Situated at the southernmost limits of the late Pleistocene Eurasian permafrost zone, the loess–paleosol sequences of the Lower Volga region contain numerous traces of cryogenesis. Cryogenic features are represented by thin vertical wedges in loess and paleosols, and involutions and wedges in alluvial deposits. Here we describe and interpret four stages of cryogenesis during the late Pleistocene, based on analysis of cryogenic structure morphology, morphoscopy of quartz grains, and micromorphology of subaerial sediments, in addition to calculation of the Cryogenic Weathering Index and a new luminescence chronology derived from published ages. These stages differ in type and distribution of cryostructures and formed in different paleogeographic conditions. Stage I, dated 95–90 ka (Marine Isotope Stage [MIS] 5b), is characterized by the existence of continuous permafrost in northern part of the Lower Volga valley. Stage II (75–70 ka, MIS 5a/MIS 4) is characterized by dry and cold conditions and widespread permafrost. During stage III (52–45 ka, MIS 3b/c), the permafrost was thin and of sporadic distribution. Stage IV (37–35 ka, MIS 3a) is characterized by thin and rare sporadic permafrost. The processes of cryogenic transformation of sediments in the region during these stages took place under both permafrost and seasonal frost conditions. The results obtained significantly improve current understanding of the extent of the permafrost in the south of the East European Plain during the late Pleistocene.

The endemic Caspian seal Pusa caspica breeds on land-fast ice or stable drift ice in the northern Caspian Sea. Breeding has rarely been reported in the ice-free south-eastern Caspian Sea. Five carcases of white-coat Caspian seal pups were recovered from the southern Caspian Sea, Iran, during 2019–2022. From survey interviews, two white-coat Caspian seal pups were also observed in Amirabad and between Ghorogh and Rudsar. These records represent very late pupping dates compared to normal pupping periods. Coastal Iran should be surveyed annually for Caspian seal pups to determine frequency of breeding and areas of importance identified in the Iran Caspian Seal Action Plan.

This article examines the Russian investigation of the death of Hedayat-Allah Khan, who ruled Gilan from the mid-1750s until his death in 1786. Using Russian consular reports and records of the 1795 Russian investigation of his death, the article offers two major conclusions. First, Hedayat-Allah Khan ruled a Caspian-centered polity linked to the Russian consulate and trade through partnerships with merchant intermediaries. As Qajar hegemony in Iran emerged by 1786, the consulate and merchants threw their support to Aqa Mohammad Khan. Second, the 1795 Russian investigation of his death was opened by Catherine II to solidify alliances in response to Qajar claims to Tiflis in 1795. Shifting Russian strategic priorities under Emperor Paul removed the need for this investigation.

This research evaluated the variability of current characteristics and seawater properties in the middle part of the southern shelf of the Caspian Sea. The effect of the coastal flow on marine debris dispreading was assessed in the southern Caspian Sea for the first time. The findings showed the existence of thermal stratification containing seasonal thermocline with thickness of about 40 m in the water column. Maximum monthly along-shore current velocities around 1.3 m s−1 were observed in November and December. Monthly variations were clearly found in both flow velocity and local wind components. However, no significant levels of correlation between wind and current speeds were observed during the study in the region. In some cases, the mean monthly cross-shore component velocities were measured at about 29 cm s−1 in November. The findings indicated that there was no upwelling phenomenon associated to the regional wind in the study area. In situ current measurements indicated dominant east and north-northeast directions, presumably related to the effect of general circulation in the southern basin. Current profiles in the water column displayed similarity in directions at 10, 15 and 20 m depths over the continental shelf. The field samples and analysis revealed that the soft and smaller-scale seawater litters can be carried long distances by the current along the coast. Most coastal based and marine litters originated from the tourist activities (in the middle and western parts of the shores) and waste emanated from the river (Tonekabon-Nowshahr).

