Biogeochemical functioning of an urbanized tropical estuary: Implementing the generic C-GEM (reactive transport) model

Abstract

Estuaries are amongst the most productive ecosystems of the land ocean continuum, but they are also under high anthropic pressures due to coastal urbanization. Too sparse observations have hindered the understanding of complex interactions between water quality and estuarine hydrodynamics and biogeochemical transformations. Until now, estuarine modelling studies have mainly focused on temperate estuarine systems in industrialized countries. This study investigates the responses of a tropical estuary to pollution load from a megacity (Ho Chi Minh City, Southern Vietnam) by applying a one-dimensional, biogeochemical estuarine model (C-GEM). The Saigon River Estuary flows through the megacity of Ho Chi Minh (HCMC) and is subject to episodic hypoxia events due to wastewater inputs from urban discharges. Good agreements are found between simulation outputs and observations for tidal propagation, salinity, total suspended sediment, and water quality variables in dry season in Saigon River Estuary. C-GEM reproduces the increases in ammonium, total organic carbon, phytoplankton and dissolved oxygen depletion in the urban section of the Saigon River as an impact of untreated wastewaters from HCMC. The steadystate version of C-GEM also reveals the formation of a pollutant cloud (30-km stretch) resulting from the combined effects of tidal fluctuation and low flushing capacity during the dry season. Furthermore, the quantification of the reaction fluxes simulated by the model demonstrates that nitrification is the main process removing NH 4 + from the Saigon River. For the first time in such a type of environment, our study demonstrates the effectiveness of C-GEM at unraveling the complex interplay between biogeochemical reactions and transport in a tropical estuary with a minimized data requirement. This is significant for tropical estuaries in developing countries, where intensive monitoring programs are rare and have thus been rarely the object of modelling investigations.

Nutrient transport and transformation in macrotidal estuaries of the French Atlantic coast: a modelling approach using C-GEM

“…Economically, aquaculture develops around the estuary; agriculture, tourism (port cities), industry and import/export logistics are also active within estuarine basins. Estuarine and coastal ecosystems throughout the world are some of the most heavily used and threatened natural systems (Barbier et al, 2011), and they are facing increasing anthropogenic impacts given that they are the receptacle of all the contaminants and nutrients from the upper river watershed (Garnier et al, 2021), from both point sources (urban and industrial wastewater) and diffuse sources (agriculture). Additionally, harbors, channelization, and flood protection structures have changed not only the geomorphology of the estuaries, but also their hydrological and biogeochemical behaviors (Romero et al, 2016).…”

“…Eutrophication is the perturbation of an aquatic ecosystem by excessive nutrient enrichment and its manifestations may take various forms, including harmful algal blooms (Glibert, 2020), which can cause damage to coastal fisheries (Husson et al, 2016). Moreover, overproduction of these harmful algal blooms, which are not suitable for consumption by zooplankton and benthic invertebrates, possibly leads to hypoxia of the bottom water layers (Garnier et al, 2021). Due to the importance of these different aspects of estuaries, their environmental situations and problems are receiving increasing attention from both researchers and stakeholders.…”

“…CC BY 4.0 License. (Arndt et al, 2009;Garnier et al, 2007;Hu and Li, 2009;Laruelle et al, 2019;Ménesguen et al, 2019;Nguyen et al, 2021;Romero et al, 2019). However, direct observations do not allow to quantify nutrient fluxes in macrotidal estuaries because the oscillatory tidal flux is several orders of magnitude larger than the ecologically and biogeochemically-relevant retention flux (Regnier et al, 1998), despite mixing curves have been useful for interpreting in situ observations and nutrient dynamics.…”

“…Moreover, the retention flux generally falls within the range of measurement uncertainties (Arndt et al, 2009;Jay et al, 1997;Regnier et al, 1998). Numerical models combined with limited observed data can fill the gap in understanding nutrient dynamics and offer insight into past and future scenarios in response to environmental and human changes (such as land use changes and agricultural practices), and climate change (Billen and Garnier, 1997;Garnier et al, 2021;Ménesguen et al, 2018b). In addition, simulations can be carried out at large spatial and temporal scales.…”

“…The generic implementation of the C-GEM model relies on a limited amount of basic information to describe estuarine geometry, hydrodynamic information, and the inputs to estuaries, and then produces annual to multi-decadal simulations. C-GEM has already been applied to one tropical estuary (Nguyen et al, 2021) and several temperate estuaries and has provided satisfactory simulations of nutrient transport and biogeochemical processes despite the simplification of the estuarine geometry (Laruelle et al, 2017(Laruelle et al, , 2019Volta et al, 2014Volta et al, , 2016a). An extensive description of C-GEM is presented in Volta et al (2014Volta et al ( , 2016a.…”