As we move in our main study from phytoplankton to zooplankton, a useful concept to learn at this point is the idea of trophic levels. Phytoplankton are known as primary producers. They build organic materials from inorganic elements. In the marine environment they are the first trophic level. Zooplankton species compose successive trophic levels. Species such as copepods and salps for instance, are herbivorous species, i.e. they feed directly on phytoplankton, therefore they are primary consumers. Chaetognaths , or arrow worms, which feed on zooplankton are secondary consumers.
Trophic levels can be expressed in terms of energy transfer. When an animal eats something, part of that energy is used to fuel respiration, motion, etc., and goes off as heat. The remainder is fixed as an increase in biomass, either through individual growth, or reproduction. The efficiency with which the biomass of one trophic level is transferred to the biomass of the next trophic level is known as the transfer efficiency. The transfer efficiency of phytoplankton to zooplankton is roughly 20%. The transfer efficiency of primary consumers to secondary consumers is roughly 10-15%. The average transfer efficiency from the primary producer to the top level predator in a food chain tends to be around 10%. This means that for each successive trophic level, there is an 80 to 90% energy loss through respiration and motion.
One consequence of this is that food chains can only afford to be a few trophic levels in length, because top level predators have only a small percentage of the original phytoplankton biomass available to them for food. To use an example, a four trophic-level food chain characteristic of some continental shelves in which the top-level predator is shark or salmon, at a 10% transfer efficiency, the top level will have less than 1% of the original primary production biomass available to them for consumption. When we add our own species to that food chain (salmon is an important commercial fishery), we become the top-level predator and the fraction of original biomass available to us is again decimated. Now consider the plight of the poor cannibal. Is it not easy to see why their practice has fallen from fashion? Note that transfer efficiency applies only at the moment of consumption. The truth is that because of mortality, not all production in one trophic level is consumed directly, thus further lowering energy transfer efficiency.
The consequences of transfer efficiency are that successive trophic levels 1) are larger, 2) are fewer, and 3) have longer generation times. Phytoplankton generation times for example, are measured in hours to days. Zooplankton generation times are measured in weeks to months. Generation times for fish are measured in years, and for marine mammals, decades. Lower population numbers suggest lower total mass (standing stock) for higher trophic levels, but due to their larger size and longer generation times, standing stocks of trophic levels in a food chain remain fairly constant, within an order of magnitude.
The reality is that the energy transfer idea of "food chains" is a simplification. Energy transfer rarely folows a linear path. Most organisms may be eaten by many species of predators and most predators eat more than one species of food. Many predators are also
The concept of trophic level energy exchange is then better expressed not as a food chain, but as a food web.
You will be examining petri dishes for this weeks lab. Each dish contains plainly visible organisms from 3 trophic levels.
Also present are
As a final note, it turns out that along with the roughly 10% transfer efficiency, the number 10 comes up frequently when comparing trophic levels. Successive trophic levels tend to be
Note however that the value "10" is in terms of order of magnitude, that is less 100, but greater than 1.
Last Updated: 17 August 1999
John H. Wormuth Department of Oceanography Eller O&M Building, Room 517A Texas A&M University College Station, TX 77843-3146
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