High-frequency sensor data capture short-term variability in Fe and Mn
cycling due to hypolimnetic oxygenation and seasonal dynamics in a
drinking water reservoir
Abstract
The biogeochemical cycles of iron (Fe) and manganese (Mn) in lakes and
reservoirs have predictable seasonal trends, largely governed by
stratification dynamics and redox conditions in the hypolimnion.
However, short-term (i.e., sub-weekly) trends in Fe and Mn cycling are
less well-understood, as most monitoring efforts focus on longer-term
(i.e., monthly to yearly) time scales. The potential for elevated Fe and
Mn to degrade water quality and impact ecosystem functioning, coupled
with increasing evidence for high spatiotemporal variability in other
biogeochemical cycles, necessitates a closer evaluation of the
short-term Fe and Mn cycling dynamics in lakes and reservoirs. We
adapted a UV-visible spectrophotometer coupled with a multiplexor
pumping system and PLSR modeling to generate high spatiotemporal
resolution predictions of Fe and Mn concentrations in a drinking water
reservoir (Falling Creek Reservoir, Vinton, VA, USA) equipped with a
hypolimnetic oxygenation (HOx) system. We quantified hourly Fe and Mn
concentrations during two distinct transitional periods: reservoir
turnover (Fall 2020) and initiation of the HOx system (Summer 2021). Our
sensor system was able to successfully predict mean Fe and Mn
concentrations as well as capture sub-weekly variability, ground-truthed
by traditional grab sampling and laboratory analysis. During fall
turnover, hypolimnetic Fe and Mn concentrations began to decrease more
than two weeks before complete mixing of the reservoir occurred, with
rapid equalization of epilimnetic and hypolimnetic Fe and Mn
concentrations in less than 48 hours after full water column mixing.
During the initiation of hypolimnetic oxygenation in Summer 2021, we
observed that Fe and Mn were similarly affected by physical mixing in
the hypolimnion, but displayed distinctly different responses to
oxygenation, as indicated by the rapid oxidation of soluble Fe but not
soluble Mn. This study demonstrates that Fe and Mn concentrations are
highly sensitive to shifting DO and stratification and that their
dynamics can substantially change on hourly to daily time scales in
response to these transitions.