Competitive outcomes are strongly influenced by variability in resources and the community context, and these outcomes directly impact species’ distributions and abundances. Resource subsidies can vary in magnitude and temporal pattern, the extremes of which are pressed (low magnitude, continuous duration) and pulsed (high magnitude, short duration) regimes. However, how resource timing affects growth and competition of primary producers is understudied. On coral reefs, macroalgae are primary producers that compete for benthic space. With additional nutrients being released into reef systems, it is important to understand how different macroalgal species respond to varying nutrient regimes because this influences their competitive abilities. We used a fully-crossed two-factor mesocosm experiment to determine how three macroalgal species respond to nutrient regime (ambient, pressed, pulsed) and community composition (alone, in pairs, or all together) in terms of their growth (change in biomass over 12 days). Our macroalgal species were Padina boryana, Sargassum mangarevense, and Galaxaura fasciculata, which are predicted to rank from high to low, respectively, in their growth responses to nutrients. We predict nutrient regime will influence their relative competitive abilities: ambient conditions should favor slower-growing species, while pulsed and pressed conditions should favor faster-growing species.
Results/Conclusions
Our results demonstrate that temporal pattern of nutrient supply and community context are key drivers influencing growth of primary producers. As predicted, Padina grew fastest in pressed nutrient conditions both when alone and with competitors. However, competitor presence mediated Padina growth responses in ambient and pulsed conditions. For example, Sargassum increased Padina growth in ambient conditions, while Galaxaura decreased Padina growth in pulsed conditions. When alone, Sargassum growth decreased from ambient, to pressed, to pulsed nutrient conditions. Sargassum growth was always reduced in the presence of competitors, with the presence of both competitors having a stronger effect in pressed and pulsed conditions compared to ambient. Finally, when alone, Galaxaura growth increased from ambient, to pulsed, to pressed nutrient conditions. The presence of Sargassum tended to facilitate Galaxaura growth in ambient and pulsed conditions, but there was no benefit of competitor presence in pressed nutrient regimes. Thus, different nutrient regimes shifted the relative growth of different macroalgal species, which can shift their competitive outcomes and ultimately the community composition. Thus, it is important to assess primary producer species responses to varying nutrient regimes in order to understand the factors structuring their communities.