Seagrass ecosystems under stress: the impact of marine heatwaves on trophic pressure and seagrass persistenceBiology & Ecophysiology

Laura Falkenberg^1,2, Alissa Bass²

¹The University of South Australia, Australia - ²The Chinese University of Hong Kong, China

Ecosystems are shaped by complex species interactions, including trophic interactions that connect habitat-forming species with higher-level consumers. These interactions are influenced by changes to the environment, with a key component being temperature. Marine temperatures fluctuate over seasonal scales with this variability also, and increasingly, overlaid by short-term extreme high temperature events known as marine heatwaves (MHWs). These changes in temperature have the potential to alter marine habitats, such as seagrass meadows, through impacts on trophic interactions. Trophic interactions can be disrupted via effects on various community members including the seagrasses themselves, as well as the ephiphitic algae and the epiphytic algae grazers with which they co-occur. Here, we examined effects of a MHW in different seasons and grazer occurrence on traits of the seagrass, ephiphytes, and grazers. We found there was an effect of season in which the MHW was implemented on seagrass response and grazer influence. That is, in winter the MHW had a positive effect on seagrass growth, and grazer pressure was minimal. In contrast, summer was associated with increased grazer effects, including some interactions with MHWs, that indicate stronger top-down control on seagrass and epiphytes during this season. Together, these results indicate that winter temperatures and MHWs can increase growth of seagrasses and have minimal impacts on grazers and their influence, while summer temperatures can enhance seagrass growth, but that this is restricted (or even negatively affected) when experienced in combination with MHWs and grazer presence. Examining responses of diverse community members under distinct seasons allows us to reveal a more representative depiction of ecosystem responses to these extreme events.