A MODEL FOR CALCULATION OF NUTRIENT-UPTAKE CAPACITY OF SEAWEEDS IN CULTIVATION

A formula has been compiled, which calculates the capacity of marine macroalgae to absorb nutrients. The formula uses the C : N : P-ratio of the alga, the capability of the alga to use the incident irradiation for net carbon growth (i. e. the quantum yield for net carbon growth; alpha(net)) and the total irradiation (PAR) reaching the alga. From known data on incident irradiation in a certain region, the formula can be used to calculate the maximal nutrient assimilation per m2 sea surface area for a certain algal species for which alpha(net) and the C : N : P-ratio has been determined. The validity of the formula was tested by comparing calculated and measured values for nitrogen uptake rates for two algal species, Ulva lactuca L. (Chlorophyta) and Chondrus crispus Stackh. (Rhodophyta), growing in a flow through system in nutrient rich sea-water under defined irradiation conditions. The efficiency of light utilisation was derived from measurements of the net carbon growth rate and the total irradiation (PAR) reaching the culture vessel, assuming a theoretical (i. e. maximal) value for the quantum efficiency of photosynthetic carbon assimilation. Ulva lactuca was found to utilise about 30% of the available light (alpha(net) = 0.031) and was about twice as efficient as C crispus (alpha(net) = 0.015). Much of this difference could be ascribed to differences in the quantum efficiency for carbon assimilation. The difference in quantum efficiency was also the major reason for a predicted two times higher nutrient absorption capacity for U. lactuca per m2 of sea surface when compared to C. crispus. A more realistic value for this quantum efficiency was calculated from fluorescence induction measurements on dark adapted algae, thus allowing a more precise calculation of different factors affecting the degree of light utilisation. It was concluded that the validity of the above formula could be improved by an incorporation of results from field and laboratory measurements with a commercial plant stress meter.