Surface Water Masses Research Articles

Terrigenous Sedimentation on the Submarine Shirshov Ridge (Bering Sea) during the Last Deglaciation

The submarine Shirshov Ridge is an independent system of terrigenous sedimentation, which is geomorphologically isolated from bottom terrigenous influx into the deep-water basin of the Bering Sea. Using the ridge as example, we studied background hemipelagic sedimentation of the finely dispersed terrigenous suspended material from water column and deposition of the coarser grained ice-rafted material in the western part of the deep-water basin. The grain-size and mineral composition of postglacial sediments of the Shirshov Ridge was studied in cores SO201-2-85KL and SO201-2-77KL taken from local basins in the central and southern parts of the ridge, respectively. Statistic treatment of uninterrupted grain-size distributions (GD) of terrigenous component of the postglacial sediments by end-member (EM) modelling revealed that the grain-size distributions of terrigenous sediments from two cores are determined by the mixing of three EMs. EM-1 and EM-2 reflect the hemipelagic sedimentation with and without bottom currents influence respectively, while EM-3 with mode at fine-grained sand characterizes GD of the ice-rafted material. Reconstructed mechanisms of terrigenous influx on the Shirshov Ridge involve advection of the suspended matter with surface and intermediate water masses and ice-rafting. The relative role of both mechanisms of the terrigenous sedimentation under the influence of varying bottom current velocities for intervals of Last Glacial Maximum, early deglaciation, Heinrich event 1, Bolling–Allerod, Younger Dryas, and Early Holocene is estimated. It is ascertained that the grain-size distribution of terrigenous component is defined by climate variations, sea ice coverage, sea ice drift pathways, conditions of fast ice melting, and mobility of bottom waters. High concentrations of drifting ice or seasonal sea ice cover likely existed above the southern part of the ridge during the second half of the Heinrich 1 event. The low mobility of bottom waters facilitated only the subice hemipelagic sedimentation of fine fractions from the background reserve of suspended material. A sharp reduction of ice-rafted flux was reconstructed for the Bolling–Allerod warming interval. Bottom currents affected sedimentation in the central part of the ridge during the entire deglaciation (in addition to the second half of the Heinrich 1 event), and in the southern part during the Bolling–Allerod, Younger Dryas, and Early Holocene.

Glider data collected during the Algerian Basin Circulation Unmanned Survey

Abstract. We present data collected in the framework of the Algerian BAsin Circulation Unmanned Survey (ABACUS) project. The main objective of ABACUS is the monitoring of the basin circulation and of the surface and intermediate water masses' physical and biological properties in a key region of the Mediterranean Sea circulation. Data presented here have been collected through deep glider cruises in the Western Mediterranean Sea during the autumns of 2014, 2015 and 2016; activities at sea are expected to be repeated during the coming years, so that the dataset will be extended. Glider missions were conducted in the Algerian Basin, between the island of Mallorca and the Algerian coast. Across the three glider missions, eight repeated transects were completed which enabled us to investigate the basin-scale circulation and the presence of mesoscale structures, utilising both the adaptive sampling capabilities of the gliders and the higher resolution of the data. After collection, all data passed a quality control procedure and were then made available through an unrestricted repository host by the SOCIB Data Centre at https://doi.org/10.25704/b200-3vf5. The actual dataset spans three autumn seasons, providing an important contribution to the data collection in the chronically undersampled Algerian Basin. Temperature and salinity data collected in the first 975 m of the water column allowed us to identify the main water masses and describe their characteristics, improving the understanding of the dynamics of the region. On the timescale of the project, data show a large variability in the surface layer and reduced variability in the intermediate and deep layers. Our measurements have been successfully compared to data previously collected in the area from 1909 to 2011. Results showed similar overall distribution, ranges and variability to the historical data, with no outliers in the surface or deep layers.

