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...

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 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...

SUMMARY 1. The response of bacterial production (measured as [3H]TdR incorporation rate) to spectral solar radiation was quantified experimentally in an oligotrophic high‐mountain lake over 2 years. Bacterial responses were consistent: ultraviolet‐B (UVB) was harmful, whereas ultraviolet‐A (UVA) + photosynthetically active radiation (PAR) and PAR enhanced bacterial activity. Full sunlight exerted a net stimulatory effect on bacterial activity in mid‐summer but a net inhibitory effect towards the end of the ice‐free period. 2. Experiments were undertaken to examine whether the bacterial response pattern depended on the presence of algae and/or was modulated by the availability of a limiting inorganic nutrient (phosphorus, P). In the absence of algae, [3H]TdR incorporation rates were significantly lower than when algae were present under all light treatments, and the consistent bacterial response was lost. This suggests that the bacterial response to spectral solar radiation depends on fresh‐C released from algae, which determines the net stimulatory outcome of damage and repair in mid‐summer. 3. In the absence of algae, UVB radiation inhibited bacteria when they were strongly P‐deficient (mean values of N : P ratio: 46.1), whereas it exerted no direct effect on bacterial activity when they were not P‐limited. 4. P‐enrichment of lake water markedly altered the response of bacteria to spectral solar radiation at the end of ice‐free period, when bacteria were strongly P‐deficient. Phosphorus enrichment suppressed the inhibitory effect of full sunlight that was observed in October, both in whole lake water (i.e. including algae) and in the absence of algae. This indicates that the bacterial P‐deficiency, measured as the cellular N : P ratio, was partly responsible for the net inhibitory effect of full sunlight, implying a high bacterial vulnerability to UVB.

Work to date has established that consumers are constrained in their chemical content variability. Such a constraint generates many different kinds of ecological relationships ranging from aspects of animal mineral nutrition to factors affecting consumer‐driven nutrient recycling. Although previous studies have shown variation in zooplankton nutrient content within and between taxa, most stoichiometric studies assume that consumers are homeostatic in their elemental composition. In this paper, this assumption is reexamined as we describe the variability in the elemental content of the calanoid copepod Mixodiaptomus laciniatus Lilljeborg throughout its ontogeny (interstage variability) and for its specific developmental stages (intrastage variability). Mean copepod carbon content as dry mass increased significantly during ontogeny from 36.3% in nauplii to 51.5% in immature copepodites and 55.8% in adults. Mean phosphorus content decreased from 0.98% in nauplii to 0.87% in immature copepodites and 0.51% in adults. The copepod molar C:N:P ratio was 99:3:1 for nauplii, 165:13:1 for immature copepodites, and 234:25:1 for adults. These ontogenetic variations in stoichiometry within a single copepod species are comparable to the previously described range between taxa. Stoichiometric and physiological approaches are used as complementary tools to explain the two‐part life history strategy of this copepod. First, the nauplii and early copepodite stages (CI–CIV) with high carbon demands adopt a growth strategy during which carbon sources are mainly allocated to increases in body size. The susceptibility of these copepodite stages to carbon limitation is reflected in the decline in the carbon content for a given stage (e.g., from 66% to 38% C in CII) and in the 40‐fold decrease in neutral‐to‐polar lipid ratio toward the end of the ice‐free season, suggesting a mobilization of food reserves toward growth. In contrast, copepodite stage CV and adults with less carbon demands for growth followed a survival–reproductive strategy, where the invariability in carbon content as dry mass (near 60%) and in neutral‐to‐polar lipid ratio through time indicates no accumulation of storage lipids, but instead an allocation of resources toward maintenance or reproduction. The basic assumption of predictions of stoichiometric models that elemental composition remains constant with time are not supported by changes in the intrastage carbon and nitrogen contents of Mixodiaptomus laciniatus. Thus, we suggest that the idea of strict homeostasis may be replaced with the concept of rheostasis, which allows the homeostatic regulatory mechanisms of an organism to adjust to a changing environment over time.

