"Impacts of agricultural water protection measures on erosion, phosphorus and nitrogen loading based on high-frequency on-line water quality monitoring"

There is an urgent need to decrease agriculturecontributed nutrient loading to surface waters. Excess amounts of phosphorus and nitrogen may lead to severe environmental problems, such as eutrophication and toxic algal blooms. Potential mitigation measures have been introduced to reduce loading, but their impacts on erosion, phosphorus and nitrogen loading are difficult to detect, due to challenging monitoring of diffuse (nonpoint) loading. New methods for defining nutrient loads are therefore needed. Accurate quantifying of diffuse loading also provides valuable information on developing adaptation strategies and efficient management practices to attain targets set by the European Union Water Framework Directive. Here, high-frequency on-line water quality and quantity monitoring (HFM) was used to detect the impacts of various agricultural mitigation measures on erosion, phosphorus and nitrogen loading on the catchment scale. Gypsum, wetland and wintertime vegetation cover in a cold climate were examined. The benefits of HFM were assessed by comparing the impacts of varying sampling frequencies on nutrient load estimations in stream waters. The effectiveness of the mitigation measures was assessed in different sized catchments under varying hydrologic conditions. Here, we 1) determined how HFM can be used to obtain more precise estimations of nutrient loads on the catchment scale, 2) tested an approach to identifying the changes in nutrient loading due to management practices conducted in the catchment and 3) studied the impacts of various agricultural mitigation measures (gypsum, wetland and wintertime vegetation), using HFM. Comparing the various sampling intervals in the load calculations clearly revealed the value of HFM. We found that with discrete water samples, phosphorus load was more likely underestimated compared to sensor-based reference load. Based on hysteresis analysis, fields were considered important source areas of phosphorus. Gypsum reduced erosion and phosphorus loading very effectively in clayey agricultural catchment. Dissolved reactive phosphorus concentrations also became lower after gypsum application. The wintertime vegetation cover decreased the total phosphorus loads under mild winter conditions, when phosphorus loading is usually major. No impact on the dissolved reactive phosphorus concentration was observed. Small constructed wetland retained phosphorus and nitrogen on a yearly basis. The wetland retained most of the incoming phosphorus and nitrogen loads during the growing season, but in spring and autumn the effectiveness was weak. The seasonal and short-term variation in nutrient removal efficiency would not have been detected without HFM. In conclusion, we provide a guideline on how to develop future water quality monitoring and how to assess the effectiveness of the various mitigation measures on the catchment scale. HFM can be used not only for estimating the impacts of agricultural mitigation measures, but also for providing more information on the water quality