Chemotaxonomy-based mapping of phytoplankton communities in the subtropical Southwestern Atlantic Ocean, with emphasis on the marine cyanobacterium Trichodesmium

The composition and distribution of the phytoplankton community on the southweastern Brazilian continental shelf and continental slope were evaluated through chemotaxonomic analyses (HPLC-CHEMTAX). The samples were collected during five oceanographic cruises from 2012 to 2014 in spring and autumn. Fourteen surface blooms of Trichodesmium (slicks) were registered during the field surveys. The Trichodesmium biomass was estimated by both pigment analyses and microscope counts, with significant correlation found between the methods, except in some slicks. However, myxoxanthophyll, a Trichodesmium pigment biomarker generally used in CHEMTAX, was detectable only in seven samples from stations with Trichodesmium slicks, indicating that their use in pigment chemotaxonomic approaches should be taken with caution, because it could lead to errors in the estimation of Trichodesmium biomass. Four water masses were identified at the surface: Plata Plume Water (PPW), Subtropical Shelf Water (STSW), Shelf Water (SW) and Tropical Water (TW). The identified water masses seem to be the main driving factor influencing the composition, distribution and biomass of phytoplankton groups in the study region. The salty-warm oligotrophic TW, associated with the Brazil Current, was the dominant surface water mass in both autumn and spring. The surface waters in this work were generally associated with low values of both total chlorophyll a (TChl a) and nutrient concentration. In no slicks conditions, surface TChl a concentration was low ranging between 0.04 and 2.44 mg m−3 (mean 0.27 mg m−3) in almost all oceanographic stations. The highest biomass concentrations were recorded in stations under influence of both PPW and STSW. Overall, Prochlorococcus, haptophytes, Synechococcus, and Trichodesmium dominated the phytoplankton community and their contributions were above 70% to total Chl a in both spring and autumn. The other phytoplankton groups (prasinophytes, cryptophytes and dinoflagellates) represented a minor fraction, with values below 15% of the total biomass. A Canonical Correspondence Analysis showed that Prochlorococcus, haptophytes, Synechococcus and Trichodesmium were related with both TW and SW and were also positively associated with weaker water column stability, a deeper mixed layer, temperature and salinity. In turn, cryptophytes, dinoflagellates, prasynophytes and diatoms, mainly associated with STSW and PPW, were positively correlated with higher stability and nutrients (nitrogen and phosphorus), and negatively associated with both temperature and salinity. Our results showed that the pigment analysis (HPLC-CHEMTAX) allowed a detailed mapping of phytoplankton communities' distribution in the south and southeastern Brazilian continental shelf and shelf-break, and that the Trichodesmium biomass, as currently estimated by this approach, should be carefully revised.

Infaunal and epifaunal benthic foraminifera species as proxies of organic matter paleofluxes in the Pelotas Basin, south-western Atlantic Ocean

Abstract One late Quaternary sediment core from the continental slope of the northern sector of Pelotas Basin (southern Brazilian continental margin, 2091 m water depth) was analyzed in order to evaluate the influence of the organic matter influx in the benthic foraminiferal assemblage composition on a regional scale. This study investigated the relationship between epifaunal species (Epistominella exigua and Alabaminella weddellensis) and infaunal species (Uvigerina peregrina and Angulogerina angulosa) integrated in a relative abundance based index, the “Organic Input Index” (OrgIn). The correlation of these species with abiotic data is also discussed, including mud content, total organic carbon (TOC) and carbon stable isotope (δ13C) in Uvigerina peregrina. The age model based on one radiocarbon dating of planktonic foraminifera and oxygen isotope composition of benthic foraminifera (Uvigerina) revealed the record spans the last 112,500 yr, covering the Marine Isotope Stages (MIS) 5 to 3. The dominance of infaunal species, mainly Globocassidulina subglobosa, boliviniids and buliminiids throughout the entire core is interpreted as an indicator of an increase of organic matter supply and dominance of the eutrophic conditions in this study area. The variations found in the proportion of the epifaunal and infaunal species indicate distinctive inputs of organic matter (phytodetritus input versus bacterial activity, respectively). An increasing trend in the importance of epifaunal species from the base of the core to uppermost samples (MIS 5 to MIS 3) was also observed, which is assumed to be evidence of the higher productivity of the surface water masses in this sector of the South Atlantic during the glacial periods.

