Salinity Stratification Research Articles

Hydrodynamic characteristics of lateral withdrawal in a tidal river channel with saltwater intrusion

Saltwater intrusions occur widely in tidal river channels, significantly changing the vertical distribution of mainstream velocity and density (salinity). They are vital to the hydrodynamic conditions of lateral withdrawal. In this study, a 3-D numerical model was constructed and validated through experimental tests, and the hydrodynamic characteristics of lateral withdrawal under saltwater intrusion were investigated. Additionally, the vertical distribution effects of mainstream velocity, withdrawal velocity, and density stratification were analyzed. Under non-uniform mainstream velocity conditions, with larger surface velocity than bottom velocity, the division width of the upper layer is smaller, while the local secondary circulation is stronger than that of the lower layer. Secondary circulation is top-shaped and weakens as the withdrawal velocity increases. When salinity stratification exists, the division width of the lower layer decreases, and the secondary circulation is stronger than that of the upper layer; meanwhile, the vertical water exchange increases. The influence of mainstream velocity is greater than that of salinity stratification. Under these two factors, the sediment entering the intake increases, and submerged vane, sill, and weir measures should be adopted. This research represents an advance in lateral withdrawal hydrodynamic research and provides support for engineering design in estuarine areas.

Influence of ocean salinity stratification on the tropical Atlantic Ocean surface

The tropical Atlantic Ocean receives an important freshwater supply from river runoff and from precipitation in the intertropical convergence zone. It results in a strong salinity stratification that may influence vertical mixing, and thus sea surface temperature (SST) and air–sea fluxes. The aim of this study is to assess the impact of salinity stratification on the tropical Atlantic surface variables. This is achieved through comparison among regional 1/4 $$^{\circ }$$ coupled ocean–atmosphere simulations for which the contribution of salinity stratification in the vertical mixing scheme is included or discarded. The analysis reveals that the strong salinity stratification in the northwestern tropical Atlantic induces a significant increase of SST (0.2 $$^{\circ }$$ C–0.5 $$^{\circ }$$ C) and rainfall (+ 19%) in summer, hereby intensifying the ocean–atmosphere water cycle, despite a negative atmospheric feedback. Indeed, the atmosphere dampens the oceanic response through an increase in latent heat loss and a reduction of shortwave radiation reaching the ocean surface. In winter, the impacts of salinity stratification are much weaker, most probably because of a deeper mixed layer at this time. In the equatorial region, we found that salinity stratification induces a year-round shoaling of the thermocline, reinforcing the cold tongue cool anomaly in summer. The concept of barrier layer has not been identified as relevant to explain the SST response to salinity stratification in our region of interest.

Upper Ocean Stratification in the Eastern Pacific During the SPURS‐2 Field Campaign

AbstractThe region between a shallow mixed layer and a deep isothermal layer resulting from salinity stratification is called a barrier layer (BL). Since BLs hinder the surface heat and momentum exchange with the ocean subsurface, they play an important role in air‐sea interaction. Synoptic features and formation of BLs and associated temperature inversions (TIs) in the Eastern Pacific Fresh Pool (EPFP) were investigated using shipboard observations. BLs and TIs were found in the high‐precipitation EPFP, a frontal transition zone (FTZ) and in the sea surface salinity (SSS) front during a 2016 boreal summer cruise. During a 2017 boreal autumn cruise, BLs and TIs were found from the southern part of the EPFP to the SSS front. The BLs in the SSS front and the FTZ were meridionally displaced from the precipitation maximum during both the 2016 and 2017 cruises. Freshening and cooling by tilting of both the isohalines and isotherms of the SSS front occurred via Ekman and geostrophic flow during both cruises, and also within the EPFP during 2016. The SSS front was a dominant contributor to the density gradient in the horizontal plane. A high dissolved oxygen content observed within BLs confirmed that the surface flow plays an important role in the BL and TI formation through the tilting process. Patchy distribution of freshening and cooling by tilting implies an intermittent BL and TI formation with short time scales.

