Algae
Growth and decay of dissolved and attached algae can be modelled as part of a Water Quality Simulation.
Dissolved algae represents microscopic organisms (plants and certain types of bacteria) that are in suspension and are liable to movement by currents.
Attached algae represent algae that lives on the surface of the bed. Because the algae are attached to sediment particles they can be put into suspension when the bed mud is eroded. They can still grow in the water column and may later be deposited back on the bed.
The variable that is modelled for both dissolved and attached algae is algal carbon, the part of the algal biomass (by dry weight) that is carbon.
To model algae, select the ALG options in the QM Parameters Dialog. Algae can only be modelled when Dissolved Oxygen (DO) 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 dissolved and attached algae are defined 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.
Algal growth
The maximum algal growth or rate of production per day (Pmax) is expressed as a function of temperature:
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where: T = Temperature (°C) m= gradient above/below critical temperature c = intercept above/below critical temperature
The critical temperature value and the two sets of the constants, m and c (gradient and intercept values above and below critical temperature), are defined in the Dissolved algae parameters group in the Water Quality and Sediment Parameters. |
The rate of growth of algae depends on the presence of adequate sunlight and sufficient nutrients in the water.
There is an optimum light intensity at which maximum primary productivity occurs for a particular species. Lower or higher intensity gives a lower productivity. By combining this optimum intensity with the actual light intensity, a light induced growth limitation factor can be calculated.
The light induced growth limitation factor is derived using the Steele (1965) formulation:
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where: mlight = light limitation factor at hydraulic mean depth, z, below the surface I= intensity of light at hydraulic mean 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-Menten 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 half saturation constants for Nitrates, Phosphates and Silicates are defined in the Water Quality and Sediment Parameters. |
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. |
Algal growth requires nutrients - nitrogen, phosphorus and silicon if silicate is being modelled. The amounts of these nutrients taken up by the algae are controlled by the nutrient to carbon ratios (rphtn, rphtp, rphts). Nitrogen is taken up from nitrate-nitrogen, phosphorus from orthophosphate and silicon from silicate. The rate of uptake is given by (taking nitrogen as an example):
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where: AC is the concentration of algal carbon
The nutrient to carbon ratios for Nitrogen, Phosphorus and Silicon are defined in the Water Quality and Sediment Parameters. |
Production can be further limited if effective growth requires more nutrients than are available.
Algal mortality
The amount of oxygen released by photosynthesis is also calculated as a proportion of the mass of carbon produced. 2.67g of oxygen are released for each gram of algal carbon produced. Algal carbon and oxygen are consumed by respiration.
Algal respiration is a function of temperature:
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where: RT = respiration constant at T°C R20 = respiration constant at 20°C Q10 = parameter which controls the temperature dependency. Its effect is to double the rate for a 10°C rise in temperature
The respiration parameters are defined in the Water Quality and Sediment Parameters. |
Algae are also depleted by death. They can die naturally due to the cell ceasing to function or due to grazing by zoo plankton. All such loss processes are combined into a single mortality process, described by a first order decay process.
Dissolved algae
Algal mortality is given by a first order process with a fixed rate constant Mp.
The nett production of dissolved algal carbon, AC, is given by:
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Mortality rate is defined in the Dissolved algae parameters group in the Water Quality and Sediment Parameters. |
Detrital carbon is produced on the death of algal carbon at a rate given by the mortality constant Mp. It is then oxidised releasing nutrients into the water column. The rate constant, Kdc, is given by:
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where: Kdc = rate constant for detrital decay at temperature q°C Kdc20 = rate constant for detrital decay at temperature 20°C adc = temperature coefficient for decay of detrital carbon
The detrital decay parameters are defined in the Detrital carbon parameters group in the Water Quality and Sediment Parameters. |
In addition, detrital carbon can settle with a settling velocity vsdc, ultimately depositing onto the bottom of the channel.
So the nett change in suspended detrital carbon is given by:
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where: d = depth of water vsdc = settling velocity of detrital carbon |
Detrital carbon on the bottom of the channel decays at the same rate as the suspended matter. The nett change in bed detrital carbon is given by:
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The decay of detritus releases nutrients into the water column (bed detritus releases nutrients into the pore water). Nitrogen is recycled as slow organic nitrogen, phosphorus as orthophosphate and silicon as silica. The proportions of the detritus that are nitrogen, phosphorus or silicon are governed by the nutrient to carbon ratios.
If macrophytes or attached algae options are also modelled then additional variables indicating the amount of nitrogen, phosphorus and silicon contained in the detritus are required, reflecting the different nutrient to carbon ratios in the algae and plants.
The production respiration of dissolved algae affects the oxygen balance as follows:
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where: the factor 8/3 represents the equivalent mass of oxygen utilised or produced per gram of carbon dioxide produced during respiration and detrital decay or consumed during photosynthesis |
Attached algae
The mortality of attached algae is given by a first order decay process with a fixed constant (dethbn).
The nett production of attached algae is given by:
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Detrital carbon is produced by the decay of attached algae. This detritus due to attached algae is added directly to the bed detritus. Thus the nett change in bed detrital carbon due to attached algae is given by:
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The contribution of attached algae to the oxygen balance in the water column is given by:
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