Shifting Trophic Structure of Rocky Shores in the Gulf of Maine: Implications forResilience of Coastal Ecosystems in a Changing Climate

Sheltered rocky shores in the Gulf of Maine previously supported three alternative community states – mussel (Mytilus spp.) beds, or fucoid communities, dominated either by Ascophyllum nodosumor Fucus vesiculosus. Our long-term experimental and observational data (1996 – 2019) show that mussels have become rare and fucoid communities resilient to perturbations have expanded to habitats formerly occupied by mussels coincident with accelerating climatic change in the Gulf of Maine this century. We used longitudinal surveys of five key species of benthic mollusks to examine whether the structure of this iconic food web of rocky shores in the Gulf of Maine shifted during this same period from 1996 to 2019 and if any change could be related to variables associated with regional climate change. We provide evidence for declines in numbers of four of the five species. Declines occurred in the abundances of the gastropods Littorina littorea(3.2% per year), Tectura testudinalis (5.9 % per year), and Nucella lapillus(3.7% per year) and in the recruitment of the mussel Mytilus edulis(14.6% per year). The declines in all four species can be explained by warmer water temperatures, pH or aragonite saturation ratio. In contrast, recruitment of the barnacle Semibalanus balanoides, which has planktonic, but non-calcifying larvae, did not shown significant changes over time and the variation over time could not be explained by temperature, pH or saturation ratio. Changing abundances of carnivorous and herbivorous benthic mollusks relative to suspension feeding sessile invertebrates associated with warming and acidification in the Gulf of Maine suggests that changing oceanic conditions is altering the trophic structure of marine food webs in coastal ecosystems.