Spatial variability of thermohaline characteristics (temperature, salinity, and dissolved oxygen) and phytoplankton structure (composition, abundance, and biomass) of the Southern Ocean during austral summer were investigated. New data were obtained for a little-studied area of Antarctica: on transects along the eastern border of the Ross Sea along W156° (T1, length of 35 km, 6 stations), near the Russkaya station (T2, 87 km, 13 stations), at the single station (Roosevelt Island area, Bay of Whales, Ross Sea), and in the Bransfield Strait (T3, 118 km, 11 stations). The relevance of the analysis of this area is due to its location on the border of the shelf and continental slope with different parameters of temperature and water structure. Low salinity and different temperature characteristics were revealed in surface waters of T1 and T2: lower values for T2 (−1.5 °C) and higher for T1 (0 °C). For the Bransfield Strait waters (T3), typical data on salinity and oxygen content were obtained against the backdrop of slightly increased temperature (up to +2 °C). For the single station, relatively fresh, cold, and oxygenated water of the upper 100-meter layer was recorded, and low temperature values of the bottom area, with high mineralization, were registered. In the phytoplankton composition of the study area, 48 taxa of microalgae from 5 divisions were identified (Bacillariophyta, 38; Dinophyta, Cyanoprokaryota, and Chrysophyta, 3 taxa each; and Haptophyta, 1) and 1 macrophyte taxon (Rhodophyta). The maximum similarity in the species composition of phytoplankton (on average, 43%) was typical for shelf stations in different areas. The total abundance of phytoplankton in the study area of the Southern Ocean ranged within 4.3–264.0 thousand cells·L−1, and biomass, 0.07–1.18 mg·L−1. The main contributors to quantitative characteristics of phytoplankton throughout the study water area were diatoms, mainly representatives of the genus Fragilariopsis Hustedt, confined to the shelf and coastal areas. At a distance and in the open sea of transects T1 and T2 deeper than 50 m and in surface waters of transect T3, Phaeocystis antarctica Karsten (Haptophyta) developed in mass. For T2, the dependence of phytoplankton abundance on water temperature and salinity was revealed.
Coastal seagrass ecosystems, particularly Zostera marina Linnaeus, 1753 ones, are capable of accumulating organic carbon by fixing carbon dioxide via photosynthesis. Seagrass biomass is considered as a short-term carbon storage, and underlying bottom sediments, as a long-term one. The research on organic matter accumulation by seagrass ecosystems is mostly carried out in areas with stable sedimentation. For such ecosystems, the importance of seagrass areas within the concept of blue carbon was shown. However, for the seas of temperate latitudes, coastal waters with unstable sedimentation and prevalence of sandy sediments are common, and the scale of carbon storage in seagrass ecosystems is not obvious. In this work, biomass and carbon stock in Z. marina leaves and roots, as well as Corg concentration and carbon stock in the upper layers of bottom sediments (0.25-m and 1-m thick), were determined for typical habitats in the semi-open Srednyaya Bight (Peter the Great Bay, the Sea of Japan), where sandy sediments prevail. Z. marina roots were characterized by 3–20 times lower biomass than its leaves. This difference increased from April to July in accordance with seasonality. Carbon concentrations in the seagrass leaves and roots were similar (33.3 and 31.3% dry weight, respectively). In the habitats with a projective coverage of 50–80%, carbon stock in Z. marina tissues was (96.8 ± 37.4) g C·m−2; with 100% coverage, the value increased to 253 g C·m−2. Corg concentration in bottom sediments of the Srednyaya Bight ranged within 0.04–0.46% and correlated with content of silt fractions. Under dense Z. marina coverage, Corg content and the fraction of silt particles in sediments were higher than under sparse ones. The vertical distribution of Corg concentration within the upper 15–35-cm layer did not reveal a downward trend in the cores. The main factor controlling Corg content was the particle-size distribution of sediments, which suggests a weak expression of reduction diagenesis and the effect of wave mixing of the upper layer of sandy sediments. Data on the bulk density and Corg concentration in sediments allowed to calculate carbon stock for the layers of 0.25 and 1 m. The quota of organic carbon in the seagrass tissues did not exceed a third of its amount in the upper layer (0.25 m) of underlying sandy sediments. When extrapolated to a 1-m thick layer, the quota of bottom sediments to Corg pool exceeds 90%. Organic carbon enrichment of sandy sediments under the seagrass beds compared to sands of similar particle size beyond the seagrass beds indicates a significant role of Z. marina in carbon storage, even in the habitats with the lack of stable and intensive sedimentation. The major factor controlling carbon stock in Z. marina ecosystems is Corg content in underlying bottom sediments which depends primarily on their particle-size distribution. In this case, the range of variation in carbon stock in the upper layer is an order of magnitude or more. Maps of the seagrass distribution in April and July 2021 were built. The absolute values of carbon stock were calculated, both accumulated in Z. marina biomass and deposited in the seagrass-covered sediments. The area of potential Z. marina distribution in the Srednyaya Bight was modelled using the MaxEnt 3.4.4 program. According to the results, areas with a predicted probability exceeding 0.5 for the seagrass occurrence occupy about a third of the total area of the bight; out of them, the area with a probability of Z. marina occurrence exceeding 0.75 accounts for 11.83 hectares. In fact, the seagrass meadows occupied > 70% of the area with a predicted probability of the species occurrence exceeding 0.5. As shown, the assessment of the contribution of seagrass ecosystems to the storage of carbon accumulating in the coastal zone requires differentiation of water areas by sedimentation regimes and types of bottom sediments. Moreover, the creation of databases with data on Corg concentration and stock per unit area is needed. Information on the areas of ecosystem distribution obtained by direct mapping and remote sensing is of high significance as well.
