Temporal and spatial differences of the under-ice microbiome are linked to light transparency and chlorophyll-a

Abstract

Under-ice community dynamics are barely understood. Temporal and spatial studies are needed to fully understand the consequences of a declining ice cover on microbial biodiversity. Here, bacterial communities of different years (2015, 2017–2021) and layers (upper and lower euphotic layer, euphotic layer, hypolimnion) were assessed by Illumina sequencing of the 16S rRNA gene. Alpha- and beta-diversity of summer and under-ice hypolimnetic communities were similar, and a seasonal difference was found only when excluding summer hypolimnetic communities. Similarly, in non-metric multidimensional scaling (NMDS), summer and under-ice communities were different even though hypolimnetic communities were similar. Investigating under-ice conditions, the year 2017 showed highest under-ice light and chlorophyll-a while 2021 showed no under-ice light and lowest chlorophyll-a. Under-ice communities were not linked to layer differences implying that a spatial distinction under ice was less important than in summer, especially in years with little or no under-ice light. Most under-ice bacterial classes and ASVs showed direct and indirect dependencies on light availability and primary production. Similarly in NMDS with only under-ice communities, light transparency and primary production were important. In the future, ice conditions with less snow cover might lead to bacterial communities similar to that of high-light years (2017, 2018, 2020).

Multi-annual comparisons of summer and under-ice phytoplankton communities of a mountain lake

“…Therefore, our first hypothesis of seasonally different phytoplankton communities was rejected based on NMDS, a multivariate analysis. These results were in contrast to Obertegger (2022) finding with NMDS different bacterial communities during summer and under-ice in the upper layers of Lake Tovel. In contrast to community composition, phytoplankton beta-diversity was higher under ice than during summer, confirming our first hypothesis of seasonal differences.…”

“…While an under-ice high-light condition does not exclude some light in the lower layer, the underlying factors for the layer similarity remained unknown and was only indicated by algal composition. Beta-diversity showed seasonal differences for the upper layer but not for the lower layer, and this might be related to the environmental stability of the lower layer as already suggested for bacterial communities (Obertegger, 2022). Also, the distribution of size classes showed similarities and differences Vol:.…”

“…In summer, 1% of light reaches a depth of approximately 23 m (Cellamare et al, 2016). Chl-a fluorescence profiles indicate that algae are widely distributed along the water column under ice (Obertegger, 2022) as postulated by the lake ice continuum concept (Cavaliere et al, 2021). Therefore, we were confident that the upper layer reflects the habitat of most phytoplankton, both during summer and under ice.…”

“…In contrast to community composition, phytoplankton beta-diversity was higher under ice than during summer, confirming our first hypothesis of seasonal differences. This, again, was in contrast to bacterial communities that showed no seasonal differences in beta-diversity over 6 years in Lake Tovel (Obertegger, 2022). Phytoplankton and heterotrophic bacteria are linked in multiple ways such as competition for nutrients, use of algal exudates, decomposition of decaying algae, or mixotrophic predation of algae on bacteria (Bižić-Ionescu et al, 2014;Salcher, 2014;Unrein et al, 2014;Ivanova et al, 2018).…”

“…Northern European lakes experience a shortening of ice-cover length with important implications for lake physics, chemistry, and biology (Sharma et al, 2020). For example, autumn mixing reaches deeper water layers with later ice-in (Flaim et al, 2020), lake surface water temperature is driven by winter ice cover (O'Reilly et al, 2015), phytoplankton composition changes in response to winter severity (Beall et al, 2016), and bacterioplankton composition responds to changes in underice conditions (Obertegger, 2022).…”