UC Irvine

Rocky shores have long served as model systems for examining the drivers of community assembly and structure, and particularly the importance of top-down versus bottom-up control. Despite growing recognition of the importance of microbial communities in these habitats, including primary production, nutrient cycling, and contributions to trophic webs, microbes have rarely been incorporated into the traditional frameworks of intertidal community ecology. Epilithic biofilms on rocky shores consist of diverse assemblages of autotrophic and heterotrophic bacteria, algae, predatory protists, and more, but are often conceptualized and investigated as phototrophs in the context of the macroscopic community. As such, these communities are hypothesized to respond, e.g., to molluscan consumers or nutrient availability, in ways that are similar to larger macroalgae. However, as I show here, biofilm communities are actually composed of multiple, co-occurring trophic levels, and consumers and nutrients likely trigger changes in trophic dynamics within the biofilm. Bottom-up processes like nutrient concentrations determine the diversity and abundance of marine organisms, limiting their growth and survival. For example, kelp populations have been declining globally as temperatures rise and nutrient availability declines. By understanding how these kelps, which act as both foundation species and conduits for nutrient inputs into marine systems, respond to variation in nutrient availability and identity, we can better understand how kelp populations may respond in the face of climate change. To examine the role of top-down and bottom-up processes in structuring epibenthic intertidal communities, I used field manipulation experiments combined with molecular tools to characterize the prokaryotic and eukaryotic biofilm communities and assess how molluscan grazers, nutrient concentrations, and temperature structured these communities. I found that biofilm community composition was largely determined by temperature and nutrient concentrations, with increased nutrient concentrations increasing the impact that grazers had on these communities, suggesting that grazers may influence these communities more through nutrient facilitation or altering dispersal processes (Ch. 1). I took a comparative experimental approach to assessing how the strength of top-down and bottom-up processes in structuring biofilm communities changes across regions in California that experience difference ambient temperatures and nutrient concentrations. I found that microbial biofilm communities were largely structured by physical distance (differences between sites and regions), with local-scale top-down and bottom-up processes having relatively weak effects on these communities in comparison. While the effects were weaker, there was regional variation in the strength of grazers and nutrients structuring these communities: grazers and nutrients had stronger effects in northern California, whereas more localized processes like establishment stage and direct grazer access had larger effects in southern California (Ch. 2). Lastly, I investigated how natural variation in nitrogen availability impacted the uptake rate of different forms of nitrogen by two species of kelp in southern California during periods of low nitrogen availability. Initial experiments included removal of biofilms from kelp blades to determine whether associated microbes might mediate nutrient uptake. The results of those experiments were inconclusive, but I subsequently found that both study species, Macrocystis pyrifera and Eisenia arborea, readily took up ammonium and nitrate, while urea uptake efficiency increased as ambient nitrogen availability decreased. Moreover, by calculating the expected rates of uptake of these nitrogen forms at various sites based on laboratory uptake experiments, I found that regenerated forms of nitrogen (ammonium and urea) contribute substantially to the nitrogen taken up by these kelps during periods of low nitrogen in the summer (Ch. 3).