{
  "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.",
  "abstract_shown": true,
  "affiliations": [
    "University of Helsinki (Helsinki, Finland)",
    "Finnish Environment Institute (Helsinki, Finland)",
    "Sun Yat-sen University (Guangzhou, China)",
    "Stockholm University (Stockholm, Sweden)"
  ],
  "article_type": null,
  "arxiv": null,
  "author_total": 9,
  "authors": [
    {
      "affs": [
        0,
        1
      ],
      "name": "Dana Hellemann",
      "orcid": "0000-0002-6967-3129"
    },
    {
      "affs": [
        2,
        3
      ],
      "name": "Xiaole Sun"
    },
    {
      "name": "Tom Jilbert"
    },
    {
      "name": "Eva Ehrnsten",
      "orcid": "0000-0002-8960-8252"
    },
    {
      "name": "Bo Gustafsson",
      "orcid": "0000-0003-1048-8452"
    },
    {
      "name": "Lora Harris"
    },
    {
      "name": "Christoph Humborg"
    },
    {
      "name": "Bärbel Müller-Karulis"
    },
    {
      "name": "Alf Norkko"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/8e01x0dxw3qhp.bib",
  "cited_by": 0,
  "doi": "10.5194/bg-23-7993-2026",
  "fulltext_url": null,
  "id": "8e01x0dxw3qhp",
  "issns": [
    "1726-4189"
  ],
  "landing_url": "https://bg.copernicus.org/articles/23/7993/2026/",
  "language": null,
  "licence": "CC-BY-4.0",
  "licence_label": "CC BY 4.0",
  "licence_source": "article",
  "licence_url": "https://creativecommons.org/licenses/by/4.0/",
  "metadata_licence": null,
  "native_subjects": [],
  "openalex": null,
  "pdf_url": null,
  "pmcid": null,
  "pmid": null,
  "published": "2026-11-09",
  "publisher": "Copernicus GmbH",
  "ris": "https://research.draconitic.com/paper/8e01x0dxw3qhp.ris",
  "sources": [
    {
      "first_seen": "2026-11-10T00:21:50Z",
      "native_id": "10.5194/bg-23-7993-2026",
      "source": "crossref",
      "url": "https://bg.copernicus.org/articles/23/7993/2026/"
    }
  ],
  "subjects": [
    {
      "field": "earth",
      "origin": "issn:1726-4189",
      "subfield": "geology-geochemistry"
    }
  ],
  "title": "Fate of legacy ammonium in the coastal Baltic Sea",
  "title_text": "Fate of legacy ammonium in the coastal Baltic Sea",
  "type": "peer-reviewed",
  "updated_at": "2026-11-10T00:21:50Z",
  "url": "https://research.draconitic.com/paper/8e01x0dxw3qhp",
  "venue": "Biogeosciences",
  "versions": [
    {
      "doi": "10.5194/egusphere-2026-959",
      "pid": null,
      "relation": "preprint",
      "title": null,
      "venue": null
    }
  ]
}