Resource subsidy regimes, which range from presses to pulses, are common structuring forces in communities, yet research contrasting their effects is lacking. Many coastal marine ecosystems, including estuaries and coral reefs, have experienced increased nutrient subsidies while concurrently shifting to macroalgal dominance; however, the role of subsidy regime in transitions remains unknown. We created concentration–frequency distributions of nutrients in Cook’s Bay, Moorea, French Polynesia, and Carpinteria Salt Marsh Reserve, CA, USA. Both showed relatively high pulses interspersed by press concentrations. We grew dominant macroalgae alone and together in microcosms approximating these subsidy regimes to quantify individual performance and competitive outcomes. Subsidy regime changed growth and competitive abilities of macroalgae from both ecosystems but with divergent effects. In nutrient-limited reefs, different species were favoured under each enrichment regime, suggesting a fluctuating nutrient environment enhances diversity. In contrast, in eutrophic estuaries, enrichment of both regimes facilitated a single competitive dominant, suppressing diversity. Functional form groups did not predict responses to subsidy regime, likely because classifications ignore temporal variability in resource supplies. Because climate change will alter rainfall patterns globally, further accelerating nutrient subsidies from land to sea, understanding species’ responses to nutrient subsidy regimes is key to predicting the fate of coastal communities.
