# Coupled hydrodynamic - water quality - ecological modelling

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Numerical models are increasingly being used together with field monitoring data as research and management tools to help understand the behaviour and functioning of a specific natural system. A common type of model used in coastal waters is a coupled coupled hydrodynamic - water quality - ecological model. Such models can be used to study historic conditions in a specific region to try to fully understand the changes that have occured (trends) and the main human factors and natural processes that can explain such changes. If a numerical model is well calibrated and validated for a specific region using historical data, it can also be used to predict future conditions. As such, computational models used for real-time and forecasting calculations can be an important component of a coastal observatory. This article on Coupled hydrodynamic - water quality - ecological modelling presents the 'effect chain' approach for modelling marine and coastal water quality and ecology.

## Introduction

This article will concentrate on an integrated modelling approach in which hydrodynamics, suspended sediment dynamics and water quality are combined in order to simulate algae primary production and nutrient concentrations. Such an integrated modelling approach generally requires a series of coupled models.

## Hydrodynamic models

Hydrodynamic models, generally 2D or 3D, calculate the circulation of coastal water based on tides and wind, and the discharge from coastal rivers. More specifically, the models calculate water velocities (speed and direction) and water levels for each grid cell in the computational model grid. The hydrodynamic models generally also calculate salinity as well as water temperature, based on heat balance equations. These last two parameters are especially relevant for 3D hydrodynamic calculations where density differences due to temperature and salinity can play an important role in the circulation of water.

The hydrodynamic model results of water level, velocities, temperature and salinity are passed on to the sediment or water quality model where they are used for calculationg the advective transport of dissolved and suspended material in the water. The temperature and salinity values are important in the calculation of many chemical and biological processes, as the rates for many of these processes are temperature and salinity dependent.

## Water quality models

The term 'water quality model' is very general and can imply many different kinds of models. However, the main component of water quality models in coupled hydrodynamic-water quality-ecological modelling systems is the calculation of suspended sediment concencentrations. These concentrations, which can be variable in time and space, are important for determining the availability of light in the water column, which in turn is critical for algae growth. Several methods are possible for suspended sediment modelling. A deterministic modelling approach takes into account all the sources of sediment into a water system (e.g. from rivers) and considers the transport, sedimentation and resuspension (erosion)of this sediment as a function of the water flow, waves and wind. Other, simplified approaches can include the use of a pre-defined function for suspended sediment concentration, possibly varying with the location, time of year and/or wind velocity. More advanced approaches incorporate optical remote sensing data (e.g. from SeaWiFS or MERIS) which can provide measured suspended sediment concentrations in the surface water layer.

## Ecological models

In coupled hydrodynamic-water quality-ecological modelling systems, the ecological model component is generally focused on the modelling of primary production and mortality, as well as concentrations of chlorophyll, nutrients and oxygen. These parameters are all linked to one another by a complex set of inter-related processes. Algae primary production (growth) is a function of the available nutrients (nitrogen, phosphorus and silica) as well as light. A major source of nutrients is river discharges, which generally carry elevated nutrient concentrations relative to sea water. Additional sources of nutrients can be direct discharges from water water treatment plants. An important component in the ecological modelling is identifying and quantifying all the sources of nutrients. The available light is primarily a function of the (inorganic and organic) suspended sediment concentrations, which are calculated in the water quality model.