Deep-sea Research Part Ii-topical Studies in Oceanography最新文献

Japan's coral reefs have changed dramatically over the last several decades due to climate change and anthropogenic impacts. Due to its dynamic location, the islands of Ryukyu Archipelago offer a unique environmental gradient to study the interactions between environmental variations, connectivity, resilience, climate change and adaptation of marine biodiversity along a wide latitudinal gradient. To obtain detailed baseline assessments of the reefs of Ryukyu Islands, we carried out Point Intercept Transect surveys to compare the assemblages of benthic communities in six regions of Ryukyu Archipelago, examining the influence of latitude gradient and emphasising on regional features. A total of 69 individual reef sites were surveyed for this study. The overall hard coral cover for Ryukyu archipelago was 22.41% ± 7.98 (Mean ± SD) from pooled dataset. The most abundant benthic assemblage for Ryukyu Archipelago was turf algae (39.72% ± 7.21) with all regions except Amami Oshima Island having overall cover above 40%. A total of 2607 individual colonies of hermatypic corals were recorded and identified up to genus level for this study where 55 genera of corals belonging to 17 families were recorded for Ryukyu Archipelago. The overall relative abundance was highest for Acroporidae family (32.22%), followed by Merulinidae family (27.69%) and Poritidae family (14.38%). Our results highlight the current condition of the reefs of Ryukyu archipelago and offers baseline data on broad scale ecosystem where future ecological change could be compared.

Japanese anchovy is used as an essential dried fish material from the larval to adult stages. In the central Seto Inland Sea, Japan, the catch of larval anchovy has markedly decreased to <3.9% of the maximum recorded in 2002 since 2013; however, the reason causing this reduction has not been well understood. The abundance of recruit fish, including larvae and early juveniles, has decreased in the last decade, despite abundant eggs, suggesting that the majority of larvae do not survive before recruitment. In contrast, the stock of Japanese Spanish mackerel, whose larvae are the major predator of larval anchovy, has increased in the Seto Inland Sea. It is hypothesized that an increase in the density of Spanish mackerel may have a top-down control on the decrease in anchovy recruitment by an increase in predation opportunities. In this study, we investigated the abundance of Spanish mackerel and anchovy larvae using a bongo net in the field in 2018 and 2019. The average densities of larvae in late May were 1.5–3.3 individuals (inds)/100 m³ and 1058–1346 inds/100 m³ for the Spanish mackerel and the anchovy, respectively; both were higher than those in 2002–2005. We constructed a Stella model, simulating the growth and survival of larval anchovy until they reached the commercial sizes by taking into account consumption by larval Spanish mackerel. The model suggested that the consumption of larval anchovy by larval Spanish mackerel accounted for <4% of the initial abundance of anchovy in 2018, which was not greater than that in 2005. In contrast, the reduction in the growth rates of larval anchovy due to reduced maternal conditions can adversely affect their survival. Thus, the results did not fully support the hypothesis mentioned above.

Holothurians are one of the key components of the deep-sea megabenthos, often forming significant aggregations with high abundance and biomass. The crucial factors determining the pattern of their spatial distribution might be their food strategies, as well as the quantity and quality of available food. Using the set of trophic markers (analysis of fatty acid (FA) composition and stable nitrogen and carbon isotope signatures), we studied the trophic preferences of four common species of deep-sea holothurians (Paelopatides sp., Pannychia henrici, Psolidium sp., and Scotoplanes kurilensis) inhabiting the Volcanologists Massif, Bering Sea, at depths from 984 m to 3395 m. The δ¹³С values in tissues of all four species varied slightly (from −16.2 to −16.6‰) and were significantly higher than those in sediment organic matter (SOM) (on average, −21.5‰), which indicates the use of organic matter of the same origin by all species. The δ¹⁵N values (10.1–12.9‰) were also significantly higher than in SOM (on average, 4.4‰). The lowest average δ¹⁵N value was recorded for the deepest-dwelling species (Scotoplanes kurilensis), and the highest value from the shallowest-dwelling ones (Psolidium sp.). All the species differed significantly in FA composition. Psolidium sp. was distinguished by the highest saturated FA content (45.5%) and the lowest polyunsaturated FA (PUFA) content (18.7%). In the rest of the species, PUFAs dominated (>35%). The ratios of major PUFAs varied among the four species studied but all holothurians species were rich in phytoplankton derived FA suggesting tight trophic coupling to phytodetritus. However, for Psolidium sp., bacteria could constitute a substantial portion in the diet. The high level of 20:4n-6 might indicate a significant contribution of protozoans to the diet of Pannychia henrici. Paelopatides sp. and Scotoplanes kurilensis were characterized by a high level of 20:5n-3 and a high value of the 20:5n-3/20:4n-6 ratio, which indicates the predominance of fresh phytodetritus in their diet. The high level of primary production in the upper layers of the Bering Sea provides the high quality of the sedimentary OM and food supply of deposit feeders even at the greatest depths.

