High-frequency diatom dynamics seen in an ice- and snow-covered temperate lake using an imaging-in-flow cytometer.

The study of winter in temperate, ice-covered lakes has largely been neglected, creating a major gap in our understanding of annual phytoplankton cycles. We assessed patterns in biovolumes of Asterionella, cf. Synedra, Urosolenia, and total phytoplankton at three depths during the winter of 2014/2015 as estimated using images from an Imaging FlowCytobot (IFCB) moored in Lac (Lake) Montjoie, Quebec (Canada). Even though the use of the IFCB under-ice presented challenges, these were outstripped by its advantages. The IFCB provided unprecedented high temporal and spatial resolution phytoplankton count and biovolume data that allowed patterns of abundance to be observed in detail. Interestingly, the IFCB captured an abundance of Urosolenia, which may be dissolved in standard Lugol's-preserved grab samples. We found Asterionella grew until mid-December and then slowly decreased towards the spring, whereas cf. Synedra diatoms decreased from early December and then rebounded towards late December as light increased before falling again towards the spring. Urosolenia peaked in late December as cf. Synedra diatoms rebounded and then began to decline towards the spring. Total phytoplankton declined towards late December, increased in the beginning of January and then declined towards the spring. The studied diatoms maintained considerable under-ice seed populations and grew when light was sufficient.

Supplementary information: The online version contains supplementary material available at 10.1007/s10750-025-05802-8.