Eco-physiological and ecological responses of Fucus algae to changes in the surrounding environment

• Online Access: http://hdl.handle.net/2047/D20211268

A fundamental goal of global climate change research is to understand how organisms will likely respond to novel environmental conditions. Determining the photosynthetic and eco-physiological response of marine macroalgae to changes in their environment is vital to characterizing the success of these habitat-forming primary producers in the face of climate change. For this dissertation research I used a combination of empirical and theoretical methods to explore the main drivers of ecophysiological performance of Fucus, and how this may translate to patterns in the field under future environmental conditions. In Chapter 1, I examined how changes in seawater temperature affected net photosynthetic production by Fucus gardneri, a northeastern Pacific species. Using empirical measurements, I determined the thermal performance optimum for non-reproductive apical tips of F. gardneri at a saturating irradiance, and the oxygen consumption rate during respiration in the dark. In conjunction with one year of environmental data (e.g., seawater temperature, irradiance, tidal height), I developed a net photosynthesis model, and used this model to explore how photosynthetic production in F. gardneri is affected by the timing of low tide at multiple intertidal elevations. Because macroalgae attain maximal photosynthesis when submerged in shallow water during peak solar radiation, the timing of when high tide occurs was found to have a significant effect on photosynthetic productivity, an observation that has not been made before. In Chapter 2, I evaluated how nutrient enrichment (i.e., nitrogen) affects macroalgal growth and photosynthesis in Fucus vesiculosus, a northwestern Atlantic species. I quantified tissue growth, photosynthetic performance (i.e., photosynthesis versus irradiance curves: P-E curves) and O₂ production for F. vesiculosus individuals incubated in either ambient or nutrient-enriched conditions. Thermal performance curves were calculated by measuring O₂ production for ambient and nutrient-enriched F. vesiculosus over a range of ecologically relevant temperatures (6 - 30°C). Though previous studies have shown the deleterious effects of nutrient enrichment in coastal habitats, this study demonstrated the potential additive effects on the growth and photosynthesis of a macroalgal primary producer. The findings from this study highlight the importance of quantifying the effects of nutrient enrichment in coastal systems near urban environments, and suggests that in some cases increased nutrients can have positive effects. In Chapter 3, I examined within- and between- site and population differences in nutrient content of F. vesiculosus and how these differences might affect grazing by an herbivorous snail Littorina littorea. Water and algal tissue samples were collected over the course of one year from three distinct sites (Deer Island, Nahant Island, and Marblehead) in Massachusetts to determine differences in nitrogen and chlorophyll a levels within seawater, and tissue nitrogen within F. vesiculosus. This study demonstrated the spatial and temporal variability in nitrogen levels in a macroalgal primary producer and grazing pressure from Littorina littorea, a dominant herbivore along the greater Boston coastline.