Ecology: Biology of Interactions. III-06. Ecosystem Production and Its Measurement
Biomass — the total mass of all organisms of the entire community or a separate population, measured in units of wet or dry mass per unit of area or volume of the environment. The biomass includes the bodies of organisms in their entirety, even if some of their parts are dead, such as wood. The totality...
III-6. Ecosystem production and its measurement
Humans are one of many consumer species inhabiting the Earth. Their existence requires organic matter produced by plants. As humanity grew, it repeatedly faced food shortages. Now, having become a planet-wide force, a natural question arises: how much organic matter, potentially suitable for human consumption, is produced on our planet?
In 1964–1974, a coordinated worldwide collection of data on ecosystem productivity and the factors affecting this productivity was conducted under the International Biological Programme of the United Nations. To this day, these results form the foundation of our knowledge about the natural resources possessed by the Earth. First, it is necessary to define the basic concepts used to study ecosystem productivity.
Biomass is the total mass of all organisms of the entire community or a separate population, measured in units of wet or dry mass per unit of area or volume of the environment. The biomass includes the bodies of organisms in their entirety, even if some of their parts are dead, such as, for example, wood. The totality of dead parts of living organisms constitutes necromass, the dead mass within organisms (note the etymological paradox: necromass is part of biomass!). The proportion of necromass is especially large in forests.
Production is the quantity of biomass reproduced per unit of area (or volume) per unit of time (as well as the biomass itself). Primary production is distinguished — the amount of organic matter produced by autotrophs (producers) — as well as secondary production — the production of heterotrophs (consumers and decomposers).
Various methods have been developed to measure ecosystem productivity. Since the formation and destruction of organic matter is closely linked to gas exchange (during the formation of organic matter in the course of the most common forms of photosynthesis, carbon dioxide is absorbed and oxygen is released), by measuring the content of these gases, production can be assessed!
Which gas content is better to measure: oxygen or carbon dioxide?
For aquatic and terrestrial ecosystems the answer will be different. The fact is that changes in the quantity of the gas that is less abundant in given conditions can be measured more accurately. Air contains relatively much oxygen and little carbon dioxide, so the production of terrestrial ecosystems is conventionally assessed by CO2. In water the situation is reversed: carbon dioxide dissolves in it very well, while oxygen is often in short supply. Therefore, the productivity of aquatic ecosystems is determined by measuring changes in oxygen concentration. How is this done? You will better understand the basic principle if you try to guess the essence of one of the simplest methods — the method of dark and light bottles.
From the side of a boat during daylight hours, a weight is lowered to a certain depth, to which a pair of bottles is attached: one transparent, and the other dark, opaque. With the help of a special device, the stoppers are pulled from these bottles at a certain depth. The bottles fill with water characteristic of that depth, together with the phytoplanktonic and zooplanktonic organisms present in this water. In addition, a sample of this water is raised to the surface and the oxygen content is determined in it. Let us designate the initial oxygen concentration as C0. The filled dark and light bottles are left at depth for some time (for example, 1 hour), then raised to the surface and the oxygen concentration in them is immediately determined. This yields two more values: Cb — the oxygen concentration in the dark bottle after keeping it at depth for the standard time — and Cw — the oxygen concentration in the light bottle after the same keeping.
Exercise. Indicate how, knowing C0, Cb, and Cw, to determine the intensity of respiration and production of the planktonic community. To help you find the answer to this question, think about why the difference in oxygen content between the dark and light bottles appears only during daylight hours.
The method of determining ecosystem productivity described here is far from the only one. Many methods of measuring productivity are based on the use of "labelled" atoms (radioactive isotopes of common nutrients).
The chlorophyll method is based on the fact that per one chlorophyll molecule there is approximately a constant amount of primary production. All biomass is collected from a certain area, chlorophyll is extracted from it, and its quantity is measured. It is even possible to assess the productivity of a particular territory by measuring from a satellite the spectral composition of light reflected by it! The fact is that chlorophyll is characterized by a well-known absorption spectrum, and from the difference between the light that entered the ecosystem and the light that left it, one can measure what fraction of the radiation was absorbed by chlorophyll.