Excessive land-derived nutrient loading has led to eutrophication of the coastal oceans globally, thereby strongly enriching these systems with organic matter (OM). The combination of high production rate and fast sedimentation of OM in coastal systems can quickly deplete oxygen (O2) in the benthic habitat, leading not only to ecosystem degradation (Breitburg et al., 2018) but also to the rapid accumulation of OM in sediments, creating a eutrophication-induced OM legacy pool (Gustafsson et al., 2012). Over time, aerobic and anaerobic mineralization of this OM generates nutrient pools in the sediment, including ammonium (NH4+) and dissolved inorganic phosphorous (DIP; Aller, 2014). The retention (DIP) and turnover (NH4+) of these nutrients within the sediment is usually high under oxic bottom water conditions; yet, when benthic hypoxia (O2 concentration <63µM or 2 mg L-1, Middelburg and Levin, 2009) develops, both nutrients are preferentially released into the water column (Koop et al., 1990; Cowan and Boynton, 1996; Norkko et al., 2015), leading to internal nutrient loading with high potential to reinforce eutrophication (Pitkänen et al., 2001; Conley et al., 2007). Research on internal nutrient loading has often focused on DIP due to its crucial role in the repeated occurrences of massive cyanobacteria blooms and close coupling to metal cycling (Conley et al., 2002; Vahtera et al., 2007; Viktorsson et al., 2013; Matisoff et al., 2016), whereas internal NH4+ loading has so far received little attention. This omission is a critical oversight, considering that excess bioavailable nitrogen (N) is a key driver of coastal eutrophication (Ryther and Dunstan, 1971).

Internal NH4+ loading may be particularly prominent in the long-term eutrophied coastal Baltic Sea (defined here as the coastal areas of the Baltic Sea extending one nautical mile seaward from territorial water baselines, following the Water Framework Directive; European Communities, 2003). Compared to coastal regions worldwide, the coastal Baltic Sea displays the highest sediment-water NH4+ effluxes of all studied sites and a highly pronounced relationship between NH4+ efflux and bottom water O2 condition (Fig. 1A). NH4+ effluxes increase significantly with decreasing O2 concentrations across all investigated sites in the coastal Baltic Sea, despite their dispersed geographic locations (Denmark, Sweden, and Finland) and only moderate number of replicates in the cited studies (n=6–14; Fig. 1B).

This clear pattern might be explained by Baltic Sea-specific environmental factors that reduce the influence of natural variability in measurements, such as low species diversity, negligible tidal influence, and a long water residence time, as well as by the presence of a significant benthic legacy pool of NH4+. Since the acceleration of Baltic Sea eutrophication in the 1950s (Zillen and Conley, 2010; Gustafsson et al., 2012), OM and eventually NH4+ have accumulated in the porewaters of muddy accumulation bottoms, visible today as characteristic NH4+ profiles of strongly increasing concentrations with depth that are consistent across seasons (Fig. 2). In a first-order estimate, using coastal boundaries as defined above (European Communities, 2003), coastal sediment classifications from Al-Hamdani and Reker (2007), and example porewater NH4+ profiles from representative sandy (site VE15, southern Bay of Gdansk, Thoms et al., 2018) and muddy (site Storfjärden, north-western Gulf of Finland, D. Hellemann Fig. 2) sediments, we suggest that >100kt NH4+ could be stored in the top 20 cm of Baltic Sea coastal sediments. This benthic pool of NH4+ is not only larger than the pelagic pool of dissolved inorganic nitrogen (DIN) in the coastal Baltic Sea, estimated at ∼61kt N using a 21 % share of DIN in total nitrogen (TN; C. Lønborg, pers. comm.) and a coastal pelagic TN pool estimate from Lønborg and Markager (2021), but, furthermore, it is only a minimum estimate of the entire NH4+ pool in coastal Baltic Sea sediments considering that concentrations continue to increase with depth below 20 cm.