SPATIAL DISTRIBUTION OF HYDROLOGICAL ELEMENTS CHARACTERISTIC IN THE MIDDLE PART OF SUKHOY LIMAN IN THE SUMMER TIME

Problem Statement and Purpose.The middle part of the Sukhoy Liman represents a perspective reservoir for development of the port of Ilyichevsk (Black Sea). Water- hydrology of a middle part Sukhoy Liman was not systematically studied now. The most studied there was a salinity.Article’s purpose is revealing spatial distribution’s features of hydrological water’s elements an average part of the Sukhoy Liman under the influence by natural and anthropogenous factors dining the summer time.Data&Methods. There were presented the results of carneral processing of field materials obtained dining expeditionary sruveys in the water area and in the valley of the mid-range of the Sukhoy Liman in 2016. Hydrological characteristics of water were studied in August 2016.In the water area of 1311000 m2, complex hydrological works were performed at 48 stations. Water upper layer temperature was measured with a water thermometer with the accuracy to 0,2 °C, the transparency was determined with a Sekki disk, the salinity was determined by the hydrometric method using water samples taken with the help of a bottle bathometer.Results. Transparency of water in the upper reaches of the Sukhoy Liman is determined by the relief of the bottom, the presence of aquatic vegetation, the hydrological regime of the liman, the absence or weak development of abrasion and erosion processes on the banks and bottom of the liman. Transparency varies from 0,8 m to 1,7 rn. The average transparency value is 1,5 m. Temperature of the water upper layer is changing from 23,6° C to 25,6° C. Average temperature is 24,6° C. The salinity of the surface water masses of the Sukhoy liman mid-range is determined by the hydr ological processes in the upper reaches and by the inflow / outflow of water masses into the Sukhoy liman during the drifts and surges. The minimum salinity is 13,6 %o, the maximum is 15,6 %o, and the average is 14,4%o.

Tracing surface water mixing and groundwater inputs using chemical and isotope fingerprints (δ18O-δ2H, 87Sr/86Sr) at basin scale: The Loire River (France)

The spatial and temporal distributions of major elements, stable isotopes of water molecule and strontium isotope ratios were investigated in the surface waters of the Loire River basin. The present study, using a coupled hydrological and geochemical approach, focuses on surface water mixing and the interactions between ground- and surface water. Stable isotopes (δ18O, δ2H) provide good evidence for the complex hydrological behavior of the Loire River system at the basin scale, and if binary mixing of surface water masses explain the longitudinal evolution of the Loire signature in the upstream part of the basin, groundwater contributions are required to explain locally the Loire signature. The water chemistry in the different zones of the basin shows large variations in major-element contents. The NO3 content in the Loire River roughly increases from up-to downstream and is mainly controlled by the input from tributaries. The highest concentrations are observed in the middle part of the Loire basin, in close relation with the diffuse agricultural sources. In terms of water-rock interaction, using Na as the reference element, the Loire Basin data are scattered between the three main end-members representing the major lithologies i.e. basalts, granite/gneiss and carbonate. Strontium-isotope ratios measured in water range from 0.70691 to 0.71395 and plotted vs. the Ca/Na ratios, the 87Sr/86Sr ratios clearly discriminate the three main lithological endmembers. In the Middle Loire section, the Loire flow rate increases without significant inputs of surface water, previous studies attributed this increase to groundwater inputs. The relationship between 87Sr/86Sr and the Cl/Sr ratios clearly shows the groundwater inputs from the Beauce carbonate aquifer along a 90 km river profile, between Orléans and Amboise. These conservative tracers can be used to calculate up to 20% groundwater mixing during low flow period.

Phytoplankton response to a weak El Niño event

The western coast of South America is a very productive upwelling zone. However, during El Niño events, phytoplankton productivity in the equatorial Pacific declines sharply. A weak El Niño event occurred in 2014. Oceanographic anomalies included an increase in sea surface temperature and water transparency and a decrease in salinity. Phytoplankton composition also changed throughout time reflecting different El Niño stages. Before the El Niño, the community was mainly dominated by species typical of nutrient rich waters, such as large diatoms or silicoflagellates. At the onset of the event, the number of species and the turnover component of beta diversity increased, indicating that previously predominant species now co-occurred with new species transported by warmer surface water masses. At this initial stage, warm water dinoflagellates were indicator species, while a diatom bloom of Dactyliosolen fragilissimus indicated nutrient availability and some degree of turbulence in the water column. Finally, the overall abundance of phytoplankton was similar to the homologous months in previous years, but under El Niño conditions the community was dominated by smaller cells. Consequently, primary production attained a minimum, as inferred from the highest values of water transparency. There was no evidence of seasonality. It is concluded that the effect of El Niño on the phytoplankton community in the eastern equatorial Pacific is appreciable even during weak events.

