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In response to the HELCOM commitment the Leibniz Institute for Baltic Sea Research Warnemuende conducts a monitoring program on listed substances of concern for the Baltic Sea environment which comprises the isomers of technical hexachlorocyclohexane (HCH). After World War II the HCH compounds were applied globally in huge amounts as insecticidal formulation in which only γ-HCH is the effective agent. Monitoring was conducted in water samples from a large Baltic Sea area. Mostly, the HCH compounds were evenly distributed in the Baltic Sea. Data from 1975 to 2015 from the Arkona Basin depict a substantial reduction of the HCH pressure (12,500 to < 400 pg/L). Longest residence is shown for β-HCH, which is currently the predominant HCH isomer in the Baltic Sea (α/β/γ:1/2/1). Half lives were determined to be 4–8 years for α- and γ-HCH and 5–20 years for β-HCH. [ABSTRACT FROM AUTHOR]

The dependence of denitrification and dissimilatory nitrate reduction to ammonium (DNRA) on different electron donors was tested in the nitrate-containing layer immediately below the oxic-anoxic interface (OAI) at three stations in the central anoxic basins of the Baltic Sea. Additionally, pathways and rates of fixed nitrogen transformation were investigated with 15N incubation techniques without addition of donors. Denitrification and anammox were always detected, but denitrification rates were higher than anammox rates. DNRA occurred at two sites and rates were two orders of magnitude lower than denitrification rates. Separate additions of dissolved organic carbon and sulfide stimulated rates without time lag indicating that both organotrophic and lithotrophic bacterial populations were simultaneously active and that they could carry out denitrification or DNRA. Manganese addition stimulated denitrification and DNRA at one station, but it is not clear whether this was due to a direct or indirect effect. Ammonium oxidation to nitrite was detected on one occasion. During denitrification, the production of nitrous oxide (N2O) was as important as dinitrogen (N2) production. A high ratio of N2O to N2 production at one site may be due to copper limitation, which inhibits the last denitrification step. These data demonstrate the coexistence of a range of oxidative and reductive nitrogen cycling processes at the Baltic OAI and suggest that the dominant electron donor supporting denitrification and DNRA is organic matter. Organotrophic denitrification is more important for nitrogen budgets than previously thought, but the large temporal variability in rates calls for long-term seasonal studies. [ABSTRACT FROM AUTHOR]

Despite the worldwide occurrence of marine hypoxic regions, benthic nitrogen (N) cycling within these areas is poorly understood and it is generally assumed that these areas represent zones of intense fixed N loss from the marine system. Sulfate reduction can be an important process for organic matter degradation in sediments beneath hypoxic waters and many sulfate-reducing bacteria (SRB) have the genetic potential to fix molecular N (N2). Therefore, SRB may supply fixed N to these systems, countering some of the N lost via microbial processes such as denitrification and anaerobic ammonium oxidation. The objective of this study was to evaluate if N2-fixation, possibly by SRB, plays a role in N cycling within the seasonally hypoxic sediments from Eckernforde Bay, Baltic Sea. Monthly samplings were performed over the course of one year to measure N2-fixation and sulfate reduction rates, to determine the seasonal variations in bioturbation (bioirrigation) activity and important benthic geo-chemical profiles, such as sulfur and N compounds, and to monitor changes in water column temperature and oxygen concentrations. Additionally, at several time points, rates of benthic denitrification were also measured and the active N-fixing community was examined via molecular tools. Integrated rates of N2-fixation and sulfate reduction showed a similar seasonality pattern, with highest rates occurring in August (approx. 22 and 880 nmol cm-3 d-1 of N and SO Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed., respectively) and October (approx. 22 and 1300 nmol cm-3 d-1 of N and SO Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed., respectively), and lowest rates occurring in February (approx. 8 and 32nmol cm-3 d-1 of N and SO Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed., respectively). These rate changes were positively correlated with bottom water temperatures and previous reported plankton bloom activities, and negatively correlated with bottom water oxygen concentrations. Other variables that also appeared to play a role in rate determination were bioturbation, bubble irrigation and winterstorm events. Molecular analysis demonstrated the presence of nifH sequences related to two known N2-fixing SRB, namely Desulfovibrio vulgaris and Desulfonema limicola, supporting the hypothesis that some of the nitrogenase activity detected may be attributed to SRB. Denitrification appeared to follow a similar trend as the other microbial processes and the ratio of denitrification to N2-fixation ranged from 6.8 in August to 1.1 in February, indicating that in February, the two processes are close to being in balance in terms of N loss and N gain. Overall, our data show that Eckernforde Bay represents a complex ecosystem where numerous environmental variables combine to influence benthic microbial activities involving N and sulfur cycling. [ABSTRACT FROM AUTHOR]

