Elevated microbial CO2 production and fixation in the oxic/anoxic interface of estuarine water columns during seasonal anoxia

Abstract

a b s t r a c tGradients of dissolved oxygen concentrations in stratified estuarine water columns directly influence microbial composition and metabolic pathways, resulting in vertical and axial chemical gradients of redox-active species. Understanding such microbial responses to changing geochemical conditions and elucidating the diversity of microbially-mediated processes are needed to comprehensively identify ecosystem functions. We tested the hypothesis that microbial metabolic processes in the vicinity of the oxic/anoxic interface in Chesapeake Bay would be elevated due to the coincidence of oxidized and reduced compounds. We measured rates of microbial redox processes associated with carbon cycles and quantified geochemical constituents. The transition from oxic to anoxic to oxic conditions was correlated with the pattern of dissolved nitrogen species, with the most reducing conditions comprising the highest concentrations of dissolved chemical compounds. The vertical distribution of community respiration was measured with changes in dissolved inorganic carbon concentrations and was related to water column stratification, with the highest median and variability (1.2 ± 1.4 mmol L À1 h À1 ) in the most stratified conditions. Rates of CO 2 fixation under dark conditions, a measure of chemoautotrophy, were elevated at the base of oxyclines and significantly correlated with a gradient of density and some reduced compounds. Although vertical interpolation must be made with caution due to high vertical variability, chemoautotrophic production averaged 0.5 mmol m À2 h À1 from May to August and added 5.8% to autochthonous organic carbon production in the mesohaline Chesapeake Bay. Overall, our results suggest that anaerobic community metabolism and chemoautotrophy in the oxic/anoxic interface exert a small impact on estuarine carbon cycles.

Chesapeake Bay Inorganic Carbon: Spatial Distribution and Seasonal Variability

“…November had a very low ratio, potentially related to coupled denitrification and nitrification, which are at maximum rates during the fall ( McCarthy et al, 1984;Kemp et al, 1990;Lee et al, 2015;Testa et al, 2018) and is positively correlated with suspended particles as in the ETM (Damashek et al, 2016). Given the non-conservative behavior of inorganic carbon in this region, and minimal buffering resulting in relatively large changes in conditions with changes in the carbonate parameters, higher resolution study is needed, particularly given the lack of measurements within the flats area, the potential for nitrogenrelated effects on alkalinity, and the analytical complications associated with the periodic presence of the ETM.…”

Nitrogen and oxygen availabilities control water column nitrous oxide production during seasonal anoxia in the Chesapeake Bay

“…No anoxic layer was observed in May and November (Fig. 1c), and previous studies showed that the water column of the Chesapeake Bay was reoxygenated following summertime anoxia during winter and spring (Lee et al, 2015a). …”

“…Since then, measures have been successfully enforced to control the nitrogen runoff into the bay from the tributaries (Boesch et al, 2001;Program, 2017). The near absence of summertime water column NO2 -+ NO3 -concentrations near the middle of Chesapeake Bay as shown in this study and others (Lee et al, 2015a) could prevent N2O emission. Contrary to the studies conducted in the 1970s Kaplan et al, 1978;McElroy et al, 1978;Elkins et al, 1981), our measurements from July 2016 showed surface N2O concentration was close to 20 air-saturation, and undersaturation of N2O within the anoxic layer (Fig.…”

“…Nitrogen (absolute and relative concentrations of NO3 -and NO2 -) and oxygen availabilities control N2O production in the water column of Chesapeake Bay. Therefore the seasonal variation of nitrogen and oxygen availabilities (Lee et al, 2015a) drive the seasonal variation in denitrifying community size, as shown by nirS gene abundance, and associated 5 potential N2O production rates. The rate and occurrence of N2O production vary greatly between seasons; thus the annual rate of N2O production and consumption by the Bay and other estuarine systems is very difficult to estimate.…”

“…In summertime (June to August), the typical depth of the oxic-anoxic interface is 10 -15 m in the 20 Chesapeake Bay (Taft et al, 1980;Kemp et al, 1992;Lee et al, 2015a). When storm events, boat traffic and surface cooling disturb the water column stratification, intermittent release of N2O to the atmosphere could occur.…”