Northwestern Black Sea Shelf Research Articles

The shelf-to-basin iron shuttle in the Black Sea revisited

Continental shelf sediments are a major source of iron (Fe) for phytoplankton in surface waters. In this study, we investigate the mechanisms that control release of Fe from shelf sediments and its lateral transport (“shuttling”) in oxic and hypoxic waters on the northwestern Black Sea shelf. We find that at two coastal stations near the outflow of the Danube river high input of organic matter drives strong reductive dissolution of Fe(oxyhydr)oxides (henceforth termed Fe oxides) in surface sediments, supporting high rates of Fe release to oxygenated bottom waters (∼0.36 mmol m−2 d−1). We suggest that bioirrigation plays a key role in the release of Fe from these sediments. At four stations further offshore organic matter deposition is lower resulting in limited mobilization of Fe2+ in the sediment and low benthic fluxes of Fe (<0.07 mmol m−2 d−1). Lateral transport of Fe from the coastal zone towards the deep basin mostly takes place in colloidal and/or particulate form (>0.2 μm) in the lower part of the water column, likely through repeated deposition and resuspension of Fe oxides from surface sediments. Using synchrotron-based X-ray spectroscopy and sequential chemical extractions, we demonstrate that the suspended matter and surface sediments are enriched in easily reducible Fe oxides (mostly ferrihydrite) and Fe associated with clay. The mobilization of Fe in the coastal zone and subsequent lateral transport of these Fe-bearing particles results in higher ratios of Fe/Al in surface sediments at outer shelf stations (ca. 1.2 to 2 wt.% wt.%−1) than at coastal stations (ca. 0.5 to 0.9 wt.% wt.%−1). However, below the sediment surface layer Fe/Al ratios are similar at all stations indicating limited burial of the laterally transported Fe. Our results highlight the critical role of organic matter input, associated biological activity and riverine Fe input as drivers of Fe shuttling on continental shelves. We also show that in shelf areas where sediments receive low inputs of organic matter, physical transport controls the ultimate fate of the shuttled Fe.This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean - GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.

Assimilyatsiya sputnikovykh nablyudeniy kontsentratsii khlorofilla a i raschetnykh dannykh o dinamike morskoy sredy v adaptivnoy modeli ekosistemy severo-zapadnogo shel'fa Chernogo morya

Assimilyatsiya sputnikovykh nablyudeniy kontsentratsii khlorofilla a i raschetnykh dannykh o dinamike morskoy sredy v adaptivnoy modeli ekosistemy severo-zapadnogo shel'fa Chernogo morya

Odessa Tsunami of 27 June 2014: Observations and Numerical Modelling

On 27 June, a 1–2-m high wave struck the beaches of Odessa, the third largest Ukrainian city, and the neighbouring port-town Illichevsk (northwestern Black Sea). Throughout the day, prominent seiche oscillations were observed in several other ports of the Black Sea. Tsunamigenic synoptic conditions were found over the Black Sea, stretching from Romania in the west to the Crimean Peninsula in the east. Intense air pressure disturbances and convective thunderstorm clouds were associated with these conditions; right at the time of the event, a 1.5-hPa air pressure jump was recorded at Odessa and a few hours earlier in Romania. We have utilized a barotropic ocean numerical model to test two hypotheses: (1) a tsunami-like wave was generated by an air pressure disturbance propagating directly over Odessa (“Experiment 1”); (2) a tsunami-like wave was generated by an air pressure disturbance propagating offshore, approximately 200 km to the south of Odessa, and along the shelf break (“Experiment 2”). Both experiments decisively confirm the meteorological origin of the tsunami-like waves on the coast of Odessa and imply that intensified long ocean waves in this region were generated via the Proudman resonance mechanism while propagating over the northwestern Black Sea shelf. The “Odessa tsunami” of 27 June 2014 was identified as a “beach meteotsunami”, similar to events regularly observed on the beaches of Florida, USA, but different from the “harbour meteotsunamis”, which occurred 1–3 days earlier in Ciutadella (Baleares, Spain), Mazara del Vallo (Sicily, Italy) and Vela Luka (Croatia) in the Mediterranean Sea, despite that they were associated with the same atmospheric system moving over the Mediterranean/Black Sea region on 23–27 June 2014.

