What headwater streams can tell us

A milestone in BIOWATER was reached when the paper “Land-use dominates climate controls on nitrogen and phosphorus export from managed and natural Nordic headwater catchments” was published in the scientific journal Hydrological Processes. In Module 2 of BIOWATER, our aim is to understand catchment processes and trends in nutrients by analysing long-term data series. In this paper, 12 authors have studied water quality and land use data from 69 Nordic catchments since 2000. The main author, Dr. Heleen de Wit, says that their results reveal that concentrations and fluxes of both nitrogen and phosphorus were highest in agricultural catchments, intermediate in forestry‐impacted and lowest in natural catchments.

When analysing for trends, De Wit and co-authors found few significant changes over time, with some exceptions. Mitigation efforts to reduce N loadings were implemented in Denmark and Sweden particularly in the 1990s, with apparent success, but after this, few further reductions were found, even though the N surplus was reduced at the national levels in all Nordic countries. For agricultural catchments, the interactions of climate, soil type, crop and management make it complicated to draw firm conclusions on effectiveness of N and P mitigation measures. In agricultural soils, large stores of P are still in place (legacy P), which also makes it difficult to detect changes in P runoff related to mitigation measures.

Figure 2 in de Wit et al. (2020) shows the differences in nutrient contents of streams draining forests with and without forestry, as well as agriculture.

Forestry-impacted catchments are more extensively managed than agricultural catchments but also here, interactions between management and soil type complicate analyses between climate drivers and nutrient runoff. The natural catchments showed a decline in nitrate runoff, which is likely to be related to reduced atmospheric N deposition, changes in winter hydrology, and increases in standing biomass.

Logging can lead to increased leaching of nutrients as well as erosion. Photo: H. de Wit.

Despite a strong focus on reductions of nutrient loadings by mitigation measures in the Nordic countries, we cannot conclude based on our data that such mitigation measures have led to widespread improvements in water quality.

The green shift implies that society will rely more on renewable biomass resources for the provision of food, fodder, fibre and fuel. Given the results in this paper, there is a clear risk for increased nutrient runoff in the future.

Nutrient losses from natural forests differ from forests under active forestry. (Photo: Eva Skarbøvik)

Feature photo: Eva Skarbøvik.