Mechanisms driving estuarine water quality: A 3D biogeochemical model for informed management
Introduction
Estuaries are amongst the most productive ecosystems of the world with the transition from fresh to salt water supporting a diverse assemblage of species. About 60% of the global population now lives along coasts and estuaries, which has resulted in widespread degradation of water quality and ecology (Wollanski, 2007). Active management is a high priority that is often hampered by limited understanding of complex multidisciplinary estuarine interactions (Davis and Koop, 2006) and multiple stressors (Cloern, 2001). In this paper we use a coupled 3D hydrodynamic, sediment and biogeochemical model to elucidate fine-scale estuarine processes in a temperate anthropogenically modified estuary for informed water quality management.
Water quality is a synthesis term for assessing the physical, chemical, biological and aesthetic characteristics of a water body against reference values to determine how ‘good’ the water is for specific uses. In a convoluted estuary local water quality can vary considerably over short spatial and temporal scales due to the interaction of the hydrodynamic circulation with strong gradients in salinity, suspended sediment, nutrient concentrations, dissolved oxygen and plankton concentrations. External forcing from catchment run off, synoptic weather, tidal cycles and point source discharges enhances variability and confounds the interpretation of sparse observations (Tappin, 2002). In many systems the fine-scale spatial and temporal physical, chemical and biological dynamics underpinning fluctuating water quality are poorly understood (Davis and Koop, 2006). Management of specific bays or reaches is challenging and it is difficult to prioritise intervention to target specific water quality issues or quantify resulting change.
Conceptual models help to synthesis understanding and provide a basis for first order nutrient budgets of a system (e.g. Fisher et al., 1988; Azevedo et al., 2008; Robson et al., 2008) however they necessarily overlook the detail and complexity required to address local issues. More finely resolved biogeochemical models have recently been applied in coastal and estuarine systems to enhance coastal ecosystem understanding with some success (e.g. Griffin et al., 2001; Byun et al., 2007; Arndt et al., 2009, 2010; Timmermann et al., 2010). Validation of model results against observations is a critical step for the successful uptake of model understanding by managers (Rykiel, 1996).
In this paper we integrate a biogeochemical model with a coupled 3D hydrodynamic (Herzfeld et al., 2005) and sediment model (Margvelashvili et al., 2005) of the Derwent Estuary and validate its performance against observations. We use the model to explore the fine-scale dynamics underpinning observed variations in local water quality and hypothesise that oscillations in the halocline downstream of the salt wedge front increase vertical exchange of nutrients across stratified layers and stimulate a persistent phytoplankton bloom in the mid estuary. The model is used as a synthesising hypothesis of cross disciplinary understanding to identify key processes controlling local water quality in the estuary for informed management.
Section snippets
Study area
The Derwent Estuary, in southeast Tasmania, (Fig. 1) is a micro-tidal drowned river valley (Edgar et al., 2000). It is approximately 52 km long, 1–10 km wide, 5–45 m deep and bordered by the state capital city of Hobart and a mixed catchment of urban, agriculture and forest drained by the Derwent River and numerous small rivulets (Butler, 2006). The influx of fresh water results in the formation of a salt wedge with stratified estuarine circulation modified by a small (<2 m) semi-diurnal tide (
Model implementation
The CSIRO Environmental Modelling Suite (EMS) comprising a coupled hydrodynamic, sediment and biogeochemical model was implemented on a high resolution 3D curvilinear grid (34,214 wet cells with spatial resolution <100–500 m and vertical resolution 0.5–5 m) extending downstream from New Norfolk at the head of the estuary to Iron Pot lighthouse off Cape Direction at the mouth. The hydrodynamic and sediment models were nested in regional and intermediate scale physical models, forced with Derwent
Seasonal cycle in biogeochemisty
The mid estuary experiences persistently elevated surface nutrient (>3 μM) and chlorophyll (>4 mg m−3) concentrations and reduced surface sediment oxygen (<50% saturation) throughout the year (Fig. 4). In winter and spring elevated nutrient concentrations are found throughout the estuary due to the influx of seasonally enriched marine water and elevated river load. During summer and autumn, surface nutrient concentrations in the upper and lower estuary are depleted, due to local productivity
Discussion
The high resolution 3D biogeochemical model presented conserves mass and reproduces the approximate timing and magnitude of the observed seasonal cycles of nutrients (nitrate ammonia, DIP), phytoplankton, dissolved oxygen and dissolved organic carbon, at most stations throughout the estuary quite well. Whilst the collated observations used in the model validation were relatively sparse in space, time and parameters, they were sufficient to assess the specified validation criteria (guided by
Conclusion
In this paper we have used a 3D coupled hydrodynamic, sediment and biogeochemical model that is dynamically consistent with sparse observations to elucidate the fine-scale 3D processes controlling local fluctuations in water quality in the estuary. Oscillations in the halocline downstream of the salt wedge front increase vertical exchange of nutrients across stratified layers and stimulate a persistent bloom of phytoplankton biomass in the mid estuary. The length of the estuary is sufficient,
Acknowledgements
Mike Herzfeld, Nugzar Margvelashvili & John Andrewartha for hydrodynamic & sediment model; Derwent Estuary Program for water quality data, sewerage treatment plant & catchment loads; Hydro Tasmania for Derwent River flow. Jointly funded by the Derwent Estuary Program & CSIRO Wealth from Oceans.
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