ПРОДУКТИВНІ ГОРИЗОНТИ КАЙНОЗОЙСЬКИХ ВІДКЛАДІВ УКРАЇНСЬКОЇ ЧАСТИНИ АКВАТОРІЙ ЧОРНОГО ТА АЗОВСЬКОГО МОРІВ

In a single methodological basis, based on complex lithologic and stratigraphic and petrophysical studies, logging interpretation, beds correlation, were selected and indexed 45 productive horizons of Cenozoic sediments at Black and Azov seas shelf (Ukrainian sector): МU-1, МU-2, МU-3, МU-4, МU-5, МU-6, МU-7, МM-1, МM-2, МL-1, МL-2, МL-3, МL-4, МL-5 and МL-6 in Miocene sediments; ОU-1, ОU-2, ОU-3, ОU-4, ОU-5, ОL-1, ОL-2, ОL-3, ОL-4, ОL-5, ОL-6, ОL-7, ОL-8, ОL-9, ОL-10 and ОL-11 – Oligocen; ЕU-1, ЕU-2, ЕM-1, ЕM-2, ЕM-3, ЕM-4, ЕM-5, ЕL-1, ЕL-2 and ЕL-3 – Eocene, PU-1, PL-1, PL-2 and PL-3 – Paleocene. The name of each productive horizon is the sum of the abbreviations of names of department and sub-department, which is the horizon, and the numeral indicating its serial number.Single scheme of indexing and correlation was composed. Key words: productive horizon, Cenozoic sediments, Ukrainian waters of the Black and Azov Seas References Atlas of oil and gas in Ukraine. V.VI. South oil and gas region. 1998. Lviv. UNGA. 224 p. (in Ukrainian). Boboshko A.V., Vakarchuk G.I., Vinnichenko L.G. et al.1974. The scheme of correlation and unified synonymy oil and gas horizons of the Lower Carboniferous of the Dnieper-Donets basin (guidelines). Kharkiv. 52 p. Vakarchuk G.I., Vinnichenko L.G., Kononenko L.P. 1990. The new scheme of indexing and correlation of the productive horizons of the Lower Carboniferous of the Dnieper-Donets basin. Geological Journal. #6. S. 109-115. Stratigraphic Code of Ukraine. 2012. Resp. editor P.F. Gozhyk. 66 p. Gozhik P.F. 2006. Oil-gas facilities in Ukraine. Scientific and practical basis for finding hydrocarbons in the Azov Sea. Kyiv. EKMO. 340 p. (in Ukrainian). Gozhik P.F. 2007. Oil-gas facilities in Ukraine. Scientific and practical basis for finding hydrocarbons in the northwestern Black Sea shelf. Kyiv. ЕКМО. 231 p. (in Ukrainian). State balance of mineral reserves Ukraine. 2012. Кiev, SSPE Geoinform Ukrainin, 3284 p. (in Ukrainian). Oil and gas potential of the northwestern Black Sea. 1995. Series «Oil-gas potencial water area of the Black Sea and the Sea of Azov». V.2. Kyiv. Ukrainian Oil-gas institute. 250 p. (in Ukrainian). Oil and gas potential of the Azov Sea 1995. Series «Oil-gas potencial water area of the Black Sea and the Sea of Azov». V 1. Kyiv. Ukrainian Oil-gas institute. 166 p. (in Ukrainian). Oil and gas potential Kerch-Taman Black Sea shelf, continental slope and deep-sea depression Black Sea. 1996. Series «Oil-gas potencial water area of the Black Sea and the Sea of Azov». V.2. Kyiv. Ukrainian Oil-gas institute. 186 p. (in Russian). Ukraine's oil and gas province. 1981. Kyiv. Naukova dumka, 172 p. (in Russian). Forecast oil and gas exploration in the south of the USSR and adjacent waters. 1981. Ed. V.V. Glushko and S.P. Maximov. Moscov. Nedra. 240 p. (in Russian). Murasta W.А., Baschkirov G.L., Karpenko О.М. 2012. On the issue of allocation of productive horizons in the Paleogene and Neogene sediments Ukrainian shelf of the Black and Azov Seas. Problems of Oil and Gas Industry. Proceedings of SE Naukanaftogaz NAK Naftogaz Ukraine. Kyiv.Vol. 10. P. 97-104 (in Ukrainian). Creation of unified directory breakdowns productive horizons Cenozoic sediments within the areas of the Black and Azov seas based on a single indexing and correlating. 2011. Report on implementation of the provision of geological and thematic work. Responsibility G.L. Bashkirov and I.I. Ishchenko. SE Naukanaftogaz. Vischneve, №16/200. Book 1,2. 503 p. (in Ukrainian). Handbook of Petroleum Geology. 1984. Ed. N.А. Yeremenko. М. Nedra. P. 480. (in Russian). Hanin А.А. 1969. Reservoir rocks of oil and gas and their study. M.: Nedra. 366 p. (in Russian).

