Historical subtidal regime shifts echoed in adjacent intertidal community

Anthropogenic stressors can trigger regime shifts, causing ecosystems to reorganize into new states. Moreover, there is growing evidence of the importance of fluxes (e.g., energy, matter and nutrients) in linking adjacent systems, suggesting that regime shifts within one system may extend to neighbouring systems through cross-ecosystem interactions. However, empirical evidence supporting this hypothesis is lacking. To address this gap, we analyzed over 40 years of community data from intertidal and subtidal ecosystems at Appledore Island in the Gulf of Maine, a region that has experienced multiple subtidal regime shifts. We identified three periods of rapid change in subtidal communities that are coherent with known regime shifts in the region. In the intertidal zone, we found that 58% of island-scale directional changes in community composition exhibited offset synchrony with subtidal regime shifts, meaning they started changing during subtidal regime shifts. This pattern was also observed in key intertidal species. We suggest that these offset synchronies may be the result of cross-ecosystem linkages through trophic interactions. Our results support the notion that regime shifts can propagate across ecosystem boundaries, demonstrating the importance of considering adjacent ecosystems as drivers of changes in any ecosystem of interest, and highlighting the need to consider the indirect effects of cross-ecosystem interactions for multiple ecological phenomena.