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Environmental changes affecting light climate in Andean Patagonian mountain lakes: Implications for the plankton community
Congreso; Global Change and the World’s Mountains; 2010
Climate warming may have important implications for the light physical structure and consequently for the planktonic microbial food web. Here, we presented evidence of the consequences of these changes in Andean North-Patagonian lakes considering three different scenarios: changes in thermocline depth, glacial melting and ultraviolet radiation (UVR) effect. In temperate lakes, summer stratification is characterized by a wind-mixed surface layer that is isolated from colder deep waters by a marked thermal discontinuity at the metalimnetic level. Wind action is important in determining mixing depth; therefore lake epilimnion can undergo periods of heating during hot and calm weather and periods of strong mixing by wind. Vertical mixing means shortage of light, because planktonic organisms are frequently mixed down to the bottom, while stratification enhances light supply through a decrease in mixing depth. Thus, temporal heterogeneity in vertical mixing can affect diversity and composition of phytoplankton communities. In the North Andean Patagonian region (around 41ºS) there is an extended lake district with both Pacific and Atlantic watersheds. In these lakes, the thermal structure, together with the low nutrient and organic carbon concentrations and the high ultraviolet transparency exert a strong effect on the composition and function of plankton communities. Stratification depth can alter the light supply to phototrophic plankton since the deepening of thermocline, which increases the thickness of the mixing layer, can drag down planktonic organisms to low light levels. Our results indicated that temporal or spatial variations in thermocline depth would imply advantages for one or other mixotrophic ciliate species that dominate the plankton assemblages in these lakes. Stentor araucanus, a UVR resistant species, was present in the epilimnion attaining higher abundances when the thermocline depth was lower and the mean irradiance higher. Ophrydium naumanni showed an opposite pattern preferring the metalimnetic layers and was more abundant in years with deeper thermoclines. S. araucanus requires high light supply to maintain endosymbiotic algal photosynthesis; thus, the lower thermocline depth implies an increase in light supply in the mixolimnion, benefitting this species. On the contrary, O. naumanni abundance decreased in relation with the increase in the light extinction coefficient caused by the numerical increment of S. araucanus. Additionally, significant warming was observed and a decrease in precipitation that corresponds to a drastic glacier recession. In particular, glacier fluctuations on Mount Tronador showed a continuous and maintained recession and these meltwaters are transported to Lake Mascardi via the River Manso Superior which carries large amounts of glacial clay. These suspended sediment particles cause a very sharp light longitudinal gradient along the western branch of Lake Mascardi (Brazo Tronador). Changes in the water transparency caused by these suspended particles were analyzed along the gradient, and an increase in light penetration resulted which was directly related to the depth of the deep chlorophyll maxima. During years with higher inputs, the conformation of chlorophyll maxima occurs at lower depth. Lakes with relatively high availability of solar radiation compared to nutrients will result in carbon rich and phosphorus poor algal biomass, and this in turn, in factors that limit growth at various trophic levels. In that sense, the Andean North Patagonian lakes have been described as high light-low nutrient environments, therefore the plankton community showed a strong nutrient limitation. The high transparency at different wavelengths exhibited by Andean lakes would imply a high exposure to UVR and Photosynthetically Available Radiation (PAR) that can lead to photoinhibition of photosynthetic species or the increase of protected mixotrophic taxa like Stentor araucanus which can benefit from high irradiances at surface levels. Finally, the microbial loop in small (maximum depth < 10 m) ultraoligotrophic alpine Andean lakes (Patagonia, Argentina), located at or above the timberline (> 1600 m a.s.l.) is exposed to high UVR. These lakes exhibited low dissolved organic matter concentrations (less than 1 mg L-1) and low Dissolved Organic Carbon specific absorbance, thus high UV irradiance and transparency. We analysed bacterial morphological distribution (cocci or rods vs. filaments >7μm) and performed a field experiment in which we measured the nanoflagellates grazing rates with natural fluorescently labelled bacteria (including cocci and filaments) in two treatments: PAR and UVR. The relative proportion of filaments to total bacterial biovolume was high in all the lakes at epilimnetic layers. Mixotrophic flagellates such as Chrysochromulina parva and Dinobryon spp were the dominant bacterivorous protists. The field experiment indicated that only bacterial cocci were ingested by protists and that UVR negatively affected the clearance rates. Thus, carbon transfer within the microbial food web will be substantially altered.