[The concept of biological production is best approached from the point of view of a product, defined as a group of organisms (not necessarily all belonging to the same species) which have similar food habits, and which are useful to man or are of special interest for some other reason. Production is defined as the sum of all organic matter added to the stock of a product (or other defined organic unit) in a unit of time, regardless of whether or not it remains alive (i.e. part of the stock) at the end of that time. Production may be expressed in various units, particularly biomass (wet or dry weight), nitrogen content, or calorific content, but the last of these is to be preferred.In order to trace the flow of energy in a body of water from solar radiation up to a given product, four types of study are needed. The first is a quantitative determination of the formation of primary organic matter — the fixation of solar energy by plankton and macrophytes. For plankton this is estimated from the oxygen produced (usually in light-and-dark-bottle experiments); for macrophytes, direct measurements of the growth of the plant body have been used.Secondly, the paths of energy transformation that lead to the chosen product must be identified. Although a complete analysis for even one product would usually be extremely complex and tedious, the complexity can be greatly reduced by concentrating on the predominant foods of each organism and ignoring second-grade components.Thirdly, the ecotrophic coefficient must be determined for each step in the food pyramid that leads from primary organic matter up to the product. The term ecotrophic coefficient is defined by Ivlev in two different ways. In discussing the flow of energy through an ecosystem, what is involved is the "dynamic" ecotrophic coefficient, the ratio of a consumer's intake of a particular food organism to the latter's production during some rather long time interval — usually a year. The energy content of the production of each food present, multiplied by the corresponding dynamic ecotrophic coefficient, gives the energy of that type ingested by the consuming organism during the time unit chosen; and the sum of these quantities for all foods eaten by this consumer gives the total energy that ascends that step in the food pyramid per unit time. Bacteria must of course be considered, both as consumers and as foods, and must be included in the picture whenever they comprise one of the steps leading to the chosen product. Unfortunately, measurements of the dynamic ecotrophic coefficient have scarcely begun; to be useful they must be carried out in real bodies of water, not in the laboratory.What may be called the "static" ecotrophic coefficient is the ratio of the quantity of a particular food consumed to the total supply of it available at a given time; it is useful, for example, in studying daily rations. Laboratory experiments in this field have demonstrated a numerical relation between the quantity of food present and the quantity consumed, and also the fact that consumption increases if the food is distributed in aggregates.Finally, there must be determinations of the energy coefficient of growth — the fraction of consumed food that is converted into body substance, for each step. This coefficient has very similar values (about 0.35) for the very young (but post-embryonic) stages of practically all animals, but it decreases with age and approaches zero as growth ceases; the course of this decline can be represented by a parabola. External factors, temperature in particular, have little effect on the magnitude of the growth coefficient. The production of any organism is equal to the quantity of food it ingests multiplied by the growth coefficient.]