Biogeochemical mass-balances (C, N, P, Si) in three large reservoirs of the Seine Basin (France)

Three major reservoirs (Marne, Seine and Aube), situated in the upstream ba sin of the river Seine represent a storage capacity of 800 10(6) m(3). In o rder to quantify the possible role of these reservoirs as a sink or source of nutrients and organic matter for the river system, an input/output mass- balance of suspended matter, organic carbon, inorganic nitrogen forms, phos phorus and reactive silica was established, providing reliable estimates of their retention/elimination and export. The study was carried out over 3 y ears (1993, 1994 and 1995) in differing hydrological conditions. The retent ion times varied from 0.3 to 0.8 year, depending on the reservoir and the y ear, but was longer in 1993 that was a drier year than 1994 and 1995, hydro logically quite similar. Regarding retention (or elimination) and export, the behaviour of the three studied reservoirs was similar. A clear loss or retention of nitrogen, pho sphorus and silica was observed in the reservoirs and represented about 40% of the incoming flux of nitrate, 50% of silica, and 60% of phosphate. The retention was lower for total phosphorus than for phosphate. The reservoirs are also sites of suspended matter deposition except during the decennial drawdown, when suspended matter is exported. For inorganic nitrogen, the av erage amount of nitrate retained in the Seine basin reservoirs upstream fro m Paris is 5000 tonnes y(-1) that is almost equal to the estimated retentio n by deposition or denitrification in river channel sediments for the whole drainage network. The retention in the reservoirs represents about 12% of the total flux of nitrate at the outlet of the basin upstream from Paris, a nd 5% at the mouth of the Seine River. We also calculated inlake C, N, P, Si budgets on the basis of direct proces s measurements. Measurements of planktonic primary and bacterial activity p roduction led to annual net production of 4200 and 580 tonnes of carbon, re spectively. A reasonable value (450 tonnes of carbon) of grazing was calcul ated. Corresponding N, P, Si fluxes were drawn from appropriate C:N:P:Si ra tios. Benthic fluxes were measured with bell jars. The retention of P and S i represents a small fraction of important internal fluxes of phytoplankton ic uptake and recycling, while inorganic nitrogen retention depends mostly on benthic denitrification. The behaviour of P and Si differs in that P is mainly recycled in the water column, while Si dissolution occurs at the sed iment interface. Nitrogen is recycled in both the planktonic and the benthi c phase.