Importance of salinity-induced stratification on flocculation in tidal estuaries

Flocculation of suspended particles in tidal estuaries exhibits large spatiotemporal variability due to an interplay of various physical and biogeochemical drivers. Salinity (S) is known to promote flocculation of fine-grained suspended particulate matter (SPM). However, the influence of salinity and salinity-induced stratification on flocculation has not been sufficiently investigated yet. This study aims to understand how these two factors, interactively with turbulent shear (G) and SPM concentration (C), control the vertical variation of floc size and flocculation process in different depth layers in a typical tide-dominated estuarine environment. Analysis of field observation data shows that flocculi (diameter < 20 μm) are mainly affected by C and originate primarily from local resuspension. Macroflocs (> 200 μm) are mainly controlled by stratification that greatly improves aggregate collision efficiency; Microflocs (20–200 μm), as a transition group between flocculi and macroflocs, are affected by dynamics of both sides. They are influenced jointly by C, G and stratification. Besides, the fresh water-dominated surface layer is dominated by small particles (flocculi and microflocs), confined in a relatively narrow particle size range between Ο (100) and Ο (101) as a result of the low level of both C (13–20 mg/L) and S (< 2 practical salinity units). Below the surface layer, floc size increases drastically along with an increased salinity-induced density gradient and achieves maximum particle size (Ο (102)) within the stratified layer. Because of its high efficiency in promoting flocculation and formation of macroflocs, the stratified layer around the halocline can be regarded as an optimal flocculation zone. The benthic layer is characterized by high C (> 30 mg/L), gentle G (~5/s), and periodic stratification, which result in a wide size range between Ο (101) and Ο (102) with microflocs as the dominant group. Finally, we found that the accuracy of flocculation modeling can be significantly improved by integrating a simple relationship between particle collision and stratification.

Source rock potential, crude oil characteristics and oil-to-source rock correlation in a Central Paratethys sub-basin, the Hungarian Palaeogene Basin (Pannonian basin)

Eocene and Lower Oligocene rocks are potential source rocks for crude oil accumulations in the Hungarian Palaeogene Basin. To enhance the understanding of the hydrocarbon system, this study (i) assesses the petroleum potential of Palaeogene formations, (ii) characterises the source rock facies of the accumulated oils, and (iii) provides an oil-to-source correlation. Rock-Eval data of samples from three boreholes (W–1, W–9 and W–12) show that most Palaeogene formations are mature at depths exceeding 2.1–2.5 km. The coal-bearing Kosd Formation includes good to excellent gas- (and oil-) prone source rocks. The overlying Buda Marl Formation is typically organic matter-lean but contains oil-prone rocks with up to 2.3 wt% TOC and a fair petroleum potential in borehole W–9. The Tard Clay Formation in W–12 reaches up to 1.9 wt% TOC and shows HI values up to 440 mg HC/g TOC, characterising the deposits as good petroleum source rocks. Based on low TOC contents, the Kiscell Clay Formation is not considered a source rock. Molecular parameters of 12 crude oil samples indicate a shaly source rock deposited in a marine/brackish environment. Salinity stratification, causing the development of oxygen-depleted conditions, is likely. The organic matter is dominated by aquatic biomass, including algae, dinoflagellates and chemoautotrophic bacteria. Minor angiosperm-dominated organic matter was transported into the basin from the shoreline. Specific V-shaped compound-specific carbon isotope patterns of n-alkanes observed in crude oils and extracts from the Tard Clay prove the dominant source rock. Minor differences between biomarker ratios are related to vertical and lateral facies variations within the Tard Clay Formation. The accumulated oils are slightly more mature than the Tard Clay in borehole W–12.

