MICROBIOLOGY OF AQUATIC SYSTEMS Seasonal Variability May Affect Microbial Decomposers and Leaf Decomposition More Than Warming in Streams Sofia Duarte 1 & Fernanda Cássio 1,2 & Verónica Ferreira 3 & Cristina Canhoto 4 & Cláudia Pascoal 1,2 Received: 8 December 2015 /Accepted: 4 May 2016 /Published online: 18 May 2016 # Springer Science+Business Media New York 2016 Abstract Ongoing climate change is expected to affect the diversity and activity of aquatic microbes, which play a key role in plant litter decomposition in forest streams. We used a before-after control-impact (BACI) design to study the effects of warming on a forest stream reach. The stream reach was divided by a longitudinal barrier, and during 1 year (ambient year) both stream halves were at ambient temperature, while in the second year (warmed year) the temperature in one stream half was increased by ca. 3 °C above ambient temper- ature (experimental half). Fine-mesh bags containing oak (Quercus robur L.) leaves were immersed in both stream halves for up to 60 days in spring and autumn of the ambient and warmed years. We assessed leaf-associated microbial di- versity by denaturing gradient gel electrophoresis and identi- fication of fungal conidial morphotypes and microbial activity by quantifying leaf mass loss and productivity of fungi and bacteria. In the ambient year, no differences were found in leaf decomposition rates and microbial productivities either be- tween seasons or stream halves. In the warmed year, phospho- rus concentration in the stream water, leaf decomposition rates, and productivity of bacteria were higher in spring than in autumn. They did not differ between stream halves, except for leaf decomposition, which was higher in the experimental half in spring. Fungal and bacterial communities differed be- tween seasons in both years. Seasonal changes in stream water variables had a greater impact on the activity and diversity of microbial decomposers than a warming regime simulating a predicted global warming scenario. Keywords Global warming . Streams . Plant-litter decomposition . Microbial activity and productivity . BACI design Introduction Due to ongoing climate change, global mean air temperature has increased by ca. 0.75 °C since the late nineteenth century and is predicted to increase by 1.5–4.6 °C by the year 2100 [1]. Such increases are likely to be mirrored in stream water temperatures, leading to alterations in flow regimes, distur- bance intensity and frequency, water chemistry, and species interactions, which can strongly compromise the services pro- vided by these ecosystems [2]. In particular, small-forest streams, where water temperature is typically low throughout the year, are expected to be highly vulnerable to temperature increases [3]. In these streams, the decomposition of organic matter provided by the riparian vegetation constitutes the main source of nutrients and energy to aquatic biota [4, 5]. Plant litter decomposition is a dynamic process that links riparian vegetation, environmental conditions, and aquatic biota, espe- cially microbes and invertebrate detritivores [4, 5]. Aquatic Electronic supplementary material The online version of this article (doi:10.1007/s00248-016-0780-2) contains supplementary material, which is available to authorized users. * Sofia Duarte sduarte@bio.uminho.pt 1 Centre of Molecular and Environmental Biology (CBMA), Department of Biology, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal 2 Institute of Science and Innovation for Bio-Sustainability (IB-S), University of Minho, Campus of Gualtar, 4710-057 Braga, Portugal 3 Marine and Environmental Sciences Centre (MARE), Department of Life Sciences, University of Coimbra, 3004-517 Coimbra, Portugal 4 Centre for Functional Ecology (CFE), Department of Life Sciences, University of Coimbra, Calçada Martim de Freitas, 3000-456 Coimbra, Portugal Microb Ecol (2016) 72:263–276 DOI 10.1007/s00248-016-0780-2