Macrophytes
Growth and decay of macrophytes can be modelled as part of a Water Quality Simulation.
The type of plant simulated by InfoWorks ICM is assumed to be one whose leaves are in contact with the atmosphere. Consequently all gas exchange is with the air - the plants do not add to or consume dissolved oxygen. The plants are modelled in terms of their carbon content per unit bed area.
To model macrophytes, check the Model macrophytes box in the QM Parameters dialog. Macrophytes can only be modelled when Algae (ALG) is also modelled.
The Nitrate (NO3) determinant is used in the calculation of a nutrient limitation factor due to nitrates. If dissolved phosphorus (TPH) and Silicates (SI) are modelled, nutrient limitation factors due to phosphates and silicates will also be calculated.
Parameters for macrophytes are defined in the Macrophytes group in the Water Quality and Sediment Parameters.
A solar radiation profile (defined in a Rainfall Event) is also required in order to calculate a growth limitation factor due to light intensity.
When conditions are favourable for growth, even though there is no existing stock, a seed concentration is set.
Equations
Plant growth is a function of temperature and is limited by sunlight (see Solar Radiation) and nutrient concentrations. The plants take up nutrients through their roots, exuding them into the water column through their stems. They can therefore act as a nutrient pump, transferring nutrients from the pore water to the overlying water. Michaelis-Menten nutrient limitation factors are calculated based on the pore water concentrations of nutrients. The exudation of nutrients to the water column is given as a proportion of the growth rate.
The maximum macrophyte growth rate, Pmax(sec-1), is expressed as a function of temperature:
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where: T = Temperature (°C) Pmac20= maximum macrophyte growth rate at 20°C (s-1) Q10 = parameter which controls the temperature dependency of the growth |
The light induced growth limitation factor is derived from the following equation:
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where: mlight = light limitation factor at depth, z, below the surface I= intensity of light at depth, z, below the surface Imax = light intensity which will produce maximum productivity |
The equation is integrated over the depth to give an average value for the whole water column. See Solar Radiation for a full description of the light intensity calculation.
Nutrient limitation factors are calculated according to a Michaelis-Menton equation:
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where: mnutrient = nutrient limitation factor Cnutrient = pore water concentration of the nutrient (mg.l) knutrient = half saturation constant for the nutrient (mg/l) |
The actual rate of production is then given by:
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where: mN, mP, mSi = nutrient limitation factors due to nitrates, phosphates and silicates respectively. Note: mSi is set to 1 when Silicate is not simulated. |
Nutrients are all obtained from the pore water. The amount of each nutrient required is given by the nutrient to carbon ratios (rnacn, rmacp, rmacs). Taking nitrogen as an example, the amount of nitrate nitrogen removes from the pore water is given by:
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where: MP = the macrophyte carbon concentration |
Uptake of nutrients will be limited by the total available amount. This in turn may limit nett production of macrophyte carbon.
The mortality of macrophytes (M) is salinity dependent. For salinities less than 3ppt the mortality (Mmac) is specified. For salinities between 3ppt and 10ppt the mortality is as follows:
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where: S = salinity (ppt) |
For salinities in excess of 10ppt the mortality (M) is set to 0.9. At this level of salinity all growth ceases.
No respiration term is included as it is assumed that oxygen is obtained from the atmosphere. Similarly, no oxygen is added to the water during photosynthesis.
A proportion of the production of macrophyte is lost due to exudation. In the model, this effectively means that a proportion of the nutrients taken up by the plant during growth is released into the water column. The proportion lost by exudation is given by a constant, pexu1. Thus the nett production of macrophyte is given by:
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Dead macrophyte material falls on to the bed where it is added to bed detritus. A proportion of the dead material produced is lost by leaching, given by the constant plea1. Thus the nett contribution of macrophyte to bed detrital carbon is given by:
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The amount of nutrient exuded or leached is determined from the amount of carbon lost via the nutrient to carbon ratios (rmacn, rmacp, rmacs).
