Recent work suggests that microbial community composition
in high-elevation lakes is significantly influenced by microbes
entering from upstream terrestrial and aquatic habitats.
To test this idea, we conducted 18S and 16S rDNA surveys of
microbial communities in a high-alpine lake in the Colorado
Rocky Mountains. We compared the microbial community
of the lake to water entering the lake and to uphill soils that
drain into the lake. Utilizing hydrological and abiotic data,
we identified potential factors controlling microbial diversity
and community composition. Results show a diverse community
entering the lake at the inlet with a strong resemblance
to uphill terrestrial and aquatic communities. In contrast, the
lake communities (water column and outlet) showed significantly
lower diversity and were significantly different from
the inlet communities. Assumptions of neutral community
assembly poorly predicted community differences between
the inlet and lake, whereas “variable selection” and “dispersal
limitation” were predicted to dominate. Similarly, the lake
communities were correlated with discharge rate, indicating
that longer hydraulic residence times limit dispersal, allowing
selective pressures within the lake to structure communities.
Sulfate and inorganic nitrogen and phosphorus concentrations
correlated with community composition, indicating
“bottom up” controls on lake community assembly. Furthermore,
bacterial community composition was correlated
with both zooplankton density and eukaryotic community
composition, indicating biotic controls such as “top-down”
interactions also contribute to community assembly in the
lake. Taken together, these community analyses suggest that
deterministic biotic and abiotic selection within the lake coupled
with dispersal limitation structures the microbial communities
in Green Lake 4.