US Long-Term Ecological Research Network
Scale in the design and interpretation of aquatic community research
The temporal and spatial scales that exist for Iimnetic communities result from basic physical and geological conditions (e.g., diurnal and annual cycles, like morphometry) and biotic dynamics (e.g., organism life spans, nutrient turnover times). For aquatic communities, these factors interact to create time scales ranging from fractions of a second to decades, and spatial scales from fractions of centimeters to tens of meters. In addition, the degree to which entities in a community are perceived or aggregated (e.g., individual species, size classes, functional groups) can also be considered as a scale perspective. In each case, the scales used to view aquatic ecosystems can have a major influence on the ways that communities are perceived. Sampling schemes for both observational and experimental studies must be designed with these characteristic scales in mind. It is possible to use a scale that is either too broad or too narrow for a phenomenon of interest. ... Investigations of the microbial loop, for example, may require sampling schemes on the scale of seconds, while phenomena associated with strong piscivore year classes may require sampling over decades. On spatial t· scales, analyses of horizontal distributions may be detected using sampling intervals greater than 1 m while vertical distributions require sampling at intervals of a few cm. In resolving community components, aggregation may be a necessity forced by the limited ability of a researcher to collect and measure things, but it may also be an advantage that can lead to the detection of greater regularity in system behavior. Scale plays an important role in the design and interpretation of experiments. For example, the most effective perturbation for a manipulation will depend upon the response time of the system components under study. Spatial scale presents special problems for experiments, which can usually only be controlled and replicated on scales much smaller than a typical lake. Since it can be demonstrated that any small-scale experimental model of a lake must rather quickly deviate from behavior of the actual lake (at least for complex community processes), methods of extrapolating from behavior on the small scale to that on the larger scale are needed. We discussed a theoretical approach to such a scaling rule. Aggregation must be considered both in terms of the community components that are manipulated in an experiment and in terms of the response variables that are measured to assess a system’s response. In some cases, choices can be made for appropriate scale in observations or experiments. In other situations, scale choice may be constrained by practical limitations. Under either circumstance, scale considerations should be incorporated explicitly into any investigation of an aquatic community. Finally, we considered the scales of manipulation that are employed in community studies, ranging from observations of natural systems through a variety of small-scale experiments to whole-lake experiments. In choosing an approach that is appropriate for a particular question, we discussed how the closeness with which an experimental system approximates natural conditions is balanced against an ability to detect a response against natural variability. Small-scale experiments have advantages in their logistics and replicability, but must be interpreted relative to their ability to reflect natural community processes. Whole-lake experiments incorporate a full range of natural conditions, but are difficult to perform and to replicate. Whole-lake experiments were considered as particularly important, and we discussed techniques for their interpretation. A hierarchical combination of approaches with smaller-scale experiments conducted in the context of whole-system manipulations would appear to provide the most powerful insights into aquatic community processes.
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