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Over the last few decades many studies have focused on the oxygen depletion of coastal and oceanic waters. An understanding of the processes involved is fundamental to assess the effects of global and climatic changes and to support an ecosystem approach to adaptive environmental management for coastal seas and ocean basins.

This timely book presents the state-of-the-art of our knowledge of the nature and chemical structure of redox interfaces in a marine water column, oxygen depletion and connected processes. The structures of the redox layers, including the distribution of certain parameters and microbiological features, are described in detail. The volume also covers studies devoted to the interannual variability of some oxygen-depleted systems, modeling and new developments in observation techniques. In addition, it identifies remaining gaps in our knowledge of the cycling of chemical elements in changing redox conditions. The chapters are based on extensive observational data, collected by the authors during sea and shore expeditions, on archive data, and on a broad range of scientific literature.



Introduction: Redox Interfaces in Marine Waters

The typical features of the structure of the redox layers in a marine water column are described. The oxic, hypoxic, suboxic and anoxic layers are discussed with respect to the definitions of terms. A classification of the redox conditions is proposed based on processes typical of the redox conditions appearing during the different stages of oxygen depletion.
Oxygen depletion events are connected with anthropogenic forcing such as eutrophication. However, climatic forcing may further stimulate the formation of oxygen-depleted zones. The redox interfaces and oxygen depletion events studies will therefore require an interdisciplinary approach, which the present book reflects.
E. V. Yakushev, A. Newton

Biogeochemical Peculiarities of the Vertical Distributions of Nutrients in the Black Sea

Analysis of the vertical distributions of the ratios between C, S, Si, N, and P revealed the layers with significant systematic differences from the theoretical Redfield and Richards values. These anomalies can testify to the presence of such processes as denitrification/anammox and the processes of the “phosphate dipole” formation. Based on the ratios to Si, which concentrations do not change under the redox conditions variability, we estimated numerically the other elements deficiencies and the rates of the processes that form these deficiencies. The calculated rates of denitrification/anammox (0.012–0.046 μM per day) correspond well to the present observations data. The calculated possible rates of the processes controlling the shallower phosphate minimum formation equal to 0.008–0.032 μM per day, and the rates of the deeper phosphate minimum formation are in the range of 0.006–0.024 μM per day. The Ct/St ratio showed that bacterial sulfate reduction was the only significant process in the anaerobic mineralization of organic matter in the anoxic zone of the Black Sea that lead to a constant stoichiometric C/S ratio close to the theoretical one of 2.
M. V. Chelysheva, E. V. Yakushev, E. L. Vinogradova, V. K. Chasovnikov

Anaerobic Microbial Community in the Aerobic Water and at the Oxic/Anoxic Interface in the Black Sea

Fluorescent in situ hybridization (FISH) was used to analyze the abundance and phylogenetic composition of physiologically active anaerobic microbial communities [sulfate-reducing bacteria (SRB) and methanogenic archaea] in the aerobic waters and in the oxic/anoxic transitional zone (chemocline) of the Black Sea. Biogenic sulfate reduction and methane formation were detected at these horizons by radioisotope techniques. Numerous SRB phylogenetically related to Desulfotomaculum (30.5% of detected bacterial cells), Desulfovibrio (29.6%), and Desulfobacter (6.7%) were detected in the aerobic zone at a depth of 30 m, whereas Desulfomicrobium-related bacteria (33.5%) were prevalent in chemocline at a depth of 150 m. In the oxic subsurface water layer, Methanomicrobiales-related archaea and subgroup 1 methanogens constituted up to 62 and 35.3% of archaeal cell, respectively. The active cells of sulfate-reducing and methanogenic microorganisms were much more abundant in the samples collected in summer than in winter from the deep-sea zone. The presence of physiologically active anaerobic microorganisms in oxic and chemocline waters of the Black Sea correlates with the hydrochemical data on the presence of sulfide and methane at corresponding depths.
N. V. Pimenov, A. L. Bryukhanov, V. A. Korneeva, E. E. Zakharova, P. A. Sigalevich, I. I. Rusanov, E. V. Yakushev, V. K. Chasovnikov