LITHOLOGICAL COMPLEXES OF THE LOWER CRETACEOUS OF NORTH-WESTERN BLACK SEA SHELF

Based on lithological research of factual data it was determined considerable variety of the Lower Cretaceous sedimentary rocks of the north-western Black Sea shelf. Using the litho-facial analysis, there were identified four lithological complexes of three series. Argillaceous series have the largest distribution, psammitic series and composite psammitic-carbonate-argillaceous series have subordinate disposition.

Аналіз літолого-фаціальних критеріїв нафтогазоносності майкопських відкладів північно-західного шельфу Чорного моря

Аналіз літолого-фаціальних критеріїв нафтогазоносності майкопських відкладів північно-західного шельфу Чорного моря

GEOLOGICAL PALEOOCEANOGRAPHY OF EARLY EOCENE SEDIMENTARY COMPLEX OF TETHYS BLACK SEA SEGMENT

The main features of Early Eocene deposition within north-western Black sea shelf have been determined. In the basin axial zone the carbonate-clayey mud of the external shelf was deposited. On its periphery the biogenic bank-reef buildups and alongshore bars were developed. The main part of the territory was occupied by the inner shelf with carbonate-clayey mud domination in eastern part and terrigenous-clay mud in central and western parts.

PALEOGEOGRAPHY OF THE NORTHWESTERN BLACK SEA SHELF DURING HOLOCENE

The features of paleoriver system forming during the Holocene in the Northwestern Black Sea shelf are presented in the paper. Late-Plaistocene – Holocene age of the paleoriver system as well as its spatial location, history of development through time and its connection with the global transgressive-regressive cycles were determined by means of geological, geomorphological, hydrological and geophysical methods. The last stage of existence of brackish basin was determined on the base of study of fauna assemblages in the bottom sediments and dates from ca. 8 – 9 ky BP which can be marked out by the appearance of the fauna from the Mediterranean Sea.

LATE-PLEISTOCENE – HOLOCENE SEDIMENT ACCUMULATION IN THE NORTHWESTERN BLACK SEA SHELF

Main purpose of this study is characterization of the facies and paleogeographical features of sedimentforming processes. The subject of the study is the Late-Pleistocene – Holocene bottom sediments of the Northwestern Black Sea shelf. Methodology includes standard statistical calculations of the thicknesses of bottom marine sediments and their description. Finding:the Late-Pleistocene-Holocene bottom sediments facies’ formation was determined by the repeated coastline position changes. The study area recent underwater landscapes were formed during the Holocene transgression, when subaerial flats were flooded. The land facies within the study area are represented by fluvial, eolian and eolian-deluvil genetic types of sediments. The basin underwent significant environmental transformations after reconnectionwith the Mediterranean Sea and within the study are such stages as lacustrine, brackish water and marine can be traced. Sediment forming processes were determined by the presence of limans during the early Holocene period, which were situated lower than the modern coastline and corresponded to the paleoriver valleys’ position. Considerable sediment thicknesses (from5 to 10 m) were accumulating during this time. The Odessa bay, Dnieper-Bug liman, Dniester coastal waters, paleo-Dniester, Dnieper depression and paleo-Sarata valley are characterized by the high rates of deposits accumulation. Thereby, sediment forming environments in the study area experienced gradual transformation due to considerable riverin waters inflow that is why brackish-water mollusks of Caspian type are wide spread even after the salt Mediterranean water entered the basin. Paleogeographical features such as coastline position and the basin bottom relief influenced strongly the bottom facies disposition and its thicknesses.

