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Certain fjord systems are characterized with a meromictic structure when a sill restricts renewal of deep water. This leads to formation of anoxic conditions in a bottom layer. The renewal of the bottom waters occurs during winter time when density of inflowing water exceeds the one of basin water. These events occur because of a combination of low temperatures and high salinity of surface water. Water renewal can be partial some time and complete another time. It can take place every year (Bunnefjord), once per several years (Drammensfjord) or once per several decades (Framvaren). Inflow events change biogeochemical structure either in appearance of oxic conditions for a long period, or in a small scale violations that lead, for instance, to oxidation of metal sulfides.and formation of metal oxides.
To study these events we used a coupled hydrophysical-biogeochemical model. Biogeochemical processes were described with a O-N-S-P-Mn-Fe ROLM model (Yakushev et al, 2007), that was designed to study cycling of processes of the following chemical elements: organic matter (OM) formation (photosynthesis and chemosynthesis), OM decay (aerobic decomposition, denitrification, sulfate reduction), nitrification, hydrogen sulfide oxidation, manganese and iron reduction and oxidation. The following variables were considered: O2, H2S, S0, S2O3, SO4, NH4, NO2, NO3, PON, DON, PO4, POP, DOP, Mn(II), Mn(IV), Fe(II), Fe(III), phytoplankton, zooplankton, aerobic heterotrophic bacteria (Bc), aerobic autotrophic Bc, anaerobic heterotrophic Bc, anaerobic autotrophic Bc. Hydrophysical processes where described with 3D General Estuarine Transport Model GETM (Burchard et al, 2004).
Numerical experiment demonstrated that intrusions caused the elimination of balanced oxic-anoxic biogeochemical structure and intensification of certain biogeochemical processes that were ?launched? by additional amount of electron-donors and electron-acceptors. During the inflow events large amounts of particulate Mn and Fe precipitate, influencing fluxes of redox sensitive metals The system returned to balanced condition after several stages characterized by successive intensification of certain processes (including heterotrophic denitrification and anammox). Frequency of appearance of such inflows can explain duration of the ?imbalanced? structures that can be often observed during field studies.
Literature
H.Burchard, K.Bolding, and M.R. Villarreal Three-dimensional modelling of estuarine turbidity maxima in a tidal estuary, Ocean Dynamics, 2004, 54, 250-265,.
E.V.Yakushev, F.Pollehne, G.Jost, L.Umlauf, I.Kuznetsov and B.Schneider Analysis of the water column oxic/anoxic interface in the Black and Baltic seas with a Redox-Layer Model. Marine Chemistry, 2007, 107, 388-410
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