Western Gyre Research Articles

Vertical distribution of picophytoplankton in the NW shelf and deep-water area of the Black Sea in spring

Structure and functions of picophytoplankton and their links to the water column density stratification and nutrient vertical profiles were studied in the Black Sea open waters along a transect from the Western Gyre to the NW shelf during spring homothermy in April of 2017. Abundances of picocyanobacteria of the genus Synechococcus (0.85 ± 0.96 (SD) × 10 3 cells ml −1 ) and eukaryotic picoalgae (5.74 ± 5.99 × 10 3 cells ml −1 ) and their intracellular pigment (chl a , phycoerythrin) contents were quantified using flow cytometry. The distribution of these variables in the upper 100-m layer was non-uniform. Along the whole transect, Synechococcus and picoeukaryote abundance maxima (up to 4 × 10 3 and 3 × 10 4 cells ml −1 , respectively) were observed at the water temperature of 8–8.5°С in the depth range between 30 and 40 m. Picoeukaryotes dominated the total community. The share of Synecococcus averaged about 22% and increased with depth up to 80%. Picophytoplankton abundance dropped abruptly through the oxycline but the cells were found in the suboxic layer at about 10 3 cells ml −1 and in a sample from the anoxic zone at 10 2 cells ml −1 . Intracellular chl a maxima were revealed in the nitrate peak at sigma- t density of 15.5. In the surface and suboxic layers, non-specific green autofluorescence (GAF) was detected in picophytoplankton cells that might provide evidence of their stress state in adverse environment. Multivariate analysis has revealed tight coupling between abiotic and microbiological variables in three statistically distinct pelagic regions of the transect, corresponding to the abyss, the continental slope and the shelf. • Picoeukaryotes dominated picophytoplankton but the proportion of Synecococcus increased with depth. • Picophytoplankton abundance dropped abruptly through oxycline but still survived in suboxic layer. • Non-specific green autofluorescence (GAF) was detected in picophytoplankton by flow cytometry. • Picophytoplankton GAF maxima were revealed in surface and suboxic layers, indicating stress conditions.

Microbial ecosystem responses to alkalinity enhancement in the North Atlantic Subtropical Gyre

In addition to reducing carbon dioxide (CO2) emissions, actively removing CO2 from the atmosphere is widely considered necessary to keep global warming well below 2°C. Ocean Alkalinity Enhancement (OAE) describes a suite of such CO2 removal processes that all involve enhancing the buffering capacity of seawater. In theory, OAE both stores carbon and offsets ocean acidification. In practice, the response of the marine biogeochemical system to OAE must be demonstrably negligible, or at least manageable, before it can be deployed at scale. We tested the OAE response of two natural seawater mixed layer microbial communities in the North Atlantic Subtropical Gyre, one at the Western gyre boundary, and one in the middle of the gyre. We conducted 4-day microcosm incubation experiments at sea, spiked with three increasing amounts of alkaline sodium salts and a 13C-bicarbonate tracer at constant pCO2. We then measured a suite of dissolved and particulate parameters to constrain the chemical and biological response to these additions. Microbial communities demonstrated occasionally measurable, but mostly negligible, responses to alkalinity enhancement. Neither site showed a significant increase in biologically produced CaCO3, even at extreme alkalinity loadings of +2,000 μmol kg−1. At the gyre boundary, alkalinity enhancement did not significantly impact net primary production rates. In contrast, net primary production in the central gyre decreased by ~30% in response to alkalinity enhancement. The central gyre incubations demonstrated a shift toward smaller particle size classes, suggesting that OAE may impact community composition and/or aggregation/disaggregation processes. In terms of chemical effects, we identify equilibration of seawater pCO2, inorganic CaCO3 precipitation, and immediate effects during mixing of alkaline solutions with seawater, as important considerations for developing experimental OAE methodologies, and for practical OAE deployment. These initial results underscore the importance of performing more studies of OAE in diverse marine environments, and the need to investigate the coupling between OAE, inorganic processes, and microbial community composition.

