Abstract
Climate change scenarios predict that lake water temperatures will increase up to 4°C and rainfall events will become more intense and frequent by the end of this century. Concurrently, supply of humic substances from terrestrial runoff is expected to increase, resulting in darker watercolor (“brownification”) of aquatic ecosystems. Using a multi-seasonal, low trophic state mesocosm experiment, we investigated how higher water temperature and brownification affect plankton community composition, phenology, and functioning. We tested the hypothesis that higher water temperature (+3°C) and brownification will, a) cause plankton community composition to shift toward small sized phytoplankton and cyanobacteria, and, b) extend the length of the growing season entailing higher phytoplankton production later in the season. We demonstrate that the 3°C increase of water temperature favored the growth of heterotrophic bacteria and small sized autotrophic picophytoplankton cells with significantly higher primary production during warmer fall periods. However, 3X darker water (effect of brownification) caused no significant changes in the plankton community composition or functioning relative to control conditions. Our findings reveal that increased temperature change plankton community structure by favoring smaller sized species proliferation (autotrophic phytoplankton and small size cladocerans), and increase primary productivity and community turnover. Finally, results of this multi-seasonal experiment suggest that warming by 3°C in aquatic ecosystems of low trophic state may cause planktonic food web functioning to become more dominated by fast growing, r-trait species (i.e., small sizes and rapid development).
Highlights
Global change poses multifaceted threats for aquatic ecosystems
Dissolved organic carbon (DOC) concentrations increased from the end of July (Fig 1C), coinciding with the onset of phytoplankton growth, and remained higher in all treatments compared to C during the entire experiment
There was no significant difference in dissolved organic carbon (DOC) concentrations among the treatments during each sampling throughout the 8 months experiment (1.5–14.5 μg L-1)
Summary
Global change poses multifaceted threats for aquatic ecosystems. It is expected that water temperatures of lakes can increase by 4°C and precipitation events will change their frequency and intensity during the century [1]. Predicted warmer fall and winter periods can induce higher and prolonged phytoplankton productivity [15] and extend the active phase of zooplankton that keep grazing on phytoplankton [10,16,17,18] as far as nutrients are not limiting. Further to these findings, it is not known yet how darker watercolor as a result of increased precipitation and recharge of drainage water (additional effect of climate change) affects phytoplankton production later in the season
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