G UILLERMO C HALAR 1 and J OSÉ GALIZIA TUNDISI 2 1 Dpt. of Limnology, Faculty of Sciences, University of Uruguay, Calle Iguá 4225. P 9. Montevideo Uruguay. E-mail: gchalar@fcien.edu.uy 2 International Institute of Ecology Rua Alfredo Lopez, 17-17 – Villa Elizabeth – Sala D.05. CEP. 13560-460 – São Carlos-SP – Brasil. E-mail: iie@parqtec.com.br Phosphorus Fractions and Fluxes in the Water Column and Sediments of a Tropical Reservoir (Lobo-Broa – SP) key words: phosphorus, sedimentation, traps, retention, reservoirs Abstract Sedimentation rates of organic and inorganic matter, chlorophyll a, P fractions, Ca, Mn, Fe and were determined by sediment traps in a tropical oligo-mesotrophic reservoir of São Paulo Vertical profiles of the sediments were analyzed for organic content, metals, P and surface P frac composition. Estimated meansedimentation rates, corrected for resuspension were:total solids, 1068 g m –2 y –1 (OM = 44.7%); chlorophyll a, 2.1 g m –2 y –1 and total phosphorus, 2.9 g m –2 y –1 . The pre- dominant P fraction in the settling flux was associated with aluminum minerals while surface sed were dominated by organic P. The reservoir exhibited low sediment retention of P (13.0%), Al (9.9 Fe (9.9%), Mn (1.4%) and Ca (traces), compared to trap sedimentation. This feature was related w a high vertical dynamics (resuspension and bottom release) and with the low retention tim system. 1. Introduction Sedimentation is a relevant process that regulates the flux of particles in reservoirs. process limits the epilimnetic nutrient regeneration and controls the food source benthic community (B LOESCH and U EHLINGER , 1986). Moreover, the sedimentation rate is an important parameter in the models used for lake restoration and mass balances. Settling particles in lakes and reservoirs contain, in addition to biological material, i manganese, oxides and hydroxides, which all offer surfaces for the (ad)sorption of ions thermore, coagulation with organic matter may induce the sedimentation of sma that would otherwise remain suspended (S IGG , 1987). In the euphotic zone inorganic dis- solved phosphorus can be adsorbed to inorganic particles or assimilated into organic c pounds by primary producers. Part of this P pool will settle while another will be recyc in the water column, according to specific sedimentation, decomposition and transform rates. P mobility is determined by its binding forms as well as the physical and environmental conditions. The fractionation of P forms allows one to determine portion is potentially mobile or not involved in exchange processes (P SENNER and P UCSKO , 1988). This technique has been used in sediment (e.g. S ØNDERGAARD et al., 1992; J ENSEN and T HAMDRUP1993; F ARMER et al., 1994; B ALDWING , 1996; K LEEBERGand K OZERSKI , 1997) and suspended particles (E NGLEand S ARNELLE , 1990; R IJKEBOER et al., 1991). Frac- tionations of material collected in sediment traps are less frequent (H UPFER , et al., 1995; U LRICH , 1997). There are more than forty reservoirs in La Plata Basin, which share several importan tures, namely: polymictic regime, shallowness and average to short retention times. Ve movements are particularly important in these systems, since they are responsible for Internat. Rev. Hydrobiol. 86 2001 2 183 –194