Measuring Productivity of a System: Explained!

Notes on some of the methods that could be used for measuring productivity in different systems.

Primary Productivity I.B. Definition: Gross Primary Productivity The quantity of organic matter produced, or solar energy fixed, by photosynthesis in green plants per unit area per unit time. i.e. total gain in energy / biomass per unit time. Primary productivity is referring to producers (plants) and so really this means how much the plant has increased in mass.

How can this be measured? If we are trying to find out how much a plant has increased in mass – first of all we need to think …… how does a plant increase in mass? Photosynthesis!! Produces glucose which the plant uses to grow. What is the equation for photosynthesis? Carbon dioxide + Water Glucose + Oxygen So to show how much growth is taking place we can also measure how much photosynthesis is taking place. We can measure how much carbon dioxide is used up or how much oxygen is produced as an indication of the rate (how much) of photosynthesis.

Marine + Freshwater Systems A technique that is often used to measure primary productivity in marine and freshwater systems is the Light and Dark Bottle Technique. This is an experiment that uses plants (usually elodea- pondweed, as it doesn’t have roots) to measure changes in oxygen concentration i.e. how much oxygen is produced. A data logger attached to an oxygen sensor are used.

Plants don’t only carry out photosynthesis – they respire too! Remember! Plants don’t only carry out photosynthesis – they respire too!

Photosynthesis and respiration take place here. Only respiration takes place here.

A Control is also needed. This is a control because neither respiration or photosynthesis are taking place here.

Oxygen produced by Photosynthesis. Oxygen also used up by respiration. Only respiration takes place and so oxygen used up and not produced.

To measure Gross Primary Productivity GPP = Oxygen in light bottle – Oxygen in dark bottle To measure Net Primary Productivity NPP = Oxygen in light bottle – Oxygen in control bottle

In Terrestrial Ecosystems: Quadrats can be used to measure the biomass of plants in a specific area. Set up 3 quadrats of a specific area. It is important that the quadrats are set up close together with very similar vegetation in them. Harvest the plants in one quadrat and record the biomass. N.B. It is important to note in your evaluaton of this method that the biomass of the roots is not being taken into account - an inaccuracy in the method. Cover the second quadrat and leave this and the final (uncovered quadrat) for a specific period of time.

Quadrat 2 Quadrat 3 Quadrat 1 Cover the quadrat to prevent light getting in. Harvest all plants after a specific period of time and record the biomass. Quadrat 3 Leave this quadrat uncovered. Harvest all plants after the same period of time as the covered quadrat and record the biomass. Quadrat 1 Harvest all plants at the start of the experiment and record the biomass. This is the control quadrat as neither Respiration or Photosynthesis will take place here during the expt. Respiration takes place but not photosynthesis. Both photosynthesis and respiration take place.

To measure the Gross Primary Productivity GPP = Biomass of uncovered quadrat – Biomass of covered quadrat To measure the Net Primary Productivity NPP = GPP – Respiration (Biomass of control – Biomass of covered quadrat)

Secondary Productivity I.B. definition: Secondary Productivity The biomass gained by heterorophic organisms, through feeding and absorption, measured in units of mass or energy per unit area per unit time.

Gross Secondary Productivity = Food Eaten – Feces Some ways that this can be measured in a lab …….. Aquatic System: An aquarium population of invertebrate herbivores (e.g. brine shrimps) Feed on known producer biomass (known amount of food) For a specific period of time Remaining food and feces are collected, dried and weighed Mass of food eaten can be calculated and mass of feces can be subtracted

Terrestrial System: Population of invertebrate herbivores (e.g. silkworms, crickets) kept in an enclosed environment. Feed on known producer biomass (known amount of food) For a specific period of time Remaining food and feces are collected, dried and weighed Mass of food eaten can be calculated and mass of feces can be subtracted

Some things to consider!! You must be careful to consider and evaluate all treatment of living organisms (including plants!). It is important to remember that organisms vary enormously in the proportion of water they contain, so the biomass should generally be given as dry biomass in order to be able to compare data from different sources. However, as a laboratory or field investigation, biomass can be estimated as a fixed percentage of wet weight to avoid the killing of organisms for dry weight measurements. Alternatively secondary data could be used.

Revision!!!!! I.B Definition: Biomass The mass of organic material in organisms or ecosystems, usually per unit area. Sometimes the term dry weight biomass is used where mass is measured after the removal of water.