The Western Pacific gastropod Rapana venosa (Valenciennes, 1846) is classified among the hundred most dangerous invasive species of the Black and Mediterranean seas. Moreover, it is recognized as a dangerous invader in a number of coastal water areas on both sides of the Atlantic Ocean that determines the relevance of the study of population characteristics and biocenotic relationships of the rapa whelk in the areas of its invasion. The analysis of a previously unexplored R. venosa population in the Donuzlav Bay (the Northwestern Crimea) of the Black Sea in 2020 showed as follows: in the occurrence of an abundant and diverse food base, the rapa whelk does not form mass aggregations and, consequently, does not significantly affect benthic biocenoses. This fact is also confirmed by the ratio of biomass of the predatory mollusc and its prey. R. venosa mean biomass in the study area was 3.8 g·m−2, and the mean biomass of its food objects (Bivalvia) was 162.8 g·m−2. The features of the population structure and biocenotic relationships of the rapa whelk in the Donuzlav Bay are considered and discussed for the first time. Direct underwater observations and indirect evidence indicate that the distribution of this invader is controlled by aboriginal predators, crabs. The main species limiting R. venosa abundance in the study area is the crab Carcinus aestuarii Nardo, 1847.
Symbioses are common relationships between organisms in marine ecosystems. Out of crabs belonging to the family Portunidae, an economically important one, Charybdis truncata (Fabricius, 1798) is a widely distributed species. However, the studies on its symbiotic assemblages are still limited. A total of 408 C. truncata were sampled in Nha Trang Bay (Khanh Hoa province, Vietnam, the East Sea) in January–August 2022. Symbionts were classified based on morphological characters. Statistical analysis was applied to compare the infestation status of symbionts. Six symbiotic species were detected, including four epibiotic barnacles [Octolasmis angulata (Aurivillius, 1894), Octolasmis alata (Aurivillius, 1894), Octolasmis warwicki Gray, 1825, and Dianajonesia tridens (Aurivillius, 1894)] and two obligate unidentified parasites [Cancrion sp. and Sacculina sp.]. Out of them, O. alata and Cancrion sp. were recorded on C. truncata for the first time. The overall prevalence of symbionts on the swimming crab hosts was 13%, and the number of infecting symbiont species on hosts ranged within 1–4. O. angulata was the dominant species, with the prevalence of 6.9%. The prevalence of this symbiotic species was significantly higher in ovigerous female crabs compared to non-ovigerous ones. Moreover, there was a noticeable rise in O. angulata prevalence rates as crab size increased. No significant differences were revealed in the mean intensity of symbionts by sex, reproductive status of females, and size of the crab hosts. The initial morphological modifications caused by infecting parasites, Cancrion sp. and Sacculina sp., were recorded and described. These findings provide data on the status of natural infection of symbiotic species on C. truncata as a basis for the management of commercial species and aquaculture development.
Chitons inhabiting natural hard substrates in the Tsemes Bay pseudolittoral from the central beach of Novorossiysk to the Cape Khako were investigated in summer and winter periods of 2022–2023. For the first time in the Tsemes Bay, a local population of Lepidochitona cinerea (Linnaeus, 1767) (Polyplacophora, Tonicellidae) was recorded on substrates of sedimentary origin. It is a rare species, and over the past 70 years, it was found in waters of the North Caucasus only three times. We registered 34 L. cinerea specimens with a maximum shell length of 8 mm. The age of the largest individuals (three years) was determined by the annual rings of the apex of the first shell shield. The species is distributed in the upper horizon of the Cape of Love pseudolittoral and is confined to supralittoral baths. A similar biotope, where the chiton was found as well, was discovered in 50 km from the Tsemes Bay. As assumed, the occurrence of the mollusc in the water area is precisely due to the presence of this bionomic type of habitat – supralittoral baths which are not expressed in other study sites of the bay. The aim of the research is to describe L. cinerea populations in a previously unspecified biotope for this species and to identify the features of this biotope and its possible effect on the chiton occurrence. Information on the geographic distribution and biotopic confinement of the species was updated. The effect of ecological, hydrochemical, and geomorphological features of the biotope on L. cinerea occurrence was considered.
