Substantial Shift in Community Composition over Two Years of Simulated Warming – A Mesocosm Study on a Temperate Coastal EcosystemResistance, Resilience and Phase Shifts

Christian Pansch¹

¹Åbo Akademi University, Finland

Shallow coastal ecosystems are hotspots of marine biodiversity, supporting high productivity and turnover while providing diverse ecosystem services. However, these vital ecosystems are increasingly threatened by rapid ocean warming and extreme events driven by climate change. In this study, we used experimental mesocosms to examine the effects of ocean warming and community stress memory on temperate coastal ecosystems initially dominated by two coastal habitat-forming macrophytes. Over 16 months, spanning two warm summers, we applied a gradient of 12 temperature treatments ranging from ambient conditions (0°C) to +5.5°C above ambient. The experiment allowed continuous recruitment of species into the system, largely driven by the prevailing heat stress gradient at any given point in time, as well as the established community within the tanks. We monitored macrophyte performance and their capacity to support associated marine fauna after the first summer (month 4), after one year (month 12), and after the exposure to a second summer (month 16). Prolonged warming caused substantial declines in macrophyte presence and biomass, particularly at the highest temperatures. This negative impact on the foundation species was associated with shifts and losses in total abundance of associated epi- and infaunal invertebrates, a pattern that manifested with time. Shannon diversity dropped overall towards the last sampling point, interestingly not so in the warmest temperature treatments, likely explained by a substantial shift in the prevailing community, opening niches for heat-tolerant (e.g., non-native) species. Our findings demonstrate that the decline in macrophyte biomass compromises their important role in sustaining biodiversity and resilience within coastal ecosystems. The potential loss of these macrophyte habitats could drive a reorganisation of community structures, favouring thermotolerant species and disrupting established ecological interactions.