The northern section of the Caspian Sea and lower reaches of the Zhaiyk (Ural) River is an important fishery for Kazakhstan. In the present study, a total of 1597 individuals of ten fish species were analysed. The fish were caught over three years, from 2018 to 2020. For each species studied – Abramis brama, Alosa saposchnikowii, Atherina boyeri caspia, Carassius gibelio, Chelon auratus, Cyprinus carpio, Leuciscus aspius, Rutilus caspius, Sander marinus and Sander volgensis – between 100 and 200 individuals were examined. A series of generalized linear models (GLMs) were used to examine the association between individual parasite intensity of infection and the Fulton index, age, year the fish was captured, where the fish was captured (northern Caspian or Zhaiyk River) and sex. For each GLM, the best-fitting probability distribution was used –either Poisson, zero-inflated Poisson, negative binomial or zero-inflated negative binomial. For some fish/parasite species, an increased Fulton index was associated with higher intensities of parasite infection, whilst, for others, the Fulton index decreased with the intensity of parasite infection. This was also true of age-related intensity of infection, with some parasites having an increased intensity of infection with age whilst others had a decreased intensity of infection with age. There was also some evidence of variation in intensity of parasite infection between different years when the fish were caught. For some species of fish that are endemic to both the fresh waters of the Zhaiyk River and the low-saline waters of the northern Caspian, there were variations in intensity of parasite infection between the two environments. The best-fitting probability distribution also gave some information about the dynamics of infection. No fish species had a Poisson distribution of parasites, which is consistent with an entirely random infection process, with all fish being potentially exposed. For some parasites, the distribution was a zero-inflated Poisson, which is consistent with either the fish being exposed to parasite infection or not; and, if exposed, infection was a random process. Other parasites had a negative binomial distribution, consistent with the entire fish population being exposed, but the infection process was clumped or there were variations in the susceptibility of infection between fish. Finally, some of the parasites had a zero-inflated negative binomial distribution, which can be interpreted as part of the fish population not being exposed and the remainder of the population being exposed to a clumped or aggregated infection process and/or a variation in individual susceptibility to infection.

The late Quaternary development of the Lower Volga region of Russia is characterized by an alternating influence of marine and continental environments resulting from fluctuations in Caspian Sea level during the last glaciation. However, sediments deposited under continental conditions have received very little research attention compared to the under- and overlying marine deposits, such that even their origin is still in debate. Detailed magnetic mineralogical analyses presented here show clear similarities to loess. The results suggest that climate during the time of loess deposition, the Atelian regression (27–80 ka, MIS 4–3), was dry and cool, similar to the modern-day Northern Caspian lowland. The magnetic properties recorded in the loess-paleosol sequences of the Lower Volga also point to short episodes of potentially more humid and warmer climate during the late Atelian. The new findings in regard to the local Caspian climate and environmental evolution support decreased river discharge from the Russian Plain and Siberian Plain as the dominant factor causing the low Caspian sea level stand during the Atelian, although local-regional climate changes might have had an additional influence.

Rove beetles (Coleoptera: Staphilinidae) are a group of diverse insects occurring in a range of different ecosystems, from the plains to the mountaintops. However, there are no reports in the literature of their presence in the tidal zone of the seashore. This is the first report of the presence of Paederus fuscipes Curtis on the coastal sands of the Caspian Sea, northern Iran. In all 14 counties of Mazandaran Province on the southern coast of the Caspian Sea, systematic monthly sampling of the beetles took place from May 2012 to April 2013. Samplings (30-min-person) were performed and specimens collected from the sandy seashore by gloved bare hands and identified using appropriate keys. To our surprise, all 1012 specimens collected were exclusively P. fuscipes Curtis, but with significant seasonal and geographical variations among the numbers of specimens collected. The distribution and activity of P. fuscipes were outstanding in Mahmoodabad and Fereidonkenar counties, with the highest abundance and frequency in September, posing a serious health threat to tourists and residents of these areas.

A verified instrumental calibration of annually resolved δ18O for a stalagmite from Gümüşhane in northeast Turkey is presented and cross-validated using a ‘leave-one-out’ technique. The amount of late autumn to winter precipitation is negatively correlated with stalagmite δ18O between AD 1938 and 2004. The observed relationship is extrapolated back to ~ AD 1500 leading to the first long winter precipitation reconstruction for this region. Modern day October to January precipitation is linked to pressure fields in Western Russia. Anomalously lower reconstructed rainfall is recorded in AD 1540–1560 at which time higher pressure over the Caspian Sea region is inferred.

