Abstract Background and Aims Climate change and bioinvasions can cause dramatic shifts in community composition followed by alterations to ecosystem functioning and services. Traditionally invaders are considered as having mostly negative impacts on the environment. However, it has been recently suggested that thermally adapted species introduced to climate change hotspots might potentially compensate for native loss preserving their ecosystem functions. To test this, we investigated the eco-physiology of two dominant invasive seaweeds of the Levantine shallow reefs, Lobophora schneideri and Galaxaura lessepsiana, and then compared them with the endemic, thermally sensitive Gongolaria rayssiae. Methods We followed structural (i.e., percent cover and biomass) and physiological traits at different ambient conditions. Ex-situ metabolic rates were assessed at ambient seawater temperature and under thermally manipulated conditions through the thermal performance curve experiments. Based on the estimated physiological performances we obtained thermal habitat suitability projections under predicted future scenarios for the Levantine basin. Key Results The invaders’ calculated annual inorganic carbon uptake was 1.5-2 times higher than the endemic G. rayssiae, mostly related to their high biomass year-round while the native is fully branched only between late winter and early summer. Furthermore, the gross primary productivity thermal optima (30.7 and 31.7°C, respectively) of L. schneideri and G. lessepsiana were higher than that of G. rayssiae (24.5°C). Habitat suitability modelling indicated a likely replacement of the native distribution by the invasive species, with potential variation between the two Levantine invaders. Conclusions Hence, some of the reef metabolic functions such as oxygen production and carbon uptake, possibly lost with the native species extinction due to warming, will be possibly maintained by dominant invasive thermophilic species. These results highlight the importance of investigating and understanding the complex role of non-indigenous invasive seaweed species in compensating and regulating ecosystem metabolic processes and services otherwise, potentially lost in tropicalized environments.

Abstract Background and Aims Warming and marine heat waves (MHWs) are responsible for global declines in kelp forest ecosystems. Research to date focuses on impacts of temperature increase based on thermal tolerances of habitat-forming kelp species. Winter MHWs do not tend to surpass the thermal maxima of macroalgae, providing the opportunity to examine impacts in the absence of large-scale biomass loss, and in conjunction with other abiotic stressors. The transformation of organic carbon by heterotrophic marine bacteria, a key component of carbon cycling and foodweb productivity in kelp forests is strongly modulated by temperature. In this study, the metabolic responses of Macrocystis pyrifera heterotrophic bacterial communities to winter MHW conditions were examined in conjunction with hydrogen peroxide (H2O2) stress, a known inhibitor of bacterial activity. Methods Carbon uptake rates of planktonic and surface-associated (biofilm) microbial communities from M. pyrifera forests were assessed with exposure to H2O2 treatments alone, and in tandem with short-term heat-spike experiments simulating ambient, moderate, and extreme heatwave conditions in winter. Key Results Carbon uptake rates of planktonic bacteria increased under extreme MHW conditions, with no significant response to increasing H2O2 concentrations. Conversely, M. pyrifera biofilm communities showed a significant decrease in carbon uptake rates at high H2O2 exposure compared to the intermediate treatment regardless of temperature regime applied. Conclusions In winter, ambient water temperatures are limiting planktonic bacterial carbon incorporation rates in kelp forests, and MHW events stimulate increased carbon turnover. Increasing bacterial metabolism during winter may affect kelp forest ecosystem services through altering turnover and maintenance of organic carbon within kelp forests and annual carbon cycling patterns. We suggest that temperature is not limiting bacterial carbon uptake at the algal surface, and other stressors are driving carbon uptake patterns in M. pyrifera biofilms.