Relationship between phytoplankton cell size and the rate of orthophosphate uptake: in situ observations of an estuarine population |
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Authors: | E. S. Friebele D. L. Correll M. A. Faust |
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Affiliation: | (1) Smithsonian Institution, Chesapeake Bay Center for Environmental Studies, Edgewater, Maryland, USA |
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Abstract: | Orthophosphate uptake by a natural estuarine phytoplankton population was estimated using two methods: (1) 32P uptake experiments in which filters of different pore sizes were used to separate plankton size-fractions; (2) 33P autoradiography of phytoplankton cells. Results of the first method showed that plankton cells larger than 5 m were responsible for 2% of the total orthophosphate uptake rate. 98% of the total uptake rate occurred in plankton composed mostly of bacteria, which passed the 5 m screen and were retained by the 0.45 m pore-size filter. There was no orthophosphate absorption by particulates in a biologically inhibited control containing iodoacetic acid. Orthophosphate uptake rates of individual phytoplankton species were obtained using 33P autoradiography. The sum of these individual rates was very close to the estimated rate of uptake by particulates larger than 5 m in the 32P labelling experiment. Generally, smaller cells were found to have a faster uptake rate per m3 biomass than larger cells. Although the nannoplankton constituted only about 21% of the total algal biomass, the rate of phosphate uptake by the nannoplankton was 75% of the total phytoplankton uptake rate. Results of the plankton autoradiography showed that the phosphate uptake rate per unit biomass is a power function of the surface: volume ratio of a cell; the relationship is expressed by the equation Y=2x10-11X1.7, where Y is gP m-3 h-1 and X is the surface: volume ratio. These results lend support to the hypothesis that smaller cells have a competitive advantage by having faster nutrient uptake rates. |
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