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The reproductibility of tank experiments concerning unicellular marine algal development was analyzed by means of parallel experiments with cultures ofThalassiosira rotula andSkeletonema costatum, using large flexible plastic tanks under semi-natural conditions. The tanks (3–4 m3, 4–5 m deep) were exposed in the German Bight at a station in the outer harbour of helgoland. The water was obtained from the open North Sea in towable tanks; it was filtered (plate filter), enriched with nitrate (20–30 μgat dm−3), phosphate (1.3–2.3 μgat dm−3) and silicate (15–23 μgat dm−3)-nearly natural springtime concentrations in this area-and inoculated with 103–105 cells dm−3. The water was mixed with non-metal stirring equipment. Within 5 days, concentrations of 106–107 cells dm−3 in an exponential growth phase were obtained. In experiments withT. rotula a parallel development was achieved in spite of some contamination by surrounding water. This is the case for nearly all parameters analyzed (nutrient salts, phytoplankton, bacteria, C, N and particulate carbohydrates). The heterotrophic bacteria, which were determined by means of the plate method, reached concentrations of up to 106 (T. rotula) and 105 (S. costatum) CFU cm−3, respectively. They showed a consistent retrograde development at diatom concentrations above a certain level. The crop did not increase again until the diatoms had reached the stationary phase. During exponential growth ofT. rotula (G=8.9–11.7 h) a partially synchronous cell division was observed. There were also rhythms with respect to cell size (pervalvar axes) and chain length (number of cells). For the experiments withS. costatum (G=10–11.4 h) diurnal variations of cell size and chain length occurred. The present results indicate acceptable reproducibility of algal development and related phenomena in enclosed water bodies.

Parallel plastic tank experiments with cultures of marine diatoms