Environmental fluctuations associated with natural events and anthropogenic impacts can alter nutrient availability in coastal systems. For example, in the Gulf of Maine, there is substantial spatial and temporal variation in availability of nutrients that fuel primary production. Seaweeds are important primary producers in intertidal habitats and play essential roles in absorbing nutrients and mediating their availability to the communities that they support. We measured levels of nitrogen in both the water-column and seaweed tissue weekly for a year, demonstrating a tight coupling between nutrient availability and seaweed tissue content. Seaweeds can exhibit remarkable variation in tissue quality depending on tidal elevation and seasonal nutrient variability, and these changes in tissue quality are associated with differences in consumption rates by herbivores. Additionally, herbivores can mediate nutrient availability to seaweeds by excreting ammonium. We experimentally separated the top-down, negative and the bottom-up, positive effects of herbivores on intertidal seaweeds and found that the combined top-down and bottom-up effect of herbivores change with season resulting in positive effects of herbivores on seaweeds in the spring, but not the summer. These results highlight not only the complex ways that herbivores can affect seaweed biomass but also how those effects can vary seasonally.