Assesement of the Trofic Level in
Portuguese Estuaries
Boxes
Approach
Transient Circulation
Residual Circulation
Residence Time
Reference
Situation
Influence of
the Nitrate Removal by Agriculture Systems
Effects of
Nutrients Removal by WWTP
IMAR has been committed by INAG to carry out a modelling study to assess the trofic level of Portuguese estuaries, using the hydro-ecological model MOHID. The results of the study will contribute for the identification of sensitive and vulnerable zones, according to Urban Waste Water Treatment Directive and Nitrate Directive.
In the estuary description a general overview of the estuary is given. This description varies from estuary to estuary, but usually includes descriptions about the size of the estuary, the size of the major adjacent basins and the location of major near by cities. To the available bathymetric data of the estuary special focus is given, because the model implementation is strongly related to the available data. The detail of the grid to use in a model must be adequate for the purpose of the study and consistent with the detail of the data available. The implementation of the hydrodynamic model uses, mostly, a bathymetry with a fine grid, so circulation patterns and residence time can be studied in detail. For the implementation of the ecological model, a bathymetry with a coarser grid is used, in order to reduce computational effort. The grid used to study hydrodynamic processes is integrated, using an integration algorithm that guarantees transport consistency. In this way, the model permits to obtain, with less computational effort, consistent results from the water quality simulations.
Usually, to study a property’s temporal variation, we compute a time series in some point of the domain. If our goal is to look at characteristics of particular areas in the estuary, the result may not be the ideal. For this reason, we integrate the results computed for each cell of that particular area. This conclusion leads us to the concept of Integration Boxes. With this method its possible, not only, to know the average property’s value in each area defined by the box, has well to compute the properties fluxes between boxes, which give us a great insight into the dynamical processes in the estuary. Hydrodynamics provides basics information to study biogeochemical processes in surface water systems. It determines vertical mixing, transport and consequently residence time in estuaries, which determine the fate of nutrients and organic matter discharged by the river in the upper estuary, by the smaller tributaries or by anthropogenic discharges. In all estuaries, the model implementation starts with the hydrodynamic model. The driving forces which are considered are tide and river discharge, since other driving forces are less important for long-term simulations. In the present study tidal forcing is imposed at the open sea boundary using tidal harmonics. River discharges were obtained from records and imposed in the upper part of the estuary.
To study the transient circulation in the estuaries, the model was run over the fine grid, using average river discharges. Simulations for neap tide and spring tide are usually presented for both, ebb and flood. These kinds of results give the reader a general overview over the flow velocities which can be expected inside the estuaries.
For a better understanding of the transport processes inside an estuary, is also presented, the residual circulation. The residual circulation is the average flow and gives an idea of the preferential transport of any property discharged in the estuary. To obtain the residual velocity, the model must run over a period of time much longer than the time periods associated to the variability of the transient flow. In the present study the residual flux is presented. It can be defined as the average of the transient water flux per unit of length (m2/s) . The residual flux gives a picture of the residual movement of the water volume, but it is difficult to visualize in figures including both deep and shallow areas, because the adequate scale is a function of the depth.
Residence time is an important indicator for global understanding of an estuary. Estuaries with a short residence time will export nutrients from upstream sources more rapidly then estuaries with longer residence time. In this study, the residence time is defined as the time required by water to leave the estuary and is computed using lagrangian tracers, which are used to label the water and to monitor its location. Different regions inside the estuary are identified by “boxes”, which are uniformly filled with tracers, representing each the same volume of water. The locations of the tracers are monitored in time and their residence time inside each part of the estuary and the time required to leave the estuary are computed. The total volume of the tracers in the estuary, at the beginning of the simulation, is equal to the total volume of the estuary. All simulations presented in this report start at high tide. Ecological processes such as primary production only occur if the adequate conditions are found (light, nutrients, temperature in this example) and if there is enough time for the ecological processes to occur. The ecological model must be run for a much longer time period then the transient flow or the residence time study. The physical principles supporting these observations (momentum and mass conservation) apply to all estuaries. The extraordinary differences observed emerge from the differences in boundary conditions and system geometry and bathymetry.
The Reference Situation represents, in this study, the nowadays system conditions, which the scenario results will be compared with. This section of the report describes (i) field data used for imposing boundary conditions and for model validation, (ii) results of the model and their comparison with field data (model validation) and (iii) space and time integration analysis of results in order to obtain a global view of the ecological functioning of the estuary.
To evaluate the effects of reducing the discharge of nutrients from agriculture, the simulations performed for the reference situation were repeated, considering a reduction of 50% of the nitrate load and maintaining all other parameters. In fact, nitrate is the most important form of nitrogen used in agriculture and phosphorous is in excess in the estuary.
To study the effects of nutrient removal by WWTP, it was tested the extreme situation of removing completely the discharge of nutrients from urban origin, when existing in the system. The test is performed following the same methodology described for removal of nitrate form agriculture. In this case the discharge of WWTP is assumed to be clean water.
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