Season‐specific trends and linkages of nitrogen and oxygen cycles in Chesapeake Bay

Abstract

A three‐decade time series of solute concentrations was combined with a box‐modeling system to analyze long‐term trends in the concentration, production, and transport of dissolved inorganic nitrogen species along the mainstem axis of Chesapeake Bay. Water‐ and salt‐balance calculations associated with box‐modeling provided regional, seasonal, and interannual estimates of net advective and nonadvective transport and net biogeochemical production rates for oxygen and dissolved nitrogen. The strongest decadal trends were observed for decreasing late‐summer ammonium concentrations in bottom layers from brackish to polyhaline bay regions. Contemporaneous trends of increasing late‐summer bottom‐layer dissolved oxygen (O2) concentration were consistent with the observed NH4+ patterns, suggesting that increasing dissolved O2 levels may also reflect declining bottom respiration and drive nitrogen loss via increased rates of coupled nitrification–denitrification. Significant (but weaker) trends of increasing nitrate plus nitrite (NO2+3−) concentration and net production were consistent with the notion that increased nitrification may be stimulated by increasing dissolved O2 concentrations. Sorting bottom water NH4+ and NO2+3− net production rates into two pools (before and after the year 2000) revealed that general seasonal patterns were similar, but recent NH4+ net production rates were consistently lower and NO2+3− and NO2− rates higher in summer and fall compared to earlier years, especially in the middle Bay regions. We conclude that late‐season replenishment of oxygen associated with declining nutrient loads induced a negative feedback process, whereby decreased hypoxia suppressed NH4+ recycling and created conditions favorable for additional nitrogen loss via coupled nitrification–denitrification.

Barriers and Bridges in Abating Coastal Eutrophication

“…They attributed this to a 23% decline in dissolved nitrogen concentrations. Testa et al (2018) reported a decline in NH 4 and an increase in NO 2+3 concentrations in channel bottom waters during late summer as dissolved oxygen concentrations have increased, suggesting a vicious biogeochemical cycle may be breaking (Kemp et al, 2005). Zhang et al (2018) demonstrated progressive improvement in a multimetric index of water quality attainment that integrates dissolved oxygen, water clarity, submersed vascular vegetation and chlorophyll a.…”

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.…”

Spatial and temporal variations of bacterioplankton in the Chesapeake Bay: A re‐examination with high‐throughput sequencing analysis

“…The adaptation to estuarine environments could also be demonstrated in variations of bacterial production, respiration, and growth efficiency in estuaries (Shish and Ducklow 1994). For instance, recent urbanization and other anthropogenic activities have increased nitrogen (N) and phosphorous (p) loadings and stimulated increase of net primary productivity and high rates of microbial metabolism (Zhou et al 2014;Testa et al 2018). Taken together, these physiological adaptations create a unique spatial and temporal distribution and dynamic balance of microbial populations in estuarine environments.…”