Sea Surface Salinity Response to Tropical Cyclones Based on Satellite Observations

Multi-year records of satellite remote sensing of sea surface salinity (SSS) provide an opportunity to investigate the climatological characteristics of the SSS response to tropical cyclones (TCs). In this study, the influence of TC winds, rainfall and preexisting ocean stratification on SSS evolution is examined with multiple satellite-based and in-situ data. Global storm-centered composites indicate that TCs act to initially freshen the ocean surface (due to precipitation), and subsequently salinify the surface, largely through vertical ocean processes (mixing and upwelling), although regional hydrography can lead to local departure from this behavior. On average, on the day a TC passes, a strong SSS decrease is observed. The fresh anomaly is subsequently replaced by a net surface salinification, which persists for weeks. This salinification is larger on the right (left)-hand side of the storm motion in the Northern (Southern) Hemisphere, consistent with the location of stronger turbulent mixing. The influence of TC intensity and translation speed on the ocean response is also examined. Despite having greater precipitation, stronger TCs tend to produce longer-lasting, stronger and deeper salinification especially on the right-hand side of the storm motion. Faster moving TCs are found to have slightly weaker freshening with larger area coverage during the passage, but comparable salinification after the passage. The ocean haline response in four basins with different climatological salinity stratification reveals a significant impact of vertical stratification on the salinity response during and after the passage of TCs.

Biophysical implications of the freshwater influx over small spatial scale in the coastal waters along the southwest coast of India during the Southwest Monsoon

This study in the coastal waters off Kochi along the southwest coast of India during the Southwest Monsoon (SWM) presents the biophysical implications of freshwater influx over a small spatial scale. Field sampling was carried out along two cross-shore transects (ST and NT), oriented ~ 8 km apart, and located to the south (ST) and north (NT) of the Kochi inlet, up to the depth of 50 m. The vertical profiles of temperature and dissolved oxygen showed clear upwelling signatures along both ST and NT, but more prominently along the latter transect. The freshwater influx and its southward advection due to the prevailing coastal currents caused stronger salinity stratification along ST and inhibited the rising of upwelled waters in the nearshore waters. The integrated phytoplankton biomass (chlorophyll a) in the upper 10 m was markedly higher along NT (50 mg m−3) compared to ST (35 mg m−3). The size-based phytoplankton biomass showed an increase in larger size (micro) fraction of phytoplankton along NT especially in the nearshore waters, wherein upwelling was prominent up to the surface waters. The more efficient surfacing of oxygen-deficient waters along NT cause more likely chances to concentrate small pelagic fishes there as compared to ST. Thus, this study presents a case, wherein, the freshwater influx and associated inhibition of coastal upwelling in the nearshore waters vary within a short spatial scale depending upon the advection of freshwater plume by the coastal currents. Also, showed that this situation might be valid to many estuarine inlet regions along the southwest coast of India during the SWM.

Variability in upper-ocean salinity stratification in the tropical Pacific Ocean

Using a gridded array for real-time geostrophic oceanography (Argo) program float dataset, the features of upper-ocean salinity stratification in the tropical Pacific Ocean are studied. The salinity component of the squared Brunt-Vaisala frequency N2 (N 2 ) is used to represent salinity stratification. Layer-max N 2 (LMN), defined as the N 2 maximum over the upper 300 m depth, and halocline depth (HD), defined as the depth where the N 2 maximum is located, are used to specifically describe the intensity of salinity stratification. Salinity stratification in the Topical Pacific Ocean has both spatial and temporal variability. Over the western and eastern equatorial Pacific, the LMN has a large magnitude with a shallow HD, and both have completely opposite distributions outside of the equatorial region. An obvious seasonal cycle in the LMN occurs in the north side of eastern equatorial Pacific and freshwater flux forcing dominates the seasonal variations, followed by subsurface forcing. At the eastern edge of the western Pacific warm pool around the dateline, significant interannual variation of salinity stratification occurs and is closely related to the El Nino Southern Oscillation event. When an El Nino event occurs, the precipitation anomaly freshens sea surface and the thermocline shoaling induced by the westerly wind anomaly lifts salty water upward, together contribute to the positive salinity stratification anomaly over the eastern edge of the warm pool. The interannual variations in ocean stratification can slightly affect the propagation of first baroclinic gravity waves.

