Response of Cyclotella species to nutrients and incubation depth in Arctic lakes

The relative abundances of small centric diatoms have increased in many Arctic lakes over the past century, with these changes commonly attributed to warming. However, the specific mechanisms by which diatom community structure is changing in response to warming remain unclear. We investigated the responses of two common centric diatoms to nutrient enrichment and incubation depth, the latter used to manipulate light availability which is a key factor that changes with altered mixing depths in lakes. We conducted 2 × 2 factorial experiments, manipulating nutrients (none added or N + P addition) and incubation depth (shallow or deep), and measured changes in growth rates and cell densities of Discostella stelligera and Puncticulata radiosa. A second set of experiments was conducted on D. stelligera in a growth chamber to separate temperature and light effects associated with incubation depth. Puncticulata radiosa was always more abundant in the shallow depth incubations, regardless of nutrient conditions. In contrast, D. stelligera responded most strongly to nutrient additions, and cell densities of this species were affected by interactions between nutrients and incubation depth or light. Our research suggests that processes that alter light availability (such as water clarity and water column stability) and nutrient concentrations are likely to play a major role in controlling the growth of small centric diatoms in Arctic lakes.