size, net proportions and composition of the plankton
plankton. Theoretically, a water bottle sample contains all but the rarest organisms in the water mass sampled and includes the whole size spectrum from the largest entities, like diatom colonies, to the smallest single cells. Similar results can be obtained by pump sampling (Sournia, 1978, p. 41), which samples much larger quantities of water allowing the collection of the rarer species. The technique has its disadvantages, however, e.g., breaking up colonies, breaking off large Chaetoceros setae, and breaking into pieces long pennate cells like Thalassiothrix spp. In contrast to these quantitative methods, sampling by plankton nets (Sournia, 1978, p. 50) is highly selective, depending on the mesh size of the gauze, net towing speed, and the species present in the water. Chaetoceros setae, for instance, may form a fine network inside the gauze, and very small single cells, which in other cases pass through the meshes, are retained. On the other hand, nets with very fine meshes (e.g., 5 or 10 ju.m) often filter too little water to provide an adequate diatom sample. As a compromise, the most useful mesh size for collecting diatoms is 25 ¡xm.
Net hauls have the advantage of a simultaneous collection and concentration of the plankton providing sufficient quantities for species identification. Water bottle and pump samples in most cases have to be concentrated. The smaller the subsample, the fewer number of rare species will be obtained. On the other hand, there is no point in concentrating large quantities of a sample rich in one or a few species. Concentration by settling (Sournia, 1978, p. 88), centrifugation (Sournia, 1978, p. 98), and filtration (Sournia, 1978, p. 108) are the most used methods.
The rich plankton in the marginal ice zone and the ice-covered waters of the polar seas has attracted particular attention during the past decades. Scuba diving has shown that at least parts of the plankton flora begin their spring development on the undersurface of the ice. In addition, there is a particular rich and highly specialized subice flora in polar waters which may become part of the plankton when the ice melts (Syvertsen, 1991). An electric suction pump or "vacuum cleaner" used in the study of ice zoo benthos (Lonne, 1988) has been modified to sample the subice flora and algae found in cracks and crevices. A plankton net is placed in front of or behind the impeller, and the water is pushed at low speed through the net. The action is gentle and the algal cells are not damaged.
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