Steep environmental gradients offer the opportunity to study organismal adaptation to local conditions. Yet these local-scale gradients are often nested within latitudinal gradients, which could mediate neutral and selective processes on the local scale. I used a classic study system, a gradient in tidal elevation on temperate rocky shores, to test for geographic variation in intraspecific differentiation across tide heights in a marine primary producer. The seaweed Fucus vesiculosus is a foundation species on rocky shores throughout the temperate North Atlantic Ocean. In the Gulf of Maine in particular, F. vesiculosus occurs from the lower to upper intertidal zones along the entire coastline, spanning local and latitudinal gradients in abiotic and biotic stressors. I conducted a series of observations and experiments across the species’ intertidal distribution at sites along the Gulf of Maine coastline to: 1) identify patterns of genetic diversity and structure from tide height to regional scales, 2) test for inter-population differences in adaptation to tide height, and 3) determine if traits important to ecosystem-level processes were associated with local adaptation.

Molecular studies using microsatellite markers showed genetic variation was significant across multiple sampling scales. Patterns of inbreeding (i.e., F_IS) revealed spatial variation in isolation. Genetic differentiation (i.e., F_ST) was attributable to distance in between-site comparisons, but by environment in between-tide height comparisons. Reciprocal transplant experiments, between the upper and lower edges of the intertidal distribution of F. vesiculosus, were conducted at sites throughout the Gulf of Maine. Relative growth rates of F. vesiculosus showed adaptive phenotypic differentiation in the northeastern Gulf of Maine, countergradient variation in the central gulf, and environmentally-driven responses in the south. Importantly, observations of nutrient physiology demonstrated that nutrient uptake and allocation of tissue nitrogen towards growth were influenced by adaptation in the northeast and driven primarily by environment in the south. By combining molecular and physiological approaches, I have identified geographic variation in genetic and environmental constraints of organismal physiology and population-level processes. Given the important bottom-up role of seaweeds as providers of food and habitat, this variation could have important consequences for the associated rocky intertidal community.