Mixing‐driven changes in distributions and abundances of planktonic microorganisms in a large, oligotrophic lake

Abstract

Temperate lakes experience variation in mixing and stratification that affects the distributions, activities, abundances, and diversity of plankton communities. We examined temporal and vertical changes in the composition of planktonic microorganisms (including Bacteria and Archaea) in oligotrophic Flathead Lake, Montana. Using a combination of approaches that included 16S rRNA gene sequencing and flow cytometric determination of cell abundances, we found that the microbial community was responsive to variations in stratification and mixing at time scales ranging from episodic (scale of days) to seasonal. However, the impact of such physical dynamics varied among taxa, likely reflecting taxa‐specific responses to environmental changes that coincide with stratification and mixing (e.g., light availability and nutrient supply). During the early spring, periods of relatively short‐term (< 7 d) intermittency in stratification and mixing influenced the vertical distributions of specific microbial taxa, notably including the cyanobacteria. These events highlight time scales of biological responses to high‐frequency variations associated with lake stratification and mixing, particularly during the transition to the growing season in the early spring.

Out of sight, but not out of season: Nitrifier distributions and population dynamics in a large oligotrophic lake

“…MAG, metagenome-assembled genome; RPKM, reads per kilobase million reads. (Klotz, 1998;Norton et al, 1996), assuming a single gene copy per cell and using total cell abundances previously reported (Evans et al, 2024), maximum abundances of each clade represent 3.4%, and 2.9% of the total cellular community, with a maximum total combined abundance of $6.0% in September at 90 m. Ammonium monooxygenase genes deriving from the archaeal Nitrososphaerota were the least abundant of the targeted groups (peak of $5000 copies mL À1 ; 1% of all cells with maxima December-March). Overall, the amoA gene distributions showed similar seasonal and depth-specific patterns as that obtained based on 16S rRNA gene amplicon sequencing, with the Nitrosomonadaceae most abundant in the hypolimnion during the summer, while Nitrososphaerota peaked in the fall and winter.…”

“…The R (R Core Team, 2020) package akima (v0.6-2.3; Akima et al, 2016) was used to interpolate temperature between depths and time points. The mixed layer depth was calculated as the depth where temperature differed by 0.5 C from the average temperature between 0.5 and 3.5 m (Evans et al, 2024). Photosynthetically active radiation (PAR; 400-700 nm) was measured using an underwater spherical quantum sensor .…”

“…Total N concentrations are typically $8 μM while total phosphorus (P) levels are $0.1 μM, leading to elevated N:P stoichiometric ratios (Elser et al, 2022). The microorganisms in Flathead Lake appear highly seasonal, with the onset of stratification leading to clear changes in community composition and diversity (Evans et al, 2024). Here, we describe the seasonality and distribution of inorganic N pools and nitrifying organisms in this largely pristine, oligotrophic lake.…”

“…Water collection and DNA extraction of samples from MLD during 2017-2018 were performed as previously described (Evans et al, 2024;Peoples et al, 2023).…”

“…PCR amplification and sequencing of 16S rRNA gene amplicons were performed as previously described (Evans et al, 2024). Briefly, the 16S rRNA gene V4-V5 region was amplified using the primers 515F-Y (5 0 -GTGYCAGCMGCCGCGGTAA) and 926R (5 0 -CCGY CAATTYMTTTRAGTTT) using reaction conditions previously recommended (Parada et al, 2016).…”