In order to monitor annual phosphorus loadings to Lake Geneva from its tributaries, during 1986 and 1987, routine samples were collected at the mouths of the Rhône, Venoge, Aubonne and Promenthouse Rivers, Switzerland. Complementary sampling was undertaken for several storm events during the same years on the Venoge River, a storm dependent system. Soluble reactive phosphorus (SRP), and total soluble phosphorus (TSP) were analysed using filtered or centrifuged waters. Total particulate phosphorus (TPP), organic phosphorus (OP), inorganic phosphorus (IP), apatite phosphorus (AP) and non-apatite inorganic phosphorus (NAIP), as well as other geochemical parameters of the samples were determined on the suspended sediment (SS) <63 μm recovered by centrifugation.
The annual low and high flow loads of these phosphorus forms are estimated for each river. The total phosphorus (TP) loads, and in particular, the bio-available phosphorus (BAP) loads of these rivers are compared to assess their contributions to Lake Geneva. The results show that, although, the Rhône River annually transports TP about 29 times and TPP 47 times more than the three small rivers together, the BAP loading from the Rhône River is only 9 times higher than for the three small rivers. The phosphorus influence of these smaller rivers on the lake is important because it is the BAP load rather than the TP or TPP load that determines the response of the lake ecosystem to phosphorus inputs.
A digital filter was developed to simulate SS transport behaviour during storm events as a fonction of discharge. The simulation curves of SS concentration for the five sampled events were tested and found to be similar in shape to the measured ones. SS loads estimated by the calculated SS concentrations for these events have a difference, as a whole, of about 11 % from the measured concentrations. Moreover, a progressive regression procedure was developed to relate the estimation of TPP concentration to the SRP and SS concentrations measured for the sampled storm events. This allows estimation of other forms of particulate phosphorus if the TPP concentration is known, based on the simple linear relationships between these phosphorus forms for the Venoge storm samples. The relative error of the calculated TPP from the measured TPP is generally no more than ±10%. These two models provide a means to approximate SS and phosphorus concentrations, and thereby their loadings using discharge and SRP and SS measured during storm events on the Venoge River. The SS and phosphorus loads derived from the models complement the sampling programme as well as resultant load estimates. For example, the 1987 TP load for the Venoge River, estimated using the filter model procedure (23.3 tonnes), is comparable to those assessed with the sampled data (26.8 tonnes) and by CIPEL (20.0 tonnes).