MORPHOLOGY AND GENESIS OF FINE GOLD IN MODERN SEDIMENTS OF THE BLACK SEA

Research the morphology and composition of the gold is one way to determine the genesis of fine gold in modern sediments. The purpose of these studies is the morphological characteristics of fine gold, to study the composition of gold, and determine the genesis of fine gold in the Black Sea sediments. The subject of the research is fine gold of modern sea sediments of the North-Western Black Sea shelf. The methodology involves the preparation of sediment samples from the sea depths up to 95 m, the depth of sampling columns – up to 4 m. Was studied over a thousand samples of different lithological types of bottom sediments. Studied the morphology of more than 300 gold particles. Composition of gold was determined of emission spectral analysis andX-ray microprobe studies. The maximum concentration of gold is set in sediments Dnieper depression. The gold particles have a size from 0.005 mm to 0.3 mm. The genesis of gold was determined by particle morphology and confirmed by the results of the analysis of its composition. Terrigenousgold has a flat or form, splices with quartz sometimes present. Authigenous gold has a spherical or round shape. Disseminated silver contained in gold, zone Cl and PbCl present. Much gold have concentric zones of different composition. This fact confirms the authigenous of gold. Processes of formation of gold in sediments are terrigenous and authigenous. Authigenous gold may bethe result of biochemical deposition and submarine hydrothermal processes that are associated with endogenous sources.

Nitrate drawdown and its unexpected isotope effect in the Danube estuarine transition zone

AbstractThe northwestern Black Sea Shelf region suffered from severe eutrophication in recent decades, mainly due to the Danube River's increased nutrient loads, as well as nutrient remineralization in sediments. During two cruises (spring 2012 and summer 2013) in the Danube River Delta–Black Sea transition zone, we analyzed dissolved nutrients, dual isotope signatures of nitrate, and δ15N of suspended particulate matter across the salinity gradient, and two pore water nutrient profiles (only 2013) in the outer river plume. We aimed to investigate nitrogen turnover and nutrient dynamics in the present Black Sea Shelf region in order to elucidate the contemporary role of nitrogen inputs of the River Danube in the light of the past, severely eutrophic, situation in the Delta region. Data indicate intense drawdown of river‐borne nitrate, phosphate, and dissolved silica in the river plume. Isotopic patterns of nitrate and suspended N illustrate exceptionally clearly the dominance of nitrate assimilation over the entire salinity gradient. An unusual 1.9 : 1 enrichment of δ18ONO3 to δ15NNO3 questions the usually uniform enrichment that is attributed to phytoplankton. Accordingly, we investigated the potential effects of nitrate regeneration and uptake by heterotrophic plankton on the isotopic composition of nitrate in the Delta. Overall, our data suggest that today, the nitrogen input from the Danube River is taken up within the salinity gradient in large quantities, and its immediate effects on the eutrophication status of the Black Sea shelf are relatively limited.

Holocene marine transgression in the Black Sea: New evidence from the northwestern Black Sea shelf

For two decades, the timing and rate of Holocene marine transgression and the level of the Black Sea prior to the transgression has been the focus of many geological, palaeoecological and archaeological studies. The potential importance of confirming or rejecting the catastrophic flood hypothesis by refining the chronology of the marine transgression and determining the water level of the early Holocene Black Sea (Neoeuxinian) lake is the aim of many ongoing Black Sea palaeoecological studies.In this report we review previous studies and present new data on the early Holocene marine transgression obtained from multidisciplinary studies of several cores from different parts of the Black Sea. Core 342 from the edge of the Dniester paleovalley on NW shelf is particularly important because it provides wood and leaf material from several peat and muddy peat beds, each up to ∼10 cm thick, inter-layered in a coastal succession with mud, clay, and shell coquina. AMS ages for wood fragments and sedge leaves in the peat layers provide critical new data for calibrating and “re-tuning” of previously published shell and bulk detrital peat ages.Our multi-disciplinary study of geological material recovered from different shelf areas of the Black Sea refines the chronology of the marine transgression and clarifies conflicting interpretations of the water level and salinity of the Neoeuxinian lake prior to the initial Mediterranean inflow (IMI) and transgression of Mediterranean water in the Holocene, We find that: (1) The level of the Late Neoeuxinan lake prior to the early Holocene Mediterranean transgression stood around −40 m bsl but not −100 m or more as suggested by advocates of catastrophic/rapid/prominent flooding of the Black Sea by Mediterranean water. (2) At all times, the Neoeuxinan lake was brackish with salinity not less that 7 psu. (3) By 8.9 ka BP, the Black Sea shelf was already submerged by the Mediterranean transgression. An increase in salinity took place over 3600 years, with rate of the marine water incursion being estimated in the order of 0.05 cm–1.7 cm a−1. (4) The combined data set of sedimentological characteristics and microfossil data establish that the Holocene marine transgression was of a gradual, progressive nature in the early Holocene.

