Seasonal diatom variability of Yunlong Lake, southwest China - a case study based on sediment trap records

Yunlong Lake, a subtropical alpine lake in southwest China, was selected for a sediment trap diatom study between September 2013 and August 2015. Combining the results of meteorological records, water temperature monitoring and water chemistry sampling, we draw the following conclusions: water temperature monitoring indicated that the entire upper 7m of the water column was mixed throughout the year, reflecting the lake's strong mixing characteristics; this ensured the year-round growth of planktonic species and the high abundance of benthic species. In addition, the diatoms exhibited a pronounced biannual warm-cold seasonal pattern, which may be related to water temperature, Si nutrient concentration and thermal stratification. During the warm season (May-November) when the lake was almost stratified, with overall higher water temperature and Si concentration, the representative species were Aulacoseira granulata var. angustissima and Achnanthidium catenatum. During the cold season (December-April), the water temperature and Si nutrient concentration were lower, but stronger lake mixing occurred, and the blooming species included Aulacoseira ambigua, Discostella asterocostata and Lindavia balatonis. After a detailed analysis, we conclude that the seasonal diatom succession was primarily driven by water temperature, and comparison with other lake records indicates that the strong mixing mechanism and Si were not the determinants of the lake diatom seasonal succession. Our data also show that during the first cold season, diatoms bloomed in the following sequence: A. ambigua - D. asterocostata - Discostella stelligera - L. balatonis - Discostella pseudostelligera, as Si decreased over the season, indicating that smaller species were more competitive in obtaining limited nutrients. Finally, there were interannual differences between diatom assemblages in the second year, especially in the abundance of L. balatonis and the occurrence of Asterionella formosa, which were probably related to the significantly increased lake trophic status.