The input of nutrients by the Rhône river into the Mediterranean Sea: recent observations and comparison with earlier data

Abstract

The nutrient input by the Rhône river into the Mediterranean Sea was measured from June 1994 to May 1995. The various fractions of N and P (dissolved and particulate, organic and inorganic) as well as chlorophyll and dissolved silicate concentrations were measured twice a month at Arles, 40 km upstream of the river mouth, in this period. In addition, some samples were taken when the river was in high flood. The study shows that nitrogen is mainly present as nitrate (76%). Dissolved and particulate organic nitrogen represent 8 and 9%, ammonium 5% and nitrite 1% of total nitrogen respectively. Almost half of the phosphate is particulate phosphate, the largest part of which is calcium-bound phosphate. Orthophosphate represents 31% of total phosphate. We estimated the total input of nitrogen, phosphate and silicate as 115–127 kt y^-1 (N), 6.5–12.2 kt y^-1 (P) and 135–139 kt y^-1 (Si) by different methods. For nitrogen an annual input of 92.3 to 96.1 kt of nitrate is found, 1.3 to 1.5 kt of nitrite, 6.3 to 6.7 kt of ammonium, 9.7 to 9.8 kt of dissolved organic nitrogen and 5.3 to 12.7 kt of particulate nitrogen; for phosphate the annual imput was 2.7 to 3.0 kt of orthophosphate, 1.3 to 1.7 kt of dissolved organic phosphate & polyphosphate and 2.5 to 7.5 kt of particulate phosphate. While the N-input was mainly in the form of nitrate, a large part of the phosphate input was particulate-P. Comparisons to previous estimations show that the mean annual nitrate concentration in the Rhône has increased by about 50% during the last two decades.