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

Abstract

Abstract.Rates of denitrification in sediments were measured with the isotope pairing technique at different sites in the southern and central Baltic Sea. The rates varied between 0.5 µmol N m −2 h −1 in sands and 28.7 µmol N m −2 h −1 in muddy sediments and showed a good correlation to the organic carbon contents of the surface sediments. N-removal rates via sedimentary denitrification were estimated for the entire Baltic Sea calculating sediment specific denitrification rates and interpolating them to the whole Baltic Sea area. Another approach was carried out by using the relationship between the organic carbon content and the rate of denitrification. The N-removal by denitrification in sediments varied between 426-652 kt N a −1 , which is around 48-73% of the external N inputs delivered via rivers, coastal point sources, and atmospheric deposition. Moreover, an expansion of the anoxic bottom areas was considered under the assumption of a rising oxycline from 100 to 80 m water depth. This leads to an increase of the area with anoxic conditions and an overall decrease in sedimentary denitrification by 14%. Overall, we show here that this type of data extrapolation is a powerful tool to estimate the nitrogen losses for a whole coastal sea and may be applicable to other coastal regions and enclosed seas.

Seasonal oxygen, nitrogen and phosphorus benthic cycling along an impacted Baltic Sea estuary: regulation and spatial patterns

“…The denitrification rates reported in the present study are similar to those measured in the northern part of the Baltic Sea, the Gulf of Bothnia, but 50 % lower than those found in the southern Bothnian Sea (Stockenberg and Johnstone 1997). Our rates are at least twice as high as rates previously measured in sediment from the open Baltic Proper (Tuominen et al 1998;Deutsch et al 2010), generally comparable to those measured in the Gulf of Finland (Hietanen and Kuparinen 2008;Jäntti et al 2011), but 1.5 to twofold lower than those in muddy sediments of the southern Baltic Sea (Deutsch et al 2010). Compared to measurements in other coastal settings worldwide, our rates and those reported in previous studies from the Baltic are generally low (Seitzinger 1988;Steingruber et al 2001).…”

“…The spreading of hypoxia and anoxia in the bottom waters since the 1960s (Elmgren 2001;Conley et al 2009) is generally thought to be responsible for significant N loss due to enhanced denitrification in the water column (Rönner 1985;Vahtera et al 2007). While an N deficit in the Baltic has been identified based on budget calculations ), coupled physical biogeochemical models (Meier et al 2012), and gas inventories (Löffler et al 2011), there have been few direct rate measurements in sediments and the water column (Deutsch et al 2010;Jäntti et al 2011;Hietanen et al 2012;Dalsgaard et al 2013). The regional and spatial coverage of benthic flux data for the Baltic is low and most of the existing studies have not investigated the key regulators of benthic N cycling such as temperature, oxygen availability and demand, presence/absence of macrofauna, sedimentary organic matter content, or variable nutrient loading from human activities.…”

“…The regional and spatial coverage of benthic flux data for the Baltic is low and most of the existing studies have not investigated the key regulators of benthic N cycling such as temperature, oxygen availability and demand, presence/absence of macrofauna, sedimentary organic matter content, or variable nutrient loading from human activities. Furthermore, for some of the most highly impacted areas of the Baltic Sea the published rates of N 2 production refer to a single campaign and do not have a temporal resolution that is high enough to catch the seasonal variability or the steep horizontal and vertical gradients of potentially regulating factors (Hietanen 2007;Deutsch et al 2010;Jäntti and Hietanen 2012).…”

Environmental Impacts—Marine Biogeochemistry

“…Together with denitrification at the pelagic redoxcline calculated from the rate measurements of Brettar and Rheinheimer (1991), this implies a total denitrification of 397 kt N year −1 for the Baltic Proper. For the remaining regions where no pelagic redoxcline exists, Deutsch et al (2010) reported a sediment denitrification of 235 kt N year −1 . Hence, the total denitrification in the Baltic Sea (823 kt N year −1 ) agrees with the upper limit given by Voss et al (2005).…”

“…18.2). Deutsch et al (2010) investigated denitrification in sediments with different physical and chemical properties. The results were extrapolated to the entire Baltic Sea by the use of maps describing sediment characteristics for the entire Baltic Sea.…”