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Winners and losers: prevalence of non-indigenous species under different simulated marine heatwaves and high propagule pressure in an oceanic island

Nuno Castro*, Patrício Ramalhosa, Eva Cacabelos, José L. Costa, João Canning-Clode, Ignacio Gestoso

*Corresponding author:

ABSTRACT: Global warming is increasing the frequency, duration and intensity of extreme events such as marine heat waves (MHWs). The effects of MHWs include a variety of negative environmental impacts. Extreme weather events can interact with other environmental stressors such as the invasion by marine non-indigenous species (NIS). The aim of this study was 1) to compare the responses of fouling assemblages recruited within a harbour (highly invaded) with those recruited in natural habitats (not invaded) to simulated gradients of MHWs with different temperatures and durations, and 2) to evaluate the legacy effects of those MHWs on the invasibility of both types of sessile assemblages by deploying them in a marina environment (high NIS propagule pressure). Experimental assemblages were sampled after exposure to two time periods at different simulated temperature conditions to examine the effects of different MHWs. Later those assemblages were deployed inside a marina facility to test the invasibility of heat-stressed assemblages. The results revealed that higher temperatures and longer MHWs had an overall negative impact on both native and non-indigenous assemblages. Shorter MHWs had greater effects on the assemblages dominated by NIS, while longer MHWs affected more native species. The duration of MHWs promoted higher variability in relation to invasibility. ‘Winner’ and ‘loser’ species and homogenisation effects could potentially alter the legacy effects of the MHWs in the pattern of NIS recruitment. This study highlights the importance of the interactions between environmental stressors to the conservation of coastal communities, crucial ecosystems in oceanic islands.