Environmental changes affecting light climate in Andean Patagonian mountain lakes: Implications for the plankton community

B. MODENUTTI; E. BALSEIRO; M. BASTIDAS NAVARRO; C. LASPOUMADERES; F. CUASSOLO

Global Change and the World’s Mountains - Extended Abstracts

Global Change and the World’s Mountains

Año: 2010; p. 633 - 634

Climate warming may have important implications for the light physical structure andconsequently for the planktonic microbial food web. Here, we presented evidence of theconsequences of these changes in Andean North-Patagonian lakes considering threedifferent 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 thatis isolated from colder deep waters by a marked thermal discontinuity at the metalimneticlevel. Wind action is important in determining mixing depth; therefore lake epilimnion canundergo periods of heating during hot and calm weather and periods of strong mixing bywind. Vertical mixing means shortage of light, because planktonic organisms are frequentlymixed down to the bottom, while stratification enhances light supply through a decrease inmixing depth. Thus, temporal heterogeneity in vertical mixing can affect diversity andcomposition of phytoplankton communities. In the North Andean Patagonian region (around41ºS) there is an extended lake district with both Pacific and Atlantic watersheds. In theselakes, the thermal structure, together with the low nutrient and organic carbon concentrationsand the high ultraviolet transparency exert a strong effect on the composition and function ofplankton communities. Stratification depth can alter the light supply to phototrophic planktonsince the deepening of thermocline, which increases the thickness of the mixing layer, candrag down planktonic organisms to low light levels. Our results indicated that temporal orspatial variations in thermocline depth would imply advantages for one or other mixotrophicciliate species that dominate the plankton assemblages in these lakes. Stentor araucanus, aUVR resistant species, was present in the epilimnion attaining higher abundances when thethermocline depth was lower and the mean irradiance higher. Ophrydium naumanni showedan opposite pattern preferring the metalimnetic layers and was more abundant in years withdeeper thermoclines. S. araucanus requires high light supply to maintain endosymbiotic algalphotosynthesis; thus, the lower thermocline depth implies an increase in light supply in themixolimnion, benefitting this species. On the contrary, O. naumanni abundance decreased inrelation with the increase in the light extinction coefficient caused by the numerical incrementof S. araucanus.633 .Additionally, significant warming was observed and a decrease in precipitation thatcorresponds to a drastic glacier recession. In particular, glacier fluctuations on MountTronador showed a continuous and maintained recession and these meltwaters aretransported to Lake Mascardi via the River Manso Superior which carries large amounts ofglacial clay. These suspended sediment particles cause a very sharp light longitudinalgradient along the western branch of Lake Mascardi (Brazo Tronador). Changes in the watertransparency caused by these suspended particles were analyzed along the gradient, and anincrease in light penetration resulted which was directly related to the depth of the deepchlorophyll maxima. During years with higher inputs, the conformation of chlorophyll maximaoccurs at lower depth. Lakes with relatively high availability of solar radiation compared tonutrients will result in carbon rich and phosphorus poor algal biomass, and this in turn, infactors that limit growth at various trophic levels. In that sense, the Andean North Patagonianlakes have been described as high light-low nutrient environments, therefore the planktoncommunity showed a strong nutrient limitation. The high transparency at differentwavelengths exhibited by Andean lakes would imply a high exposure to UVR andPhotosynthetically Available Radiation (PAR) that can lead to photoinhibition ofphotosynthetic species or the increase of protected mixotrophic taxa like Stentor araucanuswhich can benefit from high irradiances at surface levels.Finally, the microbial loop in small (maximum depth < 10 m) ultraoligotrophic alpine Andeanlakes (Patagonia, Argentina), located at or above the timberline (> 1600 m a.s.l.) is exposedto high UVR. These lakes exhibited low dissolved organic matter concentrations (less than 1mg L-1) and low Dissolved Organic Carbon specific absorbance, thus high UV irradiance andtransparency. We analysed bacterial morphological distribution (cocci or rods vs. filaments>7μm) and performed a field experiment in which we measured the nanoflagellates grazingrates with natural fluorescently labelled bacteria (including cocci and filaments) in twotreatments: PAR and UVR. The relative proportion of filaments to total bacterial biovolumewas high in all the lakes at epilimnetic layers. Mixotrophic flagellates such asChrysochromulina parva and Dinobryon spp were the dominant bacterivorous protists. Thefield experiment indicated that only bacterial cocci were ingested by protists and that UVRnegatively affected the clearance rates. Thus, carbon transfer within the microbial food webwill be substantially altered.