The encoding of wind forcing into the Pacific-Arctic pressure head, Chukchi Sea ice retreat and late-summer Barrow Canyon water masses

Barrow Canyon, which incises the northeast corner of the Chukchi Sea shelf, is a major conduit through which Pacific-origin waters carrying nutrients, biota, freshwater, and heat enter the Arctic Ocean. As such, Barrow Canyon was adopted as a long-term monitoring site for the Distributed Biological Observatory (DBO) in 2010. However, annual hydrographic surveys across Barrow Canyon, conducted during late August 2005–2015 along a location near what is the Barrow Canyon DBO survey line and in support of other research programs, extend and complement the DBO hydrographic record. These complementary hydrographic surveys show that volumes of Pacific-origin and melt water masses in Barrow Canyon are significantly-correlated with daily sea ice areas in the eastern Chukchi Sea for most of the May-August ice retreat season. Year-to-year differences in the timing and pattern of sea ice retreat across the Chukchi Sea are also shown to be well-correlated with changes in seasonally-averaged regional winds particularly as defined by the strength and longitudinal location of the Beaufort Sea High pressure cell. These interdependent wind-ice retreat-water mass relationships are largely predicated on wind stress curl-driven changes to sea level in the East Siberian Sea/northwestern Chukchi Sea. Statistically-significant correlations among wind-forced sea surface heights, ice areas, and water mass volumes suggest that, during the ice retreat season, the East Siberian Sea/western Chukchi Sea region serves as the Arctic terminus for the Pacific-Arctic pressure head.

Indo‐Atlantic Exchange, Mesoscale Dynamics, and Antarctic Intermediate Water

AbstractThis study evaluates the capability of eddy‐permitting regional ocean models to reproduce the interocean exchange south of Africa. In this highly turbulent region, we show that the vertical structure of the horizontal flows need to be appropriately resolved to realistically advect thermocline water masses into the South Atlantic. Our results point out that a grid‐spacing of 1/24° on the horizontal and 50 m on the vertical homogeneously distributed are required to account for a correct transport of surface and subsurface water masses properties and their in‐route transformation by mixing. Preliminary Lagrangian analyses highlight the primary role of the upper‐ocean mesoscale eddies on water masses transport and fate, with a particular emphasis on Antarctic Intermediate Waters (AAIWs) dynamics and characteristics. We evaluate the numerical results against observations (AVISO data and Argo floats profiles). Modeled and observed eddies were examined in number, polarity, size, trajectory, and for their contribution to AAIW properties. A clear asymmetry, in number and radius, emerges between cyclones and anticyclones. The high‐resolution simulation was the most energetic, with more abundant and smaller structures than those detected in AVISO. However, eddy statistics compare reasonably well in terms of mean pathways when restricted to Agulhas Rings, which are on average quasi‐Gaussian in shape. Regionally, the Ertel potential vorticity anomaly is marked at the surface by a temporal variability with winter intensification, directly reflected in the seasonal cycle of the eddies number. Noting the growth of the baroclinic Rossby radius in winter, this suggests baroclinic processes as essential for these eddies generation.

Responses of the picophytoplankton community to temperature fronts in the northeastern Arabian Sea during the northeast monsoon