Structural and morphological characteristics of the bottom settlements of mussels in the coastal ecosystems in the river runoff influence zone of the Ukranian shelf of Black Sea

The formation of the density, biomass, size-mass and phenotypic structure of the bottom populations of mussels Mytilus galloprovincialis (Lamarck, 1819) in three areas of the Ukrainian shelf of North-Western part of the Black Sea – near Adjiyask Cape (the water area near the Dnieper-South Bug region), near mouth of the Grigorievsky estuary and on the Odessa coast, with different gradients of salinity of sea waters due to influence of the transformation of the river runoff, were studied. The samples of mollusks were collected from surface of the seabed soil at a depth of 3.0 meters in the spring (March–April), summer (June–July) and autumn (October–November) 2016–2019. In addition to the number, biomass, size and phenotypic structure, the ratios of the individual components of the mollusk mass (wet and dry mass of the animal’s soft body and mass of its valves) were also studied. It is noted that the greatest salinity stratification of the water column, caused by the influence of fresh river runoff, was observed near Adjiyask Cape, where the salinity of the surface water layer (depth 0.5 m) in spring and early summer did not exceed 8.4±2.4 and 11.0±2.5 ppm, respectively. Nevertheless, bottom settlements of mussels in this and in two other seawater areas were relatively in favorable conditions, as the salinity at the depth of its habitat (depth 3.0 m) remained within 12–15 ppm, which corresponds to the “river–sea” ecotone. Based on the data obtained, it is concluded that such halo-tolerant mollusks as the Black Sea mussels in coastal areas affected by fresh river runoff can only slightly differ in their number and biomass indices, as well as in the phenotypic structure of settlements. As a rule, differences may be related exclusively to the individual morphological characteristics of animals, especially for its soft body weight (wet and dry mass) and mass of its valves.

Circulation and transport processes in a tidally forced salt-wedge estuary: The São Francisco river estuary, Northeast Brazil

The São Francisco river flow is regulated by a series of dams that work in cascade for hydro-power generation, maintaining the flow <1000 m3 s−1 all year long. This caused the dampening of the flood peaks and perpetuated the salt intrusion, changing the function of the system as a material source to the coastal ocean. Here we present an assessment of the estuary salt intrusion, hydrodynamics, mixing processes, and the transport of scalars based on field observations. The mains goal is to evaluate its present role as a sink or source of materials to the adjacent sea. Three tidal surveys were carried out to gather data on the water level, cross-section discharge, current velocity and direction, salinity, temperature, concentration of suspended particulate matter (SPM), dissolved oxygen, and chlorophyll. Data were recorded from moored instrumentations in different locations, from boat-based sampling in a cross-section, and also in along-estuary surveys. The estuary shows a highly stratified salinity distribution, with the salt intrusion excursion of 7 km during the spring tide, reaching ∼11 km at high tide. The salt intrusion is laterally asymmetric, influenced by the system morphology. There is a strong tidal attenuation along the estuary, even though the flow regime is sub-critical (Richardson Number > 0.25). The estuary presented lateral variability of currents modulated by the tide, being horizontally sheared during spring tide and vertically sheared during neap. The transport of scalars was modulated by the tide, and the estuary exported SPM, dissolved oxygen, and chlorophyll both during neap (44.4, 12.9 and 3.6x10−3 kg s−1, respectively) and spring tide (31.8, 7.4 and 2x10−3 kg s−1), despite the severe flow regulation, meaning that the system still functions as a source of materials to the coastal sea.

