Hypoxia and nitrogen processing in the Baltic Sea water column

Abstract

Steep redoxclines form between oxic surface water and the stagnant, sulfidic hypolimnion in the eutrophied, brackish water Baltic Sea. Nitrification, denitrification, and anammox were measured at and below the redoxcline to quantify the role of water-column nitrogen processes in the overall magnitude of nitrogen removal in the Baltic Sea. Rates of nitrification were very high (up to 85 nmol N L 21 d 21 ) at the oxic-anoxic interface, but, surprisingly, nitrification was separated from the processes reducing nitrate to N 2 (up to 810 nmol N L 21 d 21 ) by tens of meters in depth. N 2 production was dominated by chemolithotrophic denitrification, with anammox playing only a negligible role, and limited to the water layers in which nitrite or nitrate coexisted with sulfide. The alternating oxygen concentrations in the basin induce irregular bursts of nitrogen removal. Nitrification takes place after mixing of ammonium-rich deep water with oxic water, and denitrification uses the formed nitrite and nitrate once anoxic conditions re-establish. Although removal rates can be high, conditions allowing such rates are likely short-lived. While the sedimentary denitrification rates at the shallower, oxic areas are lower, they are more constant in time, highlighting the need to avoid hypoxia, which would prevent sedimentary denitrification.

Significance of archaeal nitrification in hypoxic waters of the Baltic Sea

“…This site showed an ammonium gradient with concentrations higher than those detected in the other deeps. In general, our data confirm the temporal and spatial variability in potential nitrification rates reported by Hietanen et al (2012) but also underline the persistence of nitrification in the Baltic Sea over time. Anaerobic oxidation of ammonium (anammox), however, was not detected in selected depths of the Gotland Deep (Supplementary Figure S4), and this is in accordance to detectable anammox activity only after the inflow of oxygen-rich water into the Baltic Sea (Hannig et al, 2007).…”

“…Notably, the detected nitrification potential, likely because of small oxygen additions during sampling (De Brabandere et al, 2012), of 62 nmol l À 1 per day at a sulfidic depth (Figure 3) suggests that AOA persist during sulfidic conditions and quickly become active as soon as oxygen is again available. Hietanen et al (2012) pointed out that a nitrification potential might extend into deeper layers, even into the sulfidic waters. Physiological adaptations, for example, in cell membrane composition, may enable Thaumarchaeota to tolerate the sulfide pulses encountered in the nitrification zone or sulfide accumulation in these waters.…”

“…Nitrogen transformations and the responsible microorganisms have also been investigated in the oxic-anoxic transition zones of the water column of the central Baltic Sea (Hannig et al, 2007;Schneider et al, 2010;Hietanen et al, 2012). Oxygen depletion in deep basins is accompanied by gradients of NO 3 À , NO 2 À , H 2 S and NH 4 þ that are exploited by different guilds of chemolithoautotrophic prokaryotes (Grote et al, 2008;Labrenz et al, 2010;Glaubitz et al, 2013).…”

“…As an extended oxygen-and sulfide-free zone is lacking most of the time (in contrast to the Black Sea; Lam et al, 2007), ammonia oxidation is carried out mostly aerobically because of the sensitivity of anammox bacteria toward oxygen and potentially sulfide (Jin et al, 2012;CarvajalArroyo et al, 2013). Where ammonium and oxygen gradients overlap, potential nitrification rates as high as 85-160 nmol l À 1 per day have been measured (Hietanen et al, 2012). Labrenz et al (2010) reported the dominance of a single thaumarchaeotal subcluster, termed GD2, related to the autotrophic Candidatus Nitrosopumilus maritimus (Kö nneke et al, 2005) that accounted for up to 26% of all prokaryotes in hypoxic waters.…”

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.…”