We investigated the responses of the picophytoplankton (< 3 µm) community to a temperature filament and front through high resolution spatial (~ 1 NM) sampling (November-23 to December-11, 2012) in the northeastern Arabian Sea (69°E, 18.85°N to 20.25°N). Samples were collected at discrete depths within the 100 m water column. Synechococcus dominated the picophytoplankton community numerically and in terms of biomass along the entire transect. To investigate the patterns of variability in picophytoplankton distribution, depending on the water mass characteristics, the entire transect was divided into four zones (1) south of filament (SFIL) with warm oligotrophic waters, (2) filament (FIL) with cooler and low saline waters, (3) north of filament (NFIL) with relatively cooler waters than the SFIL and (4) front (FRO) with relatively cooler and less saline waters than the FIL. Depth-integrated abundance and biomass of Synechococcus were relatively higher within the FIL and FRO whereas Prochlorococcus and picoeukaryotes were abundant in SFIL and NFIL. Redundancy analysis of environmental variables and picophytoplankton abundance showed that lower saline water mass within the mesoscale features harbored relatively higher Synechococcus abundance and biomass. Two Synechococcus ecotypes were distinguished based on the fluorescence intensity of the accessory pigment, phycoerythrin; the one with higher intensity (open ocean ecotype) dominating in the FIL and the other with lower intensity in the FRO (coastal ecotype). The relatively lower saline surface water mass at the FRO, probably a result of coastal advection, could have introduced the latter ecotype. Vertically, a positive correlation of Prochlorococcus with nutrients and Synechococcus with temperature corroborates their higher and lower abundance and biomass, respectively in the deeper waters. The positive correlation of Synechococcus with the total chlorophyll biomass indicates a similar response to environmental variables within the mesoscale features. This study shows that picophytoplankton contribution (16–24%) to the total phytoplankton carbon biomass in tropical mesoscale features is likely to have important consequences on the planktonic food web function.

Evolution of the Galicia Interior Basin over the last 60 ka: sedimentary processes and palaeoceanographic implications

ABSTRACTThe Galicia Interior Basin (GIB; NW Iberian Peninsula) is located near a critical transition between the subtropical (temperate) and subpolar (cold) gyres of the North Atlantic. It therefore witnesses oceanographic changes driven by global climatic events. This study reports on the recent (latest Pleistocene) sedimentary, palaeoceanographic and palaeoclimatic history of the basin. We integrated analysis of deep‐sea sediment cores retrieved from an E–W transect across the GIB. The analysis indicated three types of sedimentary processes recording glacial (Marine Isotope Stage 2–4) and deglacial events: along‐slope bottom currents (forming contourite deposits), pelagic and hemipelagic sedimentation, and gravitational dislocation. Variation in depositional patterns and sedimentation rates indicate distinctive transport (along‐slope and down‐slope) and depositional processes. These in turn reflect climatic and oceanographic drivers. We interpret changes in sea level from core evidence showing changes in sediment supply. The cores exhibited conspicuous sedimentary evidence of Heinrich events (HEs). The stratigraphic intervals associated with HEs showed significant lateral variation. We suggest that the lateral variation may result from the development of an oceanographic boundary between surface water masses with different temperature and salinity parameters or changes in surface currents which may have introduced relatively warmer water into the GIB during the last glacial period.

Land–sea coupling of early Pleistocene glacial cycles in the southern North Sea exhibit dominant Northern Hemisphere forcing

Abstract. We assess the disputed phase relations between forcing and climatic response in the early Pleistocene with a spliced Gelasian (∼2.6–1.8 Ma) multi-proxy record from the southern North Sea basin. The cored sections couple climate evolution on both land and sea during the intensification of Northern Hemisphere glaciation (NHG) in NW Europe, providing the first well-constrained stratigraphic sequence of the classic terrestrial Praetiglian stage. Terrestrial signals were derived from the Eridanos paleoriver, a major fluvial system that contributed a large amount of freshwater to the northeast Atlantic. Due to its latitudinal position, the Eridanos catchment was likely affected by early Pleistocene NHG, leading to intermittent shutdown and reactivation of river flow and sediment transport. Here we apply organic geochemistry, palynology, carbonate isotope geochemistry, and seismostratigraphy to document both vegetation changes in the Eridanos catchment and regional surface water conditions and relate them to early Pleistocene glacial–interglacial cycles and relative sea level changes. Paleomagnetic and palynological data provide a solid integrated timeframe that ties the obliquity cycles, expressed in the borehole geophysical logs, to Marine Isotope Stages (MIS) 103 to 92, independently confirmed by a local benthic oxygen isotope record. Marine and terrestrial palynological and organic geochemical records provide high-resolution reconstructions of relative terrestrial and sea surface temperature (TT and SST), vegetation, relative sea level, and coastal influence. During the prominent cold stages MIS 98 and 96, as well as 94, the record indicates increased non-arboreal vegetation, low SST and TT, and low relative sea level. During the warm stages MIS 99, 97, and 95 we infer increased stratification of the water column together with a higher percentage of arboreal vegetation, high SST, and relative sea level maxima. The early Pleistocene distinct warm–cold alterations are synchronous between land and sea, but lead the relative sea level change by 3000–8000 years. The record provides evidence for a dominantly Northern Hemisphere-driven cooling that leads the glacial buildup and varies on the obliquity timescale. Southward migration of Arctic surface water masses during glacials, indicated by cool-water dinoflagellate cyst assemblages, is furthermore relevant for the discussion on the relation between the intensity of the Atlantic meridional overturning circulation and ice sheet growth.

