Biological Considerations in Marine Enclosure Experiments: Challenges and Revelations

LOGISTICAL CONSIDERATIONS have usually dictated that components of estuarine systems be considered in isolation from one another whether in the laboratory or field. Hence, biological disciplines usually have specialized to a single area or process. By contrast, enclosure or mesocosm experiments force the study of all components interacting together. Since they are no longer studied in isolation, these components can be observed to behave in unpredicted, interactive patterns. Since 1976, experiments have been conducted in enclosures at the University of Rhode Island. These enclosures model various aspects of the coastal environment. They are 5 m deep, to maintain a heterotrophic benthos, 13 m 3 in volume so zooplankton can be sampled without impacting population abundances, and 2.62 m 2 in area so that the benthos may be sampled with minor impact over annual cycles. The simplicity of the enclosures together with flexibility in design and use has led to their long life. Mixing can be vertical, horizontal, intermittent, or constant at any reasonable intensity. The enclosures are sufficiently deep to allow thermal and salinity stratification to be simulated. Experiments can therefore be conducted on marine systems exhibiting different degrees of stratification. Sea water can flow constantly, or intermittently, or not at all for whatever turnover is deemed appropriate. Sediments can be present or absent; when present, they can come from various sources. Sunlight can be natural or shaded or ultraviolet enhanced. As with any new approach, the outcome of early experiments was rarely predicted correctly. Manipulations, such as installing the sediment community, often had greater effects than experimental treatments. Along the way insights have been gained on interactions between ecosystem components. Some of these insights, serendipitously discovered, were