Nitrogen limitation in Alpine lakes
In the Alps, like in other mountain regions across the world, N enrichment because of atmospheric deposition is a major environmental concern. What are the effects on microphytobenthos and grazers in high-elevation lakes? To find out, we conducted nutrient and grazing assays in two near-natural lakes in the French Alps. In each lake, nutrient-diffusing substrates, placed within cages that allowed or prevented access by herbivore invertebrates, were deployed for approximately three weeks during the summer of 2012. We tested the hypotheses that: [H1] the lakes are N limited, [H2] N enrichment alters microphytobenthos taxonomic composition, and [H3] N enrichment alters grazing pressure by invertebrates.
Supporting H1, in both lakes, by the end of the assays, substrata enriched with N had greater biomass of microphytobenthos than unenriched controls. Supporting H2, the taxonomic composition of the microphytobenthos differed between N-enriched substrata and controls. The taxa responsible for the differences varied, with N enrichment favoring the cyanobacteria Anabaena and the green algae Mougeotia in one lake and the diatoms Gyrosigma and Diatoma in the other. In contrast, H3 was not supported, as N enrichment and grazing had no interactive effect on microphytobenthos biomass. Our assays indicate that N enrichment in Alpine lakes can alter the accrual and composition of microphytobenthos, which likely contributes most primary production in these environments. Although grazing was not affected during our assays, long-term effects on consumers remain possible, and these will be investigated next.