THE INPUT OF NUTRIENTS BY THE RHONE RIVER INTO THE MEDITERRANEAN SEA - RECENT OBSERVATIONS AND COMPARISON WITH EARLIER DATA

The nutrient input by the Rhone river into the Mediterranean Sea was m easured from June 1994 to May 1995. The various fractions of N and P ( dissolved and particulate, organic and inorganic) as well as chlorophy ll and dissolved silicate concentrations were measured twice a month a t Aries, 40 km upstream of the river mouth, in this period. In additio n, 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 nitr ite 1% of total nitrogen respectively Almost half of the phosphate is particulate phosphate, the largest part of which is calcium-bound phos phate. 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 me thods. For nitrogen an annual input of 92.3 to 96.1 kt of nitrate is f ound, 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 nit rogen; for phosphate the annual imput was 2.7 to 3.0 kt of orthophosph ate, 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 i n the form of nitrate, a large part of the phosphate input was particu late-P. Comparisons to previous estimations show that the mean annual nitrate concentration in the Rhone has increased by about 50% during t he last two decades.