Ultraviolet solar radiation (UVR) and atmospheric nutrient loads to pristine ecosystems are global climate change phenomena that simultaneously affect aquatic organisms in ways not easily predicted by single factor studies. Plankton in a high mountain lake was exposed in situ to increasing phosphorus (P) concentrations (mimicking atmospheric pulses) in absence or presence of UVR in order to identify their interactive effect on functional [primary production, organic carbon (C) release (EOC), and percentage of C released (%EOC)], growth rate, structural-physiological (algal biomass, sestonic C, P content, chlorophyll a (Chl a), and Chl a : C ratio, P cell quota, cell-specific Chl a), and stoichiometric (autotroph C : P ratio) traits. The availability of P after the pulse determined the intensity of responses by primary producers to UVR stress. All structuralphysiological and functional variables significantly increased by up to two orders of magnitude in response to P enrichment. UV radiation, over a long-term scale, exerted significant deleterious effects on most structural-physiological variables when inorganic P was added at high levels (! 30 lg P L À1 ). The subsequent unexpected negative synergistic UVR Â P effect on algal development did not support our initial hypothesis that P input might buffer the harmful UVR effect. UVR exerted a weak negative effect on primary production but strongly enhanced the absolute and percentage excretion of C (up to 60%), mechanism responsible of a significant reduction in autotroph C : P ratios. We propose that low sestonic C : P ratios are the outcome of an adaptive strategy of algae in environments with high UVR exposure and extreme nutrient limitation and have important implications for C flux through grazing vs. microbial food webs in oligotrophic systems.

The responses of heterotrophic microbial food webs (HMFW) to the joint action of abiotic stressors related to global change have been studied in an oligotrophic high-mountain lake. A 2×5 factorial design field experiment performed with large mesocosms for >2 months was used to quantify the dynamics of the entire HMFW (bacteria, heterotrophic nanoflagellates, ciliates, and viruses) after an experimental P-enrichment gradient which approximated or surpassed current atmospheric P pulses in the presence vs. absence of ultraviolet radiation. HMFW underwent a mid-term (<20 days) acute development following a noticeable unimodal response to P enrichment, which peaked at intermediate P-enrichment levels and, unexpectedly, was more accentuated under ultraviolet radiation. However, after depletion of dissolved inorganic P, the HMFW collapsed and was outcompeted by a low-diversity autotrophic compartment, which constrained the development of HMFW and caused a significant loss of functional biodiversity. The dynamics and relationships among variables, and the response patterns found, suggest the importance of biotic interactions (predation/parasitism and competition) in restricting HMFW development, in contrast to the role of abiotic factors as main drivers of autotrophic compartment. The response of HMFW may contribute to ecosystem resilience by favoring the maintenance of the peculiar paths of energy and nutrient-mobilization in these pristine ecosystems, which are vulnerable to threats by the joint action of abiotic stressors related to global change.

Interactions among multiple stressors generated by global change exert cumulative effects on ecosystems. In order to identify the long-term interactive effects of ultraviolet radiation (UVR) and increasing atmospheric nutrient inputs (P-inputs) on aggregate (growth rate, chlorophyll a and abundance) and emergent (evenness and diversity) algal community variables, a 2 Â 5 factorial experiment [two levels of light quality (þUVR, 2UVR) and five levels of nutrient enrichment] was conducted using in situ mesocosms in a high-mountain lake in the Sierra Nevada (Spain) over a 70-day period. Addition of P suppressed and inverted the stimulatory UVR effect on the nonflagellate growth rate (mid-term scale). This interactive effect was propagated as increased harmful UVR effect on chlorophyll a and abundance across the P-gradient. Interestingly, P-pulse reversed the positive effect of UVR on phytoplankton evenness and diversity on the long-term scale. These findings support our hypothesis that the UVR Â P interactive effect would favor a few UVRtolerant rapid-growing species. We show that the algal community is acclimated to UVR-flux but not to high P-pulse. The loss of diversity and disappearance of mixotrophic flagellates may have important implications for the functioning of clear-water ecosystems.

The nature of the resource that limits heterotrophic bacteria, i.e. mineral nutrients or carbon (C), has consequences for biogeochemical cycles in aquatic ecosystems. Our aim was to identify the resource [C or phosphorus (P)] that mainly limits bacteria in a set of 31 Mediterranean inland water ecosystems spanning a wide trophic range. We followed an intersystem observational approach with three complementary perspectives, comparing the bacterial demand with the resource supply in terms of both the quantity (demand : supply ratio for C and P) and quality (C : P ratio of demand and supply), and assessing the relative strength of each resource in controlling bacterial production. The trophic gradient revealed a shift in the main limiting resource for bacteria, from C at the oligotrophic end (typically high-mountain, low-productivity lakes) to mainly P at the eutrophic end (typically nonmountain, high-productivity lakes). The patterns of resource limitation of bacteria found here may be related to the autotrophic nature of most of the studied ecosystems linked to a Mediterranean climate regime as representative of lakes with low inputs of allocthonous C. These patterns are consistent with the theoretical approaches and may potentially shape the contribution of this type of ecosystems to biogeochemical cycles.

In a 32-yr record in oligotrophic Lake La Caldera (Sierra Nevada, Spain) biomass of zooplankton was strongly correlated with precipitation, aerosol deposition intensity, and ultraviolet radiation (UVR). The food associatedeffects of these factors for zooplankton growth were tested in field-laboratory experiments with the aim of separating the effects of food quantity from those of food quality at low food conditions, where there is good evidence to support the existence of food quality effects. Manipulation of nutrients generated a large food quantity gradient that exerted the strongest effect on zooplankton growth, with no significant role of UVR. Growth curves were fitted to a saturation function that reached a plateau at increasing seston levels of ca. 250, 500, and 1000 mg C L 21 for the rotifer Keratella cochlearis, the copepod Mixodiaptomus laciniatus, and the cladoceran Daphnia pulicaria, respectively, and after which growth decreased in M. laciniatus. Nutrients and to a lesser extent UVR also affected seston quality, which had a minor effect on zooplankton growth. K. cochlearis growth was strongly related to the P content of seston, whereas M. laciniatus and D. pulicaria growth were positively correlated with a P-normalized v3-polyunsaturated fatty acid index (v3-PUFA:P). The increase in seston associated with more intense and frequent atmospheric depositions would adversely affect copepods, but improve growth of C-limited cladocerans and P-limited rotifers in pristine ecosystems of the Mediterranean region.