Integrated pelagic trawl surveys were conducted in the eastern sector of the Russian Arctic (Laptev Sea, Chukchi Sea and East Siberian Sea) in August–September of 2003–2018. Data were used to further describe biology and spatial distribution of Polar cod (Boreogadus saida) in this region. Polar cod in surveyed areas are characterized by similar linear size and spatial distribution. In all surveyed seas, polar cod aggregations consisted of individuals 3–29 cm in length with the age of 0+ - 6+ years. Lower growth rates of polar cod were evident in the eastern sector compared to the Kara Sea (the western sector of Russian Arctic). The lower growth rates in the eastern sector are probably due to the significant difference in environmental conditions (mainly temperature) that directly affect polar cod metabolic rates.

In the Chukchi and East Siberian seas, the main concentrations were observed within the near-bottom layer, while in the Laptev Sea they were recorded throughout the water column. The abundance and biomass of polar cod in the Chukchi Sea in different years ranged from 514 million inds. and 0.83 thousand tons (2008) to 8.26 billion inds. and 117.5 thousand tones (2003). Respective indices for the Laptev Sea amounted to 233 thousand tones and 12.75 billion individuals. The abundance and biomass of the East Siberian Sea polar cod were at a relatively low levels compared to other areas in the Russian Arctic (about 0.150 thousand tons and 20 million individuals).

The Southern Ocean is the largest high nutrient low chlorophyll (HNLC) oceanic region, where iron limits phytoplankton growth and productivity and ultimately influences the Biological Carbon Pump (BCP). Natural exceptions to the HNLC regime occur where island wakes cause iron to be mixed into surface waters from sediments, enabling large, prolonged phytoplankton blooms and increased carbon drawdown. Interactions between iron and phytoplankton are reciprocal in blooms: with plankton regulating the (re)cycling of iron through cellular uptake and remineralisation. The depth of iron remineralisation then influences either re-supply to the surface mixed layer biota or sequestration into deeper waters. Water column trace metal observations and shipboard experiments, using bioassays and radioisotope (⁵⁵Fe, ³²Si, ¹⁴C) cycling, were undertaken to investigate surface mixed layer phytoplankton iron limitation, iron uptake, and mesopelagic iron remineralisation relative to carbon and silica within the November 2017 bloom downstream of South Georgia. Surface phytoplankton residing in the iron depleted mixed layer were iron limited throughout the four-week sampling period. Experiments designed to investigate particulate water column (re)cycling revealed limited iron remineralisation from freshly produced upper ocean particles. The main pathway of iron transfer from particulates into the dissolved phase was through rapid (<2 d) release of extra-cellular adsorbed iron, which, if occurring in situ, could contribute to observed higher sub-surface dissolved Fe concentrations. This was accompanied by a small loss of cellular carbon, likely through respiration of the fixed ¹⁴C, and limited dissolution of particulate ³²Si to dissolved ³²Si. Decoupling of the remineralisation length scales for Fe, C and Si, with Fe having the fastest turnover, is thus likely in the upper mesopelagic zone beneath the bloom.

