Macrophytes

The unit models the growth, nutrient uptake and death of plants rooted on the river bed.

Description

Macrophytes are large plants that are rooted on the river bed. The type of plant simulated by the MACROPHYTES module is assumed to be one whose leaves are in contact with the atmosphere. Consequently all gas exchange is with the air so the plants do not add to or consume dissolved oxygen. The plants are modelled in terms of their carbon content per unit bed area.

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.

Dead macrophyte matter (dead leaves and plants) falls on to the river bed. A proportion of the nutrient content of the dead macrophyte matter is leached into the water column. The rest of the dead matter adds to the detritus from phytoplankton and benthic algae on the bed which when it decays releases nutrients into the pore water. Leaching and detrital decay release carbon in the form of slow BOD, nitrogen in the form of slow organic nitrogen, phosphorus in the form of phosphate and silicon in the form of silicate. Because the proportions of nutrient to carbon differ between the macrophytes and phytoplankton, it is necessary to track the detrital nitrogen, phosphorus and, where appropriate, silicon separately.

When the conditions are favourable for growth, even though there is no existing stock, a seed concentration is set.

Equations

The maximum macrophyte growth rate, P_max(sec^-1), is expressed as a function of temperature:

                     (1)

Where:
T = temperature (°C)
P_mac20 = 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:

                     (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 (J/cm²)
I_max = light intensity which will produce maximum productivity (J/cm²)

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:

                     (3)

Where:
μ_nutrient = nutrient limitation factor
C_nutrient = pore water 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_Si is set to one when the SILICATE module 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:

                    (5)

Where:
MP = is 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 (M_mac) is specified. For salinities between 3ppt and 10ppt the mortality is as follows:

                     (6)

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:

                     (7)

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 net contribution of macrophyte to bed detrital carbon is given by:

                     (8)

The amount of nutrient exuded or leached is determined from the amount of carbon lost via the nutrient to carbon ratios (rmacn, rmacp, rmacs).

General

The MACROPHYTES module must be run in conjunction with the PHYTOPLANKTON module. If silicon is known to be an important nutrient for the particular type of plant 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 plants then the ADSORBED Phosphorus module should be included in the simulation.

The MACROPHYTES module simulates the following transported variables:

• Detrital nitrogen (mg/l)
• Detrital phosphorus (mg/l)

The MACROPHYTES module simulates the following variables on the river bed:

• Macrophytes (g/m²)
• Fluffy detrital nitrogen (g/m²)
• Bed detrital nitrogen (g/m²)
• Fluffy detrital phosphorus (g/m²)
• Bed detrital phosphorus (g/m²)