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Nitrogen (N) and phosphorus (P) losses via agricultural drainage water have negative impacts on receiving water bodies and large-scale programmes to reduce nutrient losses have been established in the Nordic and Baltic countries, together with agricultural catchment monitoring programmes. This study evaluated time series (9–40 years) of data from 34 selected Nordic-Baltic catchments for spatial and temporal variations in area-specific water discharge (mm) and in concentrations and transport of total nitrogen (TN) and total phosphorus (TP). Water discharge from the catchments varied from 125 mm (Denmark) to > 1000 mm (Norway). Catchments with low TN concentrations (≤3 mg L-1) were dominated by clay or grass leys or were undrained with reduction of nitrate (NO3) in shallow groundwater. Catchments with high TN concentrations (≥10 mg L-1) had loams and cereal crops. TP concentrations were highest (≥0.45 mg L-1) in catchments with erosive soils, relatively high water discharge and cereal crops, and lowest (≤0.07 mg L-1) in catchments with permeable soils. Generalised additive mixed model (GAMM) analysis of time series of transport and flow-weighted concentrations of TN and TP for temporal patterns revealed decreases in TN concentrations in seven catchments and increases in eight, while four had periods with opposing trends. TN concentrations decreased in Denmark and Sweden in 1990–2010, following introduction of mitigation programmes. TP concentrations decreased in eight catchments and increased in six, while one showed opposing trends. Decreases in TP coincided with improved P balance in catchments with sand and loam. To further reduce N and P losses, a tailored set of mitigation measures is needed for each combination of soil, climate, geohydrology and agricultural production. Intensive monitoring of small catchments can reveal how N and P losses relate to natural conditions and to changes in agricultural production.

In most countries of the Baltic Sea region monitoring systems are in place to provide information on agricultural impacts on surface and soil/ground water quality on different levels, i.e. plot, field, small catchment and larger rivers scale. Unfortunately, the spatial and temporal coverage of monitoring sites is rather varying. Moreover, transparency of the data is not always sufficient and it can be difficult to draw a clear picture on what is actually described by the provided data. Therefore, comparison of monitoring results can be restricted. The developed guidelines attempt to find answers to these questions by highlighting the strengths, gaps and weaknesses in the assessments of the results of monitoring on different scales. The guidelines include also a list of conclusions and recommendations to further improve the existing procedures and methodologies for surface and soil water quality monitoring in agriculture-dominated areas.

There is an urgent need to decrease agriculture-contributed 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. 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. We found that with discrete water samples, phosphorus load was more likely underestimated compared to sensor-based reference load. 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 most of the incoming phosphorus and nitrogen loads during the growing season, but in spring and autumn the effectiveness was weak. Thus, a problem concerning wetlands as a mitigation measure is that they do not function efficiently during the critical times of load generation. 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 impacts of land-use changes or impacts of stormwater treatment practices, as well as for developing models to produce more reliable scenarios for nutrient loading in changing climates. Mitigation methods such as gypsum and winter time vegetation, which are implemented in large field areas, are strongly recommended for reducing erosion and phosphorus loading in boreal agricultural clayey catchments. These mitigation measures are effective, particularly in mild winter seasons, and thus will also be beneficial under future climate conditions. Hajakuormituksena vesistöihin valuvat ravinteet pilaavat ja rehevöittävät vesistöjä. Maataloutta kritisoidaan usein riittämättömistä ja tehottomista toimista vesistöihin kohdistuvan kuormituksen vähentämiseksi. Toimenpiteiden vaikutuksia ei kuitenkaan voida havaita riittävällä tarkkuudella nykyisin käytössä olevien seurantamenetelmien avulla. Yksittäisiin vesinäytteisiin perustuvat ravinnekuormitusarviot saattavat olla niin virheellisiä, että toimenpiteiden vaikutukset jäävät virheen alle piiloon. Tässä väitöstyössä hyödynnettiin sensoritekniikkaan perustuvaa automaattista tiheän mittausvälin ravinnekuormituksen seurantamenetelmää tarkentamaan maataloudesta huuhtoutuvan ravinnekuormituksen määrää. Tutkimuksessa todettiin, että nykyisin käytössä olevat yksittäisiin vesinäytteisiin perustuvat ravinnekuormitusarviot on todennäköisimmin arvioitu liian pieniksi. Menetelmää käytettiin myös kolmen erilaisen vesiensuojelumenetelmän vaikutusten mittaamiseen. Kipsin todettiin vähentävän merkittävästi peltovaltaisen valuma-alueen fosforikuormitusta ja eroosiota. Kipsin vaikutukset tulevat esiin nopeasti ja ne kestävät noin neljä vuotta. Peltojen laajaperäinen kipsikäsittely eteläisen ja lounaisen Suomen savipelloille vähentäisi merkittävissä määrin Suomen Itämereen kuljettamaa fosforikuormaa. Talviaikainen kasvipeitteisyys kirkasti peltojen halki virtaavan joen vettä ja vähensi erityisesti tulva-aikaista fosforikuormitusta. Talviaikaisen kasvipeitteisyyden liukoisen fosforin huuhtoumaa lisäävää vaikutusta on aiemmin todennäköisesti liioiteltu. Tutkimusalueen kokonaisfosforikuorma pieneni erityisesti tulvatilanteissa, eikä liukoisen fosforin pitoisuuksissa havaittu muutosta. Kosteikon havaittiin pidättävän hyvin sinne kulkeutuvia ravinteita kasvukaudella kun kosteikkokasvillisuus oli runsaimmillaan. Kevään, syksyn ja leutojen talviolosuhteiden aikana, kun kuormitus on suurimmillaan, kosteikon toiminta oli kuitenkin heikkoa. Jos ilmastonmuutos merkitsee eteläisessä Suomessa leutojen sateisten talvien lisääntymistä, on vesiensuojelutoimia kohdistettava entistä enemmän ravinnekuormituksen alkulähteille, eli pelloille. Veden laadun seurantaa suositellaan kehitettävän kohti sensoritekniikkaa hyödyntäviä menetelmiä, joilla esimerkiksi jokien ravinnekuormista ja vesiensuojelumenetelmien vaikutuksista saadaan aikaisempaa huomattavasti tarkempi kuva.