Turbidity shapes the spatial diversity of the high-frequency dynamics of bottom layer dissolved oxygen in shallow coastal oceans

Abstract

High-frequency variations in bottom layer dissolved oxygen (DO) critically influence marine ecosystem metabolism and benthic habitats, yet their controlling mechanisms and spatial heterogeneity in shallow coastal oceans remain poorly understood. Based on four years of in-situ observations and high-resolution numerical simulations, this study systematically investigates the spatial diversity of high-frequency dynamics of bottom layer DO concentrations in the coastal oceans of China. The results show that the high-frequency fluctuations of bottom layer DO concentrations are most pronounced in summer and have two dominant frequency bands at diurnal and semi-diurnal periods. Further analyses indicate that bottom-layer DO concentrations are primarily influenced by diurnal variations in shallow waters with low turbidity, whereas semi-diurnal variations exert the strongest influence in high-turbidity environments. Model experiments further confirm turbidity as the key factor shaping the spatial diversity of high-frequency bottom DO dynamics by regulating underwater light availability and the relative importance of biogeochemical and physical processes. Under low-turbidity conditions, sufficient light penetration enhances near-bottom photosynthesis, leading to diurnal dominance. In contrast, under high-turbidity conditions, strong light limitation suppresses biological regulation, allowing semi-diurnal tidal mixing to become the dominant driver of bottom-layer DO variability. Overall, this study underscores the pivotal role of turbidity in mediating the balance between tidal dynamics and biogeochemical processes, offering new insights into high-frequency oxygen variability in shallow coastal oceans.