Seaweed - epiphyte - mesograzer communities were tested for their responses to elevated seawater temperature and [CO2] in benthic mesocosms experiments across four consecutive seasons of one year in Kiel, Germany

Rising seawater temperature and CO2 concentrations (ocean acidification) represent two of the most influential factors impacting marine ecosystems in the face of global climate change. In ecological climate change research full-factorial experiments across seasons in multi-species, cross-trophic level set-ups are essential as they allow making realistic estimations about direct and indirect effects and the relative importance of both major environmental stressors on ecosystems. In benthic mesocosm experiments we tested the responses of coastal Baltic Sea Fucus vesiculosus communities to elevated seawater temperature and CO2 concentrations across four seasons of one year. While increasing [CO2] levels only had minor effects, warming had strong and persistent effects on grazers which affected the Fucus community differently depending on season. In late summer a temperature-driven collapse of grazers caused a cascading effect from the consumers to the foundation species resulting in overgrowth of Fucus thalli by epiphytes. In fall/ winter, outside the growing season of epiphytes, intensified grazing under warming resulted in a significant reduction of Fucus biomass. Thus, we confirm the prediction that future increasing water temperatures influence marine food-web processes by altering top-down control, but we also show that specific consequences for food-web structure depend on season. Since Fucus vesiculosus is the dominant habitat-forming brown algal system in the Baltic Sea, its potential decline under global warming implicates the loss of key functions and services such as provision of nutrient storage, substrate, food, shelter and nursery grounds for a diverse community of marine invertebrates and fish in Baltic Sea coastal waters.

全球气候变化背景下，海水温度上升与二氧化碳浓度升高（海洋酸化）是影响海洋生态系统的两大核心环境胁迫因子。在生态气候变化研究中，多物种、跨营养级（cross-trophic level）且覆盖不同季节的全因子实验（full-factorial experiment）至关重要，此类实验可精准评估两类主要环境胁迫因子对生态系统的直接与间接效应，以及二者的相对重要性。在底栖中型实验生态系统（benthic mesocosm）实验中，我们针对波罗的海沿岸的墨角藻（Fucus vesiculosus）群落，开展了为期一年、覆盖四个季节的实验，探究其对海水温度升高与二氧化碳浓度升高的响应。尽管二氧化碳浓度升高仅产生微弱影响，但升温对植食者群落存在显著且持久的调控作用，且这种作用随季节不同对墨角藻群落产生差异化影响。夏末时节，升温引发的植食者种群崩溃产生了自上而下的级联效应：从消费者传递至建群种（foundation species），最终导致附生藻类（epiphytes）过度覆盖墨角藻叶状体（thalli）。在秋季/冬季，即附生藻类的非生长期，升温下增强的植食作用使得墨角藻生物量显著下降。据此，我们验证了相关预测：未来海水升温将通过改变下行调控作用（top-down control）影响海洋食物网过程，同时也证实食物网结构的具体变化效应存在季节依赖性。鉴于墨角藻是波罗的海主要的建群褐藻生境系统，其在全球变暖背景下的潜在衰退，将导致波罗的海近岸海域丧失多项关键生态系统功能与服务，包括为多样的海洋无脊椎动物与鱼类群落提供营养储存、附着基质、食物来源、庇护场所以及育苗场。