Abstract
Global warming profoundly impacts the functioning of aquatic ecosystems. Nonetheless, the effect of warming on primary producers is poorly understood, especially periphyton production, which is affected both directly and indirectly by temperature-sensitive top-down and bottom-up controls. Here, we study the impact of warming on gross primary production in experimental ecosystems with near-realistic foodwebs during spring and early summer. We used indoor mesocosms following a temperate temperature regime (control) and a warmed (+4 °C) treatment to measure biomass and production of phytoplankton and periphyton. The mesocosms’ primary production was dominated by periphyton (>82%) during the studied period (April-June). Until May, periphyton production and biomass were significantly higher in the warm treatment (up to 98% greater biomass compared to the control) due to direct temperature effects on growth and indirect effects resulting from higher sediment phosphorus release. Subsequently, enhanced grazer abundances seem to have counteracted the positive temperature effect causing a decline in periphyton biomass and production in June. We thus show, within our studied period, seasonally distinct effects of warming on periphyton, which can significantly affect overall ecosystem primary production and functioning.
Highlights
One of the major processes potentially altered by global warming is primary production
Epipelon gross primary production (GPP), quantified by strips that rested on the sediment, was much lower than GPP produced by wall periphyton and showed no distinct temporal dynamics (Fig. 1)
Phytoplankton GPP was highest in March in both treatments and decreased with an earlier decline in the warm treatment, which coincided with an advanced activity by fungal parasites[41,42]
Summary
One of the major processes potentially altered by global warming is primary production. Warming can elevate primary productivity as the rate of most subcellular reactions increase exponentially with temperature following the Van’t Hoff-Arrhenius relationship, wherein the calculated activation energy quantifies the change in reaction rate with temperature (Boltzmann 1872, Arrhenius 1889, as described in Allen et al.[6]) Increases in both biodiversity and biomass of planktonic algae in direct response to warming have been reported[7]. Bottom-up effects may change during warming due to increased nutrient release from sediments[34,35] and increased nitrogen loss by denitrification[36], due to increased macrophyte surface for periphyton colonization[37], and due to decreased light availability by enhanced phytoplankton growth[3] These effects may differ in time leading to contrasting net effects of warming on periphyton biomass and production, yet studies with comprehensive within-system spatial and temporal resolution are lacking. Whether enhanced invertebrate grazing on periphyton in warmed treatments can reverse this trend leading to seasonally changing net effects of warming on GPP within the measurement period
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