Bacteriohopanepolyols across the Black Sea redoxcline trace diverse bacterial metabolisms

Redox gradients in oceanic oxygen minimum zones are hotspots of diverse bacterial populations and metabolisms. Here, we test whether bacteriohopanepolyols (BHPs), a suite of membrane-regulating pentacyclic triterpenoid lipids, are suitable tracers of major bacterial metabolisms and whether their structural diversity (BHP lipidomics) allows tracing the bacterial ecology across the Black Sea chemocline. We analyzed suspended particulate matter (SPM) sampled along depth-profiles across the redox/chemocline at two sites in the eastern and western gyres and underlying core-top sediments at the western gyre site. Our results show that BHPs abundances reflect the stratification of the Black Sea and capture ecological niches across the chemocline, being highest in the lower suboxic zone and in upper to deep sulfidic waters. BHP-inferred bacterial metabolisms in the lower suboxic zone are dominated by anammox and aerobic methane oxidizers (primarily by Type I/gammaproteobacteria). Bacterial biomass production in the upper to deep sulfidic zone is mirrored by a range of BHPs that are normally associated with aerobic metabolisms, most likely explained by either survival in oxygenated micro-niches or production by anaerobic bacteria (which could possibly include sulfate reducers and nitrite-dependent methane oxidizers). Most surprisingly, this ecological niche harbors seemingly unknown anaerobic bacteria which produce abundant C-2 and C-3 methylated BHPs. Core-top sediments reflect the BHP structural diversity found in SPM, but with notable compositional differences. Most strikingly, the export dynamics seem to result in near-absence of C-2 methylated BHPs in surface sediments. Sedimentary BHP distributions therefore underestimate the importance of C-2 methylated BHPs in the water column, which has implications for the geological record.

Biological manganese-dependent sulfide oxidation impacts elemental gradients in redox-stratified systems: indications from the Black Sea water column

The reduction of manganese oxide with sulfide in aquatic redox-stratified systems was previously considered to be mainly chemical, but recent isolation of the Black Sea isolate Candidatus Sulfurimonas marisnigri strain SoZ1 suggests an important role for biological catalyzation. Here we provide evidence from laboratory experiments, field data, and modeling that the latter process has a strong impact on redox zonation in the Black Sea. High relative abundances of Sulfurimonas spp. across the redoxcline in the central western gyre of the Black Sea coincided with the high-level expression of both the sulfide:quinone oxidoreductase gene (sqr, up to 93% expressed by Sulfurimonas spp.) and other sulfur oxidation genes. The cell-specific rate of manganese-coupled sulfide oxidation by Ca. S. marisnigri SoZ1 determined experimentally was combined with the in situ abundance of Sulfurimonas spp. in a one-dimensional numerical model to calculate the vertical sulfide distribution. Abiotic sulfide oxidation was too slow to counterbalance the sulfide flux from euxinic water. We conclude that microbially catalyzed Mn-dependent sulfide oxidation influences the element cycles of Mn, S, C, and N and therefore the prevalence of other functional groups of prokaryotes (e.g., anammox bacteria) in a sulfide-free, anoxic redox zone.

Reconstructing Western Boundary Current Stability in the North Atlantic Ocean for the Past 700 Kyr From Globorotalia truncatulinoides Coiling Ratios

AbstractDown‐core changes in the coiling direction of Globorotalia truncatulinoides in the northwestern subtropical Atlantic (KNR140‐37PC and Ocean Drilling Program Sites 1063, 1059, 1056, 1058) provide a tracer for the hydrographic conditions in the western boundary current over the past 700 kyr (Marine Isotope Stage, MIS, 1–17). A consistent association between percent G. truncatulinoides (sinistral) abundances, total test counts, and bulk sediment CaCO3 content is established by MIS 11 suggesting a response to ocean‐atmosphere interactions during the mid Brunhes event. Commencing with MIS 11, interglacial maxima are associated with high total test counts and either distinct sinistral test minima (MIS 9e, 11c) or maxima (MIS 1, 5e, 7a). High sinistral test abundances with relatively high test counts is similar to the late Holocene relationship at the study sites. Low sinistral test abundances despite high test counts means that coiling ratios are dominated by dextral forms. We interpret this pattern to indicate more intense flow in the subtropical gyre either via the western boundary current drawing toward the gyre center, or a more northern influence of the North Equatorial and Antilles Currents. This suggests that the western boundary current may have been more intense during MIS 11c and MIS 9e then during MIS 7a, MIS 5e, and MIS 1 consistent with climate warm anomalies in northern Europe at these times. Regardless of the mechanism, the observation that minima and maxima in sinistral test abundances are prolonged at these times indicates that the western gyre boundary remained stable during relative warm intervals.

