Trends in diffuse nutrient concentrations and loading in Denmark: statistical trend analysis of stream monitoring data

Abstract

The Danish Monitoring Programme reveals that diffuse sources are currently the major source of nutrient input to the aquatic environment. Nutrient concentration and loading data for about 50 small agricultural catchments and seven undisturbed catchments reveal that agriculture is the major diffuse source, average annual nitrogen (N) and phosphorus (P) concentration and loading being respectively, 5.1- and 6.8-fold, and 2.7- and 3.9-fold greater in streams draining agricultural catchments than in streams draining undisturbed catchments during the period 1989–96. A statistical trend analysis (discharge adjusted Mann-Kendall seasonal test) applied to long-term time series (1967–96) in three streams revealed an upward trend in nitrate concentration in Skjern Å (p<0.1%), Odense Å (p=5.7%) and GudenÅ (p=11%) during the 15-year period 1967-81, whereas there was no significant trend during the following 15-year period (1982-96). There was a significant downward trend in P concentration in all three streams over the 30-year period. Statistical analysis of recent trends (1989-96) in 77 (N) and 40 (P) streams draining small agricultural catchments revealed a median annual decrease of 0.073 mg N 1−1 and 0.0034 mg P 1−1 (Sen's slope estimator). The observed trend was only significant in 6 of the streams tested at the 5% level and 27 at the 10% level, however. The median annual decrease in N and P concentrations was higher in streams draining loamy catchments (0.092 mg N 1−1 and 0.0035 mg P 1−1) than in streams draining sandy catchments (0.040 mg N 1−1 and 0.0005 mg P 1−1).