Horacio Zagarese

UV Radiation as a Potential Driving Force for Zooplankton Community Structure in Patagonian Lakes

Abstract This article explores the potential role of UV radiation (UVR) as an influence on zooplankton communities. In the first section we provide a general overview of UVR effects on freshwater zooplankton, with an emphasis on Argentine and Chilean environments. In the second section we present the results of a survey involving 53 temperate lakes across a gradient of UVR exposure to determine patterns of species richness and specific diversity. These community characteristics decreased at high potential UVR exposure (i.e. high mean water column irradiance or low lake optical density). A threshold value of mean water column irradiance of approximately 10% of the surface level seems to limit both richness and diversity to minimum values. On the basis of the collected evidence it is not possible to definitely conclude that UVR rather than another covarying factor is responsible for the decrease in specific diversity observed at the lowest end of lake optical depth. However, lakes with values above the previous threshold are likely to exhibit highly depauperate zooplankton communities regardless of the mechanism. As a cautionary note we suggest that changes in the optical characteristics (i.e. changes due to atmospheric conditions, precipitation patterns or vertical displacement of the tree line) may result in sudden shifts in zooplankton community structure.

Constitutive and UV-inducible synthesis of photoprotective compounds (carotenoids and mycosporines) by freshwater yeasts

Twelve yeasts isolated from lakes of Northwestern Patagonia, Argentina, belonging to eight genera (Sporobolomyces, Sporidiobolus, Rhodotorula, Rhodosporidium, Cystofilobasidium, Cryptococcus, Torulaspora, and Candida) were analysed for their ability to produce photoprotective compounds. For this purpose, three laboratory experiments were performed to study the effect of photosynthetically active radiation (PAR) and PAR in combination with UV radiation (PAR + UVR) on the production of carotenoids and mycosporines. The synthesis of carotenoid compounds was clearly stimulated in six out of nine red yeast strains tested upon exposure to PAR or PAR + UVR; however, the latter conditions produced a stronger response than PAR alone. The increase in carotenoids in the red strains under PAR + UVR irradiation showed a negative exponential relationship with their basal carotenoid content, suggesting that cells with higher constitutive levels of carotenoids are less responsive to induction by PAR + UVR. Three red yeasts, Rhodotorula minuta, Rh. pinicola, and Rhodotorula sp., and the colourless Cryptococcus laurentii produced a UV-absorbing compound when exposed to PAR or PAR + UVR. This compound showed an absorption maximum at 309-310 nm and was identified as mycosporine-glutaminol-glucoside (myc-glu-glu). In these strains, exposure to PAR or PAR + UVR resulted in elevated concentrations of both carotenoids and myc-glu-glu. This is the first report on the production of mycosporines by yeasts. All strains that developed under PAR + UVR were able to synthesise carotenoids either constitutively or in response to PAR exposure, and a few of them also produced myc-glu-glu when exposed to PAR. Collectively, our results suggest that the presence of carotenoids, either alone or in combination with mycosporines, are required for sustaining growth under exposure to PAR + UVR in the freshwater yeast strains studied.

Ozone and UV Radiation over Southern South America: Climatology and Anomalies

Abstract Ozone and UV radiation were analyzed at eight stations from tropical to sub-Antarctic regions in South America. Ground UV irradiances were measured by multichannel radiometers as part of the Inter American Institute for Global Change Radiation network. The irradiance channels used for this study were centered at 305 nm (for UV-B measurements) and 340 nm (for UV-A measurements). Results were presented as daily maximum irradiances, as monthly averaged, daily integrated irradiances and as the ratio of 305 nm to 340 nm. These findings are the first to be based on a long time series of semispectral data from the southern region of South America. As expected, the UV-B channel and total column ozone varied with latitude. The pattern of the UV-A channel was more complex because of local atmospheric conditions. Total column ozone levels of <220 Dobson Units were observed at all sites. Analysis of autocorrelations showed a larger persistence of total column ozone level than irradiance. A decreasing cross-correlation coefficient between 305 and 340 nm and an increasing cross-correlation coefficient between 305 nm and ozone were observed at higher latitudes, indicating that factors such as cloud cover tend to dominate at northern sites and that ozone levels tend to dominate at southern sites. These results highlight the value of long-term monitoring of radiation with multichannel radiometers to determine climatological data and evaluate the combination of factors affecting ground UV radiation.

Modeling the Impacts of UV-B Radiation on Ecological Interactions in Freshwater and Marine Ecosystems

Recent data demonstrating global increases in biologically damaging UV-B radiation raise the need for knowledge of how natural communities and ecosystems will respond to these environmental changes. These responses are likely to differ in terrestrial versus aquatic environments, and in freshwater versus marine environments due to fundamental differences in the structure and function of these ecosystems.