The impact of this sediment NH4+ pool on pelagic processes is largely regulated by O2 availability, although factors such as OM availability and benthic infauna also affect the magnitude of sediment-water fluxes (Boynton et al., 2017). Commonly, under normoxic conditions, a substantial portion of the NH4+ that diffuses upwards from porewater is oxidized to nitrate (NO3-) via nitrification in the oxic surface sediment and subsequently is reduced to bio-unavailable di-nitrogen gas (N2) via denitrification in the underlying anoxic sediment (Fig. 3A). The same also happens in the numerous 3-dimensional oxic-anoxic interfaces created by benthic infauna (Andersen and Kristensen, 1991). Further, microphytobenthos, if present on the sediment surface, can intercept and assimilate nutrients effluxing from the underlying sediments (Sundbäck and Miles, 2000). Together, these factors commonly buffer against large NH4+ effluxes from the sediment, leaving only residual fluxes originating from, for example, faunal activity (Andersen and Kristensen, 1991; Karlson et al., 2007). When benthic hypoxia or anoxia develops, the oxic sediment surface is lost, benthic infauna and microphytobenthos are extirpated, and the benthic N turnover processes change from N removal to N recycling (Kemp et al., 1990; Jensen et al., 1990; Childs et al., 2002; Karlson et al., 2007; Jäntti and Hietanen, 2012). The extent of these effects depends on frequency and duration of O2 deficiency (Conley et al., 2007; Jäntti and Hietanen, 2012; Villnäs et al., 2012), but they all fuel NH4+ release into the water column compared to normoxic conditions, enriching the pelagic pool of bioavailable N (Fig. 3B). While exceptions to this pattern exist (Foster and Fulweiler, 2019; Testa et al., 2025), the available field data from the coastal Baltic Sea show a significant trend following this paradigm, resulting in internal loading of bioavailable N under hypoxia (Fig. 1). In the open Baltic Sea, such internal N load would be efficiently removed via denitrification at the redoxcline of the permanently hypoxic deep waters (Hietanen et al., 2012; Dalsgaard et al., 2013). However, in the coastal Baltic Sea, pelagic denitrification is not a common feature as density- and wind-driven water mixing usually prevent large-scale permanent hypoxia (Leppäranta and Myrberg, 2009; Conley et al., 2011) and thus the establishment of a pelagic denitrifier community (Falk et al., 2007; Kim et al., 2011). Consequently, in the coastal Baltic Sea, the internal N load from sediment-water NH4+ effluxes remains in the pelagic system and is thus available for coastal primary production, potentially enhancing coastal eutrophication.

The timeline of internal NH4+ loading affecting the Baltic Sea coastal ecosystem is uncertain, as the long-term fate and persistence of the sediment NH4+ pool is largely unknown. For the TN pool in Baltic Sea sediments, Lønborg and Markager (2021) suggest a legacy effect of several centuries. This gap in knowledge limits the capacity of eutrophication mitigation measures, such as the Baltic Sea Action Plan (BSAP), an agreement between the Baltic states on specific actions to bring the Baltic Sea back to a good environmental status (HELCOM, 2007, 2021). Coastal hypoxia is likely to increase in the future due to climate change driven water warming (Caballero-Alfonso et al., 2015; Breitburg et al., 2018), increasing the potential for internal NH4+ loading. Furthermore, present-day OM loading to coastal Baltic Sea sediments remains at ∼10mmolCm-2d-1 (estimated from the physical-biogeochemical BALTSEM model, Gustafsson et al., 2012), which is lower than the peak loading of ∼14mmolCm-2d-1 in the 1990s, but still several-fold higher than before the onset of eutrophication (Fig. 4a). As a result, new OM is continuously added to sediments still containing the OM legacy from past eutrophication, which is likely to extend the lifetime of the existing sediment NH4+ pool and its potential to impact the coastal ecosystem.

Interestingly, while data from the Chesapeake Bay and the Derwent Estuary also show a significant relationship between sediment-water NH4+ efflux and O2 deficiency (Fig. 1B), the NH4+ porewater profiles of the Chesapeake Bay do not indicate the existence of a NH4+ legacy pool (Kemp et al., 1990; Boynton et al., 2017; no porewater data available for the Derwent Estuary). Despite a eutrophication history similar to the Baltic Sea, the OM deposition environment of the Chesapeake Bay does not seem to support the build-up of benthic legacy pools, owing to higher energy of physical processes, including tidal flushing, and shorter water residence time due to a more open geomorphology. Hence, eutrophication-induced legacy effects similar to the ones from the coastal Baltic Sea might be expected more commonly in eutrophic systems of a semi- to fully enclosed geomorphology and long water residence time, such as the Black Sea (BSC, 2019), Sea of Marmara (Demirel et al., 2023; Akçay et al., 2025) or even inland waters such as Lake Erie (Watson et al., 2016). Owing to its unique data-richness, the coastal Baltic Sea can serve as a key example for these systems and their potential future evolution (Reusch et al., 2018).

