A new approach to model oxygen dependent benthic phosphate fluxes in the Baltic Sea

Highlights • A new description of sediment phosphorus dynamics was implemented in a 3D-model. • Oxygen consumption affects oxygen penetration in coastal sediments. • Low oxygen concentrations determine the oxygen penetration in deeper water sediments. • More than 80% of the phosphorus loads (1980–2008) are retained in the Baltic Sea. • Phosphorus is released from anoxic sediments and retained in oxic sediments. Abstract The new approach to model the oxygen dependent phosphate release by implementing formulations of the oxygen penetration depths (OPD) and mineral bound inorganic phosphorus pools to the Swedish Coastal and Ocean Biogeochemical model (SCOBI) is described. The phosphorus dynamics and the oxygen concentrations in the Baltic proper sediment are studied during the period 1980–2008 using SCOBI coupled to the 3D-Rossby Centre Ocean model. Model data are compared to observations from monitoring stations and experiments. The impact from oxygen consumption on the determination of the OPD is found to be largest in the coastal zones where also the largest OPD are found. In the deep water the low oxygen concentrations mainly determine the OPD. Highest modelled release rate of phosphate from the sediment is about 59 × 103 t P year− 1 and is found on anoxic sediment at depths between 60–150 m, corresponding to 17% of the Baltic proper total area. The deposition of organic and inorganic phosphorus on sediments with oxic bottom water is larger than the release of phosphorus, about 43 × 103 t P year− 1. For anoxic bottoms the release of total phosphorus during the investigated period is larger than the deposition, about 19 × 103 t P year− 1. In total the net Baltic proper sediment sink is about 23.7 × 103 t P year− 1. The estimated phosphorus sink efficiency of the entire Baltic Sea is on average about 83% during the period

Assessment of eutrophication abatement scenarios for the Baltic Sea by multi-model ensemble simulations

Peer reviewe

Eutrophication Status Report of the North Sea, Skagerrak, Kattegat and the Baltic Sea : A model study Years 2001-2005

Följande statusrapport för Nordsjön, Skagerrak, Kattegatt och Östersjön har genomförts avSMHI Sverige, IMR Norge, NERI Danmark, SPBIO Ryssland, och SYKE Finland som del avprojektet “A Baltic and NORth sea Model eutrophication Assessment in a future cLimate”(ABNORMAL), vilket finansierats av the Nordic Council of Ministers’ Sea and Air Group(NMR-HLG). De tidigare NMR-HLG projekten NO COMMENTS och BANSAI fokuseradespå etablering och underhållsstöd till operationella modeller samt utvecklingen av metoder förderas användning till utvärdering av eutrofieringstillstånd. Inom ABNORMAL har frågornavidare fokuserats på användningen av ekologiska modeller för att utvärderaeutrofieringstillståndet in framtida klimat. Viktigaste rönet från studien är det föreslagna sättetatt sammanföra observationer med resultat från en ensemble av ekologiska modeller för attutvärdera eutrofieringstillståndet i dagens klimat under fem olika år (2001-2005).Tröskelvärden och metoder från Oslo and Paris Commissionen (OSPAR) och HelsinkiCommission (HELCOM) används och möjliga förbättringar av metoder diskuteras kort.Bedömningen av eutrofieringstillståndet visar att Kattegatt, de danska sunden, Finska viken,Gotlandsbassängen, samt största delarna av Arkonabassängen, Bornholmsbassängen ochEgentliga Östersjön kan klassificeras som problemområden. Huvuddelen av Nordsjön ochSkagerrak är icke-problem områden medan huvuddelarna av Bottenhavet, Bottenviken, RigaBukten och hela sydöstra kontinentalkusten av Nordsjön kan klassificeras som potentiellaproblemområden.This joint status report for the North Sea, Skagerrak, Kattegat and the Baltic Sea area is carried out by SMHI Sweden, IMR Norway, NERI Denmark, SPBIO Russia, and SYKE Finland as a part of the project “A Baltic and NORth sea Model eutrophication Assessment in a future cLimate” (ABNORMAL), supported by the Nordic Council of Ministers’ Sea and Air Group (NMR-HLG). The previous NMR-HLG projects NO COMMENTS and BANSAI focused on the establishment and main-tenance of operational models and the use of these to develop methods for assessing the eutrophication status. Within ABNORMAL the issues are brought forward with a focus also on the use of ecological models for an assessment of marine eutrophication in a future climate. The main finding of this study is the proposed way of combining observations and results from an ensemble of ecological models to make an assessment of the eutrophication status in present climate for five different years (2001-2005). Threshold values and methodology from the Oslo and Paris Commissions (OSPAR) and the Helsinki Commission (HELCOM) are used and possible improvements of the methods are briefly discussed. The assessment of eutrophication status according to the integration of the categorized assessment parameters indicates that the Kattegat, the Danish Straits, the Gulf of Finland, the Gotland Basin as well as main parts of the Arkona Basin, the Bornholm Basin, and the Baltic proper may be classified as problem areas. The main part of the North Sea and also the Skagerrak are non-problem areas while the main parts of the Gulf of Bothnia, Gulf of Riga and the entire southeastern continental coast of the North Sea may be classified as potential problem areas