Arctic marine ecosystems are in a period of rapid change. Research in recent years has documented dramatic shifts, including an earlier ice retreat, a longer ice-free season, and the loss of multi-year ice. In the absence of physical constraints related to sea ice and cold water, barriers to exchange of marine taxa between North Pacific and Arctic marine systems have eroded. Initially, transfers were predominantly in pelagic species. More recently, groundfishes and other demersal taxa also appear to be moving north, as the intensity of warming increases temperatures throughout the water column in the shallow continental shelves of Pacific Arctic. This Special Issue volume is devoted to studies that integrate research across various components of the northern Bering Sea and Chukchi Sea marine ecosystems. The intent is to characterize and investigate these changes and their controlling processes. Research presented here integrates studies from US and Russian waters to better understand physical processes and mechanisms in the regions immediately north and south of Bering Strait. Results document shifts in the distribution and dynamics of important fish stocks. Studies here also present new methodological approaches and refinements to existing methods, including automated instrumentation and passive acoustics. These approaches are particularly relevant to sampling in this remote region. Data and analyses provide new information on ecosystem structure and linkages, including oceanographic interactions and transport, pelagic production, and benthic deposition. Results provide evidence for accelerated sea-ice decline, climate-driven shifts in the spatial distribution of marine taxa, and ecosystem transformation. These results also inform forecasts of winners and losers in a warming Arctic.

As part of comprehensive oceanographic studies on the Persian Gulf and Gulf of Oman, this article documented the seasonal and vertical distribution of nutrients, chlorophyll-a, and nitrogen deficiency in the Strait of Hormuz and northern part of Gulf of Oman. During the three consecutive research cruises of RV Kavoshgar Khalij Fars (Persian Gulf Explorer) in September–October 2018, December 2018, and May 2019, 7–18 stations were surveyed. During the southwest monsoon season, the water column is characterized by a 10–20 m shallow mixed layer and high spatial variation in nutrients and their ratio in the upper 100 m. Shallow remineralization of nitrogen-rich organic matter and lateral advection from the Arabian Sea upwelling system lead to nutrient accumulation, and enrichment of nitrate-nitrogen compared to P and Si in the euphotic zone with some sign of Si limitation. On the other hand, deepening of the mixed layer due to the cool convective mixing during the northeast monsoon, brings nutrients to the surface and stimulate the phytoplankton blooms (i.e. with surface chlorophyll-a maximum of 1.46 ± 0.57 μg l⁻¹). So, the primary production is likely to be controlled by grazing rather than nutrient availability during the northeast monsoon. In the spring inter-monsoon, the multilayer structure of the water column with a less well-characterized mixed layer has a surface layer severely depleted in nitrite and nitrate with low yet detectable concentrations of silicate (0.77 ± 0.44 μM) and phosphate (0.29 ± 0.09 μM). The distribution of surface chlorophyll-a concentration was patchy with generally low concentrations of 0.37 ± 0.10 μg l⁻¹ with highly variable subsurface maxima occurring at 10–25 m depth level. Primary production seemed to be potentially limited by inorganic nitrogen availability in spring inter-monsoon, as severe nitrogen deficiencies extended from the surface to the bottom layer (17.54 ± 4.39 μM at 400–1000 m depth level).

The biological carbon pump (BCP) contributes to the oceanic CO₂ sink by transferring particulate organic carbon (POC) into the deep ocean. The magnitude and efficiency of the BCP is likely to vary on timescales of days to seasons, however characterising this variability from shipboard observations is challenging. High resolution, sustained observations of primary production and particle fluxes by autonomous vehicles offer the potential to fill this knowledge gap. Here we present a 4 month, daily, 1 m vertical resolution glider dataset, collected in the high productivity bloom, downstream of South Georgia, Southern Ocean. The dataset reveals substantial temporal variability in primary production, POC flux and attenuation. During the pre-bloom peak phase we find high export efficiency, implying minimal heterotrophic POC consumption, i.e. productivity is decoupled from upper ocean remineralisation processes. As the bloom progresses from its peak through its declining phase, export flux decreases, but transfer efficiency within the upper 100 m of the mesopelagic increases. Conversely, transfer efficiency in the lower mesopelagic decreases in the post-bloom phase, implying that the flux attenuation processes operating in the upper and lower mesopelagic are effectively decoupled. This finding underscores an important limitation of using a single parameter, such as Martin's ‘b’, to characterise POC flux attenuation in a given location or season. Frequent pulses of export flux are observed throughout the deployment, indicating decoupling between primary production and the processes driving export of material from the upper ocean. The mechanisms underlying the observed seasonal changes in BCP magnitude and efficiency are unclear, as temperature and oxygen concentration changed minimally, although the nature of the sinking particles changed substantially as the bloom progressed. Our results highlight the difficulty of capturing temporal variability and episodic flux events with traditional shipboard observations, which affects our conceptual understanding of the BCP. The increasing use of autonomous vehicles to observe particle fluxes will be essential to characterising the temporal variability in magnitude and functioning of the BCP.

