A persistent green macroalgal mat shifts ecological functioning and composition of associated species on an Eastern Tropical Pacific coral reef

Global evidence of phase shifts to alternate community types is of particular concern because these new com-munities can provide fundamentally different and often novel ecosystem functions and services compared to the original community. Shifts of a diverse range of marine communities to dominance by green macroalgal mats have occurred worldwide, making it critical to understand their emerging functions and roles. We observed green algal mat on two reefs in the Eastern Tropical Pacific, with one persisting for >10 years on a reef with stable herbivore populations and no known sources of anthropogenic nutrients. These mats supported a more speciose macroalgal community with fewer taxa present in the adjacent coral community and facilitated growth of an associated understory macroalgal species by reducing herbivory pressure and possibly enhancing nutrient supplies within the mat community state. These results demonstrate a weakening in the processes controlling reef community structure as a result of the shift in composition associated with the macroalgal mat, creating positive feedback supporting mat persistence. These novel ecosystem functions generated by this alternate community state illustrate the importance of further research on community shifts, which will become increasingly common in the Anthropocene.