The Energetic Balance of Microbial Exploitation of Pelagic Redox Gradients

In marine environments and especially in marginal seas, pelagic redox gradients with underlying sulfidic water layers are a known phenomenon. General explanations for the observed spatial distribution and amount of chemolithoautotrophic carbon dioxide fixation within these redox zones persisted for decades. Here, we try to combine the assessment of fluxes of electron acceptors and donors which fuel chemolithoautotrophy, including energetic aspects of different reactions with observations on the microbial taxonomic structure within marine redox gradients. Although modern molecular techniques help to identify the acting organisms and verify chemolithoautotrophy on the process level, there are still gaps that need to be solved. Within the energetic frame of contributing reactions, there is still the option of the presence of hitherto undescribed physiological pathways. In this environment, characterized by strong gradients, new approaches need verification by incubation-independent methods to eliminate artifacts.
G. Jost, F. Pollehne

Manganese and Iron at the Redox Interfaces in the Black Sea, the Baltic Sea, and the Oslo Fjord

The joint analysis of the data of manganese and iron species distributions (dissolved Mn, dissolved bound Mn, dissolved Fe(II) and Fe(III), particulate Fe and Mn) obtained in the Black Sea, the Baltic Sea, and the Oslo Fjord allowed to reveal the common features that testify the similarity of the mechanism of the redox layer biogeochemical structure formation in these regions. Our investigations demonstrated that Mn bound in stable complexes with hypothetically organic matter or pyrophosphate is observed in the redox zones in significant concentrations (up to 2 μM), and is likely presented by Mn(III), an intermediate product of Mn(II) oxidation and Mn(IV) reduction. This bound Mn(III) can explain phosphate distribution in redox interfaces – formation of so-called phosphate dipole with a minimum above the sulfidic boundary and a maximum just below, and with a steep increase in the concentrations between these two. This dipole structure serves as a geochemical barrier that decreases the upward flux of phosphate from the anoxic layer. On the base of the recent data obtained in the 100th cruise of RV “Professor Shtokman” (March to April, 2009), it was found that the bound Mn could exist in two forms – colloidal (0.02–0.40 μm) and truly dissolved (<0.02 μm) that perhaps result from complexing with different types of ligands. The flushing events, river input, sporadically increased mixing, and anoxygenic photosynthesis affect the distributions of the redox zone parameters. Response time for changes in the microbial processes involved in reduction and/or reoxidation of Mn and Fe lags behind that for oxygen injection into water. Concentrations of redox-sensitive species of Mn and Fe should thus be useful as a tracer to inter prior hypoxic/anoxic conditions not apparent from oxygen levels at the time of sampling. Modeling results showed that the manganese cycle [formation of sinking down Mn(IV) and presence of dissolved Mn(III)] is the main reason of oxygen and hydrogen sulfide direct contact absence. Modeling allowed to study the role of affecting factors in the formation of the observed distributions.
S. Pakhomova, E. V. Yakushev

Role of Sulfide Oxidation Intermediates in the Redox Balance of the Oxic–Anoxic Interface of the Gotland Deep, Baltic Sea

Depth profiles of sulfur species, including sulfide oxidation intermediates (zero-valent sulfur and thiosulfate), nutrients, metals (Mn, Fe), oxygen, temperature, salinity, and turbidity, were measured in the Gotland Deep, at the eastern Gotland Basin, in July 2007. We found that the highest concentrations of more oxidized sulfide oxidation intermediate, thiosulfate, were located below the highest concentrations of zero-valent sulfur. We explain this paradox by bacterial nitrate reduction coupled with thiosulfate oxidation. The same process using zero-valent sulfur is less effective due to particulate form of the latter. Oxic water intrusions were traced in both the redox transition zone (RTZ) and deep water column by decrease in concentrations of reduced nitrogen and sulfur species (sulfide, zero-valent sulfur, and thiosulfate) as well as by increase in nitrate concentration. Two turbidity maxima were found in the RTZ. Turbidity maximum, which coincides with past oxic water intrusion, was found in the deep sulfide-rich water layer. Profiles of metals and nutrients in most of the profiles indicate as well unstability of the redoxcline and oxic water intrusions at and below the redoxcline.
A. Kamyshny, E. V. Yakushev, G. Jost, O. I. Podymov

