below the nitrocline where more favourable terminal electron acceptors
were exhausted. The differences in geochemistry and bacterial
community structure across depths suggest that organisms exploiting
similar resources should have correlated distributions across depths.
Multiple groups of spatially correlated OTUs in the lake. To
identify groups of spatially correlated organisms, we used
hierarchical clustering to group the 536 most abundant OTUs into
49 groups based on the similarity of the distributions of OTUs
across depths. We call these groups ‘operational ecological units’
(OEUs), because they are groups of organisms that we expect have
functional or ecological relationships (thus ‘ecological’) but were
defined in a purely statistical way (thus ‘operational’21).
Most OEUs contained OTUs from multiple phyla. OEUs ranged in
size from 2 to 33 OTUs and contained 1 to 10 phyla. The number of
phyla in each OEU (0.34 additional phyla per OTU beyond the first in
the OEU; Supplementary Fig. 4) was about as many as would be
expected if phyla classifications had been randomly assigned to
OTUs (0.35 ± 0.01; 1,000 permutations). To verify that the OEUs
are robust to different bioinformatic methods, extraction methodologies
and sample years, we compared the results of the OEU analysis
after varying each of these factors and found more OTUs together
than would be expected by chance (Supplementary Table 5).