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（ultraviolet radiation, UVR）与二氧化碳浓度升高会通过多种途径影响浮游植物的生长与死亡率。然而，自然浮游植物群落对气候变化的原位响应，及其对浮游植物年周期的影响，目前仍知之甚少。尽管温带纬度地区冬季的气温与紫外线辐射均呈上升趋势，但该季节在气候变化研究中仍常被忽视，过往研究常将其视为浮游植物生长与生产力的非活跃期。此外，沿海泻湖属于高生产力生态系统，且极易受到气候变化的影响。因此，本研究旨在评估紫外线辐射与二氧化碳浓度升高对温带沿海泻湖冬季浮游植物群落组成与生长的短期影响。2012年冬季，研究采用微宇宙实验，设置环境CO₂浓度与高CO₂浓度两个处理组，分别给予自然紫外线水平与光合有效辐射（photosynthetically active radiation, PAR）的复合照射，或仅给予光合有效辐射照射，以探究紫外线辐射与二氧化碳浓度升高的单独及联合效应。实验过程中，研究人员对浮游植物的群落组成、丰度、生物量及光合参数进行了测定。结果显示，特定浮游植物类群的生长发生了显著变化，进而导致群落组成发生改变。实验初期占优势的蓝藻聚球藻（Synechococcus）会受到紫外线辐射与高CO₂的负面影响。硅藻则明显从高CO₂与紫外线辐射中获益，尤以海链藻属（Thalassiosira）为甚。尽管特定浮游植物类群的丰度出现了上述变化，但整个浮游植物群落的生长与生产力并未对紫外线辐射及/或CO₂浓度升高表现出显著响应。