Biological Weighting Function for the Mortality of Boeckella gracilipes (Copepoda, Crustacea) Derived from Experiments with Natural Solar Radiation¶

Abstract We performed in situ experiments during the austral summer of 1998 to quantify the mortality of the freshwater copepod Boeckella gracilipes as a function of the UV dose. The copepods were exposed to solar radiation at the water-surface for ∼24–34 h. Long-pass cut-off filters (Schott) were used in the exposure experiments. UV radiation and PAR were measured with an IL-1700 (International Light Inc.) and a PUV-500 radiometer (Biospherical Instruments Inc.). A biological weighting function for UV-induced mortality was calculated by fitting a model based on a logistic curve. Our results show that UV damage in this species is strongly wavelength- and dose-dependent. B. gracilipes was highly vulnerable to both UV-B (290–320 nm) and UV-A radiation (< 360 nm). The shape of the BWF obtained for B. gracilipes resembles more closely the action spectra (AS)† for UV-induced erythema, than the AS for naked DNA.

UVR and its effects on species interactions

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Assessing sublethal effects of ultraviolet radiation in juvenile rainbow trout (Oncorhynchus mykiss)

This paper presents a method for assessing sublethal effects of ultraviolet radiation in fish. The method is based on the assumption that UV-B exposure is stressful to fish. We measured oxygen consumption in UV-B-irradiated rainbow trout juveniles using a UV-transparent respirometer chamber. In addition, we simultaneously monitored fish behavior by videotaping. We found a direct relationship between UV-B irradiance and the percentage increase in oxygen consumption. Increased swimming activity and restless behavior were also noted under UV-B irradiation. The actual mechanisms by which fish perceive UV radiation and a behavioral reaction is triggered are unknown. Based on the present study, a response to UV-A irradiation cannot be ruled out.

Shallow lakes from the Central Plains of Argentina: an overview and worldwide comparative analysis of their basic limnological features

The Central Plains of Argentina is a heterogeneous environment, but the lakes there share some fundamental features: they are all shallow and polymictic as being well exposed to wind. First, we provide a synthesis of the climate, geology, and hydrological network. We also discussed shallow lakes origin and their limnological and biological salient features. Second, we focus on Pampean shallow lakes from a global perspective, comparing the limnological variables: total phosphorus concentration (TP), total nitrogen concentration (TN), chlorophyll a (Chl a) concentrations, and Secchi disk reading (SD) from a compiled database. No significant differences in the Chl a vs. TP relationship were found between Pampean and other shallow lakes. Otherwise, the chlorophyll yield per unit of phosphorus of Pampean lakes is similar to the world shallow lakes average. Moreover, the relationship SD vs. Chl a differed significantly between Pampean and the remaining world lakes, about 50–60%. When confronted against other lakes worldwide, Pampean shallow lakes depart from most of them as having higher TP, TN, and Chl a concentrations and much lower SD transparency, and therefore they stand as extremes of the trophic-state continuum. Despite their highly turbid state, these lakes provide valuable ecosystem services that are highly appreciated and mobilize important economic resources.

Vertical mixing and ecological effects of ultraviolet radiation in planktonic communities.

Abstract We present a mathematical model for a phytoplankton–zooplankton system, based on a predator–prey scheme. The model considers the effects of sinking in the phytoplankton, vertical mixing and attenuation of photosynthetically active radiation (PAR) and ultraviolet radiation (UVR) in the water column. In a first approach, the model was studied under conditions of average PAR irradiance and shows fluctuations and stable equilibrium points. Secondly, we introduced the effects of photoperiod and photoinhibition by UVR and vertical mixing. Under these conditions, the phytoplankton biomass oscillates depending on the combined effects of UVR and mixing. Higher inhibition by UVR and longer mixing periods can induce strong fluctuations in the system but can also produce higher plankton peaks.

Occurrence of photoprotective compounds in yeasts from freshwater ecosystems of northwestern Patagonia (Argentina).

Abstract In this paper we present the results of research on the occurrence, induction and role of photoprotective compounds (PPCs) present in native aquatic yeasts from freshwater Patagonian ecosystems. We focus on the effect of UV radiation (UVR) as a factor that controls the level of photoprotection of yeasts, and explore its potential significance in shaping yeast distributional patterns. The research presented here combines field surveys and laboratory work, including the isolation and culture of native yeasts strains, and laboratory assays under different radiation conditions. The results obtained suggest that yeasts are common dwellers of oligotrophic Patagonian water bodies, and provide the first evidence of the distribution of PPC (carotenoid and mycosporine)–producing yeasts in temperate freshwaters. A greater proportion of carotenogenic yeasts were observed in high-elevation lakes. The yeast strains isolated from these environments were found to produce higher amounts of mycosporines (MYCs), and to present higher tolerance to UVB exposure than those from piedmont lakes. Patagonian yeasts have only one type of MYC, mycosporine-glutaminol-glucoside (myc-glu-glu), which seems common to all other yeasts. By analyzing the production of myc-glu-glu in a large number of yeasts belonging to different taxonomic groups, we propose that this compound may have potential use as a chemotaxonomic marker in yeast systematics. Collectively, our work reveals that in Patagonian freshwater yeasts there is an apparent relationship between the ability to produce PPCs, their tolerance to UV exposure and their success in colonizing habitats highly exposed to UVR.