Fate of legacy ammonium in the coastal Baltic Sea
4 affiliations
- 1University of Helsinki (Helsinki, Finland)
- 2Finnish Environment Institute (Helsinki, Finland)
- 3Sun Yat-sen University (Guangzhou, China)
- 4Stockholm University (Stockholm, Sweden)
Abstract
Eutrophication has enriched coastal sediments globally with organic matter (OM), fuelling internal nutrient loading once benthic hypoxia occurs. Internal ammonium (NH4+) loading may be particularly prominent in the coastal Baltic Sea due to the existence of a substantial NH4+ pool in its sediments, accumulated via OM deposition and mineralization over long-term eutrophication. However, despite its potential to exacerbate eutrophication, internal NH4+ loading has so far received little attention in the coastal Baltic Sea and it remains poorly understood for how long the sediment NH4+ pool could sustain internal NH4+ loading via sediment-water NH4+ effluxes, especially under varying oxygen availabilities. To reconstruct past and predict future NH4+ effluxes in response to different OM loading and bottom water oxygen conditions, we developed a reactive transport model for muddy, OM-rich sediments of the coastal Baltic Sea. Our model results suggest a strong legacy effect in the sediment NH4+ pool, driven by mineralization of OM that accumulated during past and ongoing eutrophication, as expected. The influence of the sediment NH4+ pool on the sediment-water NH4+ effluxes in our model is, however, smaller than the direct influence of the external OM loading. The sediment NH4+ pool keeps the sediment-water NH4+ effluxes 5 %–10 % elevated compared to pre-industrial baseline values for at least 140 years after OM loading has reached constant baseline levels, albeit at a small efflux magnitude. While this magnitude can increase under increasing OM loading and oxygen deficiency, the overall legacy effect on the NH4+ effluxes from coastal sediments is mild and the NH4+ effluxes react well to external OM load reductions. If strict OM load reductions following the direction of the Baltic Sea Action Plan are implemented, sediment-water NH4+ effluxes can naturally return to a level similar to the 1950s, the start of widespread eutrophication, within 110 years, given that sediments stay normoxic and bioturbated during the reduction period. Without external OM load reductions, sediment-water NH4+ effluxes stay indefinitely elevated and fuel continuous internal NH4+ loading to the coastal system. Our findings highlight the complexity of eutrophication legacy effects in regard to the sediment pool and sediment-water efflux of NH4+, and their varying impact on the coastal Baltic Sea ecosystem. The knowledge obtained is beneficial also for other anthropogenically impacted coastal seas with similar geomorphology as the Baltic Sea.
Versions
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BibTeX
@article{hellemann2026fate,
title = {{Fate of legacy ammonium in the coastal Baltic Sea}},
author = {Dana Hellemann and Xiaole Sun and Tom Jilbert and Eva Ehrnsten and Bo Gustafsson and Lora Harris and Christoph Humborg and Bärbel Müller-Karulis and Alf Norkko},
journal = {Biogeosciences},
year = {2026},
month = nov,
doi = {10.5194/bg-23-7993-2026},
url = {https://bg.copernicus.org/articles/23/7993/2026/},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Fate of legacy ammonium in the coastal Baltic Sea
AU - Dana Hellemann
AU - Xiaole Sun
AU - Tom Jilbert
AU - Eva Ehrnsten
AU - Bo Gustafsson
AU - Lora Harris
AU - Christoph Humborg
AU - Bärbel Müller-Karulis
AU - Alf Norkko
PY - 2026
DA - 2026/11/09
JO - Biogeosciences
DO - 10.5194/bg-23-7993-2026
UR - https://bg.copernicus.org/articles/23/7993/2026/
AB - Eutrophication has enriched coastal sediments globally with organic matter (OM), fuelling internal nutrient loading once benthic hypoxia occurs. Internal ammonium (NH4+) loading may be particularly prominent in the coastal Baltic Sea due to the existence of a substantial NH4+ pool in its sediments, accumulated via OM deposition and mineralization over long-term eutrophication. However, despite its potential to exacerbate eutrophication, internal NH4+ loading has so far received little attention in the coastal Baltic Sea and it remains poorly understood for how long the sediment NH4+ pool could sustain internal NH4+ loading via sediment-water NH4+ effluxes, especially under varying oxygen availabilities. To reconstruct past and predict future NH4+ effluxes in response to different OM loading and bottom water oxygen conditions, we developed a reactive transport model for muddy, OM-rich sediments of the coastal Baltic Sea. Our model results suggest a strong legacy effect in the sediment NH4+ pool, driven by mineralization of OM that accumulated during past and ongoing eutrophication, as expected. The influence of the sediment NH4+ pool on the sediment-water NH4+ effluxes in our model is, however, smaller than the direct influence of the external OM loading. The sediment NH4+ pool keeps the sediment-water NH4+ effluxes 5 %–10 % elevated compared to pre-industrial baseline values for at least 140 years after OM loading has reached constant baseline levels, albeit at a small efflux magnitude. While this magnitude can increase under increasing OM loading and oxygen deficiency, the overall legacy effect on the NH4+ effluxes from coastal sediments is mild and the NH4+ effluxes react well to external OM load reductions. If strict OM load reductions following the direction of the Baltic Sea Action Plan are implemented, sediment-water NH4+ effluxes can naturally return to a level similar to the 1950s, the start of widespread eutrophication, within 110 years, given that sediments stay normoxic and bioturbated during the reduction period. Without external OM load reductions, sediment-water NH4+ effluxes stay indefinitely elevated and fuel continuous internal NH4+ loading to the coastal system. Our findings highlight the complexity of eutrophication legacy effects in regard to the sediment pool and sediment-water efflux of NH4+, and their varying impact on the coastal Baltic Sea ecosystem. The knowledge obtained is beneficial also for other anthropogenically impacted coastal seas with similar geomorphology as the Baltic Sea.
SN - 1726-4189
ER -