Trend analysis of nutrient concentrations and loads in surface water in anintensively fertilized watershed

Nutrient concentrations and loads from 1988 to 1997 at the surface-water ou tlet of a heavily fertilized watershed in the Brittany region, west of Fran ce, were analyzed for trends. The 1240-ha watershed is characterized by mil k production and an increasing indoor pig and poultry production. A nonpara metric statistical analysis performed on mean monthly and mean annual data detected no trend in nitrate and tot al P concentrations and loads from 198 8 to 1997. When comparing data from the same month for each gear of the stu dy, a decreasing trend was detected in the November nitrate now adjusted co ncentrations, and in the October maximum nitrate now adjusted concentration s. An increasing trend was detected in the August total P now adjusted conc entrations. no trend was detected in the monthly and annual nitrate and tot al P loads. A multiple regression analysis showed that the annual nitrate c oncentration is positively correlated to the annual rainfall and to the per cent area of corn (Zea mays L.). A nutrient global mass balance was constru cted for 1988, 1991, and 1994 to explain the lack of an increasing trend in nutrient concentrations and loads despite an intensification of the pig pr oduction during these years. Results from this analysis showed that farmers have compensated for increased on-farm production of nutrients from animal manure by purchasing less fertilizers; therefore, the total nutrient budge t remained relatively stable.