Interpretation of Radiophosphate Dynamics in Lake Waters

Models of planktonic phosphorus dynamics over the last 30–40 yr depend on the steady-state distribution of isotope for the determination of compartment size. Radiophosphate data for P-deficient lakes in summer have shown a steady-state distribution of 1–15% of 32P in the filtrate within 0.5–5 h. To explain this, a phosphate back-flux term from the particulate fraction has been widely accepted (phosphate is believed to be released from the internal pools of phosphate consumers and by excretion from herbivores and bacterivores). We show that dialysis of lake water at isotopic steady state provides values for the dissolved [32P]PO4 compartment up to an order of magnitude lower than those obtained by membrane filtration and gel filtration chromatography. This apparently occurs as a result of minor cell damage during filtration when most of the [32P]PO4 is in the particulate pool. Consequently, the size of the phosphate pool and the magnitudes of phosphate uptake and back-flux may have been overestimated by up to a factor of 10. Furthermore, the turnover time of the particulate compartment lengthens from ~ 40 min to > 1 d, which is more consistent with models describing P fluxes between functional components of the plankton.