Effects of ultraviolet radiation and CO2 increase on winter phytoplankton assemblages in a temperate coastal lagoon

Increases in ultraviolet radiation (UVR) and CO2 affect phytoplankton growth and mortality in a variety of different ways. However, in situ responses of natural phytoplankton communities to climate change, as well as its effects on phytoplankton annual cycles, are still largely unknown. Although temperature and UVR have been increasing in temperate latitudes during winter, this season is still particularly neglected in climate change studies, being considered a non-active season regarding phytoplankton growth and production. Additionally, coastal lagoons are highly productive ecosystems and very vulnerable to climate change. This study aims, therefore, to evaluate the short-term effects of increased UVR and CO2 on the composition and growth of winter phytoplankton assemblages in a temperate coastal lagoon. During winter 2012, microcosm experiments were used to evaluate the isolated and combined effects of UVR and CO2, under ambient and high CO2 treatments, exposed to ambient UV levels and photosynthetically active radiation (PAR), or to PAR only. Phytoplankton composition, abundance, biomass and photosynthetic parameters were evaluated during the experiments. Significant changes were observed in the growth of specific phytoplankton groups, leading to changes in community composition. The cyanobacterium Synechococcus was dominant at the beginning of the experiment, but it was negatively affected by UVR and CO2. Diatoms clearly benefited from high CO2 and UVR, particularly Thalassiosira. Despite the changes observed in specific phytoplankton groups, growth and production of the whole phytoplankton community did not show significant responses to UVR and/or CO2.

紫外线辐射（UVR）和CO₂浓度的升高会以多种不同方式影响浮游植物的生长与死亡。然而，自然浮游植物群落对气候变化的原位响应（in situ）及其对浮游植物年度周期的影响，目前仍知之甚少。尽管温带地区冬季的温度和UVR一直在上升，但该季节在气候变化研究中仍被严重忽视，被认为是浮游植物生长和生产的非活跃期。此外，沿海潟湖是高生产力生态系统，对气候变化极为脆弱。因此，本研究旨在评估UVR和CO₂浓度升高对温带沿海潟湖冬季浮游植物群落组成及生长的短期影响。2012年冬季，研究人员通过微宇宙实验（microcosm experiments）评估了UVR和CO₂的单独及联合效应，实验设置了环境CO₂和高CO₂处理组，并分别暴露于环境UV水平与光合有效辐射（PAR），或仅暴露于PAR。实验期间对浮游植物的组成、丰度、生物量及光合参数进行了评估。结果显示，特定浮游植物类群的生长出现显著变化，进而导致群落组成改变。蓝细菌聚球藻（Synechococcus）在实验初期占据优势，但受到UVR和CO₂的负面影响。硅藻则明显受益于高CO₂和UVR，尤其是海链藻属（Thalassiosira）。尽管特定浮游植物类群发生了变化，但整个浮游植物群落的生长和生产力并未对UVR和/或CO₂表现出显著响应。