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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.

Summary Thousands of lakes in the Northern Hemisphere are experiencing a continuous increase in water temperature and colour. While increasing temperature is an effect of climate change, several factors are suggested to drive the increasing water colour, including climate change, altered land use and reversed acidification. In this mesocosm study, we study the effects on pelagic production and sedimentation of organic carbon from a 3 °C increase in temperature, a doubling of water colour and a combination of increased temperature and water colour. These manipulations correspond to the predicted development in water colour and temperature in northern temperate systems within the life time of the next generation. The water and plankton communities in the mesocosms were taken from an oligohumic and mesotrophic lake, and the increase in water colour was achieved by the additions of water from a humic lake. The experiment ran from early March to late October. In accordance with our predictions and previous cross‐system analyses, net carbon production (NCP) and organic carbon sedimentation decreased in response to increasing water colour. At odds with current theory, NCP and sedimentation increased in heated treatments, as not only respiration but also gross production increased in response to temperature. There were no synergistic effects between warming and browning. These results imply that warming may enhance pelagic delivery of organic carbon to sediments. This could potentially offset a reduction in burial efficiency of organic carbon, which has been projected based on a positive relationship between carbon mineralisation in sediments and temperature.