Effect of UVR on lake water and macrophyte leachates in shallow Andean lakes: optical features, nutrient availability and bacteria activity.

BASTIDAS NAVARRO, M.,; BALSEIRO, E.; MODENUTTI, B.

Punta del Este, Uruguay

Congreso; Shallow Lakes Congress; 2008

Univ. Rep. Uruguay

In shallow lakes with a well developed littoral zone, macrophytes could be a substantial source of dissolved organic matter (DOM).  Macrophytes generate DOM either through extracellular release or following aging and subsequent release of dissolved and particulate constituents to the surrounding water. Thus, in shallow lakes this DOM source may be important for bacteria in the carbon cycling.  The interaction of DOM and ultraviolet radiation (UVR) results in a progressive loss of colour, decreased spectrophotometric absorbance and lower average molecular size.  Molecular weight and size was indicated as an important issue influencing the microbial utilization of DOM, therefore photoalteration caused by short-wavelength solar radiation can be an important factor driving resource bioavailability for bacteria growth. In the present study, we evaluated the effect of UVR exposure on macrophyte leachates and lake water from two Andean shallow lakes, analyzing changes on: 1- optical features of DOM; 2- Soluble Reactive Phosphorous (SRP) and 3- bacteria abundance, activity and assemblage composition.The study was carried out in two shallow lakes of the Northern Patagonia (Argentina): Lakes Escondido and El Trébol, both lakes are colonized by Scirpus californicus and Potamogenton linguatus. We conducted laboratory experiments in which lake water and macrophytes leachates were exposed or protected to UVR (UVA-340 Q-Panel) in quartz tubes or aluminium foil wrapped. After the exposition we carried out spectrophotometric scans and SRP determination. Finally we performed another experiment in order to analyse bacteria response to leachate addition and UVR-DOM interaction. The lake water and macrophyte leachates changed their optical features when exposed to UVR. In both cases, the slope of the absorbance curve in the UV region of the spectrum (250-400 nm) was increased after the exposure and the absorbance at short wavelength (<300 nm) was increased. After UV exposure the a250: a365 ratio increased, indicating a decrease in the mean molecular size of DOM. We also observe an increase in the SRP release in natural lake water of El Trébol, while in Lake Escondido we observed a decrease of it. In the incubation experiments, the natural bacteria assemblages showed a positive response in abundance and production to leachate addition and UVR exposed DOM. Natural bacteria assemblages of both lakes were dominated by β-Proteobacteria. Nevertheless, we recorded an important increase in Actinobacteria at the end of the incubation in the non-exposed lake water of both lakes. Cytophaga-like bacteria also increased in non-exposed lake water from Lake Escondido, and in UVR exposed lake water of Lake El Trébol. In all cases, leachates favoured the dominance of β-Proteobacteria.