Answer Key

1. Biological productivity is the increase in organic material per unit of area or volume over time. Primary producers, which in aquatic systems include algae, bacteria, and some higher plants, are responsible for this increase by converting inorganic matter into organic matter through photosynthesis, forming the base of the food web.
2. The standing crop method measures the biomass of organisms physically present per unit area or volume at a given time. A significant limitation is that standing crop is not directly proportional to productivity, as it reflects the net effect of many events; for example, a community’s biomass may be greatly reduced by predation even while its photosynthetic rates remain high.
3. The rate of production can be estimated by measuring the decrease of inorganic nutrients, such as carbon dioxide and phosphate, in a system. This provides an indirect measurement by calculating the amount of biological production that would have been required to absorb the measured quantity of nutrients.
4. This method estimates the incorporation of carbon dioxide by measuring short-term fluctuations in the dissolved oxygen produced during photosynthesis, assuming a mole of O₂ is released for each mole of CO₂ reduced. Unlike the ¹⁴C method, this technique provides the added benefit of an estimate of community respiration by measuring oxygen decrease during periods of darkness.
5. The light and dark bottle method involves enclosing identical water samples in both a transparent “light bottle” and an opaque “dark bottle.” By calculating the difference in oxygen content between the bottles and the initial sample after incubation, one can estimate the gross and net productivity of the plankton community.
6. The primary advantage of the ¹⁴C method is its great sensitivity, which allows for much shorter incubation periods than oxygen methods. A significant technical challenge is the calibration of radioactive sources and measurement instruments, as accurate calculation requires knowing the precise amount of ¹⁴C added and recovered.
7. The trophic-dynamic model, developed by Lindeman (1942), is a model of an aquatic ecosystem. It introduced the concept of “energy flow,” which describes the efficiency of energy transfer from one trophic level to the next, up the food chain.
8. The energy cost of “information” refers to the portion of autotrophic production that a phytoplankton community must reserve for the maintenance and development of its community structure. If this cost is high, a small increase in primary production can provide a relatively larger increase in the food supply available to grazers, as they only have access to production in excess of this maintenance threshold.
9. Eutrophication is the process of increasing productivity in a body of water. The idealized succession of a lake begins as oligotrophic (low productivity), becomes mesotrophic (medium productivity), then eutrophic (highly productive), and finally dystrophic, a bog stage where productivity has greatly decreased.
10. Lake Vanda, an ultraoligotrophic lake, is one of the least productive in the world, permanently sealed under ice with photosynthesis occurring down to 60 meters. In contrast, Clear Lake is an extremely eutrophic lake with high-intensity productivity limited to the upper four meters due to blooms of blue-green algae and inorganic turbidity.

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