Hourly Observed Internal Waves by Geostationary Ocean Color Imagery in the East/Japan Sea

AbstractAs internal waves (IWs) are generated via internal tidal energy near the bottom shelf break of the Korea Strait, continuous evolutions of IWs are extremely difficult to observe using satellite observations. However, the Geostationary Ocean Color Imager (GOCI) has enabled the detection of IWs in the East/Japan Sea (EJS) on an hourly basis to investigate their propagation behavior. For an extended IW analysis, a total of seven packets were identified using GOCI chlorophyll-aconcentration (Chl-a) measurements around the Ulleung Basin and Dok Island in the EJS. The results demonstrate that the IW locations had lower Chl-avalues than non-IW-affected regions on 10, 11, 13, and 14 August 2013. The lower Chl-avalues are due to the significant wave heights of the IWs, which cause surface water masses to be dispersed. In addition, from the continuous trajectories of the waves in the hourly GOCI Chl-aimagery, quantitative physical parameters of the IWs, such as their pathway (northeasterly), phase speed (1.46–1.61 m s−1), amplitude (20.62–26.76 m), and period (12.29 h), could be obtained. Therefore, the advantage of using GOCI is the ability to detect and analyze the physical characteristics of IWs on an hourly time scale.

Temperature and isotopic composition of seawater in the epicontinental sea (Central Paratethys) during the Middle Miocene Climate Transition based on Mg/Ca, δ18O and δ13C from foraminiferal tests

Details regarding seawater temperature and isotopic composition of seawater in the Central Paratethys, during the Miocene epoch, are not well known. Here we present an insight based on results from a combination of Mg/Ca, δ18O and δ13C on foraminifera from the shallow core (LOM-1) representing the time interval of ~14.4 to 14.36Ma. Globigerinoides trilobus, Globigerina bulloides, Cibicidoides spp. and Melonis pompilioides were chosen for analysis due to their variable ecological preferences within the water column and on the sea floor. We assessed problems concerning diagenetic alteration of foraminiferal calcite by using a single test approach when analysing δ18O and δ13C. Paleotemperature ranges turned out to be comparable with modern subtropical regions with no cooling trend as was expected at this period following the Middle Miocene Climatic Optimum. However, we find an interesting offset between the benthic species temperature response that is likely related to the different life-strategies of those species. Based on a combination of mentioned proxies it was possible to partly reconstruct δ18Oseawater for the studied locality and compare it with Mg/Ca derived temperature to obtain a possible correlation between temperature and salinity. We also investigated differences in surface and bottom water hydrography in comparison with the Mediterranean, and distinguish their different hydrographies. In addition, it was possible to identify dissimilar origins of surface and bottom water masses based on comparison of their isotopic composition.

Effects of mesoscale structures on the distribution of cephalopod paralarvae in the Gulf of California and adjacent Pacific

Vertical distribution of the cephalopod paralarvae was investigated in relation to a system of two cyclonic and three anticyclonic eddies in the southern Gulf of California and a front in the adjacent Pacific Ocean. Results showed that the preferential habitat for the Sthenoteuthis oualaniensis – Dosidicus gigas “SD-complex” in both regions was the oxygenated surface mixed layer and the thermocline. The highest abundances occurred in of one of the anticyclonic eddies and a frontal zone, which are convergent structures. Enoploteuthid and Pyroteuthid paralarvae both displayed their highest abundances in the thermocline. Pyroteuthids dominated in the cyclonic eddy whereas Enoploteuthidae were less evident in the eddy system. Pyroteuthids were observed on the western (California Current) side of the frontal zone, and Enoploteuthids on its eastern (Gulf of California) side. The octopods and the complex of Ommastrephes-Eucleoteuthis-Hyaloteuthis paralarvae were present below the thermocline. Both groups had a scarce presence in the eddy system and high abundance near the frontal zone. The octopods abounded on the eastern side in association with the low dissolved oxygen concentrations (< 44µmolkg−1) of Subtropical-Subsurface Water; the complex on the western front side was immersed in California Current Water. It may be concluded that the spawning and early stages of development of these cephalopod groups are associated with particular mesoscale structures of the water masses. For example, the “SD complex” inhabits the surface water masses, preferentially in convergence zones generated by mesoscale activity.