To shed light on the long-term fate and persistence of the legacy NH4+ pool in sediments of the coastal Baltic Sea, we investigate future scenarios of sediment-water NH4+ effluxes in response to OM loading and bottom water O2 availability, using a 1-dimensional reactive transport model (RTM).

The BALTSEM-derived OM loading in both model scenarios can be divided into four phases (Fig. 4). Phase I gives the pre-eutrophication OM level (including pre-industrial baseline levels) representing natural loading. Phase II is characterized by a sharp increase in OM loading from the onset of eutrophication in the 1950s to the early 2000s, during which the loading reaches its peak of ∼14mmolCm-2d-1, tripling the OM loading of phase I. Following this peak, phase III covers the decrease in OM loading to its present-day value of ∼10mmolm-2d-1 (Present scenario), or to the projected pre-industrial baseline levels (Pre-industrial scenario). Phase IV represents the far-future return to pre-industrial OM loading and thus can only be seen in the Pre-industrial scenario.

During phase I, the model reaches a steady state of a constant NH4+ pool in the sediment and low NH4+ efflux to the water column. In phase II, the NH4+ pool and efflux respond to the fast increase in OM loading. In phase III of the Present scenario, the OM loading becomes constant in 2020, while the NH4+ pool continues to increase steadily for over two centuries before approximating a slight plateau around 2250; in contrast, NH4+ effluxes reach an actual plateau around 2100 and thereafter stay high indefinitely under all O2 conditions (Figs. 4a, 5a and c). In phase III of the Pre-industrial scenario, the NH4+ pool under normoxic conditions decreases in accordance with the drastic decrease in OM loading, albeit at a comparatively much slower rate: when OM loading reaches pre-industrial baseline values around 2160, the normoxic NH4+ pool is still as large as in the 1970s. In contrast, the NH4+ pool under O2-deficient conditions continues to increase for about 100 years despite the reductions in OM loading, before eventually starting to slowly decrease (Figs. 4b and 5b). Under neither O2 condition does the NH4+ pool of the Pre-industrial scenario reach pre-industrial baseline levels by year 2400, the end of our modelling period (Fig. 4b phase IV). Despite the difference in pool development, NH4+ effluxes in phase III of the Pre-industrial scenario follow the reduction of OM loading closely under all O2 conditions (Figs. 4b and 5d). Nevertheless, NH4+ effluxes do not reach pre-industrial baseline levels by the end of phase III, but show remaining efflux-“tails” as can be seen when zooming into the model-output (Appendix B, Fig. B1). True pre-industrial baseline levels of NH4+ efflux are reached in phase IV first after 2300 (normoxic conditions) or even around 2400 (anoxic conditions; Appendix B, Fig. B1); the flux magnitude of these “tails” under all O2 conditions is however small and comparable to the pre-eutrophication period (Appendix B, Fig. B1). In both scenarios, the NH4+ pool is smaller and increases at a lower rate under O2-deficient than under normoxic conditions, whereas the NH4+ effluxes are larger under O2-deficient than under normoxic conditions (Figs. 4 and 5). The latter one is in line with field data from the coastal Baltic Sea (Fig. 1) and the common understanding of more efficient internal NH4+ turnover under normoxic conditions (Fig. 3).

Depending on the O2 condition, the NH4+ effluxes of the Pre-industrial scenario remain elevated compared to pre-industrial baseline levels for 280–380 years after the start of drastic OM load reductions (Appendix B, Fig. B1). However, about 110 years following the reductions, i.e. in 2130, normoxic NH4+ effluxes have shrunk to a level comparable to the 1950s, the starting point of widespread eutrophication; under O2-deficient conditions, this level will be reached about 140 years following the reductions, i.e. in 2160 (Fig. 4b). This modelled timeframe of decline in NH4+ efflux is 30–60 years longer than the estimate of when BSAP-driven good environmental status is assumed to be reached (year 2100, Fig. 4; Murray et al., 2019), which could suggest that it might take longer than previously thought for the BSAP to achieve its environmental targets.

