Diffuse attenuation coefficients (&) for solar UV radiation (UVR) (305, 320, 340, 380 nm, and PAR) were measured in the mixed layer of 65 lake sites in Alaska, Colorado, and Pennsylvania and the Bariloche region of Argentina. Integrated mixed layer samples of lake water were concurrently collected, and a multivariate approach was used to model Kd with a number of optical and chemical variables.Substantial variation in transparency was observed among lakes. Attenuation depths (zlYO) for UV-B radiation ranged from several centimeters to > 10 m. In some shallow, low DOC (dissolved organic C) lakes typical of high elevation ecosystems, substantial fluxes of UVR penetrated the entire water column. In deeper lakes with low DOC concentrations, high fluxes of UVR were found in a significant proportion of the mixed layer. Much of the among-lake variation in & (87-96%) was explained by differences in DOC concentration, which strongly influenced dissolved absorbance. On average, dissolved absorbance accounted for between 33% (for PAR) and 68% (for 305 nm) of Kd measured in situ. Throughout the solar UV-A and UV-B range, Kd was best estimated with a univariate power model based solely on DOC concentration. Models are also presented that relate absorption coefficients to Kd. These models can be used with archival DOC or color data to provide approximate estimates of UV transparency of lakes.

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We experimentally tested the effect of food quality (phosphorus [P] : carbon [C] ratio) on the response of antioxidant enzymes to ultraviolet radiation (UVR) in Daphnia commutata fed with Chlamydomonas reinhardtii. Algal cultures were grown at different concentrations of phosphorus and light intensities, resulting in significant differences in the P : C ratios (mmol P?[mmol C] 21 ; 6.05, 1.70, and 0.83). After 12 d of D. commutata growth under these three food quality treatments, we observed significant differences in individual biomass and protein content of Daphnia. Subsequently, we carried out an ultraviolet exposure experiment to determine if stoichiometric constraints imposed would limit enzymatic defenses against UVR oxidative stress. The UVR-exposure experiment consisted of a factorial design with three levels of food P : C (low, medium, and high) and two levels of UVR (exposed and protected). The activities of glutathione S-transferases (GST) and catalase (CAT), enzymes involved in protection and repair of damage caused by UVR, were determined. Enzyme activities in the animals exposed to or protected from UVR showed a direct relationship with food P : C ratio that fit exponential models. Although GST and CAT differed slightly in their response to UVR, both enzymes were significantly affected by food quality: In low P : C treatments, there was significantly lower enzyme activity in response to UVR for both enzymes. Low food quality (less P for biosynthesis) may also impose a weaker antioxidant response on the organisms, a response of considerable ecological relevance in transparent Andean lakes which combine high UVR intensities with low seston P : C ratios.

Food webs from oligotrophic Andean lakes of Bariloche region (41°S) are described. Their peculiarities in comparison with Northern Hemisphere studies are noted. The endemic species composition, the extended euphotic zone, and the oxygenated bottom allow a particular structure of the pelagic and benthic food webs. Experimental work with pelagic communities indicates that models based only on zooplankton body size are questionable and that nutrient enrichment does not necessarily lead to a positive effect on herbivorous populations. Invertebrate predation effects depend on the lake and type of predator, while vertebrate predation does control crustacean populations. The characteristic low species richness of top predators is likely to change following the introduction of exotic salmonids. The benthic food web is quite distinctive with a slow rate of decaying organic matter and easily altered by the introduction of exotic tree species.

We measured biomass and primary production of picophytoplankton (PicoPhy: 0.2-2 mm) and of autotrophic size fraction >2 mm in six deep ultraoligotrophic lakes in the Andean-Patagonian region (around 41°S) during summer stratification. Surface Photosynthetically Active Radiation (PAR) ranged from 1277 to 1849 mmol photons m À2 s À1 , and the euphotic zone, generally deeper than the mixed layer, varied between 28 m and 49 m. We found a strong photoinhibiting effect of high PAR and UV-A at surface levels, whereas UV-B radiation (<320 nm) had low extra contribution in the photosynthesis inhibition. As a consequence, cell numbers, Chl a and primary production rates of both fractions increased towards deep layers in all lakes. The photosynthetic efficiency (Chl-specific production per photon unit) of both fractions increased with depth, although this increase was higher in PicoPhy, indicating a higher fitness to low-light. The per cent contribution of PicoPhy production to total production, showed an inverse significant relation with total dissolved phosphorus (TDP). Moreover our data fitted the existing database showing a significant trend towards a decrease of PicoPhy biomass and an increase of its relative contribution to total biomass with decreasing trophic state. At very low-phosphorus concentration, typical of north Patagonian lakes, we found good evidence of the competitive advantage of PicoPhy. Low-light and low TDP may interact to create the most favourable conditions for the smaller photosynthetic organisms. In conclusion, we found that at low-light and very low nutrient regime PicoPhy achieves higher photosynthetic efficiency than the larger autotrophic organisms.

We studied the phylogenetic diversity of nonmarine picocyanobacteria broadening the sequence data set with 43 new sequences of the 16S rRNA gene. The sequences were derived from monoclonal strains isolated from four volcanic high-altitude athalassohaline lakes in Mexico, five glacial ultraoligotrophic North Patagonian lakes and six Italian lakes of glacial, volcanic and morenic origin. The new sequences fall into a number of both novel and previously described clades within the phylogenetic tree of 16S rRNA gene. The new cluster of Lake Nahuel Huapi (North Patagonia) forms a sister clade to the subalpine cluster II and the marine Synechococcus subcluster 5.2. Our finding of the novel clade of 'halotolerants' close to the marine subcluster 5.3 (Synechococcus RCC307) constitutes an important demonstration that euryhaline and marine strains affiliate closely. The intriguing results obtained shed new light on the importance of the nonmarine halotolerants in the phylogenesis of picocyanobacteria.

Introduction of salmonids is a common and widespread practice in rivers and lakes of Patagonia, but their impacts remain poorly understood. We analyse the effect of exotic introduced salmonids (rainbow trout Oncorhynchus mykiss) on the benthic macroinvertebrate community of low order streams. We conducted a field survey in three headwater streams (Challhuaco, Cascada and Pescadero streams) in the northern Andean-Patagonian region (around 41˚S and 1500 m above sea level). The streams are canopied by deciduous Nothofagus pumilio forest. On each system, we established fishless and contiguous fish sites separated by waterfalls that limit fish access to upper sections. At each site we determined benthic macroinvertebrate size, taxonomic and functional structure in addition to trout gut contents. In the presence of trout, we observed significant shifts in invertebrate body size towards smaller individuals, thus a decrease in total macroinvertebrate biomass was observed. We found that large taxa (Klapopteryx kuscheli, Tipula sp.) and active swimming species (Metamonius anceps and Hyalella curvispina) were reduced in abundance or were absent in reaches with trout. At the same time, we found that trout positively selected large size classes of invertebrates. As a consequence of species losses, benthic community structure changed drastically in the presence of trout: shredders were the feeding group most affected negatively (-68 %) followed by scrapers. The reduction of shredders may suggest a potential effect on Nothofagus leaf litter breakdown and hence, supply of FPOM to downstream reaches.

Volcanic eruptions that shape the earth's surface can have major effect on ecosystems and, as natural