Micropaleontological studies of the Black Sea, including ostracod records, have suggested that early Holocene salinity values were between ~5 and 10 practical salinity units (psu), contrasting with present values of 18–22 psu. However, more precise paleoenvironmental reconstructions based on ostracod assemblages require additional information related to their modern ecological affinities. This study uses modern species information collected from samples with living fauna to interpret the fossil Holocene assemblages of two sediment cores, Ak-2575 and Ak-521, collected from the northeastern outer shelf of the Black Sea. A total of 37 ostracod species are recorded in the fossil assemblages, with 2 related to freshwater/oligohaline environments, 23 from Caspian-type environments, and 12 from environments similar to the Mediterranean. Three distinct assemblage zones are identified from the Caspian type dominating in the early Holocene up to 7.4 cal ka BP, a mixed assemblage of Caspian type and Mediterranean type from 7.4 to 6.8 cal ka BP, and a progressive dominance of Mediterranean species from 6.8 cal ka BP. It is very likely that the dominant control of ostracod species occurrence during the period up to ~6.8 cal ka BP is salinity. A range of factors including temperature, biotope, and sedimentation rates influenced the species distribution over the last 6.8 cal ka BP.

We studied the communities of the invasive Balanus improvisus and native Cerastoderma glaucum populations in the south-western Caspian Sea. The massive movement of live Bivalvia attached to Cirripedia colonies along the studied coastline strengthens the hypotheses asserting the possible negative effects of exotic species on endemic species. Different live stages of both animals including meroplankton and macro-invertebrates were considered in the analysis. Bivalvia larvae showed a downward trend in population, in contrast with an upward trend of Cirripedia larvae from 1996 to 2013. The abundance of C. glaucum decreased west to east along the sea shore in contrast with increasing biomass of B. improvisus. Both Bivalvia and Cirripedia larvae did not show any overlapping temporal abundance. The Cirripedia larvae showed its highest abundance in winter while the bloom of Bivalvia larvae occurred in April and May during 2004–2013. The biomass of B. improvisus reported in this study was higher than those reported for the northern parts and for the middle parts. Distribution patterns of both species were described based on temperature, salinity gradient and local nutrient content. A non-linear growth model of Bivalvia showed the short-term effects of Cirripedia on Bivalvia growth. The controversy between the effects of Cirripedia on the movement of two different Cardiidae (C. glaucum, which is affected by the presence of B. improvisus, and Adacna vitrea with no attached Cirripedia) highlights the contributing role of several other factors including ecosystem degradation.

The mesozooplankton of the south-western Caspian Sea, off Anzali, sampled from 1996–2010, had undergone severe changes, especially after the year 2001, when the invasive ctenophore Mnemiopsis leidyi bloomed for the first time. Native species vanished or decreased, while invasive species such as Pleopis polyphemoides, Acartia tonsa, M. leidyi and the larvae of Balanus sp., plus Hediste diversicolor dominated the zooplankton. It could be stated that the increasing amount of nutrients since the early 1980s led to a decrease of endemic species and smoothed the way for opportunistic invader species, which outcompeted and depleted the endemic species in their turn. However, the major changes in the zooplankton community and possibly the blooming of M. leidyi during 2001–2002 were triggered by changing weather patterns, when a period with heavy rain at the end of the 1990s was followed by a prolonged drought (2001–2002). It is not clear to what extent M. leidyi was responsible for the disappearance of endemic Copepoda and Cladocera species such as Eurytemora grimmi, Limnocalanus grimaldii, Cercopagis pengoi, and Polyphemus exiguous, because the mesozooplantic invader species seemed to be more successful competitors than the endemic Caspian Sea fauna. Compared with other areas of the Caspian Sea, the development of the M. leidyi stock was moderate in the area under investigation. Mnemiopsis leidyi numbers and biomass increased from the beginning of sampling in 2001 to about 600 n.m−3 and 40–60 g wet weight m−3 until 2003. Since then the stock oscillated in this range until 2010.

