The plea for using more "realistic," community-level, investigations to assess the ecological impacts of global change has recently intensified. Such experiments are typically more complex, longer, more expensive, and harder to interpret than simple organism-level benchtop experiments. Are they worth the extra effort? Using outdoor mesocosms, we investigated the effects of ocean warming (OW) and acidification (OA), their combination (OAW), and their natural fluctuations on coastal communities of the western Baltic Sea during all four seasons. These communities are dominated by the perennial and canopy-forming macrophyte Fucus vesiculosus-an important ecosystem engineer Baltic-wide. We, additionally, assessed the direct response of organisms to temperature and pH in benchtop experiments, and examined how well organism-level responses can predict community-level responses to the dominant driver, OW. OW affected the mesocosm communities substantially stronger than acidification. OW provoked structural and functional shifts in the community that differed in strength and direction among seasons. The organism-level response to OW matched well the community-level response of a given species only under warm and cold thermal stress, that is, in summer and winter. In other seasons, shifts in biotic interactions masked the direct OW effects. The combination of direct OW effects and OW-driven shifts of biotic interactions is likely to jeopardize the future of the habitat-forming macroalga F. vesiculosus in the Baltic Sea. Furthermore, we conclude that seasonal mesocosm experiments are essential for our understanding of global change impact because they take into account the important fluctuations of abiotic and biotic pressures.This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.Additional Supporting Information may be found in the online version of this article.

Marine organisms in the Mediterranean Sea experience the highest temperatures, salinities and oligotrophic conditions in its easternmost part along the eastern shores of the Levantine basin. Over the past three decades this region has warmed by ca. 1.5-3.08C with current winter and summer extremums of 178C and 318C, respectively. In this study, we tested the response of the native abundant articulated coralline red alga Ellisolandia elongata to this warming. Coralline algae play a key role in coastal ecosystems by structuring marine habitats, providing shelter for a myriad of species, and substantially influencing the coastal carbon budget. Despite being ubiquitous along the Levantine coasts, coralline's ecology, physiology, and biogeochemical role are nearly unknown as well as their performance under different temperatures. Measurements of primary production, respiration and calcification in the temperatures range 15-358C, which represent past, present and predicted local annual conditions, indicated two physiological tipping points: 1) metabolic breakdown above 318C; 2) metabolic shift at 238C, possibly promoting seasonal algal heterotrichy (perennation of the alga without its fronds). Annual production rates were evaluated under the current and predicted temperature regimes indicating a loss of ca. one third of the organic carbon and carbonate production by corallines contributed to the shallow Levantine coast in the upcoming decades. We predict that with continued warming, Eastern Mediterranean corallines will experience a westward range contraction, initiating with phenological shifts, followed by performance declines and population decreases, ending with local extinctions.

Human-induced ocean warming and acidification have received increasing attention over the past decade and are considered to have substantial consequences for a broad range of marine species and their interactions. Understanding how these interactions shift in response to climate change is particularly important with regard to foundation species, such as the brown alga Fucus vesiculosus. This macroalga represents the dominant habitat former on coastal rocky substrata of the Baltic Sea, fulfilling functions essential for the entire benthic community. Its ability to withstand extensive fouling and herbivory regulates the associated community and ecosystem dynamics. This study tested the interactive effects of future warming, acidification, and seasonality on the interactions of a marine macroalga with potential foulers and consumers. F. vesiculosus rockweeds were exposed to different combinations of conditions predicted regionally for the year 2100 (+∆5°C, +∆700 μatm CO ) using multifactorial long-term experiments in novel outdoor benthic mesocosms ("Benthocosms") over 9-12-week periods in four seasons. Possible shifts in the macroalgal susceptibility to fouling and consumption were tested using consecutive bioassays. Algal susceptibility to fouling and grazing varied substantially among seasons and between treatments. In all seasons, warming predominantly affected anti-fouling and anti-herbivory interactions while acidification had a subtle nonsignificant influence. Interestingly, anti-microfouling activity was highest during winter under warming, while anti-macrofouling and anti-herbivory activities were highest in the summer under warming. These contrasting findings indicate that seasonal changes in anti-fouling and anti-herbivory traits may interact with ocean warming in altering F. vesiculosus community composition in the future.

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