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.
The effects of ultraviolet radiation (UVR) under future expected conditions of acidification and increase in nutrient inputs were studied on a post-bloom phytoplankton and bacterioplankton community of Patagonian coastal waters. We performed an experiment using microcosms where two environmental conditions were mimicked using a cluster approach: present (ambient nutrients and pH) and future (increased nutrients and acidification), and acclimating the samples for five days to two radiation treatments (full solar radiation [+UVR] and exclusion of UVR [–UVR]). We evaluated the short-term (hours) sensitivity of the community to solar UVR through chlorophyll a fluorescence parameters (e.g. the effective photochemical quantum yield of PSII [ΦPSII]) at the beginning, at the mid-point and at the end of the acclimation period. Primary production and heterotrophic bacterial production (HBP) were determined, and biological weighting functions were calculated, at the beginning and at the end of the acclimation period. Mid-term effects (days) were evaluated as changes in taxonomic composition, growth rates and size structure of the community. Although the UVR-induced inhibition on ΦPSII decreased in both clusters, samples remained sensitive to UVR after the 5 days of acclimation. Also, under the future conditions, there was, in general, an increase in the phytoplankton carbon incorporation rates along the experiment as compared to the present conditions. Bacterioplankton sensitivity to UVR changed along the experiment from inhibition to enhancement of HBP, and future environmental conditions stimulated bacterial growth, probably due to indirect effects caused by phytoplankton. Those changes in the microbial loop functioning and structure under future global change conditions might have important consequences for the carbon pump and thus for the carbon sequestration and trophodynamics of Patagonian coastal waters.
The short-and mid-term effects of a simulated global change scenario (i.e., Future) of increased nutrients, acidification, and solar radiation, in the presence or absence of grazers, were evaluated on a freshwater plankton community of Patagonia, Argentina. We used a cluster experimental design with microcosms incubated outdoors simulating the in situ (i.e., Present) and the Future conditions. Short-term changes in net productivity and respiration, together with mid-term changes in the community (abundance, biomass, and phytoplankton cell size) were measured. Phytoplankton had lower net productivity and higher respiration and zooplankton had, in general, higher respiration under the Future than that under the Present condition when organisms were exposed to UVR. The mid-term impacts of the Future condition were neither significant on zooplankton abundances, nor in phytoplankton abundances, biomass, and cell size. Nevertheless, the zooplankton-phytoplankton interaction strength was greater under the Future condition. Zooplankton exerted a strong top-down pressure, regardless of the experimental scenarios, grazing preferentially on small phytoplankton cells, thus decreasing their abundances and biomass. Overall, there were significant short-term impact of our Future global change scenario; however, its effects on midterm time scales were not significant, and indeed, the zooplankton top-down pressure was the main driver that shaped the phytoplankton community.
The original version of this Article contained missing bar chart outlines and missing error bars in all figures. Additionally, in Figure 1 the figure key was incorrect. The individual groups for "Crysophyceae", "Bacillariophyceae" and "Dinophyceae" are now in one group "Others". As a result, the legend of Figure 1 was incorrect. "Sestonic N:P ratio (on a molar basis), phytoplankton abundance (PA) and chlorophyll a (Chl a) concentration under full sunlight (UVR + PAR) and photosynthetically active radiation (PAR) and under ambient phosphorus (P) concentration and Padded conditions. Bars represent the mean values and error bars represent the standard deviation (SD) (n = 3). " now reads: "Sestonic N:P ratio (on a molar basis), phytoplankton abundance (PA) and chlorophyll a (Chl a) concentration under full sunlight (UVR + PAR) and photosynthetically active radiation (PAR) and under ambient phosphorus (P) concentration and Padded conditions. In Fig. 1b, Others group includes Cryptophyceae, Bacillariophyceae and Dinophyceae, and represents a small proportion of the total phytoplankton abundance. Bars represent the mean values and error bars represent the standard deviation (SD) (n = 3). " This has now been corrected in the PDF and HTML versions of the original article.
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