We measured diel fluctuations in surface water chemistry of two high marsh ponds in July 2016. The ponds are shallow and experience day-night swings in oxygen concentrations, from super-saturation to anoxia. Our goal was to characterize changes in geochemical properties and dissolved organic carbon concentrations that accompany wide swings in oxygen concentrations. These data provide information about the responsiveness of microbial communities and how the dominance of redox-sensitive metabolisms changes over short time scales (e.g., hours) to feed back on pond water chemistry.
Diel changes in surface water chemistry were measured over 2 summer days (25- and 26-July, 2016) in ponds 1 and 4. We focused on these ponds because we had previously characterized seasonal changes in primary producer communities, metabolism, and biogeochemical processes, which provide context for the current measurements [Spivak et al., 2017; Spivak et al., 2018]. The ponds were hydrologically connected to nearby creeks shortly before sample collections began at 17:45 on 25-July, but then remained hydrologically isolated for the rest of the sampling period, which ended at 12:00 on 26-July 2016. Discrete surface water samples were collected every 1.5 h from two locations in each pond. Samples were collected at mid-depth (~15 cm), 3 m from pond edges by pulling water through tygon tubing into 60 mL syringes. The tubing was flushed before water was collected for salinity, pH, and concentrations of dissolved nutrients (NH4+, NO3-, PO43-), organic carbon (DOC), and sulfide. Samples were passed through pre-combusted glass fiber filters and put on ice. Samples for DOC were preserved with hydrochloric acid (HCl, pH 2) and sulfide with an antioxidant buffer solution. Upon return to the lab, pH and sulfide samples were analyzed immediately using ion selective electrodes while the remaining samples were stored cold (4 °C: DOC) or frozen (-20 °C: NH4+, NO3-, PO43-). Dissolved inorganic nutrient concentrations were measured on an autoanalyzer using standard methods [Gordon et al., 1994]. Surface water DOC was determined by high-temperature combustion [Lalonde et al., 2014]. Salinity was measured using a handheld refractometer.
Gordon, L., J. Jennings, A. Ross, and J. Krest (1994), A suggested protocol for continuous flow analysis of seawater nutrients (phosphate, nitrate, nitrite, and silicic acid) in the WOCE hydrographic program and the joint global ocean fluxes studyRep., Woods Hole, MA.
Lalonde, K., P. Middlestead, and Y. Gélinas (2014), Automation of 13C/12C ratio measurement for freshwater and seawater DOC using high temperature combustion, Limnology and Oceanography: Methods, 12(12), 816-829, doi:10.4319/lom.2014.12.816.
Spivak, A. C., K. Gosselin, E. Howard, G. Mariotti, I. Forbrich, R. Stanley, and S. P. Sylva (2017), Shallow ponds are heterogeneous habitats within a temperate salt marsh ecosystem, J. Geophys. Res.-Biogeo., 122(6), 1371-1384, doi:10.1002/2017JG003780.
Spivak, A. C., K. M. Gosselin, and S. P. Sylva (2018), Shallow ponds are biogeochemically distinct habitats in salt marsh ecosystems, Limnol. Oceanogr., 0(0), doi:doi:10.1002/lno.10797.
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