This study focuses on spatial and temporal distribution and species composition of phytoplankton in the south-western Caspian Sea between July 2009 and March 2010. Samples were collected from 11 stations along three transects: Lisar, Anzali and Sefidrood. Among 44 identified phytoplankton species, diatoms (70.2%) and cyanophytes (25.0%) were dominant. The average phytoplankton abundance was calculated as 1.085E + 05 cells l−1. Among the phytoplankton groups, diatom density was higher (70.2%) than two out of three of total abundance in density of 1.085E + 05 cells l−1. Cyanophytes were the second important group (25.0%) contributing to total phytoplankton. Dinoflagellates, chlorophytes and euglenoids were other contributors to total phytoplankton. Diatoms Dactyliosolen fragilissimus and Skeletonema costatum and cyanophyte Oscillatoria sp. numerically dominated in the system. There were major changes in phytoplankton composition and average phytoplankton density was higher than those documented in 1996–1997 and 2005. The average concentrations of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP) and dissolved inorganic silicate were 14.5 ± 6.32, 1.14 ± 0.44 and 5.10 ± 3.98 μM, respectively, and these concentrations were strikingly high. Increases in DIN and DIP concentrations were more than twofold compared to recorded values during the last 2 decades due to the eutrophication. Fluctuations in nutrients played an important role in the variation of phytoplankton composition and abundance. Chlorophyll-a concentrations varied between 3.22 and 16.1 μg l−1 and there was a significant increase in chlorophyll-a (8.13 ± 5.72 μg l−1) compared to the values in 1996–1997 (1.44 ± 1.48 μg l−1), 2001 (2.62 ± 1.48 μg l−1) and 2005 (2.14 ±1.94 μg l−1).

For studying community structure and biodiversity of macrofauna, seasonal samplings were carried out along three different depths (5, 15 and 30 m) in four transects (12 stations) at Noor coast during 2005. Four higher taxa were determined. Polychaeta was the dominant group comprising 96.61 of the total individuals, followed by Bivalvia 1.35%, Oligochaeta 1.15% and Amphipoda 0.87%. Of the six encountered species, the polychaete, Streblospio gynobranchiata accounted for 84.95 of the total population. Also, Tubificoides fraseri was observed in the Caspian Sea for the first time. During this study, density of macrofauna increased with depths, total organic matter and percentage of silt–clay. The highest density was obtained in winter and the lowest was observed in summer. Maximum diversity, richness, and evenness were obtained, 0.91, 1.05 and 0.88, respectively. Also, multivariate analysis separated differences of density between seasons and depths.

The LS5096 h of prawn, Palaemon elegans, was determined at various salinities (0–45 ppt) with the Caspian Sea salt proportion. In addition, growth indices, survival and moulting at salinities 8, 13 and 18 ppt were also studied during four weeks. All the experiments were carried out in the laboratory (24±1°C). The results showed that more than 50% of the prawns survived at 1 to 30 ppt salinity range, while above and below this range, less than 50% survived within 24 hours. No significant differences (*P > 0.05) in growth parameters such as carapace length (CL) increase, specific growth rate (SGR) and also in survival and moulting rates were observed in prawns reared in salinities 8, 13 and 18 ppt. Mean intermoult period in each salinity was 8.12 ± 3.07, 8.61 ± 2.96 and 8.92 ± 3.47 days, respectively. Salinity 8–18 ppt was found to be the optimum range for P. elegans in the studied length range (CL, 6–9 mm).

Population changes and reproduction of the alien polychaete, Streblospio gynobranchiata, living in 5, 15 and 30 m depths of the south Caspian Sea (Noor coast) was studied seasonally during 2005. Density and biomass of S. gynobranchiata increased with increasing depth, total organic matter and decreased with sand percentage of the substrate. Maximum density was 10311.11 ± 1596.4 ind m−2 and biomass 2320 ± 359.19 mg m−2 were observed in 30 m depth in winter. The maximum and minimum densities and biomass were obtained in winter and summer, respectively. Mature females were observed in all seasons, but reproduction peaks were observed in summer and autumn.

One of the most controversial contests over the allocation of shared natural resources is now being waged by four former Soviet Union republics (Azerbaijan, Kazakhstan, Russia, and Turkmenistan) and Iran over the riches of the Caspian Sea. The bilateral regime established by the former USSR and Iran governing the Caspian Sea, though technically still existing today, is inadequate to deal with the present-day complex issues of the use and allocation of natural resources. The situation is aggravated by unilateral claims of the states bordering the Sea. Although the coastal states are currently discussing how the regime of the Caspian Sea might be resolved, the issue is not yet settled and the positions of the parties remain divergent.