Allometric estimation of the productivity of phytoplankton assemblages

Abstract

An allometric model is used to estimate the productivity of natural phytoplankton assemblages. The 14C fixation rates of > 5 ~m phytoplankton cells, measured over a 5 yr period in the Celtic Sea, are used to test the relationship between cell size and productivity of natural phytoplankton assemblages. Phytoplankton biomass, as estimated from cell volume, explains between 85 and 90 % of the variance in 14C fixation rate under optimum growth conditions; in comparison, chlorophyll a concentration explains only 59 % of the variance. An allometric model, applied to the estimated phytoplankton biomass, is used to calculate the potential productivity of the phytoplankton under optimum growth conditions; this estimate of phytoplankton productivity explains 93 % of the variance in the in situ 14C fixation rate. When applied to data obtained over a 4 d period in the Celtic Sea, differences are found in the growth rate of individual species as well as in presumed losses due to grazing and sedimentation. This approach has the potential to estimate the productivity of each of the individual phytoplankton species which comprise a natural assemblage. where rate is growth rate or some other metabolic function: W = a measure of the mass of the organism; and (a) and (b) are constants (Peters 1983).