Vulnerability of mixotrophic algae to nutrient pulses and UVR in an oligotrophic Southern and Northern Hemisphere lake

P Carrillo,M S Souza,B E Modenutti,M Villar-Argaiz,J M Medina-Sánchez,C Durán,F J Bullejos,E G Balseiro,M Bastidas-Navarro

doi:10.1038/s41598-017-06279-9

Abstract

Nutrient inputs and ultraviolet radiation (UVR) are global factors affecting the structure and functioning of aquatic ecosystems, particularly clear-water ecosystems. We performed experiments in two model lakes highly exposed to UVR fluxes in order to test the effect that future increases in mineral nutrients transported by dust aerosol might exert on primary producers depending on the likelihood of atmospheric inputs. Lake La Caldera (Northern Hemisphere) has been receiving recurrent dust inputs from the Sahara Desert while lake Los Cántaros (Southern Hemisphere) has been less affected by dust aerosol. UVR × Nutrient synergistically stimulated primary production (PP), chlorophyll a (Chl a), with a smaller increase in phytoplanktonic biomass in La Caldera, but not in Los Cántaros, where nutrient addition unmasked the UVR inhibitory effect on phytoplankton. The proportional decrease of mixotrophic nanoflagellates (MNFs) after the nutrient pulse (in Los Cántaros) and the long-term decline of MNFs in La Caldera associated with the increase in aerosol-dust intrusions from the Sahara during the last 40 years suggest that a future scenario of intensified aerosol events from desert and desertified areas would not only reduce functional diversity with the decline of MNFs, but would ultimately alter the C flux towards the grazing chain in oligotrophic ecosystems.

Highlights

Because information on the interaction modes and target sites for many stressors is still controversial, future studies should focus on variables that, by representing key biological processes, help identify early ecosystem responses to multiple stressors[15]
The most striking feature of the time-series for the percentage of mixotrophic nanoflagellates (MNFs) in La Caldera, UVR324nm irradiance, and events aerosol index (AI) > 1 is the opposite trend found between MNFs and events AI > 1 in La Caldera lake (Fig. 1)
This prediction is based on (i) the proportional decrease of MNFs after a nutrient pulse (Fig. 2 inset) along with the decrease of the bacterivory rates after nutrient pulse (Supplementary Fig. S3); both findings are in agreement with the notion that MNFs are better adapted to P-limited and stressful light conditions than are strict autotrophs[12, 41]; (ii) the loss of MNFs after P pulses reported in previous studies through an experimental nutrient-addition gradient[12, 42] or along a natural gradient of increasing trophic state[43]; and (iii) the inverse relationship between MNFs and the intensity of aerosol events registered over a long-term scale (1973–2014 years) in lake La Caldera (Fig. 1)

Summary

Introduction

Because information on the interaction modes and target sites for many stressors is still controversial, future studies should focus on variables that, by representing key biological processes, help identify early ecosystem responses to multiple stressors[15]. Relevant global factors affecting the C flux in ecosystems are, among others, ultraviolet radiation (UVR) and changes in nutrient inputs brought about by both climate change (e.g. atmospheric deposition, wind, precipitation) and increased human activity[20, 21] In remote areas such as Alpine lakes, the main nutrient inputs are dust-aerosol transport by wind from source areas (e.g. a desert[22]). It has been shown that the P-addition unmasks the deleterious effect of UV on primary producers in clear oligotrophic freshwater[24, 34,35,36] and marine ecosystems[37] These results open new questions concerning the joint role of UVR and nutrients in clear-water ecosystems that demand further research: Do ecosystems differ in their responses based on their likelihood to receive atmospheric inputs? These results open new questions concerning the joint role of UVR and nutrients in clear-water ecosystems that demand further research: Do ecosystems differ in their responses based on their likelihood to receive atmospheric inputs? What are the mechanistic bases for the generated responses? How might changes in climatic regimes (e.g. drought frequency) affect biodiversity in response to nutrient inputs to aquatic ecosystems?

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Discussion

Conclusion