Warming has stronger direct than indirect effects on benthic microalgae in a seaweed system in spring

Using outdoor mesocosms we investigated the relative importance of the direct and indirect (here: altered grazing) effects of seawater warming on benthic microalgae in a Baltic Sea Fucus vesiculosus (Phaeophyceae) system during the spring season. Seawater warming had a direct positive effect on microalgal total biomass accrual and growth rate and on total mesograzer abundance and biomass. Moreover, under the existing resource-replete conditions in spring the direct positive effect of warming on microalgae was stronger than the indirect negative effect through enhanced grazing. The outcome of this study contrasts previous observations from the summer and winter season, where indirect effects of warming mediated by altered grazing were identified as an important driver of primary biomass in the Fucus system. In this context, the results from the spring season add mechanistic information to the overall understanding of the seasonal variability of climate change effects. They suggest that the relative importance of the underlying direct and indirect effective pathways of warming and the overall effect on the balance between production and consumption are influenced by the trophic state of the system, which in temperate regions is related to season.本研究借助室外中型实验生态系统（outdoor mesocosms），于春季在波罗的海墨角藻（Fucus vesiculosus，褐藻门Phaeophyceae）群落生境中，探究了海水升温对底栖微藻的直接与间接（本研究中指摄食行为改变）影响的相对重要性。海水升温对微藻总生物量积累、生长速率，以及中型牧食者的丰度与生物量均产生了直接正向效应。此外，在春季现有资源充足的环境条件下，升温对微藻的直接正向效应强于其通过增强牧食作用引发的间接负向效应。本研究结果与夏、冬季的既往观测结果相悖：在夏、冬季的墨角藻群落生境中，由摄食行为改变介导的升温间接效应，被证实是调控初级生物量的关键驱动因子。在此背景下，本春季研究的结果为气候变化效应的季节变异规律的整体认知补充了机制层面的信息。研究结果表明，升温所依托的直接与间接作用通路的相对重要性，以及其对生产与消费间平衡的整体效应，均受系统营养状态的调控；而在温带区域，系统营养状态与季节紧密相关。