In a rapidly changing Arctic, effects of taxonomic and functional composition of infaunal benthic communities on ecosystem function are poorly characterized for many areas of the continental shelf environment. Infaunal macrobenthos perform essential ecosystem services in soft-sediment habitats, including sediment oxygenation, organic-matter burial, and nutrient regeneration. Changes in the distribution and composition of infauna are likely to impact these important processes. We investigated spatial patterns in macrofauna on the northern Bering and southern Chukchi Sea continental shelves in conjunction with a suite of environmental variables as part of the Arctic Shelf Growth, Advection, Respiration, and Deposition (ASGARD) project. Depth-stratified macrofauna samples (>500 μm) were collected from multi-cores in June 2017 and 2018. Abundance and biomass were recorded for broad taxonomic categories (phylum or class). Polychaetes were identified to family to capture well-described functional-trait information. Functional guilds were assigned to polychaete families, reflecting feeding mode, motility, and feeding structures. Macrofauna exhibited no significant spatial patterns when examined at higher taxonomic levels; however, analysis of polychaete families yielded three distinct station groups, and polychaete functional guilds yielded four distinct groups. These groups were aligned with variations in key habitat characteristics and suggest variation in ecosystem function across the study area. At sandy stations, tube-dwelling, particle-selective suspension and surface deposit feeders were relatively abundant, reflecting an advective system with moderate current speeds and low deposition. Sites with gravelly substrate and moderate levels of organic matter in deeper sediment layers likely experience sporadic organic-matter deposition and disturbance from scouring and were inhabited by larger-bodied species, including bivalves and subsurface deposit-feeding or carnivorous polychaetes. Muddy, offshore stations in the southern Chukchi Sea also contained large bivalves and large carnivorous polychaetes, with evidence of high bioturbation. At more coastal Chukchi Sea stations, macrofauna were dominated by small, motile polychaetes concentrated in the 1-cm surface layer, with indications of anaerobic conditions below. Overall, polychaete functional traits and vertical distribution reflected the quality and quantity of organic matter input and the depositional environment inferred from grain size and current speed, with implications for biogeochemical cycling in sediments.

Recently, sea surface temperatures (SSTs) near the Korean Peninsula have increased rapidly due to global warming; this phenomenon can lead to high water temperatures and extensive damage to Korean fish farms. To reduce such damage, it is necessary to predict high water temperature events in advance. In this study, we developed a method for predicting high water temperature events using time series SST data for the Korean Peninsula obtained from the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 product and a long short-term memory (LSTM) network designed for time series data prediction. First, the SST prediction model was used to predict SSTs. Predicted SSTs exceeding 28 °C, which is the Korean government standard for issuing high water temperature warnings, were designated as high water temperatures. To evaluate the prediction accuracy of the SST prediction model, 1-to 7-day predictions were evaluated in terms of the coefficient of determination (R²), root mean square error (RMSE), and mean absolute percentage error (MAPE). The R², RMSE, and MAPE values of the 1-day prediction SST model were 0.985, 0.14 °C, and 0.38%, respectively, whereas those of the 7-day prediction SST model were 0.574, 0.74 °C, and 2.26%, respectively. We also calculated F1 scores to evaluate high water temperature classification accuracy. The F1 scores of the 1- and 7-day SST prediction models were 0.963 and 0.739, respectively.