Ultraviolet solar radiation (UVR) and atmospheric nutrient inputs associated with aerosols are major worldwide stressors that simultaneously affect species and the interaction among them. A 2 3 5 field experimental design was used to determine how variations in light regimes (presence and absence of UVR [+UVR and 2UVR]) and nutrients might influence the strength of phytoplankton-zooplankton coupling (PZC). We observed unimodal curves for zooplankton biomass in response to increased food supply from nutrient enrichment. These results challenge the ''more is better (or at least never worse)'' concept, since high food levels resulted in weakened PZC. The effect of UVR on zooplankton was nutrient dependent, significantly reducing zooplankton abundance at intermediate phosphorus (P) supplied levels but not at the two ends of the trophic gradient generated (control and highest P level). Neither food quantity nor food quality explained observed differences in zooplankton biomass between light treatments, suggesting a deleterious direct effect of UVR on zooplankton at intermediate food ranges, resulting in a weakening of PZC. The location of this lake in the Mediterranean region has shown an increasing intensity and frequency of aerosol depositions over the past three decades , resulting in higher phytoplankton biomass. A combination of these higher atmospheric dust depositions with the high UVR levels characteristic of high mountain lakes might underlie the interannual decoupling between phytoplankton and zooplankton dynamics observed in these oligotrophic ecosystems.

An in situ experimental assessment was made of the effects of the interaction between spectral composition of solar radiation and the limiting nutrient (phosphorous, P) on the algae‐bacteria link, measured over the short term (1.5 h after P enrichment) in an oligotrophic and autotrophic high mountain lake. Variables related to the metabolism of algae (primary production, excretion of photosynthetic carbon) and bacteria (bacterial production, percentages of photosynthetic exudates assimilated and used for bacterial production) were studied. P enrichment suppressed or attenuated the negative effects exerted by ultraviolet radiation on algae when their elemental composition was P deficient. This was reflected in antagonistic interactive effects between P enrichment and solar radiation (P × R), which were triggered by a decrease in primary production and an increase in organic carbon excretion due to metabolic adjustments to growth. P enrichment also suppressed or attenuated the main effects exerted by ultraviolet radiation on bacteria. This was reflected in antagonistic P ×R interactive effects triggered by an enhancement of the dual control (resource and predation) that algae exerted on bacteria and by the growth stimulation of P‐deficient bacteria after P enrichment. All of these observed responses contribute to improving the food quality for herbivores and reinforce the flux of carbon and nutrient to the grazing chain, which explains its development in this and other clear‐water ecosystems.

The food web structure in some high mountain lakes deviates from the established tendency of high heterotrophic bacteria: phytoplankton biomass ratios in oligotrophic ecosystems. Thus, the microbial food web in La Caldera Lake is weakly developed, and bacteria constitute a minor component of the plankton community in terms of abundance, biomass and production. Autotrophic picoplankton is absent, and heterotrophic microbial food web is weakly developed compared to a grazing chain dominated by calanoid copepods and a phytoplankton community mainly composed of mixotrophic flagellates. In order to explain the singular food web structure of this lake, functional, stoichiometric and taxonomical approaches are followed to assess, on various temporal and spatial scales, the relevance of stressful abiotic factors (ultraviolet solar radiation and P-limitation) on the structure and functioning of this ecosystem. P-availability was the main factor controlling the algal biomass whereas bacterial P-limitation was a transient phenomenon. The algae-bacteria relationship was predominately commensalistic. In contrast to algae, full-sunlight radiation had no negative effect on bacterial growth but rather enhanced bacterial dependence on the carbon released by algae. The prevalence of the commensalistic-mutualistic relationship and the development of a more complex microbial food web were related to the stoichiometry of algae and bacteria (N:P ratios). The microbial food web only developed at balanced algal and bacterial N:P ratios, with the appearance of ciliates after a nutrient pulse. However, mixotrophic algae dominated the planktonic community under P-deficit conditions, and they were the main factor controlling bacterioplankton. Their regulatory effect has a dual nature: (i) a resource-based control, where bacteria depend on the photosynthetic carbon released by algae, i.e., a commensalistic interaction ("without you I cannot live"); and (ii) a predatory control, where bacteria is a prey for mixotrophs ("with you I die"). Hence, the niche of microheterotrophs (nanoflagellates and ciliates) is occupied by mixotrophs, and there is a resulting simplification of the planktonic structure. With respect to the carbon cycle, mixotrophic bacterivory constitutes a "by-pass" for the flux of C towards the grazing chain, precluding the development of a complex heterotrophic microbial food web. Mixotrophs thereby improve the energetic transfer efficiency in high mountain lakes through a reduction in the number of trophic levels. Antagonistic effects of UVR x P interactions on the algae-bacteria relationship were caused by an enhancement of dual (resource and predation) control. Based on these results, an alternative model for the flux of C in autotrophic high mountain lakes has been proposed.

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