ALBERT

Description: Competition experiments with phytoplankton under steady state conditions have largely verified the competitive exclusion principle. Coexistence of species limited by different resources contributes little to the explanation of the natural diversity of phytoplankton. It is shown by multispecies experiments in flow-through cultures that pulsed input of a key nutrient allows the coexistence of species competing for the same resource.

Description: (1) Interactions between silicon concentration and diatom growth have been studied in Lake Constance with data on number and volume of cells per unit volume of water and concentration of dissolved reactive silicon. (2) Concentration of dissolved reactive silicon was found to decrease markedly during the spring and summer growth period of diatoms in 1979 and 1980. (3) Amounts of silicon taken up per cell and per unit cell volume have been calculated from the changes in silicon concentration and the increases in population density for several diatom species. The silicon to cell quotient (mg per million cells) varied over a wide range from 0.005 for Stephanodiscus hantzschii to 0.72 for Synedra acus, while that of silicon per unit cell volume (mg mm-3) varied over a narrower range from 0. 10 for Stephanodiscus hantzschii and Fragilaria crotonesis to 0.36 for Synedra acus. (4) A four-compartment model, that allows estimates of the rates of growth and of loss of diatom populations to be calculated, is outlined. (5) Summer succession from Asterionellaformosa to either Fragilaria crotonensis (in 1979) or Stephanodiscus binderanus (in 1980) is explained by interspecific differences in the effects of silicon concentration on growth and death. For Asterionellaformosa, silicon depletion did not stop cell-division but led to the death of most of the population. For Fragilaria crotonensis and Stephanodiscus binderanus cell division stopped but cells did not die and population growth continued after the concentration of dissolved silicon had increased again

Description: P-limited chemostat cultures of the green alga Scenedesmus acutus were inoculated with the herbivorous zooplankter Daphnia galeata after steady state had been reached. At low dilution rates (up to 0.5 d-l) and consequently low P content of the food algae (P: C 〈 0.0011) Daphnia could not grow. At higher dilution rates (from 0.55 d-l upward) and higher P content ofthe food, Daphnia developed dense populations which were able to reduce algal biomass more than IO-fold. This reduction was accompanied by an increase in the P content of the algae which gave rise to increased birth rates of Daphnia. The birth rates of P-limited Daphnia depended on algal cell quotas in a saturating fashion which was mathematically similar to Droop’s growth equation. During P limitation the birth rates of Daphnia were positively correlated to population density, which suggests intraspecific facilitation instead of competition.

Description: Phytoplankton species composition and the availability of potentially limiting resources were investigatcd in five northern German lakes (Suhrer See, Kellersee, Behler See, PluBsee, and Krummsee) during spring 1988. Species composition was related to resource ratios to test Tilman’s resource-ratio hypothesis. The clearest results were obtained for Aulacoseira spp. Both the time-courses within the lakes and the comparison among the lakes suggest a strong tendency of this genus to become dominant at high Si : light ratios. Fragilariaceae occupied the next position on the Si : light gradient. Such conditions permitted them to become important even when Si : P ratios were quite low. With the onset of stratification diatoms were replaced by flagellates.

Description: I tested the extent to which differences in light supply could influence the outcome of nutrient (Si and N) competition between marine phytoplankton. Competition experiments were performed with 11 species of marine phytoplankton at Si: N ratios from 16 to 124 : 1, light intensities from 28 to 225 µmol quanta m−2 s−1, and three different daylengths. Thus, light supply was the composite result of two components: photoperiod and intensity. Diatoms were dominant competitors at higher Si: N ratios, nonsiliceous flagellates at lower ones. Light had no impact on the transition from flagellate to diatom dominance along the Si: N gradient. However, species within those groups were separated along the light gradient. Contrary to theoretical expectations, changes in light intensity and changes in daylength led to similar shifts in species dominance. Therefore, it was possible to describe the light climate by the integral parameter “daily light dose.”

Description: During a 10-month study in Plusssee Si, N, and light were found as potentially limiting resources for phytoplankton growth rates. Therefore, three ratios of essential resources (Si : N, Si : light, N : light) and one ratio of substitutable resources (nitrate: ammonium) were compared to changes in species composition to test the hypothesis that the seasonal change of phytoplankton species composition was a response to changing resource ratios. The relationship was analyzed by a rank correlation analysis between the relative contribution of individual species to total biomass and resource ratios. Allowance was made for time lags between changes in resource ratios and changes in relative biomass. Of 16 species, 14 showed a significant response to at least one resource ratio. Time lags ranged from 0 to 6 weeks. Most species seemed to be favored either by minimal or maximal ratios; optimal ratios in the middle of the range were rare.

Description: The extent of nutrient limitation of phytoplankton in eutrophic Pluβsee was studied by enrichment bioassays and by analysing the cellular stoichiometry of monospecific fractions obtained by size fractionation and density-gradient separation. In this lake silicate and nitrogen, but not phosphorus, at times limit the reproductive rates of phytoplankton. The dependence of nutrient-limited reproductive rates on the cellular content of the limiting nutrient (cell quota) could well be described by the Droop model. Biomass specific minimal cell quotas of nitrogen ranged from 0.014 to 0.061 mol N mol-1 C, minimal cell quotas of silicon ranged from 0.055 to 0.127 mol Si mol-1 C. The cell quotas of the non-limiting nutrients usually increased with the cell quotas of the limiting nutrient. In contrast to the Droop model, the Monod model which relies on ambient concentrations of limiting nutrients was a much poorer predictor of growth rates.