Predicted warming and browning affect timing and magnitude of plankton phenological events in lakes: a mesocosm study ALICE NICOLLE, PER HALLGREN, JESSICA VON VON EINEM, EMMA SOFIA KRITZBERG, WILHELM GRANE ´ LI, ANDERS PERSSON, CHRISTER BRO ¨ NMARK AND LARS-ANDERS HANSSON Institute of Biology ⁄ Aquatic Ecology, Lund University, Lund, Sweden SUMMARY 1. Aquatic ecosystems in Northern Europe are expected to face increases in temperature and water colour (TB) in future. While effects of these factors have been studied separately, it is unknown whether and how a combination of them might affect phenological events and trophic interactions. 2. In a mesocosm study, we combined both factors to create conditions expected to arise during the coming century. We focused on quantifying effects on timing and magnitude of plankton spring phenological events and identifying possible mismatches between resources (phytoplankton) and consumers (zooplankton). 3. We found that the increases in TB had important effects on timing and abundance of different plankton groups. While increased temperature led to an earlier peak in phytoplankton and zooplankton and a change in the relative timing of different zooplankton groups, increased water colour reduced chlorophyll-a concentrations. 4. Increased TB together benefitted cladocerans and calanoid copepods and led to stronger top-down control of algae by zooplankton. There was no sign of a mismatch between primary producers and grazers as reported from other studies. 5. Our results point towards an earlier onset of plankton spring growth in shallow lakes in future with a stronger top-down control of phytoplankton by zooplankton grazers. Keywords: climate change, lakes, phenology, phytoplankton, spring, temperature, water colour, zooplankton Introduction Zooplankton play a key role in lake ecosystems, as they transfer energy from primary production to higher trophic levels and thereby control the abundance of phytoplankton (Sommer et al., 1986). It is therefore important to explore the vulnerability of this group of organisms to future environmental changes. Tempera- ture-related effects of climate change on both phytoplank- ton and zooplankton have been investigated in a rapidly increasing number of studies (Straile, 2000; Straile & Adrian, 2000; Weyhenmeyer, Westoeoe & Willen, 2008). While early-spring development of phytoplankton seems to primarily depend on light as mediated through the effect of mixing depth in deeper lakes (Straile, 2000; Berger et al., 2007; Sommer & Lengfellner, 2008), zoo- plankton development is to a large extent dependent on temperature and resource availability (Lampert, 1977; Orcutt & Porter, 1983; Mueller-Navarra, Guess & Von Storch, 1997; Berger et al., 2007). Several authors argue that this discrepancy in temperature sensitivity might lead to a situation of mismatch between phytoplankton density and the need for resources by zooplankton in deep lakes, as early lake stratification and phytoplankton blooms are decoupled from zooplankton spring growth (George & Harris, 1985; Winder & Schindler, 2004; Winder et al., 2009). Whether shallow lakes, where stratification pro- cesses are of little importance for phytoplankton growth, are in danger of developing a mismatch situation between phytoplankton and zooplankton is unclear. Correspondence: Alice Nicolle, Institute of Biology ⁄ Aquatic Ecology, Lund University, Ecology Building, SE-22362 Lund, Sweden. E-mail: alice.nicolle@biol.lu.se Freshwater Biology (2012) 57, 684–695 doi:10.1111/j.1365-2427.2012.02733.x 684 Ó 2012 Blackwell Publishing Ltd