Influences of bag dimensions on the development of enclosed plankton communities during POSER

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Abstract
During POSER natural plankton communities were enclosed by large plastic bags anchored in situ. Enclosures of different dimensions, ranging in depth from 3-40 m and containing 1.5-30 m3 of water, facilitated the study of dimension effects of the enclosure on the development of the plankton inside. When comparing mean values in small bags with those of the total mixed layer of large bags, phytoplankton development was very similar in large and small enclosures. Although large and small enclosures had been filled simultaneously, bacterial numbers increased faster in small than in large enclosures, probably because of closer contact with the substrate. Copepod populations suffered high mortalities, especially in small bags. Their mortality may have been influenced by the extremely cold weather prevailing during the experiment. Fluctuations in environmental factors (temperature, light, nutrients, etc.) appeared to be much more important for plankton development than the dimensions of the enclosure. The suitability of enclosures for studying natural plankton ecosystem is discussed. Optimum dimensions depend on the aim of the experiment, number of trophic levels enclosed, population density at these levels and on the species present at these levels. For ecotoxicological experiments with marine phyto- and zooplankton commmunities in eutrophic waters, enclosures of 1-2 m3 are sufficiently large, and for practical reasons should be preferred over larger ones. In more oligotrophic waters, enclosures with a volume of circa 10 m3 are preferable, so that larger zooplankton samples can be taken.