On Interannual Variability of Chemical Characteristics of Redox Layer and Cold Intermediate Layer of the Black Sea

Based on the measurements on more than 1,700 stations during last three decades in the northeastern Black Sea, analysis of seasonal and interannual variability of chemical characteristics of Cold Intermediate Layer (CIL) and redox layer is given. Studied species include main nutrients, dissolved oxygen, hydrogen sulfide, and carbon system elements. Our studies showed that surface layers ventilation with dissolved oxygen down to the depth of the CIL occurs in winter. The intensity of ventilation is determined by climate forcing, which may be regulated by large-scale climate formations like the NAO. This ventilation sets the upper boundary conditions for the downward transport of O2. The Black Sea hydrogen sulfide boundary oscillates in the density field with an amplitude of σ θ = 0.05–0.15 kg m−3 depending on the climate variability, which is well related to the NAO index. The position of the sulfide boundary indicates on the volume of the oxic layer in the Black Sea and plays a major role in the functioning of the Sea ecosystem. No clear trend of pH decrease in the Black Sea CIL, testifying to progressive acidification, was revealed. The CIL pH variability was significantly correlated with the CIL oxygen changes, which were logically consistent with the interannual variability of the winter vertical mixing intensity.
O. I. Podymov, E. V. Yakushev, A. V. Kostyleva

Large-Scale Dynamics of Hypoxia in the Baltic Sea

Large-scale hypoxia is an inherent natural property of the Baltic Sea caused by geographically and climatically determined insufficiency of oxygen supply to the deep water layers. During 1961–2005, the hypoxic zone covered by waters with oxygen concentration less than 2 mL L–1 extended on average over a huge area of about 50,000 km2, albeit with large seasonal (a few thousand km2) and, especially inter-annual (dozens of thousand km2) variations, the later caused by an irregular ventilation with sporadic inflows of saline oxygen-enriched waters. The expansion of hypoxia induces a reduction of dissolved inorganic nitrogen pool due to denitrification and an increase of dissolved phosphate pool by internal loading, these changes reaching hundred thousand tonnes of N and P. The resulting excess of phosphate pool over the “Redfield” demand by phytoplankton is favourable for the dinitrogen fixation by cyanobacteria in amounts sufficient to compensate for denitrification and to counteract possible reductions of the nitrogen land loads.
Oleg P. Savchuk

Biogeochemical Characteristics in the Elefsis Bay (Aegean Sea, Eastern Mediterranean) in Relation to Anoxia and Climate Changes

Historical data on physico-chemical and biological characteristics during the last 30 years were used to examine the seasonal nutrient and plankton dynamics in a semi-enclosed area of the Mediterranean Sea, Elefsis Bay, in phase with the development of intermittently hypoxic and anoxic conditions. Sediment records covering most of the Holocene showed that the area was affected by hypoxia and/or anoxia in the past. However, the occurrence of hypoxia in Elefsis Bay need not necessarily be attributed to anthropogenic activities but could be naturally driven by oceanographic – climate forcing.
The ecosystem of the Elefsis Bay seems to be very complicated and variable. Its variability over the last 30 years can be attributed to the differences in anoxia intense and the amount of the accumulated organic material. The first observations in the temporal variations of environmental parameters in the bay primarily reflect the impact of decreasing pollution during the last decade rather than climate variability. On the contrary, the observed variation in the intensity of the hypoxia/anoxia developed in the bay appears to be related to local climate variability. However, this variation, together with the decrease in pollution levels in the bay during the last years, seem to have an effect on the N:P ratio which controls planktonic production.
A. Pavlidou, H. Kontoyiannis, Ch. Anagnostou, I. Siokou–Frangou, K. Pagou, E. Krasakopoulou, G. Assimakopoulou, S. Zervoudaki, Ch. Zeri, J. Chatzianestis, R. Psyllidou-Giouranovits

