LB 10-123 - Coastal sand dynamics structure the spatial synchrony of kelp forest communities

Background/Question/Methods:

Spatial synchrony, correlated fluctuations of populations in different locations, is a central aspect of population dynamics that impacts population stability, persistence, and resilience. Despite its importance, synchrony remains poorly resolved in marine ecosystems, including giant kelp (Macrocystis pyrifera) forests. Specifically, what are the physical drivers of giant kelp synchrony, how are these drivers correlated across space, and what are the consequences for the diverse communities supported by giant kelp? This project investigates the synchrony of reef substrate change and giant kelp population dynamics, and the relationship between them over space in sites across Southern California.

Using long-term kelp forest surveys from Santa Barbara Coastal LTER and the SONGS Mitigation Monitoring Program, we examine the relationship between kelp forest communities and rocky habitat. First, we apply quantile regression to quantify the response in giant kelp and understory guilds to rocky reef availability. Additionally, we measure the spatial correlation of kelp populations and reef substrate composition using non-parametric correlation functions. Using pairwise spearman correlation between observed points, these functions provide estimates of synchrony across space. We performed this analysis over large (0-40 km, LTER) and small (0-4 km, SONGS) scales to examine local and regional spatial synchrony.

Results/Conclusions:

Our quantile regression analysis revealed a non-linear, positive relationship between giant kelp abundance and rocky cover within the upper 70% ( > 30th percentile, P < 0.05) of abundance observations. Likewise, sessile and mobile invertebrate abundances displayed a non-linear, positive relationship within the upper 99% ( > 1st percentile, P < 0.05) of observations. However, understory algae abundance did not exhibit a significant relationship with rocky cover, likely due to the diverse growth morphologies within the guild. This analysis establishes the importance of rocky habitat to guilds spanning multiple trophic levels in these communities.

Next, our spatial synchrony analysis found reef composition to be substantially less correlated over space than giant kelp. Specifically, at the regional scale, giant kelp populations exhibit a spatial decay over 0-15 kilometers, followed by a gradual decline at longer distances. This contrasts with rocky cover, which declines to 0 correlation over distances of 0-10 kilometers. Similarly, the local scale analysis reveals spatial correlation in giant kelp exhibits a spatial decay over 0-4 kilometers, while rocky cover remains low over the same distances. This indicates that reef composition contributes to desynchronizing local giant kelp population dynamics, operating over smaller scales compared to typical oceanographic processes.