Identifying distribution and accumulation patterns of floating marine debris in the Black Sea

The distribution and accumulation of floating marine debris in the Black Sea during the last few decades are analysed by the help of numerical modelling. An approach based on a mesoscale circulation model combined with a particle tracking model is applied. It is established that the litter distribution is nearly independent of the source location and is mainly controlled by the basin circulation system. The western gyre predominantly accumulates floating debris in summer. After the integration of the main cyclonic current in winter, the debris in the inner basin moves east. Retention zones along the south-western coast persist in time. The mean particle stranding time is estimated at about 200 days. Accumulation zones along the south-eastern and eastern coast are abundant in summer, and then move further northeast and north. Simulations demonstrate an increasing litter accumulation in summer on the North Western Shelf and shelf break.

A first description of the summer upwelling off the Bay of Algeciras and its role in the northwestern Alboran Sea

A local upwelling process is observed in the mouth of the Bay of Algeciras showing a singular pattern, partially disengaged of the neighboring coastal upwelling variability. The physical mechanisms which drive this local upwelling are not clear, but the Ekman pumping seems to be the most important forcing factor. The surface chlorophyll-a variability associated with this local upwelling is uncorrelated with the concentration registered in the inner Bay of Algeciras or in the surrounding coastal and shelf waters. Secondary, the emerged waters in the mouth of the Bay of Algeciras flow eastwards into the northwestern Alboran Sea dragged by the northern edge of the Atlantic Jet, forming a narrow and elongated nutrient-enriched area (hereafter called filament) usually detached from the coast. A sampling station located at 14.6 km from the coastline and 68 km far from the mouth of the Bay of Algeciras was employed to monitor the presence of the filament in the Alboran Sea. The filament is most frequently presented during the summer (in fact, it was detectable in more than 50% of the days of august). When the filament is distinguishable, its chlorophyll-a concentration doubles in average the values registered in neighboring locations, and in annual average also higher nitrate and phosphate surface concentrations (about 30%) are registered at the filament location. These results indicate that the filament provides nutrients to the surface waters of Alboran not only when it is optically detected. Our analyses indicate that the trajectory of the filament follows the northern edge of the western gyre of Alboran although its width and speed are variable depending on the Atlantic Jet properties and the related sub-mesoscale activity.

Contrasting Wind Regimes Cause Differences in Primary Production in the Black Sea Eastern and Western Gyres.

12-year time series of SeaWiFS chlorophyll a (Chl-a), primary production (PP), sea surface temperature (SST), and meteorological wind speed were used to examine decadal changes in these parameters in the eastern and western Gyres of the Black Sea. In both Gyres, low wind speeds and SST led to higher PP. After 2004, there was a progressive decrease in PP and Chl-a, which co-varied with increasing SST. Chl-a and PP were significantly higher in the western Gyre compared to the eastern Gyre, especially from 1998 to 2004. Wind speed negatively correlated with PP in both Gyres, but the higher wind speed prior to relaxation in the western Gyre led to higher PP during spring and autumn. Variability in annual PP in both Gyres was coupled to fluctuations in the Multivariate ENSO Index (MEI), which affected the wind regime more in the eastern than in the western Gyre. The data suggest that localised wind regimes in the western gyre that are uncoupled from MEI, sustains higher PP in this area.

Stimulation of Autotrophic Denitrification by Intrusions of the Bosporus Plume into the Anoxic Black Sea

Autotrophic denitrification was measured in the southwestern coastal Black Sea, where the Bosporus Plume injects oxidized chemical species (especially O2 and ) into the oxic, suboxic, and anoxic layers. Prominent oxygen intrusions caused an overlap of and sulfide at the same station where autotrophic denitrification activity was detected with incubation experiments. Several bacteria that have been proposed to oxidize sulfide in other low oxygen environments were found in the Black Sea including SUP05, Sulfurimonas, Arcobacter, and BS-GSO2. Comparison of TRFLP profiles from this mixing zone station and the Western Gyre (a station not affected by the Bosporus Plume) indicate the greatest relative abundance of Sulfurimonas and Arcobacter at the appropriate depths at the mixing zone station. The autotrophic gammaproteobacterium BS-GSO2 correlated with ammonium fluxes rather than with sulfide fluxes and the maximum in SUP05 peak height was shallower than the depths where autotrophic denitrification was detected. Notably, anammox activity was not detected at the mixing zone station, though low levels of DNA from the anammox bacteria Candidatus Scalindua were present. These results provide evidence for a modified ecosystem with different N2 production pathways in the southwest coastal region compared to that found in the rest of the Black Sea. Moreover, the same Sulfurimonas phylotype (BS139) was previously detected on >30 μm particles in the suboxic zone of the Western Gyre along with DNA of potential sulfate reducers, so it is possible that particle-attached autotrophic denitrification may be an overlooked N2 production pathway in the central Black Sea as well.

