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.

Salt marshes remove terrestrially derived nutrients en route to coasts. While these systems play a critical role in improving water quality, we still have a limited understanding of the spatiotemporal variability of biogeochemically reactive solutes and processes within salt marshes, particularly nitrogen species. To investigate this knowledge gap, we implemented a high-frequency sampling system to monitor sub-hourly nitrate (NO3) concentrations in salt marsh porewater at Elkhorn Slough in central California, USA. We instrumented three marsh positions along an elevation gradient subjected to different extents of tidal inundation, which we hypothesized would lead to varied biogeochemical characteristics and hydrological interactions. At each marsh position, we continuously monitored NO3 concentrations at depths of 10, 30, and 50 cm with subsurface water levels measured from 70 cm wells over seven deployments of ~10 days each. We quantified tidal event hysteresis between NO3 and water level to understand how NO3 concentrations and sources fluctuate across tidal cycles. There was significant differences in the NO3-subsurface water level hysteresis patterns across seasonal wet/dry periods common to Mediterranean climates. In dry periods, the NO3-subsurface water level relationship indicated that the source was likely estuarine surface water that flooded the transect during high tides. In wet periods, the NO3-subsurface water level relationship suggested the salt marsh was a source of NO3. These findings suggest that tidal and seasonal hydrologic fluxes control NO3 porewater dynamics and influence ecological processes in coastal environments.