Mesocosm composition

+/0/-

Engineering components

I

Ecological components

+/0/-

Figure 1 Interactions between engineering and ecology within mesocosms.

Table 1 A few examples of interactions between engineering and ecological components in a typical aquatic mesocosm

Sign of the

Interaction interaction

From engineering to ecology

Pumps causing water flow +

Pumps damage planktonic larvae -

Walls of tank absorb certain chemicals -

Walls of tank release certain chemicals -

Artificial lighting supports photosynthesis +

From ecology to engineering

Growth of biofilm on transparent walls -

reduces lighting Growth of organisms clog pipes -

Biomass clogs pumps -

aquatic mesocosm. Many direct and indirect interactions between ecology and engineering occur in mesocosms and they must be designed and/or managed to maintain the integrity of the model ecosystem.

The size range of mesocosms is somewhat arbitrary but, within the spectrum of experimental ecology, mesocosms fall between small-scale microcosms (such as contained in laboratory beakers) and whole, natural ecosystems (such as paired lakes or watersheds). They must be large enough to contain a fairly complex ecosystem but small enough to be constructed and manipulated for experiments. Mesocosms are usually within the range of 1-10 m in diameter or length and typical examples are artificial ponds, tanks, or raceways for aquatic systems, and large pots, plots, or greenhouses for terrestrial systems. Use of mesocosms is an extension of the microcosm method, so some replication of experimental units is usually employed. However, because of their size, meso-cosms are often costly which practically limits the opportunity for replication. Thus, a gradient of tradeoffs exists between microcosms and mesocosms: the small scale of microcosms makes them easy to replicate but not very realistic, while the larger scale of mesocosms makes them realistic but difficult to replicate.

Ideas on the scale of the mesocosm in terms of space and time must be considered as part of the design process. Some scaling rules for the design of pelagic, aquatic mesocosms have been investigated that indicate the optimal size and shape of tanks to support a realistic balance between the benthos of the sediments with the suspended plankton in the water column. Although the size of mesocosms is large relative to microcosms, it is very small relative to the natural analog ecosystems. Because of this fact, mesocosms can only represent or simulate fragments or patches of the larger natural analogs. But then, ecosystems are generally made up of patches that change with time. Ecosystem characterization and mesocosm representation can represent a mean of those everchanging patches. For biodiversity of the mesocosm, scaling can be evaluated with the standard species-area relationship, S = kAz, where S is the number of species in the system, A is the area of the system, and k and z are coefficients characteristic of the analog ecosystem.

The complexity of mesocosms can take many forms: longer food chains and biogeochemical cycles, broader food webs, and greater diversity of species and chemical constituents. Supporting this ecological complexity within the containment structure is the challenge of ecological engineering. Some ecosystem types are relatively easy to model with mesocosms, such as simple ponds or tanks containing lentic water columns with strong benthic influence. These kinds of experimental aquatic ecosystems have been widely used for both study of basic ecology (such as the role of predation in structuring communities) and of applied ecology (such as the effect of toxins on ecological structure and function). In these kinds of relatively simple mesocosms the interactions between the engineering components and the ecological components are few in number and easy to deal with. These kinds of mesocosms will likely continue to play significant roles in experimental ecology in the future. Other ecosystem types, such as those with water flows, species migrations, and organisms with complex life cycles, are much more difficult to model with mesocosms. Examples are tidal estuaries and coral reefs. Creating and managing these systems requires great ingenuity and a deep understanding of the ecosystems being modeled. Mesocosms of these ecosystems are costly and much less common than, for example, simple aquatic ponds and they will likely continue to play limited, special-purpose roles in experimental ecology in the future.

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