Managing Phytoplankton Blooms In A Reservoir By Coupled Models

The linking or coupling of a model enables the outputs of one model to become the inputs to another. Through linking models, the effects of changes in one part of a system can be simulated in the linked model. Without such links, a model may suffer from the effects at the boundary of the model. One such case is a plankton model, which is influenced by complex processes in the surrounding catchment, but these catchment processes are not often explicitly simulated.

In lakes and reservoirs, excessive phosphorus loads from external sources are a prime cause of eutrophication (Vollenweider 1980). A significant portion of the external phosphorus load originates from the land around these waters, in addition to specific point sources such as municipal wastewater-treatment plants or factories. Lake eutrophication leads to increased biomass of phytoplankton and periphyton (that is, mostly diatom

Figure 9.3 Integrated modelling process for formulating and assessing management scenarios. The reservoir modelling contains two models, the nutrient-phytoplankton-zooplankton (NPZ) model, and one for vertical mixing.

films growing on any hard substrate), reduced water clarity, elevated pH and dissolved oxygen depletion in the water column (Smith et al. 1999). In particular, bloom-forming cyanobacteria, such as Microcystis and Anabaena, can change the taste and odour of water, release toxins and clog water filtration systems.

The Ben Chifley reservoir is a medium-sized multi-purpose reservoir located in eastern New South Wales, Australia (Box 9.1). To develop management strategies to reduce the biomass and blooms of problematic phyto-plankton (Kobayashi and Church 2003), we conducted a study to assess the environmental and socioeconomic effects of a range of catchment management options, by using a catchment-and-in-stream model, linked with a reservoir model (Figure 9.3), based on measurements of the physics, chemistry and biology in the catchment and reservoir.

The model relies on a comprehensive range of data being collected at appropriate spatial and temporal scales within the catchment and reservoir, and on long time series of data being sourced for these areas where possible. Data for the catchment, included stream water quality and quantity, stream and gully physical dimensions and rates of stream-bank erosion. Data on water quality phytoplankton, zooplankton, climate and hydrology were collected for the reservoir.

BOX 9.1 BEN CHIFLEY CATCHMENT AND BEN CHIFLEY RESERVOIR

Ben Chifley catchment has an area of approximately 985 km2, with the highest altitude at the eastern and south-eastern margins of the catchment (up to 1330 m above sea level). The Campbells River is the main stream draining the western half of the catchment. Sewells Creek is the main stream draining the eastern side of the catchment. The dominant land use is agriculture, with 65% of the land used as pasture for sheep and cattle grazing and 15% of the catchment is covered with Pinus radiata plantations - the remainder is covered by native forest.

Ben Chifley reservoir (149°33'E, 33°34'S) is located at the northern end of the catchment, approximately 20 km south-east of Bathurst. The reservoir was built in 1957. It has an average depth of 5.5 m and volume of 9.2 x 109 L (volume at full supply level is 16 x 109 L). The reservoir is the primary source of potable water for the city of Bathurst and is also used for recreational fishing and water sport activities. Nutrient concentrations of the reservoir indicate that the reservoir is meso-eutrophic (Kobayashi and Church 2003).

A catchment-and-in-stream model - Catchment Scale Management of Distributed Sources (CatchMODS) - estimates pollutant source and transport under current conditions and a variety of changed management scenarios (Figure 9.4). To provide the broad catchment scale perspective required, CatchMODS is based on a series of linked river reaches and associated sub-catchment areas (Newham et al. 2004a). In this manner, upstream tributaries provided input for downstream nodes to enable pollutants to be routed through a stream network. Outputs from the model are available for each river reach and are evaluated at the downstream end of the reach. Management recommendations can extend down to these individual river reach and sub-catchment scales. Outputs from CatchMODS include:

• estimates of daily stream flow at each node in the stream network

• a series of summary hydrologic variables (including mean annual stream flow, bank-full stream flow and mean annual base flow)

• total suspended solids (TSS)

• the cost (ongoing and fixed) for each management scenario.

Results are reported for each reach in the river network and also the total input to the Ben Chifley Dam. This enabled the outputs of CatchMODS to be linked with the reservoir modelling.

Figure 9.4 Structure of the CatchMODS model. The dashed horizontal line in the centre of the diagram represents the division between catchment and in-stream process modelling. The model links several components - a hydrologic model based on a rainfall-run-off model (Jakeman and Hornberger 1993), a sediment model (modified from Prosser et al. 2001), simple total P and total N models and an economic cost component. The downstream output feeds into the reservoir model M2PM.

Figure 9.4 Structure of the CatchMODS model. The dashed horizontal line in the centre of the diagram represents the division between catchment and in-stream process modelling. The model links several components - a hydrologic model based on a rainfall-run-off model (Jakeman and Hornberger 1993), a sediment model (modified from Prosser et al. 2001), simple total P and total N models and an economic cost component. The downstream output feeds into the reservoir model M2PM.

The reservoir model consists of a nutrient-phytoplankton-zooplankton model (Berryman 1992) and a vertical-mixing model (Chen et al. 1994). Air-water fluxes were calculated from meteorological observations. The coupled Mixing Model and Plankton Model, (M2PM) was configured with boundary conditions and initial conditions obtained from measurements made in the dam. In this study, the coupled model was integrated with observations that enabled easy comparison with observations and testing of hypotheses specific to Ben Chifley Dam. Outputs from the M2PM include:

• nutrient concentrations

• plankton biomasses

• dissolved oxygen

• vertical mixing

• water column temperature

• vertical shear in the horizontal currents.

The catchment and in-stream modelling identified priority sites for remediation of diffuse source pollution (Table 9.2). Several sub-catchments have been identified that have high pollutant source and transport potential relative to the remainder of the catchment. These sub-catchments generally have the greatest stream-flow volumes, and hence potential stream-bank erosion rates, and are also the sites of the highest incidence of gully erosion. They have the potential to contribute large volumes of sediment (and associated pollutants) to the stream network. Because of the proximity of these catchments to the Ben Chifley Reservoir, and minimal floodplain development in these areas, these pollutants also have the highest potential (relative to the remainder of the catchment) to be transported to the reservoir. Simple scenarios constructed for the catchment demonstrate that remediation effort focused on these areas is appreciably more effective than effort that is randomly or uniformly spread throughout the catchment. Remediation efforts could include stock exclusion and establishment of riparian vegetation in gully and riparian zones and more broad land use and pasture management changes. With reducing loads of nutrients from the catchment, the M2PM indicated a concomitant reduction of phytoplankton biomass (Table 9.2). Note that the use of any plankton model for analysis of a management scenario is only sensible providing one carefully considers the ecological theory upon which the model is based. For the present modelling, the M2PM was configured with boundary conditions and initial conditions obtained from measurements made in Ben Chifley reservoir.

There is a recognised need to implement integrated approaches to natural resource management (Jakeman and Letcher 2003). An integrated modelling approach to total catchment management is a useful one - improving an understanding of the interactions between terrestrial and aquatic systems. In managing the eutrophication of lakes and reservoirs, linking the catchment-in-stream modelling with the modelling of the plankton population dynamics is of fundamental importance.

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