The role of environmental conditions in regulating long-term dynamics of an invasive seaweed

The mechanisms underpinning long-term dynamics and viability of invader populations in the receiving environment remain largely unknown. We tested the hypothesis that temporal variations in the abundance of a well-established invasive seaweed, Caulerpa cylindracea, in the NW Mediterranean, could be regulated by inter-annual fluctuations in environmental conditions. Abundance data of C. cylindracea, sampled repeatedly between 2005 and 2020 at the peak of its growing season (late summer/early fall), were related to interannual variations in seasonal seawater temperature, wind speed and rainfall recorded during different growth phases of the alga, in both subtidal and intertidal habitats. In both habitats, higher peak of C. cylindracea cover was associated with lower seawater temperature in spring and summer, when the seaweed exits the winter resting phase and starts a period of active growth. In addition, the peak abundance of subtidal C. cylindracea was positively associated with higher autumn wind speed intensity and spring daily total precipitation. Our study reveals the importance of seasonal and interannual variation of abiotic factors in shaping temporal patterns of abundance of C. cylindracea, in both subtidal and intertidal habitats. Identifying the factors underpinning invasive population temporal dynamics and viability is essential to predict the time and conditions under which an invader can thrive, and thus guide management strategies aimed to containing invasions under current and future climates.