Norwegian Institute for Water Research, Oslo, Norway 14th AOMIP Workshop, WHOI, 20-22.10.2010 On modeling of biogeochemical processes in oxygen-deficient and anoxic conditions Yakushev E.V. Norwegian Institute for Water Research, Oslo, Norway

Why the Arctic? “Dead zones in the coastal oceans have spread exponentially since the 1960s and have serious consequences for ecosystem functioning” (Diaz, Rozenberg, 2008) Fjord systems: existing oxygen depletion in the coastal regions.

Oxygen depletion Oxygen-deficient < 80% of sat. Hypoxic < 30% of sat. Suboxic < 10 μM (~3%) O2 Oxygen detection limit “formal” 3 μM “reachable?” 1 μM Anoxic/Sufidic (H2S >0.3 μM) A "healthy" aquatic environment should seldom experience DO less than 80%

Anoxic conditions in the water column Rivers and rains low So/oo high So/oo fresh water outflow Sill marine water Inflow Formation of Halocline / Pycnocline

Anoxic conditions in the water column OM Mixing occurs down to halocline Maximum depth of mixing (CIL in the Black Sea) Weak turbulent transport of O2 exists to the deeper layer O2 is using up for the OM decomposition

Anoxic conditions in the water column Formation of anoxia H2S + NH4, Mn(II), Fe(II), CH4… Flux of OM is not balanced by the flux of O2 and OM is decomposed with other electron-acceptors (NO3, Mn(IV), Fe(III), SO4, CO2…)

Redox interfaces structure gradO2, NO3max, PO4max PO4min, NO2min, O2,NO3 Mn2+, NH4, CH4, S0 H2S, gradPO4max Black Sea Turbidity layer

Anoxic conditions in the water column Baerumsbassenget Drammensfjord (Richards, 1965) Bunnefjord

Oslofjord

Oslo Fjord

Oslo Fjord

Hunnbunn

Anoxic conditions in the water column H2S

Hunnbunn vertical structure May 2o09 August 2o09

Process dependent of oxygen conditions OXIC > 62.5 μM HYPOXIC <62.5 μM >10 μM SUBOXIC <10 μM >0 μM ANOXIC =0 μM Oxic mineralization of OM and nitrification + Denitrification /anammox Oxydation of reduced species of S, Mn, Fe, C, N Sulfatereduction, resuction of oxydised species of S, Mn, Fe, C Mortality caused by low oxygen Synthesis of OM

N+O model NO3 NO2 NH4 Norg N2 O2 Goal: to study the process of formation of anoxic (O2=0 ) conditions Am = KAm [Norg] – ammonification Nf1 = KNf1 [NH4] fNf(O2) – nitrification 1 Nf2 = KNf2 [NO2] fNf (O2) – nitrification 2 Nr1 = KNr1 [NO3] fNr (O2) – nitratereduction 1 Nr2 = KNr2 [NO2] fNr (O2) – nitratereduction 2 Dn = KDn[NO2] fDn (O2) – denitrification Sources: RNorg= -Am RNH4 = Am - Nf1+ Nr2 RNO2 = Nf1 - Nf2 + Nr1 RNO3 = Nf2 - Nr1 – Dn RO2 = -m12 Am - m10 Nf1 – -m11 Nf2 + m21 Nr1+ m21Nr2 Dependence of rates of processes on О2

N+O model Anoxic conditions due to OM flux and restricted aeration of deep waters. Necessity of parameterization of cycles of several elements (Yakushev, 1992)

ROLM biogeochemical model NO3 NO2 NH4 PON N2 O2 DON

ROLM biogeochemical model NO3 NO2 NH4 PON N2 O2 SO4 S2O3 S0 H2S DON

ROLM biogeochemical model Phy Zoo NO3 NO2 NH4 PON N2 O2 SO4 S2O3 S0 H2S DON Bacteria Ox.Aut Ox.Het Anox.Aut Anox.Het

ROLM biogeochemical model Phy Zoo PO4 POP DOP NO3 NO2 NH4 PON N2 O2 SO4 S2O3 S0 H2S DON Bacteria Ox.Aut Ox.Het Anox.Aut Anox.Het

ROLM biogeochemical model Phy Zoo PO4 POP DOP NO3 NO2 NH4 PON N2 O2 Mn2+ Mn3+ SO4 S2O3 S0 H2S DON Fe2+ Fe3+ Bacteria Ox.Aut Ox.Het Anox.Aut Anox.Het Mn4+

Scheme of calculations: T,S, weather conditions 1D GOTM T,S, light Turbulence, sinking Integration Biogeochemical changes Parameterized biogeochemical properties of the intrusions ROLM GOTM (General Ocean Turbulence Model), Burchard et al., 1999 (http://www.gotm.net).

Vertically balanced structure: Gotland 250-400 μM 4-5 μM m-1 O2 25 μM 3-4 μM m-1 Mn(II) Mn(III)+Mn(IV) 0.1-1.5 μM 300 μM 3-4 μM m-1 H2S Black Sea Model

Vertically balanced structure 3 m suboxic zone PO4 minimum Oxidized Mn maxima between O2 and H2S Mn-oxidation: 0.15-1.62 μM d-1 (Tebo, 1991, Schippers et al.,2005) Kz=3.5x10-6, m2 s-1 WMn = 7.5 m d-1 Mn(II)|150 m = 10 μM Mn-reduction: 0.96-3.6 μM d-1 (Nealson et al.,1991)

Low mixing: Kz=1x10-6, m2 s-1 well-defined suboxic layer small PO4 minimum small Mn(III) and Mn(IV) maxima low Mn-transformation rates ~ Central Black Sea

Large mixing: Kz=3.5x10-5, m2 s-1 c- layer large PO4 minimum large Mn(III) and Mn(IV) maxima high Mn-transformation rates ~ Bosphorus region and the Black Sea RIM current

Modeling of oxygen-depletion processes in the Arctic: Conclusions Modeling of oxygen-depletion processes in the Arctic: Open Arctic – no (climate will probably not create oxygen-minimum zones …) Coastal regions and fjord system –yes (climate forced decreased mixing, permafrost …) Open for collaboration/coupling with 3D regional models http://www.gotm.net/pages/documentation/manual/html/node258.html

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