Changes in coastline positions during the Holocene in the shelf of the Northwestern Black Sea

The first evidence for the reconnection of the Black Sea with the Mediterranean Sea are euryhaline mollusc species that appear in the bottom sediments. Lacustrine conditions existed over the larger part of study area at the beginning of the Holocene. Early Holocene beds are characterized by a wide distribution of continental facies (e.g. lacustrine-marshy and alluvial). The lacustrine phase was followed by a marine phase and the transition occurred around ca. 8.9–8.5 ka BP. The Holocene was marked by a gradual increase of Mytilus sp. shells in sediments. Reconstructions of coastline positions during the Holocene in the Northwestern Black Sea shelf are presented. Extensive sampling of the study area during several decades allowed the acquisition of a comprehensive data base for paleogeographic reconstructions. Positions of ancient coastlines are discussed. Analysis of relief features, lithological composition and bottom sediments distribution and faunal complexes allowed identification of paleofacies in the northwestern part of the Black Sea. Facial and paloegeographic maps of the Holocene discrete time intervals such as the Bugazian (from ca. 10.5–10 to 8.9–8.5 ka BP), the Vityazevian (from ca. 8.9–8.5 to 7.1–6.2 ka BP), the Kalamitian (from ca. 7.1–6.2 to 4.1–4.0 ka BP), and the Dzhemetinian (from ca. 4.1–4.0 to present) are presented. Dating of the sediments is based on uncalibrated radiocarbon determinations. Study of the granulometry of sediments provided evidence that supports the proposed ancient coastline positions. The samples from the cores recovered from different facial zones enabled the characterization of sedimentological environments for each time interval. The position of the early Holocene coastal sediments is marked out on the level of modern isobaths approximately from −25 to −35 m. In the range of time from ca. 8.9–8.5 to 7.1–6.2 ka BP, the coastline was located at depths from 20 to 25 m. The position of the coastline which existed from ca. 7.1–6.2 to 4.1–4 ka BP is allocated on the level of modern isobaths from −0 to −15 m. Comparison of the various positions of the coastline and their facies changes through time indicates that the Holocene transgression had a progressively and oscillatory course.

Drivers, mechanisms and long-term variability of seasonal hypoxia on the Black Sea northwestern shelf – is there any recovery after eutrophication?

Abstract. The Black Sea northwestern shelf (NWS) is a shallow eutrophic area in which the seasonal stratification of the water column isolates the bottom waters from the atmosphere. This prevents ventilation from counterbalancing the large consumption of oxygen due to respiration in the bottom waters and in the sediments, and sets the stage for the development of seasonal hypoxia. A three-dimensional (3-D) coupled physical–biogeochemical model is used to investigate the dynamics of bottom hypoxia in the Black Sea NWS, first at seasonal and then at interannual scales (1981–2009), and to differentiate its driving factors (climatic versus eutrophication). Model skills are evaluated by a quantitative comparison of the model results to 14 123 in situ oxygen measurements available in the NOAA World Ocean and the Black Sea Commission databases, using different error metrics. This validation exercise shows that the model is able to represent the seasonal and interannual variability of the oxygen concentration and of the occurrence of hypoxia, as well as the spatial distribution of oxygen-depleted waters. During the period 1981–2009, each year exhibits seasonal bottom hypoxia at the end of summer. This phenomenon essentially covers the northern part of the NWS – which receives large inputs of nutrients from the Danube, Dniester and Dnieper rivers – and extends, during the years of severe hypoxia, towards the Romanian bay of Constanta. An index H which merges the aspects of the spatial and temporal extension of the hypoxic event is proposed to quantify, for each year, the intensity of hypoxia as an environmental stressor. In order to explain the interannual variability of H and to disentangle its drivers, we analyze the long time series of model results by means of a stepwise multiple linear regression. This statistical model gives a general relationship that links the intensity of hypoxia to eutrophication and climate-related variables. A total of 82% of the interannual variability of H is explained by the combination of four predictors: the annual riverine nitrate load (N), the sea surface temperature in the month preceding stratification (Ts), the amount of semi-labile organic matter accumulated in the sediments (C) and the sea surface temperature during late summer (Tf). Partial regression indicates that the climatic impact on hypoxia is almost as important as that of eutrophication. Accumulation of organic matter in the sediments introduces an important inertia in the recovery process after eutrophication, with a typical timescale of 9.3 yr. Seasonal fluctuations and the heterogeneous spatial distribution complicate the monitoring of bottom hypoxia, leading to contradictory conclusions when the interpretation is done from different sets of data. In particular, it appears that the recovery reported in the literature after 1995 was overestimated due to the use of observations concentrated in areas and months not typically affected by hypoxia. This stresses the urgent need for a dedicated monitoring effort in the Black Sea NWS focused on the areas and months concerned by recurrent hypoxic events.

