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

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 (O-2) concentration were consistent with the observed NH4+ patterns, suggesting that increasing dissolved O-2 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 O-2 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.