Sediment-water oxygen and nutrient exchanges along a depth gradient in the Baltic Sea

Abstract

Bottom water and sediment characteristics and net sediment-water fluxes of oxygen, nitrogen, phosphorus and hydrogen sulphide were measured under oxic a n d experimentally induced anoxic conditions along a depth gradient (47 to 130 m) during summer in the Baltic proper. Temperature, salinity and dissolved nutrient concentrations (particularly phosphorus and nitrate) in bottom waters increased with depth while oxygen concentrations decreased sharply. Sediment organic content was much higher in sediments located beneath the permanent pycnocline (ca 65 m). Sediments at all stations were somewhat depleted in total N relative to total C (C.N -10). Sediments at the shallow station (47 m ) were highly enriched with total P relative to C or N (C:N P = 18:2:1) but were somewhat depleted in P at the deeper stations (C:N:P = 120:12:1). Under oxic conditions oxygen fluxes ranged from 214 to 777 pm01 0 m Q-' and decreased with depth. Phosphorus and nitrite fluxes were always very small and were directed either into or out of the sediments. Ammonium fluxes were small (1 to 30 pm01 N m-' h-') at all stations and did not exhibit a clear pattern with depth; 0 : N -N H , flux ratios were close to expected 'Redfield ratios' at the deep station (16:l) but were much higher (>55:1) at the shallower sites indicating that processes other than only an~monificatlon were taking place in sediments. Nitrate was always consumed by sediments (ca 1 to 16 ktn~ol N m-' h-') and fluxes were proportional to nitrate concentrations in the overlying water. Under anomc conditions there was a dramatic increase in P-PO4 fluxes (2 to 40 pm01 P m-' h-') and a smaller lncrease In N-NH, fluxes (14 to 35 pm01 N m-2 h ' ) . Large hydrogen sulphide fluxes (>40 pm01 S m-' h-') were observed in sediments from the deepest station only. Under oxic conditions sediment recycling of N and particularly of P were small compared to estimated rates of burial of P and burial plus denitrification of N However, under anoxic conditions, sediment recycling of both N and P were similar to or much greater than sedlment loss terms of burial and denitrification. Sediment regeneration under oxic conditions could supply 1 to 8 % and 0 to 2 % of estimated phytoplankton demand of N and P, respectively; under anoxic conditions 12 % of N and u p to 200 O/o of P demand could be met via sediment recycling.

Denitrification in sediments as a major nitrogen sink in the Baltic Sea: an extrapolation using sediment characteristics

“…For all other stations, our sediment-specific denitrification rates are in the range reported from other studies in the Baltic Sea (Koop et al, 1990;Tuominen et al, 1998;Hietanen and Kuparinen, 2008; Table 6). …”

Indications of Recovery from Hypoxia in the Inner Stockholm Archipelago

“…1) had extremely high concentrations of iron-bound P (Fe-P) in the surface layer (4 mg kg -1 dw, see further Karlsson et al 2009) indicating an effective trapping of phosphorus. Fe-P is mostly immobile in oxidized sediments but is typically dissolved in the pore water and released to the water column through diffusion if the sediments turn reduced (Mortimer 1941;Balzer 1984;Koop et al 1990;Eilola et al 2009). An interesting question is, therefore, what would happen if the system again deteriorated toward less oxic conditions?…”

Nitrogen as the limiting nutrient for eutrophication in coastal marine ecosystems: Evolving views over three decades

“…This, in combination with N lost through denitrification, is a major reason that primary production in Narragansett Bay is predominantly N limited (Nixon et al 1980;Howarth 1988). On the other hand, the P released from sediments in Chesapeake Bay is less than the amount released during decomposition (Boynton and Kemp 1985), and there is evidence of P adsorption and storage in surface sediments in several other estuaries (van Raaphorst et al 1988;Koop et al 1990;Sundby et al 1992). Many of the differences in P uptake and release from sediments in both estuaries and lakes may be explained in part by the extent of eutrophication in those systems.…”