Transport and transformation of surface water masses across the Mascarene Plateau during the Northeast Monsoon season

The Mascarene Plateau lies in the south-west Indian Ocean between the islands of Mauritius and the Seychelles Bank, and is characterised by a series of shallow banks separated by deep (>1 000 m), narrow channels. The plateau acts as an obstruction to the general ocean circulation in this region, separating the westward-flowing South Equatorial Current (SEC) into two branches downstream of the plateau. In this article, we present the results of a survey conducted along the entire Mascarene Plateau during the Northeast Monsoon, in October–November 2008. In addition, data from Argo floats were used to determine the origin of water masses entering this region. The plateau contains three gaps through which branches of the SEC are channelled. The northern, central and southern gaps receive 14.93 Sv, 14.41 Sv and 6.19 Sv, respectively. Although there are differences in water-mass properties to the west and east of the Mascarene Plateau due to mixing, the SEC acts as a sharp boundary between water masses of southern and northern Indian Ocean origin. Mixing occurs in the central gap between intermediate water masses (Red Sea Water [RSW] and Antarctic Intermediate Water [AAIW]) as well as in the upper waters (Subtropical Surface Water [STSW] and Indonesian Throughflow Water [ITW]). Through the northern gap, mixing occurs between Arabian Sea High-Salinity Water (ASHSW), ITW and Tropical Surface Water (TSW), while through the southern gap, mixing occurs between STSW and ITW. North Indian Deep Water (NIDW) is present in the region but the plateau appears to have no effect on it.

Effects of the Blob on settlement of spotted sand bass, Paralabrax maculatofasciatus, to Mission Bay, San Diego, CA.

The West Coast of the United States experienced variable and sometimes highly unusual oceanographic conditions between 2012 and 2015. In particular, a warm mass of surface water known as the Pacific Warm Anomaly (popularly as “The Blob”) impinged on southern California in 2014, and warm-water conditions remained during the 2015 El Niño. We examine how this oceanographic variability affected delivery and individual characteristics of larval spotted sand bass (Paralabrax maculatofasciatus) to an estuarine nursery habitat in southern California. To quantify P. maculatofasciatus settlement patterns, three larval collectors were installed near the mouth of Mission Bay, San Diego CA, and retrieved weekly from June–October of 2012–2015. During ‘Blob‘ conditions in 2014 and 2015, lower settlement rates of spotted sand bass were associated with higher sea surface temperature and lower wind speed, chlorophyll a (chl a) and upwelling. Overall, the number of settlers per day peaked at intermediate chl a values across weeks. Individual characteristics of larvae that settled in 2014–2015 were consistent with a poor feeding environment. Although settlers were longer in length in 2014–15, fish in these years had slower larval otolith growth, a longer larval duration, and a trend towards lower condition, traits that are often associated with lower survival and recruitment. This study suggests that future settlement and recruitment of P. maculatofasciatus and other fishes with similar life histories may be adversely affected in southern California if ocean temperatures continue to rise in the face of climate change.

Evidence of Diel Vertical Migration of Mesopelagic Sound-Scattering Organisms in the Arctic

