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

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 the 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.