ORIGINAL PAPER Climate change effects on phytoplankton depend on cell size and food web structure Toni Klauschies • Barbara Bauer • Nicole Aberle-Malzahn • Ulrich Sommer • Ursula Gaedke Received: 18 October 2011 / Accepted: 17 February 2012 Ó Springer-Verlag 2012 Abstract We investigated the effects of warming on a natural phytoplankton community from the Baltic Sea, based on six mesocosm experiments conducted 2005–2009. We focused on differences in the dynamics of three phytoplankton size groups which are grazed to a variable extent by different zooplankton groups. While small-sized algae were mostly grazer-controlled, light and nutrient availability largely determined the growth of medium- and large-sized algae. Thus, the latter groups dominated at increased light levels. Warming increased mesozooplankton grazing on medium-sized algae, reducing their biomass. The biomass of small-sized algae was not affected by temperature, probably due to an interplay between indirect effects spreading through the food web. Thus, under the higher temperature and lower light levels anticipated for the next decades in the southern Baltic Sea, a higher share of smaller phytoplankton is expected. We conclude that considering the size structure of the phyto- plankton community strongly improves the reliability of projections of climate change effects. Introduction Marine phytoplankton contribute to the biological regula- tion of the climate and provide half of the world’s primary production (Baumert and Petzoldt 2008; Boyce et al. 2010). As primary producers, phytoplankton supplied the energy basis of pelagic and benthic food webs, and potential changes in the structure and dynamics of marine phytoplankton communities under climate change are a reason for concern. According to the IPCC report (2007), regions in the higher latitudes of the Northern Hemisphere are expected to experience the most pronounced changes during the next 100 years, including earlier warming in the spring, which is the most decisive season in the yearly phytoplankton development. It is therefore particularly relevant to study the response of phytoplankton commu- nities to increased temperature during the spring, to esti- mate effects of future climate change on aquatic food webs. Boyce et al. (2010) proposed that increasing sea surface temperatures resulted in the recent global decrease in phytoplankton biomass. However, the observed patterns strongly differed at the regional scale, for example in the western Baltic Sea. Phytoplankton biomass decreased by 50 % since 1979 in the Kattegat (Henriksen 2009), whereas it increased by a factor of two in the Kiel Fjord over the past 100 years (Wasmund et al. 2008). This example points to the need to better understand the processes that regulate Communicated by R. Adrian. T. Klauschies (&)  B. Bauer  U. Gaedke Institute of Biochemistry and Biology, University of Potsdam, Am Neuen Palais 10, 14469 Potsdam, Germany e-mail: tklausch@uni-potsdam.de B. Bauer e-mail: barbara.bauer@uni-potsdam.de U. Gaedke e-mail: gaedke@uni-potsdam.de B. Bauer  U. Sommer Helmholtz Centre for Ocean Research Kiel (GEOMAR), Du ¨sternbrooker Weg 20, 24105 Kiel, Germany U. Sommer e-mail: usommer@ifm-geomar.de N. Aberle-Malzahn Alfred Wegener Institute for Polar and Marine Research at Biologische Anstalt Helgoland, Kurpromenade C-47, 27498 Helgoland, Germany e-mail: Nicole.Aberle-Malzahn@awi.de 123 Mar Biol DOI 10.1007/s00227-012-1904-y