Ecoenzyme activity ratios reveal interactive effects of nutrient inputs and UVR in a Mediterranean high-mountain lake

To understand how atmospheric dust deposition and ultraviolet radiation (UVR) can affect remote, freshwater ecosystems through changes in their microbial metabolism, it is important to have tools that allow us detecting alterations and anticipating potential shifts in the functioning of microbial communities. Ecoenzyme activities (EA) are easy to measure and their ratios can be used to assess system microbial metabolism of freshwater bodies, thus evaluating the effects of global change stressors. We carried out an in situ full factorial experiment to determine how the interaction between the addition of C and P, and UVR affect the microbial metabolism of a Mediterranean high-mountain lake. Overall, activities of five ecoenzymes involved in the degradation of C-compounds and in the acquisition of N and P revealed that, under natural conditions, the growth of heterotrophic prokaryotes was dependent on organic compounds released by algae, which is consistent with a higher constraint of bacterial carbon production by C than by P or N, as suggested by EA ratios. Accordingly, the addition of a labile C source did not lead to any significant response of microbial communities, but the addition of P provoked a clear change in the microbial metabolism of the lake, promoting the growth of phytoplankton and leading heterotrophic prokaryotes to be more constrained by P, and to a lesser extent by N, in relation to C. UVR played a secondary role, probably because microbial communities inhabiting high-mountain lakes possess several evolutionary adaptations to high UVR levels. Changes in the microbial metabolism of our model lake under different scenarios of nutrient inputs and UVR can therefore be evaluated by EA ratios.