The roles of nutrient dynamics, oceanography, and light in the structure and functioning of rocky intertidal macrophyte assemblages

approved: _____________________________________________________________________ Bruce A. Menge The influence of the physical environment on organisms has long been a subject of ecological research. But, the complex drivers of environmental variation, and the multiple scales at which this can occur, make studying this topic a difficult challenge. In rocky intertidal habitats, oceanographicand climate-scale variability influence benthic communities primarily via changes in temperature, propagule delivery, and nutrient availability. Nutrient availability especially is thought to influence the diverse and productive macrophyte assemblages in these communities. While bottom-up regulation of communities provisioned by resource subsidies is known to play a role in community dynamics in this system, we are still developing an understanding of how nutrients influence macrophytes. In this dissertation, I investigated the environmental drivers of intertidal macrophyte ecology at both the biogeographic (regional) and the local scale. Using data spanning 10 years and over 900 km of coastline within the California Current Large Marine Ecosystem (CCLME), I first evaluated the contributions of nutrient availability, upwelling, spatiotemporal variation, and climate variability to macrophyte tissue nutrient content. The insights gained from this investigation show a strong influence of nutrient availability on nutrient status, although that relationship varied among species. I also observed remarkably similar interannual variability in tissue nutrient content, which is consistent with regional and basin-scale oceanographic processes. My results in Chapter 2 point to a role of nutrient availability in modulating macrophyte nutrient content which is consistent with regional forcing in the CCLME. In Chapter 3 I asked how nutrient and light availability drive growth, and how this relates to nutrient content. Combining insights from Chapter 2 on macrophyte nutrient content with data on growth rates in two dominant low intertidal species, Saccharina sessilis and Phyllospadix scouleri, I found contrasting drivers of growth in the two species. Growth in S. sessilis was positively related to nutrient availability, consistent with our expectations, but negatively related to light availability, confounding predictions. Growth in P. scouleri, on the other hand, showed more complicated dynamics in relation to nutrients and light, suggesting that the abundance of these resources modulates the growth response. These patterns nonetheless suggest that growth in these species is strongly influenced by the upwelling process, although the mechanisms behind such influences may vary. Finally, in Chapter 4 I approached the question of how the lower limits of one high zone intertidal species are set, that of Fucus distichus, to determine local scale environmental drivers of macrophyte distribution. Through a field experiment, I manipulated F. distichus across an intertidal gradient and found that it had high mortality at low intertidal elevations. Using a laboratory mesocosm experiment, I found that the mechanism behind the elevational pattern appears to be the reduced light availability in the low intertidal zone. Combined with the insights from Chapter 3, which suggests that light influences macrophyte growth, the result that light could play a role in setting the elevational limit in this species also underscores the importance of considering this resource in studies of macrophyte ecology. © Copyright by Sarah L. Close January 17, 2014 All Rights Reserved The Roles of Nutrient Dynamics, Oceanography, and Light in the Structure and Functioning of Rocky Intertidal Macrophyte Assemblages