Impact of saltwater inflows on phosphorus cycling and eutrophication in the Baltic Sea : a 3D model study

The impact of dense saltwater inflows on the phosphorus dynamics in the Baltic Sea is studied from tracer experiments with a three-dimensional physical model. Model simulations showed that the coasts of the North West Gotland Basin and the Gulf of Finland, the Estonian coast in the East Gotland Basin are regions where tracers from below the halocline are primarily lifted up above the halocline. After 1 yr tracers are accumulated at the surface along the Swedish east coast and at the western and southern sides of Gotland. Elevated concentrations are also found east and southeast of Gotland, in the northern Bornholm Basin and in the central parts of the East Gotland Basin. The annual supplies of phosphorus from the deeper waters to the productive surface layers are estimated to be of the same order of magnitude as the waterborne inputs of phosphorus to the entire Baltic Sea. The model results suggest that regionally the impact of these nutrients may be quite large, and the largest regional increases in surface concentrations are found after large inflows. However, the overall direct impact of major Baltic inflows on the annual uplift of nutrients from below the halocline to the surface waters is small because vertical transports are comparably large also during periods without major inflows. Our model results suggest that phosphorus released from the sediments between 60 and 100 m depth in the East Gotland Basin contributes to the eutrophication, especially in the coastal regions of the eastern Baltic Proper

Projected future climate change and Baltic Sea ecosystem management

Climate change is likely to have large effects on the Baltic Sea ecosystem. Simulations indicate 2-4 degrees C warming and 50-80 % decrease in ice cover by 2100. Precipitation may increase similar to 30 % in the north, causing increased land runoff of allochthonous organic matter (AOM) and organic pollutants and decreased salinity. Coupled physical-biogeochemical models indicate that, in the south, bottom-water anoxia may spread, reducing cod recruitment and increasing sediment phosphorus release, thus promoting cyanobacterial blooms. In the north, heterotrophic bacteria will be favored by AOM, while phytoplankton production may be reduced. Extra trophic levels in the food web may increase energy losses and consequently reduce fish production. Future management of the Baltic Sea must consider the effects of climate change on the ecosystem dynamics and functions, as well as the effects of anthropogenic nutrient and pollutant load. Monitoring should have a holistic approach, encompassing both autotrophic (phytoplankton) and heterotrophic (e.g., bacterial) processes

Improving the multiannual, high-resolution modelling of biogeochemical cycles in the Baltic Sea by using data assimilation

The impact of assimilating temperature, salinity, oxygen, phosphate and nitrate observations on marine ecosystem modelling is assessed. For this purpose, two 10-yr (1970-1979) reanalyses of the Baltic Sea are carried out using the ensemble optimal interpolation (EnOI) method and a coupled physical-biogeochemical model of the Baltic Sea. To evaluate the reanalyses, climatological data and available biogeochemical and physical in situ observations at monitoring stations are compared with results from simulations with and without data assimilation. In the first reanalysis, only observed temperature and salinity profiles are assimilated, whereas biogeochemical observations are unused. Although simulated temperature and salinity improve considerably as expected, the quality of simulated biogeochemical variables does not improve and deep water nitrate concentrations even worsen. This unexpected behaviour is explained by a lowering of the halocline in the Baltic proper due to the assimilation causing increased oxygen concentrations in the deep water and consequently altered nutrient fluxes. In the second reanalysis, both physical and biogeochemical observations are assimilated and good quality in all variables is found. Hence, we conclude that if a data assimilation method like the EnOI is applied, all available observations should be used to perform reanalyses of high quality for the Baltic Sea biogeochemical state estimates