# Northern Nordic river mouth N:P:Si stoichiometry shows limited evidence of Si depletion

## TL;DR

This study examines the N:P:Si ratios in river mouths across Northern Nordic countries to assess eutrophication risks and harmful algal bloom potential.

## Contribution

The study introduces a multi-element approach using N:P:Si stoichiometry to better identify nutrient limitations and guide management actions.

## Key findings

- Most rivers showed P or NP depletion relative to Si, with no overall Si depletion observed.
- Seasonal Si depletion was detected, suggesting potential for harmful algal blooms.
- Multi-element approaches like ICEP can improve eutrophication management compared to single-nutrient assessments.

## Abstract

River mouth nitrogen: phosphorus: silicon (N:P:Si) stoichiometry can predict marine eutrophication potential. Furthermore, deviations from Redfield molar N: P:Si = 16:1:20 ratios can offer insight into limiting nutrients and risks for harmful algal blooms (HAB). Here, we present N:P:Si stoichiometry based on total P, dissolved inorganic N and dissolved Si for 88 coastal river mouths in Norway, Sweden and Finland between 2017 and 2024. Rivers ranged from ultra-oligotrophic to hyper-eutrophic. N and P concentrations increased from north to south, with no latitudinal Si gradient. Most rivers were either P or jointly NP depleted relative to Si, with no overall evidence of Si depletion. However, there was some evidence of seasonal Si depletion. We show how using multi-element approaches, e.g., the Index of Coastal Eutrophication Potential (ICEP) can help to guide management actions. Specifically, Using Si depletion relative to N and P concentrations as an indicator of HAB potential may offer a means to identify catchments where nutrient load reductions can have the largest benefit on marine ecosystem health. Such multi-element approaches can complement single nutrient assessments based on, e.g., the Carlson Trophic Status Index or Water Framework Directive status class assessments.

The online version contains supplementary material available at 10.1038/s41598-025-34052-w.