Synergistic nutrient colimitation across a gradient of ecosystem fragmentation in subtropical mangrove‐dominated wetlands

Abstract

We examined benthic algal response to nutrient enrichment by nitrogen (N), phosphorus (P), and N + P in mangrove wetlands of The Bahamas, to test the hypothesis that human impacts (fragmentation) on these ecosystems altered nutrient limitation, thereby changing the frequency and/or magnitude at which ecological synergies occurred. Fragmentation occurred due to road construction, resulting in reduced hydrological connectivity between the wetlands and marine environment. Strong, persistent, and synergistic nutrient colimitation occurred in both pristine and fragmented estuaries. Ecosystem fragmentation did not alter the biomass response to dual nutrient enrichment, but did alter the relative magnitude of the nonadditive response. That is, synergistic responses were less extreme in fragmented systems. This was supported by the strong, negative relationship between ambient algal biomass (a surrogate for background productivity) and the strength of synergistic responses (R2 = 0.69 and 0.79, year 1 and year 2, respectively). Bahamian coastal ecosystems exhibited the greatest synergistic responses reported for a marine ecosystem, suggesting that the benthic algal communities associated with Bahamian wetlands are among the most nutrient‐limited marine ecosystems. Our findings provide a case study illustrating how altered nutrient dynamics associated with land‐use change may decrease the frequency and/or magnitude of synergistic responses to nutrients in aquatic ecosystems.