Seasonal variations of diatoms in a low-latitude mountain lake: a case study from Douhu lake-Southeast China

In the scientific literature on diatom ecology whether temperature drives changes in diatom assemblage directly or indirectly remains a subject of debate. In order to clarify the relationship between temperature and diatom assemblage composition, we focused on a region with significant seasonal changes but without a freezing period. From March 2016 to March 2018, sediment trap samples were collected monthly to monitor the seasonal succession of diatoms in Douhu Lake, in subtropical southeast China. The results show that in 2016 the variations in abundance of Aulacoseira granulata and Aulacoseira ambigua followed an obvious seasonal pattern, corresponding to the seasonal variation in water and air temperature. In 2016, the thermophilic taxon A. granulata was dominant in the warm season, and in the cold season its abundance declined, while that of A. ambigua, a taxon better adapted to cooler conditions, increased relatively. In 2017, however, there was no such typical seasonal variation. In the cold season of 2017, A. granulata did not decline and there was no significant increase in the cold-preferring A. ambigua. This was related to the large drop in the lake level associated with human activity (substantial water extraction) that occurred during the winter of 2017, which disturbed the natural seasonal fluctuation in water temperature. Thus, in 2017, while air temperature followed the regular seasonal cycle, lake water temperature did not decrease significantly in winter, which allowed A. granulata to maintain a relatively high abundance in the cold season. From these results and an extensive survey of the literature, we conclude that water temperature, more than the seasonal succession, directly drives the fluctuation in abundance of these two Aulacoseira species.