In both scenarios, the NH4+ effluxes seem to respond more rapidly to the external OM loading than to the internal NH4+ pool (Figs. 4 and 5). This result is interesting, as the NH4+ pool shows a strong legacy effect and thus creates high internal NH4+ availability, even long after strong external OM loading has ceased, such as in the Pre-industrial scenario (Fig. 5b). This legacy effect is caused by NH4+ being continuously regenerated from the OM that accumulated in the sediment over subsequent years of strong eutrophication (Fig. 4 phase II), and results in (i) the increase of the NH4+ pool in the Present scenario, despite constant OM loading (Figs. 4a phase III and 5a), as well as (ii) the slow decline (normoxic condition) and initial increase (O2-deficient conditions) of the NH4+ pool in the Pre-industrial scenario, despite rapid OM load reductions (Figs. 4b phase III–IV and 5b). The different evolution of the normoxic and O2-deficient NH4+ pools can be attributed to the more efficient mineralization and internal NH4+ turnover under normoxic than O2-deficient conditions (Fig. 3). The influence of the NH4+ pool on the NH4+ effluxes eventually becomes visible in the Pre-industrial scenario, where the NH4+ effluxes remain 5 %–10 % above pre-industrial baseline values after OM loading has returned to constant baseline levels (Appendix B, Fig. B1, Fig. 4b), creating NH4+ efflux-“tails”. While these tails are small in magnitude, they last for 140–240 years, depending on the O2 condition (Appendix B, Fig. B1), clearly indicating the influence of the NH4+ pool and its strong legacy effect. In consequence, the NH4+ effluxes are subject to a eutrophication legacy effect, which is highly persistent, but only small in effect size. Hence, the majority of the NH4+ effluxes remain controlled by the external OM loading, which suggests that NH4+ effluxes and their subsequent impact on the coastal environment can be mitigated in a straightforward manner by reducing the external OM loading, such as in our Pre-industrial scenario.

While our modelled NH4+ effluxes in both scenarios show distinctly different responses between normoxic and O2-deficient conditions, confirming the common assumption of O2 as a key control of sediment-water NH4+ effluxes (Fig. 3), the magnitude of our modelled O2-deficient fluxes is below the range of field-observations from the coastal Baltic Sea (Fig. 1). This underestimation of O2-deficient NH4+ effluxes can be explained by the resolution of our RTM, which runs on an annual basis and thus does not capture seasonal fluctuations and anoxic extreme events that can drastically increase the efflux. Such events are generally challenging to incorporate in a model, as they originate from individual combinations of OM loading and loading history, anoxia duration, temperature, and more. Without the addition of extreme values to the RTM's annual average, the modelled O2-deficient NH4+ effluxes are lower than they can periodically be in nature, and our model consequently provides an estimate of O2-deficient NH4 effluxes at the lower range of their annual value. This underrepresentation of seasonal extreme values in our model affects only the O2-deficient, but not the normoxic fluxes, which fit well to the range observed in the coastal Baltic Sea (Fig. 1).

Our model results represent one site in the northern coastal Baltic Sea, which is considered representative for a muddy, OM-rich accumulation bottom in low-saline, eutrophic conditions. To test the sensitivity of our modelled NH4+ pool and efflux to different environmental conditions, we applied a range of OM loadings, salinities (representative of different SO42- concentrations in bottom water), sedimentation rates, bioturbation coefficients, C/N ratios of OM, and different shares of reactive material in OM to our RTM, both under normoxic (Fig. 6) and anoxic (Appendix B, Fig. B2) bottom water conditions. As a baseline for comparison, we used the conditions of our model setting under the Pre-industrial OM loading scenario, i.e. a salinity of 6 with a SO42-concentration of 5 mmolL-1, a sedimentation rate of 6 mmyr-1, strong bioturbation under normoxic conditions, and a C/N ratio of 6.6 in the reactive fraction and 13 in the less reactive fraction. The OM loadings were generated by the BALTSEM simulations.