Abstract. Global change, together with human activities, has resulted in increasing amounts of organic material (including nutrients) that water bodies receive. This input further attenuates the penetration of solar radiation, leading to the view that opaque lakes are more "protected" from solar ultraviolet radiation (UVR) than clear ones. Vertical mixing, however, complicates this view as cells are exposed to fluctuating radiation regimes, for which the effects have, in general, been neglected. Furthermore, the combined impacts of mixing, together with those of UVR and nutrient inputs are virtually unknown. In this study, we carried out complex in situ experiments in three high mountain lakes of Spain (Lake Enol in the National Park Picos de Europa, Asturias, and lakes Las Yeguas and La Caldera in the National Park Sierra Nevada, Granada), used as model ecosystems to evaluate the joint impact of these climate change variables. The main goal of this study was to address the question of how short-term pulses of nutrient inputs, together with vertical mixing and increased UVR fluxes modify the photosynthetic responses of phytoplankton. The experimentation consisted in all possible combinations of the following treatments: (a) solar radiation: UVR &amp;plus; PAR (280–700 nm) versus PAR (photosynthetically active radiation) alone (400–700 nm); (b) nutrient addition (phosphorus (P) and nitrogen (N)): ambient versus addition (P to reach to a final concentration of 30 μg P L−1, and N to reach N:P molar ratio of 31); and (c) mixing: mixed (one rotation from surface to 3 m depth (speed of 1 m 4 min−1, total of 10 cycles)) versus static. Our findings suggest that under ambient nutrient conditions there is a synergistic effect between vertical mixing and UVR, increasing phytoplankton photosynthetic inhibition and excretion of organic carbon (EOC) from opaque lakes as compared to algae that received constant mean irradiance within the epilimnion. The opposite occurs in clear lakes where antagonistic effects were determined, with mixing partially counteracting the negative effects of UVR. Nutrient input, mimicking atmospheric pulses from Saharan dust, reversed this effect and clear lakes became more inhibited during mixing, while opaque lakes benefited from the fluctuating irradiance regime. These climate change related scenarios of nutrient input and increased mixing, would not only affect photosynthesis and production in lakes, but might also further influence the microbial loop and trophic interactions via enhanced EOC under fluctuating UVR exposure.

Work to date has established that consumers are constrained in their chemical content variability. Such a constraint generates many different kinds of ecological relationships ranging from aspects of animal mineral nutrition to factors affecting consumerdriven nutrient recycling. Although previous studies have shown variation in zooplankton nutrient content within and between taxa, most stoichiometric studies assume that consumers are homeostatic in their elemental composition. In this paper, this assumption is reexamined as we describe the variability in the elemental content of the calanoid copepod Mixodiaptomus laciniatus Lilljeborg throughout its ontogeny (interstage variability) and for its specific developmental stages (intrastage variability). Mean copepod carbon content as dry mass increased significantly during ontogeny from 36.3% in nauplii to 51.5% in immature copepodites and 55.8% in adults. Mean phosphorus content decreased from 0.98% in nauplii to 0.87% in immature copepodites and 0.51% in adults. The copepod molar C:N:P ratio was 99:3:1 for nauplii, 165:13:1 for immature copepodites, and 234:25:1 for adults. These ontogenetic variations in stoichiometry within a single copepod species are comparable to the previously described range between taxa. Stoichiometric and physiological approaches are used as complementary tools to explain the two-part life history strategy of this copepod. First, the nauplii and early copepodite stages (CI-CIV) with high carbon demands adopt a growth strategy during which carbon sources are mainly allocated to increases in body size. The susceptibility of these copepodite stages to carbon limitation is reflected in the decline in the carbon content for a given stage (e.g., from 66% to 38% C in CII) and in the 40-fold decrease in neutral-to-polar lipid ratio toward the end of the ice-free season, suggesting a mobilization of food reserves toward growth. In contrast, copepodite stage CV and adults with less carbon demands for growth followed a survival-reproductive strategy, where the invariability in carbon content as dry mass (near 60%) and in neutralto-polar lipid ratio through time indicates no accumulation of storage lipids, but instead an allocation of resources toward maintenance or reproduction. The basic assumption of predictions of stoichiometric models that elemental composition remains constant with time are not supported by changes in the intrastage carbon and nitrogen contents of Mixodiaptomus laciniatus. Thus, we suggest that the idea of strict homeostasis may be replaced with the concept of rheostasis, which allows the homeostatic regulatory mechanisms of an organism to adjust to a changing environment over time.