant non-native species are introduced to ecosystems, such as estuaries. I predicted that predation would decrease with distance into the estuary but that the introduction of non-native species would extend the importance of predation further along this stress gradient than predicted by the Environmental Stress Model. Using a predator exclosure experiment, I evaluated how fouling communities and effects of predators differed across three sites in the summer of 2019. Fouling community composition differed significantly across sites and predation treatments, but the effects of predators differed significantly across sites. In general, the effect of predation was to reduce abundance, richness, diversity, and the abundance of specific morphotypes. The greatest effect of predation was in the middle of the bay where both native and non-native predators co-occur and was similarly low near the mouth and head of the estuary. This pattern was likely influenced by the abundance of solitary ascidians, which are highly susceptible to predation and were most abundant in the middle of the bay. The results differed slightly from the predictions of the Environmental Stress Model and suggest that ecosystems with large numbers of stress-tolerant introduced species may experience predation at higher levels of stress than predicted by the model.

In my second chapter, I investigated how fouling communities and effects of predators differed inside and outside of seagrass at one site in Tomales Bay, CA. Biogenic habitat, such as seagrass, could directly and indirectly influence fouling communities. Direct effects could occur when the structure associated with seagrass reduces flow or modifies water chemistry, resulting in physiological influences on fouling species. Indirect effects could occur when seagrass provides a habitat for predators, thereby increasing risk of predation for fouling species. To better understand the mechanisms in which seagrass influences fouling communities, I conducted a predator exclosure field experiment in 2018 and a predator exposure field experiment in 2020. Community composition differed significantly inside and outside of seagrass, with abundance, richness, and diversity being higher outside of seagrass than inside, suggesting a strong direct effect of seagrass. Predation differed significantly inside and outside of seagrass with predation being higher outside, though this effect was likely driven by differences in recruitment patterns of specific morphotypes and not differences in predator habitat use. These experiments provided evidence for both direct and indirect effects of seagrass on fouling communities; however, indirect effects of predators could be more variable than what has previously been documented. I caution against the overgeneralization about effects of seagrass on biological communities, and additional research is needed to better understand the mechanisms behind the relationship between biogenic habitat and biodiversity.

Finally, in my third chapter I examined how the effects of predators and seagrass on fouling communities vary over multiple years at Sacramento Landing in Tomales Bay, CA. While fouling communities have been used as a model system to test ecological theories, most previous studies have taken place over short time scales and have not focused on benthic processes in natural habitats. Given that estuaries experience high interannual variability in temperature, salinity, chlorophyll a, and other water quality parameters, ecological patterns are likely to vary along with these abiotic changes. This study tested how the importance of predation and seagrass in structuring fouling communities change over three years using results from predator exclosure experiments conducted in 2018, 2019, and 2020. Fouling community composition varied significantly across years, and this was likely due to interannual variability in recruitment, predation, and water quality. While there were some consistent effects of predators (on solitary ascidians), predation had variable effects on community metrics across years, which was likely driven by recruitment variation in specific morphotypes. Seagrass reduced abundance, richness, and diversity in 2018, but there was no significant effect of seagrass when averaging across all years. This pattern is likely due to interannual variability in seagrass bed characteristics.

Overall, processes influencing fouling communities are variable over space and time, and future research should account for this by taking place across greater spatio-temporal scales and should utilize laboratory experiments to isolate mechanisms of change.

One of the most ecologically significant invaders in the coastal waters of the U.S.is the European green crab, Carcinus maenas. It is a voracious, generalist predator in its native range in Europe as well as invaded regions which include including South Africa, Australia, Japan, and both coasts of North America (reviewed in Grosholz and Ruiz 1995, Cohen et al. 1995, Geller et al. 1997). Previous work in its native range and in the eastern U.S. has provided much evidence for its strong control over the abundance of benthic organisms in soft sediment communities (e.g. Ropes 1968, Reise 1977, 1985, Morgan et al. 1980, Jensen and Jensen 1985, Tettlebach 1986). Furthermore, work in Bodega Bay Harbor, CA has documented significant changes in the local food web as the direct result of green crab predation: Grosholz et al (2000) found 5- to 10-fold declines in the abundances of previously abundant invertebrate taxa. Lafferty & Kuris (1996) estimated that the annual net value of crab, mussel, oyster, and bait harvests threatened by green crabs in Western North America (WNA) is $43.7 million.

In coastal wetlands, invasive plants often act as ecosystem engineers altering flow, light and sediments which, in turn, can affect benthic animal communities. However, the degree of influence of the engineer will vary significantly as it grows, matures and senesces, and surprisingly little is known about how the influence of an ecosystem engineer varies with ontogeny. We address this issue on the tidal flats of San Francisco Bay where hybrid Spartina (foliosa x alterniflora) invaded 30 years ago. The invasion has altered the physico-chemical properties of the sediment habitat, which we predicted should cause changes in macrofaunal community structure and function. Through mensurative and manipulative approaches we investigated the influence of different growth stages of hybrid Spartina on macrobenthos and the underlying mechanisms. Cross-elevation sampling transects were established covering 5 zones (or stages) of the invasion, running from the tidal flat (pre-invasion) to an unvegetated dieback zone. Additionally, we experimentally removed aboveground plant structure in the mature (inner) marsh to mimic the 'unvegetated areas'. Our results revealed four distinct faunal assemblages, which reflected Spartina-induced changes in the corresponding habitat properties along an elevation gradient: a pre-invaded tidal flat, a leading edge of immature invasion, a center of mature invasion, and a senescing dieback area. These stages of hybrid Spartina invasion were accompanied by a substantial reduction in macrofaunal species richness and an increase in dominance, as well as a strong shift in feeding modes, from surface microalgal feeders to subsurface detritus/Spartina feeders (mainly tubificid oligochaetes and capitellid polychaetes). Knowledge of the varying influence of plant invaders on the sediment ecosystem during different phases of invasion is critical for management of coastal wetlands.

The introduction of nonnative invasive pests is among the many threats facing coastal ecosystems worldwide. Managing these pests often requires considerable effort and resources, and community scientists can be essential for providing the capacity needed for management and monitoring activities. In response to the invasion of a Northern California estuary by the predatory European green crab, a collaborative team of academic researchers and community scientists initiated a local eradication program. The green crab is listed among the world’s 100 worst invaders, and threatened both native species and commercial shellfisheries. The program dramatically reduced the green crab population over a 5-year period, but it rebounded, which necessitated a switch in project goals from eradication to population suppression. Community scientists were essential for facilitating this switch by providing the necessary capacity to quantify population characteristics and maintain reduced crab populations. The result was a sustainable program that successfully maintained low green crab densities, which will likely improve habitat for native species.