Nutrient limitation affects biofilm enzymatic activities in a glacier-fed river

In the Patagonian Andes, glaciers of Tronador Mountain are headwaters of Manso River that drains glacial waters into Lake Mascardi. The river receives different tributaries that differ in turbidity and canopy. In this scenario we performed a field study to elucidate the effect of the input of tributary streams on the elemental limitation of the biofilm of the main glacier-fed river. We analyzed through general additive models and standardized major axis regressions the activities of two extracellular enzymes, β- glucosidase (GLU) and alkaline phosphatase (APA). We hypothesized that their ratio would reflect stoichiometric constraints to changes in the relative availability of carbon and phosphorus due to tributary streams’ influence. Along the glacier-fed river, we observed a decrease in the glacial influence and a spatial shift in P and dissolved organic carbon availability, that affected biofilm enzymatic activity. High enzymatic activity of GLU and APA in biofilm was related to low nutrient fluxes in stream water. In addition, the analysis of GLU and APA showed a switch in elemental limitation from Climitation in headwaters to P-limitation downstream. Finally, our results showed that biofilm enzymatic activity is a good ecological indicator to analyze changes in glacier retreat and their consequences in glacier-fed streams.