COS 173-2 - Sand dynamics drive kelp forest community structure and stability

Abstract: Rocky substrate is an essential physical requirement for many coastal foundation species. Despite its importance, aspects of reef substrate composition remain poorly resolved in some ecosystems, including giant kelp (Macrocystis pyrifera) forests. Specifically, what are the consequences of sand inundation on giant kelp and what are the implications for the diverse communities supported by kelp forests? Furthermore, how does temporal variation in substrate influence the stability of these communities? Using long-term kelp forest survey data from Santa Barbara Coastal LTER, we apply statistical modeling methods to examine the response in abundance and diversity of kelp forest guilds to rocky reef availability. Additionally, we describe the change in community composition across levels of rockiness. Lastly, we quantify how the temporal variability in abundance and composition relates to the variability of rocky substrate over time. Our analysis shows a positive relationship between giant kelp abundance and rocky cover. Likewise, understory algae, sessile invertebrate, and mobile invertebrate abundances display a positive relationship with rocky cover. The magnitude of the effect varies across community guilds, likely do to varying morphologies across the community. Next, stacked composition plots visualize the change in the relative abundance of species groups across increasing levels of rockiness. Grouping by morphology, certain types of species comprise a greater share of community biomass when rock is scarce (red algae, anemones, carnivorous gastropods), while other groups are more dominant in rocky conditions (stipitate and calcareous algae, sea cucumbers, sea urchins). Lastly, temporal variability in reef substrate is positively correlated to variability in abundance and composition. These results establish the importance of rocky habitat to guilds spanning multiple trophic levels in these communities, documenting its impact on the abundance, diversity, composition, and stability of kelp forest ecosystems. Given these findings, sand dynamics on reefs should be considered a significant driver of local kelp forest dynamics.