This article focuses on the ecological role of benthic macrofauna on nutrient dynamics and benthic-pelagic coupling in the Baltic Sea with relation to eutrophication. Generally, benthic macrofaunal activities have large effects on sediment biogeochemistry and often with stimulatory effects on processes that counteract eutrophication, i.e., denitrification and increased phosphorus retention of the sediment. The degree of faunal impact on such processes varies depending on faunal density and functional group composition. The effect of macrofaunal activities on sediment nutrient dynamics can also result in a higher nitrogen : phosporus ratio of the sediments efflux compared with sediments without macrofauna. Increased internal nutrient loading during eutrophication-induced anoxia is suggested to be caused both by altered sediment biogeochemical processes and through reduced or lost bioturbating macrofauna and thereby a reduced stimulatory effect from their activities on natural purification processes of the Baltic Sea ecosystem. [ABSTRACT FROM AUTHOR]

The aim of this study was to compare structural differences in the denitrifying microbial communities along the environmental gradients observed in the water column and coastal sediments of the Baltic Sea. To link community structure and environmental gradients, denitrifier communities were analyzed by terminal restriction fragment length polymorphism (T-RFLP) based on nirS as a functional marker gene for denitrification. NirS-type denitrifier community composition was further evaluated by phylogenetic analysis of nirS sequences from clone libraries. T-RFLP analysis indicated some overlap but also major differences of communities from the water column and the sediment. Shifts in community composition along the biogeochemical gradients were observed only in the water column while denitrifier communities were rather uniform within the upper 30 mm of the sediment. Specific terminal restriction fragments (T-RFs) indicative for the sulfidic zone suggest the presence of nitrate-reducing and sulfide-oxidizing microorganisms that were previously shown to be important at the suboxic-sulfidic interface in the water column of the Baltic Sea. Phylogenetic analysis of nirS genes from the Baltic Sea and of sequences from marine habitats all over the world indicated distinct denitrifier communities that grouped mostly according to their habitat. We suggest that these subgroups of denitrifiers had developed after selection through several factors, i.e. their habitats (water column or sediment), impact by prevalent environmental conditions and isolation by large geographic distances between habitats. [ABSTRACT FROM AUTHOR]

Chemical profiles of nutrients at the sediment–water interface were measured in the northern Baltic Sea. A whole core squeezer technique capable of mm-scale resolution was used to obtain the vertical profiles of NO3-, NO2-, o-P, NH4+ and Si in the soft bottom sediments. The profiles were compared with nutrient flux and denitrification measurements. In the Gulf of Finland, the profiles revealed a marked chemical zonation in NO3- and NO2- distribution indicating strong potential of nitrification just under the sediment surface followed by a layer of denitrification down to a depth of 30 mm. Below the depth of 20 mm NO3- was usually absent, whereas other nutrients were increasing steadily in concentration. A distinct minimum of NO3- was observed at the sediment–water interface, suggesting NO3- uptake by a microbial biofilm and/or active denitrification at the suboxic microniches usually present in organic-rich sediments. At the deep stations in the Baltic Proper, the NO3- concentration in pore water, as well as denitrification, were very low. The concentrations of NH4+, o-P and Si were usually increasing steadily with depth. [ABSTRACT FROM AUTHOR]

Incubation experiments with 15N-labeled compounds (NO3- and NH4+) were performed during three cruises (2002, 2004, and 2005) to study the loss of inorganic N as dinitrogen gas (N3) via denitrification and anammox in the water column of the Gotland Deep (central Baltic Sea). 15N incubations did not provide evidence for direct conversion of NO3- reduction to N3 (heterotrophic denitrification) in the suboxic (O3 < 10 µmol L-1) sulfide-free waters. Substantial denitrification rates (up to 2.7 µmol L-1 d-1 N2) were measured in water samples collected from the NO3--H2S interface (redoxcline) in 2002 and in water from the sulfidic zone in 2004, which indicates chemolithotrophic denitrification as the dominant N-loss process in both years. Massive inflows of oxygenated North Sea water from 2002 to 2003 caused a complete ventilation of the Baltic Sea with high oxygen concentrations in the Gotland Deep bottom water. After the reestablishment of the redoxcline in 2004, a newly formed suboxic zone above sulfidic waters-with NO3-, NO2-, and NH&sub4;+ at the detection limit-was observed in spring 2005. The development of this zone was associated with a several-fold increase in reduced and oxidized manganese and with a shift from denitrification to anammox as the main N-loss process. Fluorescence in situ hybridization analysis confirmed the presence of anammox bacteria and the number of anammox cells was consistent with the observed N2 production rates in 2005 [ABSTRACT FROM AUTHOR]