Investigations of the distribution and flux of phosphorus (P) among the compartments using radioisotope tracers, selective filtration and gel chromatography have provided an insight into the general nature of the epilimnetic P cycle in lakes. Under P-limited conditions, enzymatic hydrolysis and ultraviolet (UV) mediated photo-oxidation of dissolved organic phosphorus (DOP) are important rate-limiting processes in the P cycle. The objective of this study was to investigate the dynamics of P in flood plain lakes and rivers in the Amazon River Basin near Manaus, Brazil. Radioisotope tracers, selective filtration, ultrafiltration and gel chromotagraphy were used to characterize the different components of P and their dynamics in these systems. Laboratory assays were also used to charactize the lability of different DOP fractions to enzymatic hydrolysis and UV photo-oxidation.

When carrier-free 32P was added to surface waters from Lago Camaleao, a flood plain lake associated with the Solimoes River, the labeled P fractionated slowly (12 hours) into 4 distinct P compartments: a particulate P pool (>0.2millimeters, 83%), a phosphate pool (9.4%), a high molecular weight DOP component (~66,000 Da, 7.3%) and a very high molecular weight DOP component (>200,000 Da, 0.3% ). The accumulation of activity in the PP pool was greatest in the 0.2-1 millimeter-size fraction suggesting that bacterial uptake may control the flux between the PO4 and PP pools. Turnover times for P in the PO4 pool ranged from 8.3-11.1 hours. Enzymatic hydrolysis of DOP was found to occur in both lakes and rivers.and the low molecular weight fractions (<1,000) appeared to be more susceptable to hydrolysis. Significant levels of ultraviolet-mediated photo-oxidation was only encountered in the Negro River and only in the DOP fraction from 1,000 - 30,000 Da.

The dynamics of P in Amazon flood plain lakes and rivers is apparently quite different from that encountered in glacial lake basins of the north temperate zone. The long turnover rates encountered for phosphate in Lago Camaleao, suggest that P may not be as limiting in it is in north temperate lakes. Seasonal inputs of P-rich waters and sediments from the Solimoes River may help to reduce P-limitation in this case. The DOP compounds in these tropical flood plain lakes and rivers also appear to be much larger and more refractory. This may reflect the predominance of refractory terrestrially derived organic matter in these riverine environments. Photo-oxidation of POD appears to occur only in high DOC systems such as the Negro River.