Denitrification in the water column of the central Baltic Sea

Abstract

Removal of fixed nitrogen in the water column of the eastern Gotland Basin, central Baltic Sea, was studied during two cruises in September 2008 and August 2010. The water column was stratified with anoxic sulfidic bottom water meeting oxic nitrate containing water at the oxic-anoxic interface. Anammox was never detected whereas denitrification was found in all incubations from anoxic depths and occurred immediately below the oxic-anoxic interface. Sulfide (H 2 S + HS À + S 2À ) was in most cases the only electron donor for denitrification but, in contrast to previous findings, denitrification was in some situations driven by organic matter alone. Nitrous oxide (N 2 O) became an increasingly important product of denitrification with increasing sulfide concentration and was >80% of the total N gas formation at 10 lM sulfide. The potential rates of denitrification measured in incubations at elevated NO À 3 or sulfide concentrations were converted to in situ rates using the measured water column concentrations of NO À 3 and sulfide and the actual measured relations between NO À 3 and sulfide concentrations and denitrification rates. In situ denitrification ranged from 0.24 to 15.9 nM N 2 h À1 . Assuming that these rates were valid throughout the anoxic NO À 3 containing zone, depth integrated in situ denitrification rates of 0.06-2.11 mmol N m À2 d À1 were estimated. The thickness of this zone was generally 3-6 m, which is probably what can be maintained through regular turbulent mixing induced by internal waves at the oxic-anoxic interface. However, layers of up to 55 m thickness with low O 2 water (<10 lM) were observed which was probably the result of larger scale mixing. In such a layer nitrification may produce NO À 3 and once the O 2 has been depleted denitrification will follow resulting in enormous rates per unit area. Even with an active denitrification layer of 3-6 m thickness the pelagic denitrification per unit area clearly exceeded sediment denitrification rates elsewhere in the Baltic Sea. When extrapolated to the entire Baltic Proper (BP) denitrification in the water column was in the range of 132-547 kton N yr À1 and was thus at least as important as sediment denitrification which has recently been estimated to 191 kton N yr À1 . With a total external N-input of 773 kton N yr À1 it is clear that denitrification plays a significant role in the N-budget of the BP.

Key processes in the coupled carbon, nitrogen, and phosphorus cycling of the Baltic Sea

“…These matters will not be discussed in any detail in the present study, instead we choose to highlight a few recent estimates: Noffke et al (2016) estimated the benthic denitrification rate in the Baltic Proper to *0.43 mmol m -2 day -1 , or some 500 kton year -1 , while Deutsch et al (2010) estimated the benthic denitrification rate in the EBS to be in a range 426-652 kton year -1 . Dalsgaard et al (2013) estimated that pelagic denitrification in the Baltic Proper amounts to 132-547 kton year -1 . Our simulated average total denitrification rate (benthic ?…”

Influence of Natural Oxygenation of Baltic Proper Deep Water on Benthic Recycling and Removal of Phosphorus, Nitrogen, Silicon and Carbon

“…Nitrogen-fixing cyanobacteria are limited by P, and the blooms are triggered by low N:P ratios in the nutrient pool (e.g., Nausch et al, 2012). The low N:P ratios are caused by removal of fixed nitrogen (denitrification and anammox) at both the oxycline/nitracline in water column (Dalsgaard et al, 2013) and in sediments underlying oxygenated or at least nitrate containing bottom water (Tuominen et al, 1998;Deutsch et al, 2010) together with enhanced benthic P regeneration due to oxygen depletion in deep waters (Jilbert et al, 2011;Viktorsson et al, 2013a). The change in the bottom area covered by hypoxic water in the Baltic Sea, but not the external P load, has been found to correlate well with variations in the dissolved water column P pool size (Conley et al, 2002;Stigebrandt et al, 2014).…”

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

“…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.…”