Abstract
Global-change stressors act under different timing, implying complexity and uncertainty in the study of interactive effects of multiple factors on planktonic communities. We manipulated three types of stressors acting in different time frames in an in situ experiment: ultraviolet radiation (UVR); phosphorus (P) concentration; temperature (T) in an oligotrophic Mediterranean high-mountain lake. The aim was to examine how the sensitivity of phytoplankton and bacterioplankton to UVR and their trophic relationship change under nutrient acclimation and abrupt temperature shifts. Phytoplankton and bacteria showed a common pattern of metabolic response to UVR × P addition interaction, with an increase in their production rates, although evidencing an inhibitory UVR effect on primary production (PP) but stimulatory on bacterial production (HBP). An abrupt T shift in plankton acclimated to UVR and P addition decreased the values of PP, evidencing an inhibitory UVR effect, whereas warming increased HBP and eliminated the UVR effect. The weakening of commensalistic and predatory relationship between phyto- and bacterioplankton under all experimental conditions denotes the negative effects of present and future global-change conditions on planktonic food webs towards impairing C flux within the microbial loop.
Highlights
Global-change stressors act under different timing, implying complexity and uncertainty in the study of interactive effects of multiple factors on planktonic communities
This study was designed to assess the vulnerability of phytoplankton and bacterioplankton communities from a Mediterranean high-mountain lake to the interactive effects of global-change stressors acting at different timing
The low abundance of mixotrophs and absence of ciliates in the lake during this experiment led us to disregard bacterivory as a major control mechanism of bacterial abundance, we cannot rule out a constraint on Bacterioplankton abundance (BA) development imposed by viral lysis[46,47]
Summary
Global-change stressors act under different timing, implying complexity and uncertainty in the study of interactive effects of multiple factors on planktonic communities. The duration of stressors should be taken into account, as these can be chronic (e.g. UVR), pulsed [e.g. nutrient-pulses (pulse = combination of low frequency, large magnitude, and short duration, sensu Yang et al.2)] or abrupt (e.g. heat waves) Changes in these multiple factors may potentially trigger complex interactive effects among them; it is critical to determine their combined impact, which might be stronger (synergistic effect) or weaker (antagonistic effect) than the sum of their individual effects[3,4]. Few experiments have focused on the interactive effects of UVR, nutrients, and temperature on phytoplankton and/or bacterioplankton[37,38,39], and only one report has described the effects of these three factors on phytoplankton-bacterioplankton relationship in Mediterranean high-mountain lakes[39]. The strength of phytoplankton-bacterioplankton commensalistic relationship is quickly modulated by the interactive effect among UVR, nutrient inputs and warming by mismatching phytoplankton-bacteria coupling[39]
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