Lodgers or tramps? Aporrhais pespelecani and Turritella communis on the north-western Black Sea shelf

Lodgers or tramps? Aporrhais pespelecani and Turritella communis on the north-western Black Sea shelf

LITHOLOGIC FEATURES OF MIDDLE EOCENE DEPOSITS OF THE BLACK SEA NORTH-WESTERN SHELF

The territory of researches include the Black sea north-western shelf, where Middle Eocene deposits are opened by numerous wells on seventeen structures. In connection with small coring deposits of Middle Eocene age are studied insufficiently. On a basis of lithologic studying of this complex three types of sections are distinguished: carbonate, terrigenous-carbonate and terrigenous. Areas of dominate development of terrigenous (Odessko-Bezimenna) and carbonate deposits (Gamburtsivsky, Shmidtivsky, Olimpic) are established. Petrography features of the rocks are studied. These results are a base for geology paleoceanographic reconstructions of the Carpathian Black sea segment of Tethys ocean. They allow correlating Tethydian Eocene deposits developed in adjoining regions of the East European platform and Carpathian orogene.

HOLOCENE BOTTOM SEDIMENTS THICKNESS VARIATION IN THE NORTHWESTERN BLACK SEA SHELF

Data about Holocene bottom sediments thickness were obtained. Sedimentation rates on the elevations and depressions of Northwestern Black Sea shelf were calculated. Sediments distribution and accumulation features on the basis of geomorphologic and paleogeographic data were characterized.

NEW DATA OF EOCENE DEPOSITS OF THE NORTH-WESTERN BLACK SEA SHELF

Territory of researches includes the north-western Black Sea shelf, where Eocene deposits are opened by numerous wells on 17 structures. On a basis of lithological studying of this complex the types of lithological sections for each Eocene substages are established. Areas of dominating development of terrigenous and carbonate deposits are established. Results are given is a basis for geological paleooceanography reconstructions of the Carpathian-Black Sea segment of the Tethys ocean. They give the chance to realize correlation with one-age Eocene deposits spreading in adjoining regions of the East European platform and Carpathian orogene.

The features of transparency field generation in the upper sea layer of the northwestern Black Sea in the winter-spring period

The features of transparency field generation in the upper sea layer of the northwestern Black Sea in the winter-spring period are analyzed on the basis of long-term data of specific marine observations from 1978–1995. The statistical assessments of the seasonal variations of the beam attenuation coefficient associated with fluctuations of river discharge and open sea water characteristics are calculated for the region under study. It is shown that the amount of suspended matter, which is most responsible for seawater transparency, depends both on rivers’ discharge volumes, the area of their interaction with seawater over the northwestern Black Sea shelf, and regional hydrometeorological conditions, which in turn depend on the hydrometeorological conditions in the catchment areas of the European rivers of the Black Sea basin.

Abrupt transition of the northwestern Black Sea shelf ecosystem from a eutrophic to an alternative pristine state

Long-term (1960-2005) ecological data were used to identify a regime shift in the north- western Black Sea shelf and to assess whether it has recovered from its former eutrophic state. Fol- lowing the collapse of fish stocks and the population explosion of the ctenophore Mnemiopsis leidyi between 1988 and 1991, anthropogenic P-PO4 loads from the River Danube dropped strongly in 1992-1993. This decline in P-PO4 levels was caused by reductions in fertilizer use and emissions from land-based point sources during the economic recession of former Eastern Bloc countries, as well as by the low discharge rate of the River Danube. Commencing in 1993, the phosphate limitation apparently maintained a low-energy, inefficient food web dominated by the dinoflagellate Noctiluca scintillans and jellyfish, and relatively low levels of phytoplankton, bacterioplankton, mesozoo- plankton and fish. This 'post-eutrophication' regime was markedly different from the classical phytoplankton-mesozooplankton-fish chain of the similarly low nutrient 'pre-eutrophication' regime prior to 1970. Therefore it appears that the food web can attain 2 alternative regimes during periods of low productivity of the ecosystem. The post-eutrophication state cannot be considered as a major improvement or restoration of the northwestern coastal ecosystem.