Benthic Algae

The unit models the growth through photosynthesis and nutrient uptake, and decay through respiration and death of benthic algae.

Description

Benthic algae represent those algae that live on the surface of the bed. The variable modelled is their carbon content. Because benthic 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 to the bed.

The growth of benthic algae is determined by the same relationship as for phytoplankton. Growth is limited by light intensity and nutrient uptake. Benthic algae take up their nutrients from the water column. The nutrient composition of the benthic algae can differ from that of the phytoplankton so their rates of nutrient uptake may differ.

On dying, benthic algae contribute to the detritus in the fluffy bed layer. If benthic algae die while in suspension they contribute to the detritus in the water column. Because the proportions of nutrients to carbon differ between the benthic algae and phytoplankton it is necessary to track the nutrients in their detrital form. Consequently, concentrations of detrital nitrogen, detrital phosphorus and where appropriate detrital silicon are traced as different variables.

For a full description of the photosynthesis, respiration and mortality of algae, see PHYTOPLANKTON section.

Equations

The maximum benthic algal growth rate, P(s^-1), is expressed as a function of temperature:
                    (1)

Where:
T = temperature (°C)
m, c = constants set in the PHYTOPLANKTON module

The light induced growth limitation factor is derived from the following equation:

                    (2)

Where:
μ_light = light limitation factor at some depth, z, below the surface
I = intensity of light at some depth, z, below the surface (kJ/cm²)
I_max = light intensity which will produce maximum productivity (kJ/cm²)

The light intensity at the bed is used to simulate the growth of benthic algae. See SOLAR RADIATION section (Chapter 5.5.16) for a full description of the light intensity calculation.

Nutrient limitation factors are calculated according to a Michaelis-Menten equation:

                    (3)

Where:
C_nutrient = concentration of the nutrient (mg/l)
k_nutrient = half saturation constant for the nutrient (mg/l)

The actual rate of production is then given by:

                    (4)

Where:
μ_N , μ_P , μ_Si = nutrient limitation factors due to nitrates, phosphates and silicates respectively. Note that μ is set to one when the SILICATE module is not simulated.

For a description of the uptake of nutrients during growth, see the manual entry on PHYTOPLANKTON.
Benthic algal respiration is a function of temperature:

                    (5)

Where:
R_T = 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 mortality of benthic algae is given by a first order decay process with a fixed constant (dethbn). This also gives the rate of production of detrital carbon due to benthic algae. Thus the net production of benthic algae is given by:

                    (6)

Detrital carbon is produced by the decay of benthic algae. This detritus due to benthic algae is added directly to the bed detritus. Thus the net change in bed detrital carbon due to benthic algae is given by:

                    (7)

The processes involved with the production and decay of detrital carbon are discussed in the manual entry for PHYTOPLANKTON. The contribution of benthic algae to the oxygen balance in the water column is given by:

                    (8)

General

The BENTHIC ALGAE module must be run in conjunction with the PHYTOPLANKTON module. If silicon is known to be an important nutrient for the particular type of benthic algae being simulated then the SILICATE module should be included. If the adsorption of phosphorus is thought to limit the supply of phosphates available for uptake by the benthic algae then the ADSORBED Phosphorus module should be included in the simulation.

The BENTHIC ALGAE module simulates the following transported variable names:

• Suspended benthic algae (mg/l)

• Detrital nitrogen (mg/l)

• Detrital phosphorus (mg/l)

The BENTHIC ALGAE module simulates the following variables on the river bed:

• Benthic algae (g/m²)

• Fluffy detrital nitrogen (g/m²)

• Bed detrital nitrogen (g/m²)

• Fluffy detrital phosphorus (g/m²)

• Bed detrital phosphorus (g/m²)