Impacts of anthropogenic inputs on hypoxia and oxygen dynamics in the Pearl River estuary

Abstract

Abstract. In summer, the Pearl River estuary (PRE) experiences hypoxia, largely driven by the high input of fresh water with low dissolved oxygen (DO), abundant nutrients and particulate organic carbon from the Pearl River network. In this study, we used a well-validated physical–biogeochemical model together with a DO species-tracing method to study the responses of hypoxia and oxygen dynamics to the anthropogenic perturbations of riverine inputs (i.e. DO, nutrients, and particulate organic carbon) in July–August 2006. Model results showed that hypoxia in the PRE was most sensitive to riverine inputs of particulate organic carbon, followed by DO concentrations and nutrients. Specifically, a 50 % decrease (increase) in riverine input of particulate organic carbon led to a 47 % decrease (64 % increase) in hypoxic area, with the sediment oxygen demand and water column production being the two most important processes contributing to changes in DO concentration. Changes in the riverine inputs of DO and nutrients had little impact on the simulated hypoxia because of the buffering effects of re-aeration (DO fluxes across the air–sea interface); i.e. the re-aeration responded to the changes in surface apparent oxygen utilization (AOU) associated with river-induced variations of oxygen source and sink processes. The PRE features shallow waters (with averaged depth of 10 m) in which oxygen provided by the re-aeration could penetrate to bottom waters via vertical diffusion and largely offset the changes in DO contributed by other oxygen source and sink processes. This study highlights the importance of re-aeration in reducing hypoxia variability in shallow estuaries.

Effects of Physical Forcing on Summertime Hypoxia and Oxygen Dynamics in the Pearl River Estuary

“…A more detailed description of the methodology and implementation of the coupled hydrodynamic model is given in Hu and Li [1]. The 1D-3D coupled model has been validated using the observed water levels and/or river discharges at the eight outlets [1], and has been applied to study the water-nutrients-sediment budgets [1,32], the oxygen budget [30,33], and the nutrient fluxes between the sediment and overlying waters in the PRE [35].…”

“…Then, the sediment module calculates the SOD returned to the water quality module as one of the source/sink terms of DO in Equation (1). A more detailed description of the sediment kinetic equations can be found in Brady et al [39] and Wang et al [30].…”

“…Hypoxia in the PRE is significantly affected by both natural and anthropogenic processes [29]. Wang et al [30] investigated the impact of anthropogenic inputs of nutrients and organic carbon on Hypoxia in the PRE is significantly affected by both natural and anthropogenic processes [29]. Wang et al [30] investigated the impact of anthropogenic inputs of nutrients and organic carbon on the summertime hypoxia and oxygen dynamics in the PRE.…”

“…Wang et al [30] investigated the impact of anthropogenic inputs of nutrients and organic carbon on Hypoxia in the PRE is significantly affected by both natural and anthropogenic processes [29]. Wang et al [30] investigated the impact of anthropogenic inputs of nutrients and organic carbon on the summertime hypoxia and oxygen dynamics in the PRE. Their model results, based on a wet year scenario, showed that the simulated hypoxia in the PRE was most sensitive to the riverine inputs of particulate organic carbon (POC), which was the main driver of oxygen depletion at the bottom waters.…”

“…Their model results, based on a wet year scenario, showed that the simulated hypoxia in the PRE was most sensitive to the riverine inputs of particulate organic carbon (POC), which was the main driver of oxygen depletion at the bottom waters. In this study, using the same well-validated hydrodynamic and water quality model with a sediment module in Wang et al [30], we will further explore the influences of natural processes on hypoxia in the PRE. To understand the effect of physical forcing on hypoxia in the PRE, a suite of numerical simulations were conducted to specifically examine the impacts of river discharge, winds, and tides.…”