3.2 Seasonal Dynamics in Aqueous Geochemistry and Microbiome
Membership
Using a principal component analysis, we identified which geochemical
indicators were most representative of connectivity and generated a
reduced dimensional space with uncorrelated components. The primary
principal component (PC1) corresponded to bulk ionic strength and
explained 62.2% of the variance in water chemistry and the secondary
principal component (PC2) explained 17.8% of variance and was strongly
driven by SO42- concentrations (Figure
1c, Table S2). All ion concentrations were negatively related to PC1
with Na+, Ca2+,
Cl-, Mg2+, and K+having moderate loadings (between -0.39 to -0.48).
SO42- had a strong positive loading on
PC2 (0.84) while Ca2+and Mg2 had
moderate negative loadings (-0.36 and -0.30, respectively).
Seasonal geochemical patterns at Inflow followed a snowmelt dilution
pattern where geochemical ion concentrations (e.g.,
Na+) were lowest during peak flows (Figure 1, S1 &
S2). This geochemical pattern propagated strongly to sites with surface
connections to the river during high flows resulting in high geochemical
connectivity strength (σg) (Figures 2 & 3). As stage
declined, geochemical composition diverged between Inflow and most
floodplain target sites, resulting in lower σg values
(Figure 2 & 3). Floodplain target sites crossed the 0.5
σg value across a wide range of Inflow stages
(Igeo) from 351 to 650 mm, demonstrating substantial
heterogeneity in connectivity dynamics across the river-floodplain
system. Connectivity strength at Main-Mid and Outflow never declined
below 0.5 while Pond-Iso had distinct geochemistry from Inflow
throughout the season resulting in low σg across the
study period that never exceeded 0.5. A sharp decline in
σg at Pond-Iso was observed at low Inflow stages during
the period that the pond was going dry, which may be the result of
evapo-concentration (Figure 3),
Using a principal coordinates analysis (PCOA), we explored seasonal
dynamics in microbiome membership and examined the potential to utilize
microbiome membership as an indicator of connectivity. The PCOA of
microbiome membership identified a major axis PCOA-1 that explained
32.5% of the variance in microbiome membership and a secondary axis
that explained 15.9% of the variance (Figure 1c). All additional axes
explained less than 10% of the variance. Microbial membership at Inflow
was relatively stable between the rising limb and falling limb, with
more observable shifts in membership at Inflow observed during the
recession period (Figure 1c, S3 & S4). During peak flows, sites with
structural surface connections to the river had microbiomes similar to
Inflow, resulting in high microbial connectivity strength
(σm) values (Figure 2 & 3). Major channel sites
maintained their similarity to Inflow for most of the study period with
some divergence at the lowest flows later in hydrograph recession. At
side channels and connected pond sites, microbiome membership started
diverging from the seasonal pattern at Inflow in either the falling limb
or recession hydro-period, resulting in lower σm values
later in the season (Figure 2 & 3). Floodplain target sites crossed the
0.5 σm value at Inflow stages (Imicro)
ranging from 308 to 577 mm. Like with the geochemical metric, the model
fit for σm at Main-Mid and Outflow never declined below
0.5 while Pond-Iso had distinct microbial membership from Inflow
throughout the season with low σm across the study
period that never exceeded 0.5.