Under normoxic conditions and compared to the baseline, an increase in OM loading would strongly increase the NH4+ pool and efflux and both their legacy effects (Fig. 6I). While the increase in legacy effect in the NH4+ pool is very visible, its increase in the NH4+ efflux is more subtle in larger magnitudes of the efflux-“tails” (Fig. 6a I), proportionally to the level of OM loading: the highest OM loading (5-times the BSAP OM loading) shows the strongest effect on the NH4+ effluxes (Fig. 6b I). An increase in salinity does not change the size of the NH4+ pool and only marginally increases the NH4+ efflux (Fig. 6a II), with the largest change observed when salinity changes from 6.6 to 10 (Fig. 6b II). This result is caused by the comparatively low availability of OM that limits the reduction of SO42-; hence, higher salinity can only increase the NH4+ pool and efflux, if also the OM availability is higher, such as can be expected under the Present scenario. A change in sedimentation rate affects the size of the NH4+ pool but not the NH4+ efflux (Fig. 6 III). The presence or absence of bioturbation has a strong effect on both NH4+ pool and efflux (Fig. 6 IV). In the presence of bioturbation, the NH4+ pool increases, likely because bioturbation promotes the degradation of OM (Andersen and Kristensen, 1991) and thus the release of NH4+ to the sediment porewater. However, at the same time, the NH4+efflux decreases, as bioturbation generally increases the availability of O2 in surface sediments, thereby facilitating sediment internal NH4+ turnover (Fig. 3). If bioturbation is absent, the opposite happens. This result highlights the importance of bioturbating infauna for sediment O2 and N-turnover. An increase in C/N ratio towards less bioavailable and more refractory OM will decrease both the NH4+ pool and efflux (Fig. 6V), as the OM becomes less rich in N and thus less NH4+ is produced in mineralization. The C/N ratio is also affected by different shares of reactive OM, thereby affecting the NH4+ pool and efflux (Fig. 6VI). In this test, the share of refractory OM is kept at 23 %, and the share of reactive OM is 47 % and 10 %. Lower shares of reactive OM lead to a larger NH4+ pool and a lower NH4+efflux (Fig. 6VI), which is caused by in turn higher shares of less reactive OM that degrade slowly but last longer in the sediments. Under anoxic bottom water conditions, similar patterns as the ones described can be observed but with slightly stronger effects on the NH4+ pool and efflux (Appendix B, Fig. B2).

While these results point to a general direction for NH4+ cycling in eutrophied coastal systems of different environmental conditions, they need to be interpreted while considering the geomorphological context for each system and the understanding that parameters occur in combination not in isolation. For instance, high salinity might often occur together with a low sedimentation rate, given that many coastal systems of high salinity are connected to the open ocean and thus experience high water energy from waves and tidal force, which enhances dispersion of OM and ultimately affects the sedimentation rate. In our sensitivity analysis, we do not consider such parameter combinations. Hence, understanding a habitat's geomorphological setting and environmental conditions are highly important when aiming to realistically model NH4+ flux predications of coastal sediments.

Our model results highlight the presence of legacy effects from past eutrophication in present-day and future NH4+ cycling in coastal sediments of the Baltic Sea. While the legacy effects are highly persistent in both the sediment NH4+ pool and the sediment-water NH4+ effluxes, the legacy's effect size is much smaller in the NH4+ effluxes than in the NH4+ pool. This means, that the NH4+ pool affects the NH4+ effluxes for a long time, but potentially plays a minor role in its ecological impact. The impact of the NH4+ efflux legacy on the coastal ecosystem is ultimately determined by the external OM loading, which not only strongly drives the effluxes (Fig. 4b) but also determines the magnitude and thus ecological significance of the legacy effluxes (Fig. 6I). The strong connection between external OM loading and NH4+ effluxes gives a clear direction for the restoration of eutrophied coastal ecosystems. Our model results show that if strict OM load reductions are implemented, sediment-water NH4+ effluxes will decrease promptly and nearly proportionally to the load reductions. About 110 years after the start of OM load reductions, NH4+ effluxes could reach the same level as at the start of eutrophication in the 1950s. This is a relatively short time-frame for natural ecosystem recovery of a heavily eutrophied system, such as the coastal Baltic Sea. The time-frame can be met when bottom waters are normoxic and sediments are strongly bioturbated, meaning that the benthic system stays as healthy as possible during the duration of OM load reductions. If bottom waters become O2-deficient and the bioturbating infauna dies, the duration to reach 1950s levels of NH4+ effluxes will be extended by approximately 30 years, during which also the magnitude of NH4+ effluxes will be higher than under normoxic conditions (Fig. 4b), thereby sustaining the strain on the ecosystem. Hence, the key factors affecting internal N loading via sediment-water NH4+ effluxes in the coastal Baltic Sea are the external OM loading and the bottom water O2 conditions. Consequently, internal N loading and its impact on the coastal environment cannot be mitigated if OM loading remains on its present-day level (Fig. 4a).

The limited direct impact of the sediment NH4+ pool on the sediment-water NH4+ effluxes in our model is unexpected, but important knowledge for ecosystem management, as it suggests that ecosystem recovery in regard to N overloading is achievable within realistic time-frames. A stronger influence of the NH4+ pool on the NH4+ effluxes than what we see in our modelling results would increase both the magnitude of the NH4+ effluxes and their resistance to external management measures, potentially extending their impact on the coastal ecosystem by many years. Further, it is worthwhile to keep in mind that, despite its limited direct impact, our modelled sediment NH4+ pool nevertheless maintains the long-lasting legacy effect of the NH4+ effluxes, whose magnitude and thus ecological impact can increase under O2-deficient (Appendix B, Fig. B2) and OM enriched (Fig. 6a I) conditions. The estimated duration of legacy effects in the NH4+ effluxes is similar to Lønborg and Markager (2021), who suggested a TN legacy in Baltic Sea sediments of several centuries.

