Alpine waters are sentinels of environmental change. The Macun lakes form a high-alpine ( >2,600 m asl) cirque landscape (3.6 km2) in the Swiss National Park, comprising 26 small lakes together with a number of temporary ponds. Streams interconnect the four largest lakes, forming the drainage network consisting of a north and a south basin that overlie an ortho-gneiss, meta-granitoid bedrock. The south basin is influenced by various rock glaciers. Diatom assemblages and physico-chemistry of surface waters at 10 sites along the network have been monitored annually in mid-summer since 2001. Sites included lake inlets and outlets as well as the outlet stream of the drainage. Diatoms were identified to species using standard protocols. Water physico-chemistry was analyzed for N, P, POC, DOC, TIC, pH, silicate and spot measures recorded temperature, conductivity, and turbidity. The main objective was to examine long-term patterns of diatom assemblages of sites over the 19 year study period. Diatoms were compared between basins, lake inlets and outlets, and for lake order effects using various multivariate analyses and in relation to environmental measures.
Results showed clear diatom and physico-chemical differences between the two basins, which mostly reflect rock-glacier inputs in the south basin. Diatoms comprised 355 species in the network. Nitrogen values were two-fold higher and particulate phosphorus values two-fold lower in the south basin than in the north basin. Over time, the physico-chemistry in the two basins became more homogeneous, with a reduction in rock-glacial inputs in the south basin and an overall decrease in nitrogen in the catchment. Continuous temperature records showed north basin streams to be ca. 3°C warmer than south-basin streams, but with high inter-annual variation that reflected annual differences in weather and no evidence of a consistent change over time (increase or decrease). A major temporal shift in diatom assemblages occurred between pre-2010 samples and post-2010 samples, suggesting a tipping point occurred in the catchment. Diatom results also showed differences between lake inlets and outlets, north basin sites in particular, as well as a lake order effect. The results highlight the importance of long-term monitoring for understanding ecosystem state changes in alpine freshwaters, especially during periods of rapid environmental change.