Impacts of Tide Gate Modulation on Ammonia Transport in a Semi-closed Estuary during the Dry Season—A Case Study at the Lianjiang River in South China

Recovery of tide-receiving is considered to improve the water quality in the Lianjiang River, a severely polluted and tide-influenced river connected to the South China Sea. A tide-receiving scenario, i.e., keeping the tide gate open, is compared with the other scenario representing the non-tide-receiving condition, i.e., blocking the tide flow during the flood phase, by numerical simulations based on the EFDC (Environmental Fluid Dynamics Code) model. The impacts of tide receiving were evaluated by the variation in the concentration of ammonia and its exporting fluxes, mainly in the downstream part of the river. With more water mass coming into the river, in the tide-receiving scenario, the averaged concentration of ammonia reduced by 20–40%, with the most significant decrease of 0.64 g m−3. However, the exporting flux of ammonia has decreased in the tide-receiving scenario, as the consequence of the back–forth oscillation of tidal current. In the tide-receiving scenario, the time series of ammonia concentration approximately followed the tidal oscillation, with increased concentration during the ebb tide and reduction in the flood tide. In the non-tide-receiving scenario, the ammonia concentration decreases when the tide gate is open which results in further intrusion of seawater. This was followed by an increase in ammonia concentration again after the currents shift seaward and water mass with higher concentration from the upstream part is transported downstream. Given the identical ammonia input and river runoff, the ammonia concentration stays lower in the tide-receiving scenario, except for short periods after the tide gate opening and neap tides in the downstream part which lasts for around half a day. This study highlights the importance of hydrodynamic condition, specifically tidal oscillation, in the semi-diurnal and fortnight cycles, for the transportation of waterborne materials. Furthermore, the operation of the tide gate was additionally discussed based on potential varied practical conditions and evaluation criteria.

Responsiveness of the human airway in vitro during deep inspiration and tidal oscillation

In healthy individuals, deep inspiration produces bronchodilation and reduced airway responsiveness, which may be a response of the airway wall to mechanical stretch. The aim of this study was to examine the in vitro response of isolated human airways to the dynamic mechanical stretch associated with normal breathing. Human bronchial segments ( n = 6) were acquired from patients without airflow obstruction undergoing lung resection for pulmonary neoplasms. The side branches were ligated and the airways were mounted in an organ bath chamber. Airway narrowing to cumulative concentrations of acetylcholine (3 × 10−6 M to 3 × 10−3 M) was measured under static conditions and in the presence of “tidal” oscillations with intermittent “deep inspiration.” Respiratory maneuvers were simulated by varying transmural pressure using a motor-controlled syringe pump (tidal 5 to 10 cmH2O at 0.25 Hz, deep inspiration 5 to 30 cmH2O). Airway narrowing was determined from decreases in lumen volume. Tidal oscillation had no effect on airway responses to acetylcholine which was similar to those under static conditions. Deep inspiration in tidally oscillating, acetylcholine-contracted airways produced potent, transient (<1 min) bronchodilation, ranging from full reversal in airway narrowing at low acetylcholine concentrations to ∼50% reversal at the highest concentration. This resulted in a temporary reduction in maximal airway response ( P < 0.001), without a change in sensitivity to acetylcholine. Our findings are that the mechanical stretch of human airways produced by physiological transmural pressures generated during deep inspiration produces bronchodilation and a transient reduction in airway responsiveness, which can explain the beneficial effects of deep inspiration in bronchial provocation testing in vivo.