Figure 4. Power spectra of deep water oxygen concentrations for different cases of imposed cyclic variation of the vertical exchange rate (amplitude in m y-1 as deviation from 11 m y-1, the MCAHI default value), representing variability of climatic forcing. Each row represents a given period (T) and the columns different amplitude of the oscillation. The dashed line graphs show the power spectra of the inherent oscillation with constant vertical exchange rate (cf. Fig 3c).
Although enhanced nutrient loading to the Baltic during the MCA has been suggested (Zillén & Conley, 2010), little evidence for eutrophication has been found in coastal paleoenvironmental data from this period (Ning et al., 2016; Jokinen et al., 2018; Norbäck Ivarsson et al., 2019). Therefore we imposed an external loading of 8000 tonnes P/year for both the MCAHI and HTMHI simulations, as derived from estimates of the pre-industrial loading to the Baltic Sea (Gustafsson et al., 2012). Because this value is at the lower limit of the range yielding oscillations in most parameterizations (Fig. 3c,d), we adjusted the vertical exchange parameter D in MCAHI and HTMHI simulations to approximately 20% lower than the modern value (Table S1). Lower values of D favor stagnation and the development of deep water hypoxia, and tip the system into an oscillatory state at external loading of <8000 tonnes P/year (Supporting Information Figs. S8 and S9). Our simulations therefore suggest that climatic factors controlling vertical mixing may be capable of triggering hypoxia under pre-anthropogenic P loading, despite a recent study indicating otherwise (Meier et al., 2019).
3.7 Implications for future management of the Baltic Sea
Under the BSAP (HELCOM, 2007; 2013), riparian countries agreed to limit external P loading to the Baltic in order to reduce the occurrence of hypoxia and cyanobacterial blooms in the future. As part of the plan, maximum allowable inputs of P to the whole Baltic Sea were set at 21716 tonnes P/year (about 18500 tonnes P/year in the region covered by the model). Our findings suggest that such targets, if met, will improve water quality but are unlikely to force the system to a fully oxic state (Fig. 3e). Rather, deep water oxygen concentrations are likely to remain close to the hypoxia threshold of 63 µmol/L. This finding is consistent with existing simulations from more complex coupled physical-biogeochemical models for the Baltic Sea (e.g. Meier et al., 2018). To achieve a transition beyond hypoxia to quasi-permanent oxic conditions would require a further load reduction and our model shows that this will likely proceed via an interval during which the system is vulnerable to multidecadal oscillations in hypoxia, creating a challenge for assessments of the success of management efforts. These results highlight the need to include more details of Fe-P interactions in models used for management decisions in the Baltic Sea (e.g. BALTSEM; Savchuk et al., 2012). We suggest that the extent to which oscillations may impact upon the actual recovery trajectory in the coming centuries will depend on the speed with which loading reduction targets are achieved. More rapid reductions are more likely to starve the system of P and therefore to promote a more linear recovery trajectory.
4 Conclusions
Our study shows that feedbacks in the coupled cycling of Fe and P drove multidecadal oscillations in the intensity of oxygen depletion during past hypoxic intervals in the 8000 year history of the Baltic Sea. In-phase oscillatory profiles of Mo/Al, Br/P and Fe/Al in sediments suggest large-scale synchronicity in deep water hypoxia, P regeneration and Fe shuttling, respectively. A simple box model of coupled Fe and P cycling suggests that internal multidecadal instability in hypoxia intensity is controlled by non-linear recharging and discharging of the sedimentary Fe-P reservoir in deeper areas. Crucially, our model simulations show that oscillatory behavior is sensitive to external P loading. While low external loads favor stable oxic conditions, and high loads sustain stable hypoxia, intermediate loads lead to unstable oscillatory behavior on multidecadal timescales. This observation has implications for predicting the trajectory of recovery from modern hypoxia in the Baltic Sea, since external P loading is expected to decline in accordance with the BSAP. Variable external climate forcing may influence the frequency and amplitude of oscillations through its impact on vertical water mass exchange.
Acknowledgments, Samples, and Data
This work was supported by European Research Council (ERC) Starting Grant (Consolidator Level) 278364 and Netherlands Organisation for Scientific Research (NWO) Vici grant 865.13.005 to Caroline Slomp, Academy of Finland Research Fellowship 317684 to Tom Jilbert, the EU BONUS-HYPER project, Baltic Bridge collaboration between University of Helsinki and Stockholm University and the program of the Netherlands Earth System Science Center (NESSC), financially supported by the Ministry of Education, Culture and Science (OCW).
The raw LA-ICP-MS data and time series analysis outputs are available free of charge (CC-BY 4.0) on Zenodo at DOI: 10.5281/zenodo.5222925. The box model (v1.0.2) used for investigating the mechanisms of oscillations is available (MIT Licence: Copyright Bo Gustafsson) at DOI: 10.5281/zenodo.5235401. The model code was developed using Xcode IDE (https://developer.apple.com/xcode/) in Swift language.
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