High‐frequency dynamics of bottom dissolved oxygen in temperate shelf seas: The joint role of tidal mixing and sediment oxygen demand

Abstract

Dissolved oxygen (DO) in the bottom layer is essential for benthic organisms, and its temporal variations are widely concerned. However, previous studies have primarily focused on the long‐term variations of bottom DO, leaving its high‐frequency (HF) dynamics inadequately understood. This study addresses this gap by utilizing two seafloor monitoring systems that provide 3‐year‐long HF records in a typical temperate shelf sea, the Bohai Sea, China. During the stratified period each year, bottom DO exhibits notable HF fluctuations superimposed on its seasonal cycle. These HF signals originate from tide‐induced vertical mixing, showing peaks at various tidal frequencies. Notably, significant shallow‐water tidal signals are observed in bottom DO due to the frequency doubling of semi‐diurnal and diurnal tidal currents. Moreover, bottom DO demonstrates strongly asymmetric responses to tidal mixing on HF time scales. To be specific, the bottom DO increases with the intensity of tidal mixing, with this process being exceptionally rapid under conditions of weak tidal mixing. The underlying cause of this asymmetry is the markedly stronger vertical DO gradient near the seabed due to sediment oxygen demand. A process‐oriented biological model successfully reproduced observational features, further supporting our theoretical inference. These findings highlight the joint role of tidal mixing and sediment oxygen demand in modulating the HF dynamics of bottom DO in temperate shelf seas, underscoring their significance for the refined prediction of bottom DO in the future.