RedOx Layer Model: A Tool for Analysis of the Water Column Oxic/Anoxic Interface Processes

The goal of the elaboration of the RedOx Layer Model (ROLM) was to create an instrument for a complex analysis of the structures of the pelagic redox-interfaces in the seas with anoxic conditions. The processes of formation and decay of organic matter (OM), reduction and oxidation of species of nitrogen, sulfur, manganese and iron, and transformation of phosphorus forms were parameterized. This chapter is devoted to the detailed description of the assumptions and parameterizations of the processes considered. Examples of the ROLM application are given in other chapters of this volume.
E. V. Yakushev

Modelling of the Meromictic Fjord Hunnbunn (Norway) with an Oxygen Depletion Model (OxyDep)

A biogeochemical model OxyDep coupled with three-dimensional hydrodynamic model GETM was used to simulate the hydrophysical and biogeochemical regimes of the meromictic Fjord Hunnbunn over the summer period. The main goal was to parameterize the oxygen depletion processes resulting in formation of suboxic and anoxic conditions in the water column. OxyDep considered five state variables: dissolved oxygen, inorganic nutrient, dissolved organic matter, particulate organic matter, and biota. This model parameterized the main processes responsible for the changing of the water column oxygen conditions – i.e. organic matter (OM) synthesis; OM decay due to oxic mineralization and denitrification; consumption of oxygen from sulphur and metals oxidation; and the processes at the boundaries (air–water exchange and the sediment–water exchange). Results of numerical experiments have reproduced the main features of the observed structure and have allowed to reveal the main components responsible for the formation of biogeochemical structure of the meromictic water objects. With the hydrodynamical model block used it was impossible to reproduce the presence of a permanent pycnocline. We suppose that special attention must be paid when using terrain following vertical coordinates (i.e. GETM) to avoid spurious vertical mixing. The results also showed that an application of simplified biogeochemical model blocks can be used as a useful tool for analysing and forecasting oxygen and nutrient regime changes.
E. V. Yakushev, E. I. Debolskaya, I. S. Kuznetsov, A. Staalstrøm

Numerical Modelling of Biogeochemical Regime Response to Decadal Atmospheric Variability During 1960–2000 in the Black Sea

Based on an analysis of observations and one-dimensional coupled hydrophysical biogeochemical model, long-term variability of the physical and biogeochemical structure of oxic and suboxic layers in the Black Sea is studied here. The correlation between large-scale atmospheric forcing [2 m air temperature, surface level pressure, surface wind and North Atlantic oscillation (NAO) index] and local responses is the main point. The comparison of model performance with respect to spatial and temporal distribution of biogeochemical variables against observed vertical distribution patterns is quite good. It is demonstrated that during 1960–2000, the long-term variability of winter-mean-simulated SST in the Black Sea is reasonably well correlated with the variability of 2 m air temperature. Furthermore, it is demonstrated that the thermal state of the upper ocean impacts largely the variability of concentration of biogeochemical variables, such as oxygen and nitrate. The teleconnection between NAO and Black Sea biogeochemistry manifests differently for the periods 1960–2000. The corresponding regime shifts are also associated in a vital way with the large-scale forcing.
Yunchang He, Emil Stanev, Evgeniy Yakushev, Joanna Staneva


This book is aimed at a systematic description of the knowledge accumulated on studying of chemical structure of pelagic redox interfaces, oxygen depletion and connected processes. It presents the principal particular features of the water column redox structure, including the distributions of certain parameters and microbiological features. The book also presents studies devoted to the inter-annual variability of some oxygen-depleted systems and modelling. The monograph is written by a team of scientists joined by a common understanding of the complicated phenomena and processes that are connected with the redox-interfaces structure formation and oxygen depletion development. The book is based on numerous observational data, collected by the authors of the chapters during sea and shore expeditions, on the archive data, as well as on a wide scientific literature.
E. V. Yakushev


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