Press versus pulse nutrient supply and species interactions mediate growth of coral reef macroalgae

Abstract

Globally, ecosystems are experiencing dramatic alterations in the supply of resources, including nutrients. How the temporal regime (press versus pulse), independent of total resource supply, affects growth and species interactions of primary producers remains unexplored. Coral reefs experience anthropogenic modifications to nutrient regimes, making it critical to understand impacts on primary producers, such as macroalgae. In mesocosms, we examined how three macroalgae respond to temporal pattern in nutrient regime (ambient, press, pulse) and species interactions (alone, pairwise, or all together) in terms of their individual growth and assemblage productivity. We found nutrient regime and species interactions influenced individual growth and total assemblage productivity. Press regimes promoted the highest productivity of total assemblages. We observed species interactions ranging from competitive to facilitative varied between macroalgal species and nutrient regimes. Ours is the first study to demonstrate the temporal regime of nutrient delivery, independent of total nutrient supply, strongly impacts the productivity of species assemblages, the nature and outcome of species interactions, and the relative growth rates of individual producer species. As nutrient regimes increasingly fluctuate for coastal marine ecosystems in the Anthropocene, our findings imply macroalgal community composition may also fluctuate. More broadly, our study highlights the importance of assessing primary producer species' responses to varying nutrient regimes to understand factors structuring their communities.