Ecological synergies are of particular interest because their unpredictable nature hinders accuracy of ecological forecasting.  Synergies can occur in response to enrichment by multiple nutrients (typically nitrogen (N) and phosphorus (P)), whereby the response to simultaneous enrichment by N+P is greater than that predicted by their additive responses. Because producers typically need N and P for growth, it is predicted that a synergy would occur most strongly in an environment with low availability of both key nutrients. Most human activities enhance rather than reduce the availability of nutrients and thus may dampen the frequency and/or magnitude of synergies.  Here we explored the frequency at which synergies occurred in previously published studies and provide a case study to test how land-use change altered the magnitude of synergistic responses.  To determine patterns in synergistic responses to N and P, we applied a novel metric, the Interaction Effect Index (IEI), that quantifies the relative degree of non-additive responses to multiple nutrient enrichment (nitrogen - N, phosphorus - P, and both - N+P), to 653 studies across diverse ecosystem types and latitudes.  We then quantified benthic algal response to enrichment by N, P and N+P across a gradient of ecosystem fragmentation (blockage of tidal flow to shallow estuaries in The Bahamas) to assess the implications of land-use change for synergistic responses. 
Results/Conclusions

We found that synergies occurred less commonly than alternative responses (e.g., antagonisms) across all ecosystem types.  But when they did occur, they were often great in magnitude.  Synergistic responses were strongest in freshwater and temperate systems, and weakest in terrestrial and arctic systems.  We found that ecosystem fragmentation resulted in greater algal biomass and reduced magnitude of synergisms than in unfragmented sites.  A strong negative correlation (R2 = 0.78) was found between algal biomass under control conditions (a surrogate for base ecosystem productivity) and the magnitude of synergism (as quantified by the IEI).  Our findings demonstrate anthropogenic changes to coastal systems may be shifting nutrient limitation from historical baselines, thereby affecting our basic understanding of how relatively pristine ecosystems function.