Discussion for Freshwater Energetics

Ecological pyramids

In many ecosystems, organisms can readily be sampled, identified and counted.

Different organisms can be assigned to different trophic levels (such as primary and secondary consumers), and the data collected in the field used to construct ecological pyramids.

Pyramids of number

These show the number of individuals at each trophic level. The producers are always put at the bottom of the pyramid, with each successive trophic level placed on top.

This is not always pyramid-shaped. In flowing water, invertebrates can be carried downstream in the flow, which is known as invertebrate drift. The habitat sampled may also produce an inverted pyramid of numbers, such as one oak tree (primary producer) supporting thousands of caterpillars (primary consumer).

Pyramids of biomass

These show the biomass at each trophic level, ideally using the total dry mass of organisms. Dry mass is found by placing specimens in an oven for 4 hours at 80°C to drive off all moisture. Historic data on dry biomass of specimens may be obtained from academic sources. Biomass is measured in grams per square metre for terrestrial ecosystems and grams per cubic metre for water.

Alternatively, you can directly measure wet biomass without harming the animals as follows:

• sort the animals into primary producers, primary consumers, secondary cosumers and tertiary consumer groups
• empty each pot into a fine sieve, so that you are just left with the mass of animals in each sieve
• weight the sieve plus animals
• return the animals to the white tray
• weight the sieve only, then subtract this from the weight of the sieve plus animals to get the wet biomass at each level.

Inverted pyramids of biomass are fairly frequent in freshwater ecosystems. This could be due to:

Cold blooded animals in freshwater environments do not need to use up energy regulating their body temperature. This means there is potentially more energy available to be converted into biomass, which can then be transferred on to the next trophic level.

Microorganisms like algae and other primary producers in freshwater have short lifecycles. At any one point in time, their biomass might be smaller than that of primary consumers. Over a longer period of time, such as one year, the biomass of primary producers will always be larger than that of consumers.

As with pyramids of number, pyramids of biomass are snapshot pictures in time, rather than examples of productivity over a longer period of time, such as a year.

Pyramids of energy

These show the total energy content of organisms at each trophic level in kilojoules per metre squared per year (kJ m^-2 yr^-1). Energy content is found by placing specimens in a bomb calorimeter, and measuring the amount of energy released through their combustion. Historic data on energy content of specimens may be obtained from academic sources.

Productivity

Productivity is the rate at which energy passes through each trophic level in a food chain. it is measured in units of energy per area per period of time, typically kJ per m² per year.

• Gross Primary Productivity (GPP) is the rate at whih plants convert light energy into chemical energy
• Net Primary Productivity (NPP) is GPP minus respiration loss. NPP represents the energy available to the primary consumer.

Efficiency of energy transfer

The efficiency of an energy transfer at any stage in the food chain can be calculated as

\(\mathsf{energy\; efficiency = 100\; \times\; \frac{energy\; or\; biomass\; available\; before\; the \;transfer}{energy\; or\; biomass\; available\; after\; the\; transfer}}\)

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