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 role of spectral composition of solar radiation has seldom been considered as a critical factor controlling the algae-bacteria relationship. A coupled algae-bacteria relationship mediated by C released from algae was observed during a 2-yr period (1996)(1997) in an oligotrophic high mountain lake, except at upper depths. The intensity of photosynthetically active radiation was negatively related to primary production, and the highest percentages of excretion of organic carbon (%EOC) from algae were found at upper depths of the water column. The effect of different spectral regions of solar radiation on the algae-bacteria relationship was assessed by in situ experiments, in which the exposure, tracer uptake by target organisms, and interactions among abiotic and biotic factors were simultaneous. Primary production was ultraviolet radiation (UVR) inhibited by 33-83% depending on depth and date, with ultraviolet-A radiation (UVA) exerting the main effect. EOC and %EOC yielded highest values under UVR exposure. Sunlight affected bacterial production (BP) only at upper depths. UVB inhibited BP by 39-82%, whereas UVA ϩ photosynthetic active radiation (PAR) and PAR enhanced BP three-to fourfold. Full sunlight increased BP 2.5-fold in midsummer but inhibited it (37%) in the late open-water period. The percentage of photosynthetic exudates assimilated by bacteria, and photosynthetic carbon use efficiency by bacteria, showed a similar pattern to that of BP. The experimental results support our hypothesis that increased organic C release from UV-stressed algae stimulates bacterial growth if the bacteria are relatively well adapted to sunlight, determining a coupled algae-bacteria relationship. Thus, sunlight may play a key role as the underlying abiotic factor that regulates algae-bacteria interaction in shallow and clear-water ecosystems.The degradation of Earth's ozone layer, one of the main causes of global climate change, has allowed an increase in solar ultraviolet-B (UVB) radiation fluxes on the Earth's surface (Crutzen 1992) and aquatic ecosystems (Karentz et al. 1994; Häder 1997). The pelagic planktonic community functions through a web of energy and nutrient exchanges, mediated by a diverse array of producers and consumers that ultimately depend on the energy supplied by sunlight. Thus, alterations in spectral composition of solar radiation can modify the structure (Vinebrooke and Leavitt 1999) and functioning of the pelagic food web. In order to understand the C cycle in aquatic ecosystems, we need to explore the 1 To whom correspondence should be addressed. Present address: Departamento de Biología Animal y Ecología, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain (pcl@ugr.es). AcknowledgmentsWe are grateful to R. Sommaruga for his useful criticism on an earlier draft of this study and for the spectral UV measurements and DOC determinations, and to F. Figueroa for making the air UV radiation measurements available. We also thank I. Reche and Morales-Baquero for their comments on me...

The literature proposes that the microbial loop is a key link for ecosystem functioning, particularly in oligotrophic conditions. From original and published data for a period spanning 1986 to 1999, we examined the food web structure of a clear-water, oligotrophic, high mountain lake. The heterotrophic microbial food web was weakly developed in a grazing food chain dominated by copepods and phytoplankton, the latter mainly composed of mixotrophic flagellates. Bacteria constituted a minor component of the plankton community in terms of abundance, biomass, and production, in contrast to the situation usually reported in oligotrophic conditions. Abiotic and biotic factors that regulate bacterioplankton biomass and production were assessed, including the bacterivory capability of mixotrophic flagellates experimentally quantified by using 3 H-thymidine as a tracer. Algae were the main factor controlling bacterioplankton. Their regulatory effect has a dual nature: (1) a resource-based control, through the dependence of the bacteria on photosynthetic carbon released by algae, i.e., a commensalistic interaction (''without you I cannot live'') and (2) a predatory control, with bacteria as prey for mixotrophs (''with you I die''). Mixotrophic metabolism can constitute an adaptive strategy for algae to overcome ultraviolet (UV) stress, by using bacteria as a source of carbon and mineral nutrients in conditions of inhibited photosynthesis and mineral nutrient uptake. Mixotrophy acts as a bypass of carbon flux toward the grazing food chain, explaining the scarce development of the heterotrophic microbes in this and other high mountain lakes.The role of bacteria in ecosystem functioning, as a link between dissolved organic matter (DOM) and higher trophic levels, appears to strengthen in more oligotrophic conditions, where bacterial : algal biomass and bacterial : primary production ratios tend to be higher than in eutrophic ecosystems (Gasol et al. 1997;Biddanda et al. 2001;Cotner and Biddanda 2002).Within an oligotrophic gradient, high mountain lakes over the tree line are exposed to low temperatures, high ultraviolet irradiation, long ice and snow cover period, and fluctuating hydrology. These abiotic factors may influence the development and dynamics of bacterioplankton (Straskrabová et al. 1999). The generally low concentration of dissolved organic carbon (DOC) in clear-water high mountain lakes (commonly Ͻ1 mg L Ϫ1 , see Laurion et al. 2000), and its potentially low quality or bioavailability due to photoreactivity and photoalteration processes (Benner and Biddanda 1998;Reche et al. 2001), may make carbon the main limiting nutrient to bacterial growth. Moreover, the interaction of low mineral nutrient and/or DOC concentrations with 1 Corresponding author (jmmedina@ugr.es). AcknowledgmentsWe sincerely acknowledge L. Cruz-Pizarro, R. Morales-Baquero, P. Sánchez-Castillo, and I. Reche for their contribution to the database. We thank M. J. Villalba and J. A. Delgado for assistance in the field and Richard Davies for...

