Experimental Systems in Aquatic Ecology


The main experimental approaches in aquatic community and ecosystem ecology will be presented along a gradient of scale: unenclosed field experiments, mesocosms with a natural mix of species, microcosms with an artificial mixture of species from cultures, and single species experiments in batch and continuous culture. Experimental manipulations usually consist of the addition or removal of (supposed) key organisms, addition of chemicals or alterations of the physical environment. Selecting the appropriate scale of experimentation is not only a question of technical and financial feasibility but also it has to consider the inevitable trade‐offs between realism and control. Conclusions will be more widely accepted, if they are supported by experiments at a variety of scales.

Key Concepts:

  • The choice of the appropriate experimental scale has to face an inevitable trade‐off between realism and control.

  • Unenclosed field manipulations have the highest degree of realism but the least degree of control.

  • Mesocosms operate with natural species assemblages, permit replication and control of experimental manipulations but are limited in temporal scale (weeks).

  • Microcosm experiments operate with artificial communities assembled from cultures, offer a high degree of control, but lack the pre‐history a mutual adaptation of the participating species.

  • Experiments with single or few species rely on the ‘model organism’ concept.

  • Small‐scale experiments are biased against the detection of slow and space requiring processes.

Keywords: experiment; field experiment; mesocosm; microcosm; scale

Figure 1.

Example of a mesocosm study on the top‐down impacts of two different freshwater zooplankton types (copepods versus Daphnia) on different phytoplankton species. Biomass (measured as biovolume in 103 μm3 mL–1) of one large alga (Ceratium hirundinella (blue dots); c. 150 μm cell length) and one small alga (Stephanodiscus parvus (red dots); c. 6 μm) after 9 days in different mesocosms with logarithmically scaled seeding densities (individual L–1) of copepods and Daphnia (original graph after data in Sommer et al., ).

Figure 2.

KOSMOS mesocosm system: left – sketch of single mesocosm unit, consisting of floating frame, mesocosm bag and sediment trap; right – mesocosms during 2010 campaign in Kongsfjord, Spitsbergen. Courtesy of Ulf Riebesell, contact: usriebesell@ifm‐geomar.de.

Figure 3.

The BENTHOCOSM‐facility at Kiel (Courtesy of Martin Wahl, contact: mwahl@ifm‐geomar.de).

Figure 4.

Circular‐flow compartmentalised microcosm system to study the impact of zooplankton on phytoplankton competition via differential excretion of limiting nutrients (Sommer, ). The light chamber (left) contains phytoplankton consuming and competing for the limiting nutrients, silicon (Si) and phosphorus (P); the dark chamber (right) contains zooplankton (Daphnia), which consume algae and excrete nutrient. Removal of algae and loading of the light chamber with excreted nutrients can be calculated from concentration differences between the forward and the backward flow. Graphs on bottom: The bigger efficiency in P‐recycling leads to a decline of Si:P ratios and a subsequent decline of diatoms. Reproduced after Figure 8.22 from Sommer , with permission of Springer.



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Further Reading

Carpenter SR (1988) Complex Interactions in Lake Communities. New York: Springer.

Lampert W and Sommer U (1997) Limnoecology. New York: Oxford University Press.

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Tilman D (1982) Resource Competition and Community Structure. Princeton, NJ: Princeton University Press.

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Sommer, Ulrich(May 2012) Experimental Systems in Aquatic Ecology. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0003180.pub2]