While sound scattering layers have been described previously from ice-covered waters in the Arctic, the existence of a viable mesopelagic community that also includes mesopelagic fishes in the Arctic has been questioned. In addition, it has been hypothesized that vertical migration would hardly exist in these areas. We wanted to check if deep scattering layers (DSLs) was found to the west and north of Svalbard (79°30’N - 82°10’N) during autumn 2015, and if present; whether organisms in such DSLs undertook vertical migrations. Our null hypothesis was that there would be no evidence of diel vertical migration. Multi-frequency acoustic observations by hull mounted echosounders (18, 38 and 120 kHz) revealed a DSL at depths ~210-510 m in areas with bottom depths exceeding ~600 m. Investigating eight geographical locations that differed with respect to time periods, light cycle and sea ice conditions, we show that the deeper layer of DSL displayed a clear ascending movement during night time and a descending movement during daytime. The high-light weighted mean depth (343-514 m) with respect to backscattered energy was statistically deeper than the low-light weighted mean depth (179-437 m) for the locations studied. This behavior of the DSL was found to be consistent both when the sun was continuously above the horizon and after it started to set on 1 September, and both in open water and sea ice covered waters. The weighted mean depth showed an increasing trend, while the nautical area backscattering strength from the DSL showed a decreasing trend from south to north among the studied locations. Hydrographic observations revealed that the diel migration was found in the lower part of the north-flowing Atlantic Water, and was disconnected from the surface water masses above the Atlantic Water during day and night. The organisms conducting vertical migrations were studied by vertical and oblique hauls with zooplankton nets and pelagic trawls. These data suggest that these organisms were mainly various mesopelagic fishes, some few larger fishes, large zooplankton like krill and amphipods, and various gelatinous forms.

Hidrografía de la entrada al Golfo de California: La frontera superior del mínimo de oxígeno

We used historical data to describe the hydrograph variability at the entrance to the Gulf of California, particularly of the limit with the depth of the oxygen minimum. At the entrance to the Gulf of California three surface waters converge: California Current, Tropical Surface and Gulf of California water masses. The historical data includes observations of 10 cruises between 1991 and 2011. The Gulf of California water mass does not appear to have a significant presence at the entrance, we only found it at the northern end of the entrance. The water of the California Current is typically found at the surface but in this region it is often at about 50 m depth, below the Surface Tropical Water mass. In contrast, the Subtropical Subsurface Water is often very close to the surface and because of this, the oxygen minimum is also shallower. The confluence of water masses at the entrance of the gulf results in changes in vertical composition between oceanic and coastal regions. In the oceanic region we find two water masses (Tropical Superficial and California Current) above the oxygen minimum. Whereas near the coastal the oxygen minimum occurs immediately below the Tropical Surface water mass. However, the stratification here is more intense than in the open ocean hindering the mixing of low oxygen values ​​with the surface. At smaller scales, the density surfaces limiting with the layer of oxygen minimum are influenced by the mesoscale structures in the region. They deepen/rise below the hot/cold-core eddies and rise close to the surface below coastal currents.

Regional variations in early life stages response to a temperature gradient in the northern shrimp Pandalus borealis and vulnerability of the populations to ocean warming

In order to define the relative vulnerability of northern shrimp (Pandalus borealis) populations to the ongoing global warming, we compared the thermal performance curves for survival and growth in the first three pelagic larval stages from three populations of the Northwest Atlantic. Egg carrying females were obtained from different regions characterized by distinct sea surface temperature (SST) conditions for larval development in spring. Two independent experiments were conducted in two different years. In spring 2012, larvae from females captured in the Lower St Lawrence Estuary (LE) and in the Northeast Gulf of St Lawrence (GSL) were compared. In spring 2014, larvae from females captured in the LE and on the Labrador–Newfoundland Shelf (Northwest Atlantic, NWA) were used. The LE larvae were used both years and served as the reference population for comparisons. In 2012 and 2014, groups of 25 newly hatched northern shrimp larvae from each source population were incubated at six temperatures (0, 3, 6, 9, 12, and 15°C) to monitor and compare survival and growth at moult. Northern shrimp larvae from the LE (warmer May–June SST) had a higher optimal temperature range for survival compared to larvae from the GSL and the NWA (colder May–June SST) populations. However, in 2012 growth performance at moult was reduced at higher temperatures for the LE population compared to the GSL population. The differences in thermal performance curves observed may suggest the presence of a certain level of local adaptation in response to the different regional SST regimes in spring–early summer. Northern shrimp larvae in the Northeast Gulf of St Lawrence and Northwest Atlantic shelf could benefit from warmer early-spring temperatures; however, larvae from the Lower Estuary may be closer to their upper tolerance limits and thus more likely at risk of negative impact of future warming of surface water masses.