Trace metal composition of suspended particulate matter in the water column of the Black Sea

We analyzed the chemical composition of suspended matter from the Black Sea for a complete suite of elements used as tracers for various sources and processes. Samples of suspended matter were collected at four stations from the Bosporus to the center of the western gyre during a R/V Bilim cruise in September 2000. Sample depths were from the oxic surface layer, the suboxic zone and the upper part of the anoxic (sulfidic) layer. All samples were analyzed for Al, Ti, V, Cr, Mn, Fe, Co, Cu, Zn, Mo, Ag, Cd, Ba, Pb, U and P using ICP-MS, AAS, and spectrophotometer. The main objective of this study was to determine the processes that influence the distributions of particulate trace metals in the water column under various redox environments. The composition of the particulate matter was influenced by lithogenic input from surrounding rivers, biological production in the surface layer, biogeochemical (e.g. redox) processes and anthropogenic inputs. Almost all elemental concentrations decreased from the coastal to the offshore stations to the center of the western gyre. Metal/Al ratios were compared with the ratios of average crust and Danube and Turkish rivers. Deviations from these lithogenic sources were used to examine the anomalies due to geochemical, biological and anthropogenic processes. In general, the largest metal enrichments relative to crust were observed inthe surface samples away from coastal sources. Metal/P ratios were used to compare data for some of the elements with previous data from the open ocean. The Me/P ratios for Cu, Zn, Mo and Ni were much higher than open ocean studies, while the ratios for Cd and Co were comparable. The high Me/P (and Me/Al) for several elements suggest anthropogenic input may contribute to these high surface concentrations. The composition of particulate matter from the suboxic zone showed that some elements were strongly enriched. Enrichments of Mn and Fe in the suboxic zone were observed, as expected. Mn and Fe oxides play an important role in scavenging processes. The redox dependent processes in the suboxic - anoxic interface influence the vertical distribution of U, V, Ni, Co, Cu, Zn, Ba and possibly Mo, Cr. Elements influenced by sulfide formation in the anoxic layer were Fe, Cr, Ni, Co, Mo, and Ag.

The cyclonic circulation in the Australian–Antarctic basin simulated by an eddy-resolving general circulation model

Flow structure in the Australian–Antarctic basin is investigated using an eddy-resolving general ocean circulation model and validated with iceberg and middepth float trajectories. A cyclonic circulation system between the Antarctic Circumpolar Current and Antarctic Slope Current consists of a large-scale gyre in the west (80–110° E) and a series of eddies in the east (120–150° E). The western gyre has an annual mean westward transport of 22 Sv in the southern limb. Extending west through the Princess Elizabeth Trough, 5 Sv of the gyre recirculates off Prydz Bay and joins the western boundary current off the Kerguelen Plateau. Iceberg trajectories from QuickScat and ERS-1/2 support this recirculation and the overall structure of the Antarctic Slope Current against isobath in the model. Argo float trajectories also reveal a consistent structure of the deep westward slope current. This study indicates the presence of a large cyclonic circulation in this basin, which is comparable to the Weddell and Ross gyres.

Cross-basin differences in particulate organic carbon export and flux attenuation in the subtropical North Atlantic gyre

