Spatial and Temporal Variability in Processes Regulating Estuarine Fouling Communities
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.