Rapid assessment of multiple-limiting factors of phytoplankton biomass : bioassays, in vivo chlorophyll-a fluorescence, and factorial design

An approach is described here that has been designed to determine the fac- tors that control growth and standing crop in natural populations of phytoplankton. This approach involves the use of 1) small volume bioassays of natural phytoplankton samples, incubated in semi-natural conditions, 2) indirect determination of phyto- plankton biomass during growth by in vivo chlorophyll-a fluorescence measurements, and 3) computer-designed factorial plans. A commercial computer program was used to achieve optimal planning of the assays, statistically sound and rapid data analysis, and forecasting of phytoplankton responses to inputs. An example of field application of the protocol is described, in which a phytoplankton population of a Western African Lake was studied. This approach can be used to rapidly identify the main bottom-up control factors and their interactions, and further refining of analysis with optimization of sample number. Modelled responses also make it possible to predict how phyto- plankton biomass would change in reaction to changes in environmental conditions. The pros and cons of the approach are discussed taking into account the use of frac- tional design and the reduction of sample numbers.