We compared in- situ and satellite-derived measures of the biological carbon pump efficiency at the two seemingly similar subtropical North Atlantic gyre time series sites, the Bermuda time series (BATS, Bermuda Atlantic time-series study and OFP, ocean flux program) in the western gyre and the ESTOC time series (European station for time-series in the ocean, Canary Islands) in the eastern gyre. Satellite-derived surface chlorophyll a was slightly lower at Bermuda compared to ESTOC (annual average of 0.10±0.04 vs. 0.14±0.05-mg-m −3), as was satellite-derived primary production (annual average of 380±77 vs. 440±80-mg C-m −2 d −1). However, export production normalized to primary production (export ratio) was higher at Bermuda by a factor of 2–3 when estimated using mesopelagic traps moored at 500-m depth and by a factor of 3–4 when estimated using surface-tethered drifting traps. When averaged seasonally, flux at BATS was highest in spring (March, April, May) at all depths followed by summer (June, July, August) and decreasing towards fall, but this seasonality was less visible at ESTOC. Seasonal comparison showed the fastest flux attenuation at Bermuda in winter and spring, coinciding with the highest POC flux. POC/PIC ratios derived from the moored traps were significantly higher at BATS than at ESTOC in fall and winter, but this difference was not significant in spring ( p>0.05). This study shows that while the western and eastern Atlantic subtropical gyres have similar rates of primary production, the biological carbon pump differs between the two provinces. Higher new nutrient input observed at Bermuda compared to ESTOC might explain part of the difference in export ratio but alone is insufficient. Greater winter mixed-layer depths and higher mesoscale eddy activity at Bermuda resulting in pulsed production events of labile organic matter might explain both the higher export flux and export ratios found at Bermuda.

Warm intrusions in the intermediate layer (150–500 m) of the Black Sea eastern gyre interior

The new CTD data from the R/V Akvanavt 2002 cruise presented and discussed in this study, show that lateral limits of the Bosphorus lens penetration into the Black Sea interior are substantially wider than the earlier observed ones. The data reveal unexpectedly thick warm lenses in the 150–500 m depth range within the eastern gyre of the Black Sea. Thus, we provide observational evidence that the Bosphorus warm water penetration into the sea interior is not a local phenomenon confined to the southwestern and central parts of the western gyre (the intrusions have been previously observed only there), but rather a fundamental basin‐wide process determining the intermediate layer structure in the Black Sea, as was recently assumed on the basis of the numerical modeling results.

Alkalinity of the anoxic waters in the Western Black Sea

Abstract Measurements of total alkalinity and nutrients were made on samples from discrete depths, aboard the R/V Knorr , May 2001 in the Black Sea. Determination of macronutrient concentrations permits inclusion of these minor bases in alkalinity calculations and thus improved determination of the carbonate system. The permanent pycnocline maintains vertical separation of the oxic, suboxic and anoxic waters of the Black Sea. Results show an apparent excess in alkalinity and/or deficit in hydrogen sulfide from the stoichiometric ratio predicted from the sequential oxidation of Redfield organic matter by sulfate in the anoxic waters near the center of the western gyre. The lateral ventilation of the main pycnocline by Bosporus plume waters is responsible for consuming or removing sulfide from the anoxic zone. The removal of sulfide via intrusion of oxygen-enriched waters and generation of alkalinity and total inorganic carbon dioxide via dissolution of calcium carbonate results in differing stoichiometry between the upper and lower anoxic zones in the center of the western gyre. Changes in the distribution of nutrients and hydrogen sulfide are due to the oxidation of particulate organic matter apparently due to increased productivity in surface waters.

Foraging destinations and marine habitat use of short-tailed albatrosses: A multi-scale approach using first-passage time analysis

Abstract We used satellite telemetry, remotely sensed data (bathymetry, chlorophyll a (chl a), sea-surface temperature (SST), wind speed) and first-passage time (FPT) analysis to determine the distribution, movement patterns, and habitat associations of short-tailed albatrosses (Phoebastria albatrus) during the non-breeding season, 2002 and 2003. Satellite transmitters were deployed on birds immediately prior to their departure from a breeding colony at Torishima, Japan ( n = 11 ), or at-sea in the Aleutian Islands ( n = 3 ). Tracking durations ranged from 51 to 138 days for a total of 6709 locations after filtering (131 – 808 per bird). FPT (time required to transit a circle of given radius) revealed the location and spatial scale of area-restricted search (ARS) patterns along flight paths. On average, ARS occurred within 70 km radii. Consequently, the fit of the habitat use models increased at spatial scales beyond a 40 km FPT radius ( R 2 = 0.31 ) and stabilized for scales of 70 km and larger (R2=0.40– 0.51). At all scales, wind speed, depth or depth gradient, and chl a or chl a gradient had a significant effect on FPT (i.e., residence time). FPT increased within regions of higher gradients of depth and chl a. In contrast, FPT decreased within regions of greater depth and wind speed, with a significant interaction of wind speed and depth at some scales. Sea-surface temperature or its interactions were only significant at large spatial scales (⩾160 km FPT radius). Albatrosses engaged in ARS activities primarily over the shelf break and slope, including Kuroshio and Oyashio regions off the western subarctic gyre. Occasionally, birds transited the northern boundary of the Kuroshio Extension while in-route to the Aleutian Islands and Bering Sea, but overall spent little time in the western gyre. In the Aleutian Islands, ARS occurred within straits, particularly along the central and western part of the archipelago. In the Bering Sea, ARS occurred along the northern continental shelf break, the Kamchatka Current region, and east of the Commander Islands. Non-breeding short-tailed albatross concentrate foraging in oceanic areas characterized by gradients in topography and water column productivity. This study provides an understanding of the foraging ecology for a highly migratory, imperiled seabird, and confirms the importance of shelf break and slope regions as hot spots for a variety of top marine predators in the North Pacific.

