The roles of evolutionary history and ecological interactions in the maintenance of a high-diversity macroalgal assemblage

Abstract

Communities often comprise non-random samples of regional species pools, whether in phenotypic or phylogenetic terms. A major focus of community ecology has been to determine the relative contributions of environmental and organismal variables to species interactions and community assembly. As ecological communities are increasingly subjected to the effects of climate change and intensifying environmental disturbance, researchers have focused on how to use these variables to predict community responses to environmental change. In this dissertation, I expand studies of drivers of community structure to a species-rich system that experiences strong local abiotic and biotic disturbance gradients: the sessile marine macroalgal community of the Northeast Pacific temperate marine rocky intertidal zone. Three natural axes of variation are apparent in this system: 1) distinct temperature, wave and light stress gradients; 2) pulse habitat-generating disturbances; and 3) press herbivory disturbance. I focused on a key organismal trait in this system: resource acquisition in the form of inorganic carbon acquisition by macrophytes for photosynthesis. I used pH* drift assays to identify intertidal algal species that can access not only dissolved CO 2 , but can also use HCO 3 - . Thirty-two of 40 species depleted HCO 3 - , indicating a carbon concentrating mechanism. Fifteen of 23 species tested also changed seawater total alkalinity, advantageously shifting local carbon resources towards increasing CO 2 and HCO 3 - . While pH* assays represent a snap-shot assessment of HCO 3 - use, tissue C isotope ratios (δ 13 C) provide a more time-integrated assessment of HCO 3 - use due to differing isotopic signals of CO 2 and HCO 3 - . My meta-analysis found that species pH* and δ 13 C measurements are correlated, and that macrophytes rely more on CO 2 further from the equator, but have increased use of HCO 3 - at sites with high temperature variance. To investigate factors driving local macroalgal assemblage structure in situ, I observed intertidal macroalgal assemblages over four years under differential abiotic and biotic disturbance treatments and generated a phylogeny for the red macroalgal community. Contrary to expectations, macroalgal assemblages were not increasingly phylogenetically clustered with increasing environmental stress; the mid intertidal zone had the highest phylogenetic diversity. Reducing grazers in disturbance plots in the mid zones reduced phylogenetic diversity.