Phytoplankton cages for the measurement in situ of the growth rates of mixed natural populations

We have designed and tested phytoplankton “cages” in which captured natural populations can be re-suspendedin situ at their original site or at different sites. The “bars” of the cage consist of a membrane whose pores are sufficiently large to permit relatively rapid exchange of “inside” and “outside” nutrient-containing waters which ensures that the contained phytoplankton are always subjected to the “outside” (in situ) nutrient regime. The pores are sufficiently small to retain the captured phytoplankton. In order to measure growth rates precisely the cages are incubatedin situ for times greater than the doubling times of the population (ca. 3 days). The population changes within the cages reflect the integral net primary production. Therefore the technique can be used to characterize geographical locations in terms of their potential for supporting the growth of natural phytoplankton populations. A possible specific application of this technique is the comparative study of the effects on the growth of phytoplankton of near field and far field water masses adjoining power plants or sewage treatment plants.