Biotests with phytoplankton assemblages. Growth limitation along temporal and spatial gradients

The development of eight different species (populations) along temporal and vertical gradients in several lakes was studied. Many populations had an exponential growth phase and a decline phase. The growth rate was often high during the exponential phase. Some species, e.g. Oscillatoria spp. and Synedra cf. acus, often also had a long stationary phase. The growth rate and the sinking rate of these populations were often very low. Laboratory batch experiments with dilute phytoplankton populations were carried out to estimate the degree of growth limitation (L) for different populations sample from different lakes during the three growth phases. L was always low and often zero for populations initially in the exponential phase and always high for populations initially in the decline phase. The biotests also gave results that can help to explain the vertical distribution of Oscillatoria or Asterionella in three lakes investigated. The results indicate that the growth rates and the development of the populations were dependent on the external chemical and physical conditions. The transition between the different growth phases seemed often to be dependent on the external nutrient conditions. P, N, Si and Fe were probably the most growth-limiting nutrients. The growth rate of some diatoms was probably limited directly or indirectly at high pH. Laboratory biotests with natural populations may give valuable information on the growth-properties of different populations in the lakes. The biotests should, however, be carried out in combination with chemical and physical measurements and quantitative determinations of population densities.