Considering that our model is 1-dimensional and not directly coupled to the water column, we cannot estimate any direct feedback between sediment and water column. In a natural system, a large part of the N released to the water column via sediment-water NH4+ efflux would be taken up in primary production and eventually settle back to the sediment to become new OM, which adds complexity to the internal nutrient loops in both modelling scenarios and is not included in the current model. It is likely that the coastal current of the Baltic Sea would keep OM and nutrients for a certain time in the coastal zone before both end up in the open Baltic Sea (Radtke et al., 2012), which would increase the chance that the coastal N load is ultimately removed from the aquatic system via denitrification at the permanent redoxcline. How long it would take to remove the internal N loading from the coastal zone and thus from the potential to favour coastal primary production, remains, however, unclear. Furthermore, the impact of internal N loading on the coastal ecosystem may be localized, as despite a predicted rise in hypoxia it is highly unlikely that the entire coastal zone of the Baltic Sea will eventually become O2-deficient. Localized hypoxic or anoxic events can lead to increases in N availability and subsequent primary production, yet, such increases are likely masked by the overall coastal eutrophication. Nevertheless, for management efforts to mitigate coastal eutrophication and restore a healthy ecosystem, every addition of bioavailable nutrients that is not accounted for in mass balance budgets should be noteworthy. In the big picture, internal loading adds nutrients to a system and even if these nutrients are transient, they can cause ecological harm and impede eutrophication mitigation measures.

Following OM load reductions, NH4+ effluxes in our model decrease slightly faster and with a smaller flux magnitude under normoxic than under O2-deficient conditions (Fig. 4b), which might raise the question whether artificial aeration methods could be a justifiable option to aid the ecosystem recovery. However, the effects of artificial aeration on benthic N-cycling in the coastal Baltic Sea are not straight forward and sediment-water NH4+ effluxes both did (site Lännerstasundet, Lehtoranta et al., 2022) and did not (site By Fjord, De Brabandere et al., 2015) significantly decrease under aeration, or even increased (site Sandöfjärden, Lehtoranta et al., 2022) due to aeration induced sediment warming and a return of anoxia despite aeration. Similar conflicting results were found also outside the coastal Baltic Sea (e.g. Harris et al., 2015, Testa et al., 2025), including increased air-water methane exchange under conditions of aeration (Lapham et al., 2022), which suggests that engineered aeration approaches often have unexpected and unintentional impacts on both nutrient cycling and greenhouse gas emissions. Given these variable responses, we do not consider artificial aeration to be a clear-cut solution to shorten the duration of NH4+ effluxes in the coastal Baltic Sea.

Muddy sediments in the coastal Baltic Sea commonly contain large NH4+ accumulations from past and ongoing eutrophication, which are favourably released to the water column under O2-deficient conditions, resulting in internal N loading with the potential to facilitate coastal primary production. To better understand the fate and persistency of this NH4+ legacy and its impact on the coastal ecosystem, we modelled the past to future development of the sediment NH4+ pool and sediment-water NH4+ effluxes in the coastal Baltic Sea under two contrasting OM loading scenarios and three O2 conditions. Our model results clearly show a strong eutrophication legacy effect in the development of the sediment NH4+ pool, which affects the sediment-water NH4+ effluxes for a long time, but with only a mild effect size. The key driver of the sediment-water NH4+ effluxes is the external OM loading, which continuously adds to the OM- and NH4+ accumulations in coastal surface sediments. This result strongly indicates, that present-day elevated sediment-water NH4+ effluxes can be effectively reduced if strict OM load reductions are realized, such as proposed by the BSAP. Oxic bottom water and strong bioturbation aid the reduction of NH4+ effluxes, as both aspects increase the sediment internal turnover of NH4+. However, these factors are secondary behind the reduction in OM loading: following our model results, internal N loading to the coastal Baltic ecosystem cannot be mitigated if OM loading continues as at present, which has also been suggested in other studies (Pitkänen et al., 2001, Saraiva et al., 2019). Our results are beneficial also for other coastal areas of similar geomorphology as the coastal Baltic Sea that face similar challenges in nutrient reduction and ecological management.