Experiment on impacts of ocean warming and biotic stress in a coastal seaweed ecosystem

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.近年来，学界呼吁采用更具“现实性”的群落水平研究来评估全球变化的生态影响的呼声日益高涨。相较于简单的个体水平室内实验，这类实验通常更为复杂、周期更长、成本更高，且结果解读难度更大。这类额外投入是否值得？本研究依托室外中宇宙实验（outdoor mesocosms），在四季周期内探究了海洋变暖（OW）、海洋酸化（OA）、二者联合胁迫（OAW）及其自然波动对波罗的海西部沿岸群落的影响。该沿岸群落的优势种为多年生冠层大型藻类囊泡墨角藻（Fucus vesiculosus），其是波罗的海全域重要的生态系统工程师。此外，本研究通过室内台面实验（benchtop experiments）测定了生物体对温度与pH的直接响应，并探究了个体水平的响应能否有效预测群落水平对核心驱动因子海洋变暖（OW）的响应。相较于海洋酸化，海洋变暖对中宇宙实验群落的影响显著更强。海洋变暖引发了群落的结构与功能转变，且这类转变的强度与方向随季节而异。仅在冷热热胁迫条件下，即夏季与冬季，生物体对海洋变暖的个体水平响应，才能较好地匹配对应物种种群的群落水平响应。而在其余季节，生物相互作用的转变会掩盖海洋变暖的直接效应。海洋变暖的直接效应与海洋变暖驱动的生物相互作用转变的共同作用，很可能会威胁波罗的海造栖大型藻类囊泡墨角藻（F. vesiculosus）的未来存续。此外，本研究得出结论：季节性中宇宙实验对于我们理解全球变化的生态影响至关重要，因为此类实验充分考虑了非生物与生物胁迫因子的重要波动。