PHOSPHORUS UPTAKE KINETICS OF SPIROGYRA FLUVIATILIS (CHAROPHYCEAE) IN FLOWING WATER1,2

The inorganic phosphorus (P_i) uptake kinetics of Spirogyra fluviatilis Hilse were examined as a function of phosphorus cell quota (Q^P) and flow velocity in a laboratory stream apparatus. Short-term uptake and the acclimation of the uptake mechanism to flow were measured by the disappearance of P_i pulses in a recirculating flow cell. Short-term P_i uptake was biphasic. When the alga was P-deficient, Phase 1 and 2 half-saturation constants and maximum uptake rates were 11.0 and 47.2 μg P·L^?1 and 473 and 803 μg P·g dry wt^?1 h^?1, respectively. Flowing water altered short-term uptake when the alga was P-deficient, but not when it was P-replete. When Q^P was less than 0.21%, increases in flow velocity from 3 to 15 cm·s^?1 enhanced uptake with maximum uptake for any P_i pulse at 12 and 15 cm·s^?1. At 22 and 30 cm·s^?1, uptake was reduced by 12% or more relative to the maxima. If, however, the alga was cultivated at 22 and 30 cm·s^?1 and short-term P_i uptake was measured at 12 cm·s^?1, uptake was on average 33% greater than when the alga was cultivated at the latter velocity. Apparently, the alga could adjust short-term uptake to compensate for the suboptimal conditions of the faster velocities. Long-term P_i uptake and net phosphorus efflux were estimated by a non-steady state application of the Droop equation. Long-term uptake of very low P_i concentrations was not reduced by fast flowing water. Instead, uptake increased proportionately with flow velocity. Maximum phosphorus efflux from S. fluviatilis was 3% of cellular P per hour and occurred when Q^P was greater than 0.2%. At lower Q^P, the hourly efflux rate was typically less than 1%. Flowing water did not greatly enhance efflux, although when P_i was undetectable, efflux did tend to increase slightly with velocity. The data show that the effects of flowing water on P_i uptake were varied and not always beneficial. If the effects of flowing water on nutrient acquisition by other lotic algae are similarly varied and complex, flow may be an important determinant of nutrient partitioning among benthic algae in streams.