1. The inter‐ and intra‐annual changes in the biomass, elemental (carbon (C), nitrogen (N) and phosphorus (P)) and taxonomical composition of the phytoplankton in a high mountain lake in Spain were studied during 3 years with different physical (fluctuating hydrological regime) and chemical conditions. The importance of internal and external sources of P to the phytoplankton was estimated as the amount of P supplied via zooplankton recycling (internal) or through ice‐melting and atmospheric deposition (external). 2. Inter‐annual differences in phytoplankton biomass were associated with temperature and total dissolved phosphorus. In 1995, phytoplankton biomass was positively correlated with total dissolved phosphorus. In contrast, the negative relationship between zooplankton and seston biomass (direct predatory effects) and the positive relationship between zooplankton P excretion and phytoplankton biomass in 1997 (indirect P‐recycling effects), reinforces the primary role of zooplankton in regulating the total biomass of phytoplankton but, at the same time, encouraging its growth via P‐recycling. 3. Year‐to‐year variations in seston C : P and N : P ratios exceeded intra‐annual variations. The C : P and N : P ratios were high in 1995, indicating strong P limitation. In contrast, in 1996 and 1997, these ratios were low during ice‐out (C : P < 100 and N : P < 10) and increased markedly as the season progressed. Atmospheric P load to the lake was responsible for the decline in C : P and N : P ratios. 4. Intra‐annual variations in zooplankton stoichiometry were more pronounced than the overall differences between 1995 and 1996. Thus, the zooplankton N : P ratio ranged from 6.9 to 40.1 (mean 21.4) in 1995, and from 10.4 to 42.2 (mean 24.9) in 1996. The zooplankton N : P ratio tended to be low after ice‐out, when the zooplankton community was dominated by copepod nauplii, and high towards mid‐ and late‐season, when these were replaced by copepodites and adults. 5. In 1995, the minimum demands for P of phytoplankton were satisfied by ice‐melting, atmospheric loading and zooplankton recycling over 100%. In order of importance, atmospheric inputs (> 1000%), zooplankton recycling (9–542%), and ice‐melting processes (0.37–5.16%) satisfied the minimum demand for P of phytoplankton during 1996 and 1997. Although the effect of external forces was rather sporadic and unpredictable in comparison with biologically driven recycle processes, both may affect phytoplankton structure and elemental composition. 6. We identified three conceptual models representing the seasonal phosphorus flux among the major compartments of the pelagic zone. While ice‐melting processes dominated the nutrient flow at the thaw, biologically driven processes such as zooplankton recycling became relevant as the season and zooplankton ontogeny progressed. The stochastic nature of P inputs associated with atmospheric events can promote rapid transitional changes between a community limited by internal recycling and one regulated by external load. 7. The elem...

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