Uncoupled phytoplanktonbacterioplankton relationship by multiple drivers interacting at diferent temporal scales in a highmountain Mediterranean lake

Abstract

Global-change stressors act under diferent timing, implying complexity and uncertainty in the study of interactive efects of multiple factors on planktonic communities. We manipulated three types of stressors acting in diferent 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 efect 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 efect, whereas warming increased HBP and eliminated the UVR efect. The weakening of commensalistic and predatory relationship between phyto- and bacterioplankton under all experimental conditions denotes the negative efects of present and future global-change conditions on planktonic food webs towards impairing C fux within the microbial loop.