A community approach to understanding patterns and processes on rocky subtidal reefs in the Salish Sea
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The structuring of marine benthic communities is driven by a diverse set of ecological factors, each exerting its influence to differing degrees across all spatial and temporal scales. Here we’ve leveraged a decade of ecological data of the algal, sessile invertebrate, and mobile fauna assemblages on rocky subtidal reefs to present a community approach to understanding the patterns and underlying processes shaping the subtidal seascape in the Salish Sea (inland waters of Washington State). Specifically, we investigated at multiple spatial scales the structure of these communities along a depth gradient, on surfaces with diverse substrate orientations, and across a spectrum of flow regimes to reveal the associations of hundreds of organisms interacting in a mosaic of patchy habitats. We also strongly advocated for the continued efforts to quantify ecological change in our coastal oceans and its long-term consequences. In the first chapter, we compared the rocky subtidal communities of the San Juan Archipelago to similar temperate habitats worldwide and discovered similarities, such as the characteristic zonation patterns of a shallow subtidal macroalgal-dominated zone shifting to an epifaunal invertebrate-dominated deep subtidal zone. We hypothesized that sessile invertebrate communities would exhibit vertical zonation patterns in diversity and percent cover across a depth gradient and now provide evidence for depth as a strong ecological axis in determining community composition. Contrary to our further hypothesis, the mobile fauna assemblages showed very little evidence of strong zonation patterns across depth in either abundance or diversity; this is perhaps the most surprising result from this study. After our initial bottom-up approach to understanding depth zonation patterns on our reefs, we took a top-down approach to investigating scale-dependent patterns and processes in the second chapter. Starting first with the big picture at the habitat scale (hundreds of meters), then at the individual substrate-slope scale, and finally focusing in to sub-meter reef features, we now see a highly nuanced image of the communities across the underwater seascape. Assemblages on horizontal surfaces change much more dramatically with increasing depths than those found on vertical surfaces, although the diversity and abundance of taxa across all substrate orientations on converges in the deep zone, a condition referred to as the ‘depth emergence’ phenomenon. Regardless of depth, assemblages at the extreme ends of the substrate orientation spectrum (i.e. horizontal vs. vertical) have significantly different community structure owing largely to the increased cover of crustose coralline and foliose red algae on horizontal surfaces. And sub-meter vertical features in shelf habitats took on the characteristics of the surrounding surfaces, a pattern we did not expect to find but lends more evidence to the overall identification of light levels as a strong driver of community composition in all habitat types, substrate orientations, and at all spatial scales. Lastly, at all 60 transect locations, we deployed alabaster dissolution blocks in an effort to connect small-scale localized flow patterns to community composition. In the third chapter we designed flow-mediated species response models and then fit long term ecological monitoring data of algal and invertebrate assemblages to these models to characterize the communities along the flow regime spectrum. As we suspected, differences in community structure are greatest between the lowest and the highest flow rates, with little overlap in the taxa found in the lowest and highest flow rates. Coherent species curves were generated and show clear abundance patterns related to local flow regimes with the taxa found together having the strongest association to each other in terms of showing a similar type of response (abundance in this case). We did not previously identify habitat-wide patterns of zonation in the mobile fauna, but abundance patterns in these taxa do appear to be flow related due in large part to an organism’s ability to adhere to the substrate and its phenotypic plasticity. In conclusion, we found that many subtidal benthic species flourish in particular flow conditions and fit within predicted flow-mediated response curves based largely on organismal body plans, attachment to and elevation from the substrate, and feeding strategies. This study encompasses multiple years, sites, and community components and as such we anticipate this study will contribute to a better understanding of subtidal community patterns locally and provide comparisons globally for years to come, especially as we have barely scraped the surface of the massive underlying data set. We believe we have created a solid foundation for quantifying ecological change in our coastal oceans and its long-term consequences.
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