Alteration of Macroalgal Subsidies by Climate-Associated Stressors Affects Behavior of Wrack-Reliant Beach Consumers

Connectivity between ecosystems is of ecological relevance, especially when adjacent areas of contrasting productivity are compared. High-productive rocky shores dominated by macroalgae are one of the most important sources of wrack subsidies linked to low-productive sandy beaches. Rocky bed communities from nearshore environments are affected by shifts in ultraviolet radiation (UVR) and temperature. Therefore, any alteration to macroalgal traits, in terms of nutritional quality, pigments, or phlorotannins, due to environmental stress could trigger cascading changes in the food web of recipient ecosystems. To examine the effects of climate-associated variables, we mimicked a rocky intertidal ecosystem by constructing a set of mesocosm tanks harboring two macroalgal species, the native Laminaria ochroleuca and the non-indigenous Sargassum muticum , subjected to a combination of UVR and temperatures. We used the manipulated macroalgae to explore the effects of climate stress variables on the wrack-reliant amphipod Talitrus saltator . The macroalgae displayed differential and variable responses to UVR and temperature manipulations. L. ochroleuca nutrient quality and phlorotannins decreased with elevated UVR and at warm temperatures. S. muticum seemed to be more tolerant to stress conditions, and phlorotannin production was induced by elevated UVB. We documented concomitant effects induced by the treated macroalgae on the food consumption of T. saltator . We suggest that macroalgae became less palatable to T. saltator because chemical defenses were gained rather than a significant change in the nutritive value occurring. We hypothesize that the effects of warming and enhanced UVR on macroalgae might shift source-sink dynamics between connected ecosystems. Understanding the way in which climate-associated variables interact and influence subsidies in recipient ecosystems is of paramount relevance to assess the broad consequences of climate change and its proper management.