The northeastern Black Sea redox zone: Hydrochemical structure and its temporal variability

Detailed studies of the vertical structure of hydrochemical parameters in the northeastern Black Sea near Gelendzhik are presented and discussed. This work is based on a set of systematic observations carried out in this region during R/V Akvanavt and R/V Ashamba cruises from 1997 to 2005 and also on data from previous studies from 1984 to 1997. The Black Sea region near Gelendzhik is far from the influences of the Bosporus input and Danube River inflow. Therefore, the vertical structure in this region is more stable compared to the western Black Sea and reflects integrated, rather than local, changes of the Black Sea. Seasonal variations in the distribution of chemical parameters at the redox interface are connected to seasonal variations in hydrophysical processes and organic matter production. In winter, maxima of organic phosphorus and urea were absent in the vicinity of the onset of hydrogen sulfide. The concentrations of nitrate were lower in winter than in summer. Winter mixing in the anticyclone eddies led to reduced vertical gradients in the redox layer and to the disappearance of an upper phosphate minimum. Study of the interannual dynamics of oxygen concentrations in the Cold Intermediate Layer and anoxic waters boundary in the density field position revealed climatic-scale changes that may be connected with changes in the North Atlantic Oscillation (NAO) index. © 2006 Elsevier Ltd. All rights reserved

A structured jet explains the extreme GRB 221009A

Long duration gamma-ray bursts (GRBs) are powerful cosmic explosions, signaling the death of massive stars. Among them, GRB 221009A is by far the brightest burst ever observed. Due to its enormous energy ($E_\textrm{iso}\!\approx$10$^{55}$ erg) and proximity ($z\!\approx$0.15), GRB 221009A is an exceptionally rare event that pushes the limits of our theories. We present multi-wavelength observations covering the first three months of its afterglow evolution. The X-ray brightness decays as a power-law with slope $\approx\!t^{-1.66}$, which is not consistent with standard predictions for jetted emission. We attribute this behavior to a shallow energy profile of the relativistic jet. A similar trend is observed in other energetic GRBs, suggesting that the most extreme explosions may be powered by structured jets launched by a common central engine.Comment: Submitted version. 53 pages, 9 figures, 6 table

State of the Marine Ecosystem Near the Mouth of the Agoy River (Black Sea)

Aim. To study the state of the marine ecosystem of the recreational‐tourist zone of the Caucasian sector of the Black Sea through the example of a beach near the mouth of the Agoy River.Material and Methods. Phytoplankton, heterotrophic bacterioplankton, infusoria, holoplankton, meroplankton, ichthyoplankton, zoobenthos of loose bottom sediments and hydrochemistry samples of the water and bottom sediments were collected in June 2012 on three sections from the mouth of the Agoy River to the coastal runoff zone (depths 2.5–7.5 m). The identification of species of plankton and of the zoobenthos and of the chemical parameters of water and sediments was carried out according to standard methods. Results. It was revealed that most of the beach area, where psammophilic biocenoses of Lucinella divaricate and Chamelea gallina (Bivalvia) were located, was in satisfactory condition. An increase in the density of Lucinella divaricate, a rare species in the late 1990s, was noted. In the runoff zone, there was observed the appearance of cyanobacteria and the suppression of zoobenthos, expressed through the replacement of mollusc biocenoses by the biocenosis of the polychaete, Capitella capitate, with a biomass two orders of magnitude lower than the average for the area. High numbers of heterotrophic bacterioplankton (4.5 million cell/ml) and infusoria (64 million ind./m3) could indicate bacterial contamination of this zone. The negative impact of waste water on plankton is manifested in a decrease in the population of netted zooplankton, their abnormal development, and the increasing role of microheterotrophs.Conclusions. The results obtained give an image of the state of marine coastal ecosystems of recreational‐tourist and cordoned areas of the Caucasus and can be useful for the further monitoring of this region

Understanding phytoplankton successions in the northeastern black sea through long-term data series.

In order to understand of the processes controlling phytoplankton successions in the NE Black Sea, long-term data series are needed. We compiled 15 years (2002-2017) of measurements from which the existence emerges of a tight link between phytoplankton species dominance and nutrients concentrations. The latter is strongly influenced by wind direction. The link between algal dominance and nutrients is mediated by the growth strategy adopted by algal species. In spring, when nutrients are abundant, small diatoms such as Pseudo-nitzschia pseudodelicatissima, with a “rapid growth strategy”, prevail. In late spring and early summer, when N is low and P and Si are high, coccolitophorids such as Emiliania huxhleyi dominate, thanks to an “affinity growth strategy”. Large diatoms, especially Pseudosolenia calcar-avis, dominate in summer and autumn, when their “storage growth strategy” allows the exploitation of discontinuous upwelling of nutrients. These seasonal changes of dominant species influence the structure of the food web