Surface ventilation of the Black Sea's cold intermediate layer in the middle of the western gyre

Understanding the origin of the shallow temperature minimum, known as the Cold Intermediate Layer (CIL), in the Black Sea has long been hampered by the scarcity of winter observations. During March 2003, we observed a cold‐air outbreak over the center of the Black Sea's Western Gyre. Freezing winds drove convection that cooled the surface mixed layer to 6.1°C and deepened it to 40 m, directly ventilating the upper 80% of the CIL, whose lower boundary was at 49 m. Concentrations of dissolved oxygen were 350 μM in the mixed layer and decreased rapidly to 70 μM at the base of the CIL, 9 m below the mixed layer. A few meters deeper, at the top of the Sub‐Oxic Layer (SOL), both oxygen and hydrogen sulfide became undetectable (<5 μM and <1 μM, respectively). Microstructure profiles revealed intermittent turbulence in the oxycline below the mixed layer. Average rates of turbulent dissipation were 10−9–10−8 W kg−1. The accompanying mixing produced diapycnal diffusivities, Kρ, that were only (1–4) × 10−6 m2 s−1. Consequently, turbulent fluxes were too weak to renew significantly either the lower 20% of the CIL or the SOL, whose top was 4 m below the bottom of the CIL and hence well‐removed from direct surface replenishment.

Phytoplankton growth and microzooplankton grazing in the subarctic Pacific Ocean and the Bering Sea during summer 1999

Phytoplankton growth and microzooplankton grazing rates were measured by the dilution technique in the subarctic North Pacific Ocean along a west–east transect during summer 1999. Average phytoplankton growth rates without added nutrients ( μ 0) were 0.33, 0.41, 0.20 and 0.49 d −1 for the four regions sampled: the Western Gyre, the Bering Sea, the Gulf of Alaska gyre and stations along the Aleutian Trench. Average grazing mortality rates ( m) were 0.34, 0.27, 0.20 and 0.49 d −1. Limitation of phytoplankton growth by macronutrients, such as NO 3 and SiO 2, was identified only at a few stations, with overall μ 0/ μ n ( μ n is nutrient-enhanced growth rate) averaging 0.9. Phytoplankton growth and microzooplankton grazing were approximately balanced, as indicated by high m/ μ 0 ratio, except in the Bering Sea, where the m/ μ 0 ratio was 0.65, indicating the relative importance of the diatom-macrozooplankton grazing food chain and possible higher export flux to the deep layer. Flow cytometric analysis revealed that the growth rates of picoplankton ( Synechococcus and picoeukaryotes) were usually much lower than the total phytoplankton community growth rates estimated from chlorophyll a, except for stations in the Gulf of Alaska Gyre, where the growth rates for different populations were about the same. Lower than community-average growth rate for picoplankton indicates larger phytoplankters, presumably diatoms, were growing at a much faster rate. Suppressed phytoplankton growth in the Gulf of Alaska was probably a result of iron limitation.

Biomass, Abundance, and Vertical Distribution of Micronekton and Large Gelatinous Zooplankton in the Subarctic Pacific and the Bering Sea during the Summer of 1997

The biomass, abundance, and vertical distribution of micronekton, including enidarians, mysids, euphausiids, decapods, thaliaceans, and fishes, were studied on the basis of samples collected with an 8-m2 opening-closing rectangular midwater trawl (RMT-8, mesh size: 4.5 mm) at three stations in the subarctic Pacific (the western subarctic gyre, the central Subarctic, and the Gulf of Alaska) and one station in the oceanic Bering Sea. The total biomass in the 0–1000 m water column ranged from 2.9 to 5.1 gDW m−2. Except for primary consumers that showed highly variable biomass (thaliaceans and euphausiids), biomass was highest in the oceanic Bering Sea followed by the central (boundary between eastern and western gyres), western gyre, and eastern Gulf of Alaska. The biomass compositions by higher taxa were basically similar between regions: fishes were most dominant, followed by enidarians at all stations, except for the marked predominance of thaliaceans in the Gulf of Alaska. High biomasses of gelatinous animals (31% of overall dry weight), occasionally comparable to those of fishes and crustaceans, suggest their potential importance in the subarctic Pacific. Characteristics in vertical patterns of micronekton biomass common in all stations were: (1) a mesopelagic peak around 500–600 m both day and night, (2) a layer of low biomass in the cold intermediate water and/or in the upper mesopelagic zone, (3) a nighttime shift of biomass to upper layers, and (4) an highly variable biomass of epipelagic/interzonal migrants (euphausiids and thaliaceans).

Sea level variations and their dependency on meteorological and hydrological forcing: Analysis of altimeter and surface data for the Black Sea

TOPEX/Poseidon (T/P) altimeter data in the Black Sea are analyzed for almost 5 years in parallel with available hydrological and meteorological data with the aim of studying the water balance and the dependency of sea level oscillations on meteorological and hydrological forcing. This forcing induces seasonal variations of mean sea level with oscillations of ∼10–15 cm. The consistency between satellite and tidal gauge data is demonstrated in several coastal locations, and a mean ascending trend of ∼3 cm yr−1 is found in the two data sets. The variability in all components of water balance, including the Bosphorus outflow calculated as the difference between the fresh water flux and the time rate of sea level change estimated from altimeter data, is analyzed. The T/P data give very clear signals in the patterns of amplitudes of oscillations at intraannual, seasonal, and interannual timescales that help in understanding the variability of circulation. The intraannual variations are well pronounced on the continental slope and shelf and reach highest amplitudes in the areas of Sevastopol and Batumi quasi‐permanent eddies. The clearest representation of oscillations with seasonal periodicity exists in the area of Batumi Eddy. This variability is associated with the transition between states with intense cyclonic circulation in winter and weaker (sometimes anticyclonic) circulation in summer‐fall period. The Sevastopol Eddy is not clearly resolved in the seasonal variability. The interannual variability has the strongest signature in the area of western gyre and southeastern Black Sea. The analysis of satellite data supports some earlier studies on the circulation based on dynamic computations and numerical modeling. They make it possible to estimate the amount of water exchanged between coastal and open ocean areas caused by the time variability in the Ekman drift. The good quality of altimeter data and the high level of signals could ensure more accurate numerical simulations by means of data assimilation.

Mesozooplankton in the eastern and western subarctic Pacific: community structure, seasonal life histories, and interannual variability

The zooplankton community of the subarctic Pacific is relatively simple, and contains a similar set of major species in all deep water areas of the subarctic Pacific. Their role in the food web varies considerably between coastal and offshore locations. In the oceanic gyres, microzooplankton and other mesozooplankton taxa replace phytoplankton as the primary food source for the dominant mesozooplankton species. Micronekton and larger zooplankton probably replace pelagic fish as major direct predators. Productivity and upper ocean biomass concentrations are intensely seasonal, in part because of seasonality of the physical environment and food supply, but also because of life history patterns involving seasonal vertical migrations (400–2000 m range) and winter dormancy. During the spring–summer season of upper ocean growth, small scale horizontal and vertical patchiness is intense. This can create local zones of high prey availability for predators such as planktivorous fish, birds, and marine mammals. On average, the cores of the subarctic gyres have lower biomass and productivity than the margins of the gyres. There is also some evidence that the Western Gyre is more productive than the Alaska Gyre, but more research is needed to confirm whether this east–west gradient is permanent. There is increasing evidence that the pattern of zooplankton productivity is changing over time, probably in response to interdecadal ocean climate variability. These changes include 2–3 fold shifts in total biomass, 30–60 day shifts in seasonal timing, and 10–25% changes in average body length.