Organic petrology and geochemistry of Lower Cretaceous lacustrine sediments in the Chaoyang Basin (Liaoning Province, northeast China): Influence of volcanic ash on algal productivity and oil shale formation

Airfall volcanic ash deposited in the form of tuff layers is widely distributed in oil-shale bearing basins in western Liaoning Province, northeast China. In order to investigate the impacts of volcanic ash on algal productivity and the formation of organic-rich oil shales, the organic petrology and organic geochemistry of lacustrine sediments of the Lower Cretaceous Jiufotang Formation were investigated. Molecular geochemical data (the pristane/phytane ratio, the gammacerane index and the relative amount of β-carotane) indicate that they were formed in suboxic/anoxic depositional environments, with salinity fluctuations from freshwater to saline. Oil shales in the semi-deep to deepwater lake facies were mainly deposited under still and anoxic bottom water conditions, concomitant with water column salinity stratification. The relative abundances of diterpenoids and regular steranes indicate a limited terrigenous conifer plant contribution and a predominantly algal contribution to the organic matter. Blooms of algae contributing to high primary productivity, and anoxic depositional environments contributing to organic matter preservation, were key to organic-rich oil shale formation. Fish bones, ostracod shells, and alginite are present below the thick tuff layer. Alginite richness gradually increased from very low just above the tuff layer, to a higher concentration further above the tuff layer, indicating that during major volcanic eruptions, large quantities of volcanic ash led to water column poisoning, causing the sudden mass mortality of some aquatic organisms that were sensitive to turbid, acidified and toxic water. The increased proportion of brackish/saline-water dinoflagellates, which is corroborated by the high concentration of telalginite, abundant dinoflagellate-specific biomarkers (C31 4,22,23,24-tetramethylcholestanes), high values of the C27/C29 ααα-20R and C28/C29 ααα-20R sterane ratios, 4-methylsterane index, dinosterane index, and tetramethylsterane index, suggest that brackish/saline-water dinoflagellates show a positive response to volcanic ash input. Some specific brackish/saline-water dinoflagellates are present in organic-rich tuff laminae, indicating that during periods of minor volcanic eruptions or in intermissions between major eruptions, a minor volcanic ash input was more likely to result in rapid and efficient volcanic fertilisation effects and high-quality oil shale formation. Although the lacustrine oil shale system of the Jiufotang Formation shares some similar petrological and geochemical characteristics with the classical saline lacustrine oil shale system of the Green River Formation in the United States of America, the significant influence of airfall volcanic ash on algal productivity makes the formation mechanism of oil shales within the Jiufotang Formation strata quite different from that of oil shales within the Green River Formation.

Application of satellite observations to study the changes of hypoxic conditions in near-bottom water in the western part of Peter the Great Bay (the Sea of Japan)

In this paper we explore the possibilities of applying satellite ocean colour (OC) observations and SST to study the changes in the conditions of hypoxia in the near-bottom water in the western part of Peter the Great Bay. Near-bottom water hypoxia occurs in water bodies with increased organic matter influx when the dissolved oxygen (DO) consumed at its oxidation is not restored. Consumption of most DO is usually attributed to the oxidation of organic matter formed as a result of increased algae growth during water eutrophication. Satellite data on indicators of phytoplankton (chlorophyll-a concentration (Chl) and fluorescence (FLH)) allow to analyze the spatial-temporal changes of this substation. Coloured dissolved organic matter (CDOM), non-algal particles (NAP) influence on satellite Chl estimates and also on near-bottom water hypoxia formation. This study analyzes daily, seasonal, and inter-annual changes in the distributions of indicators (Chl, FLH, the coefficients of light absorption by coloured detrital matter (aCDM) and light backscattering by suspended particles (bbp)), based on the instant satellite OC data from MODIS-Aqua. Data on the Chl, the sea surface temperature (SST) from the MODIS-Aqua, the precipitation from the TRMM satellite and the hydrometeorological stations (HMSs), the wind speed and direction from HMS “Vladivostok” are used to study the influence of hydrometeorological conditions on the Chl values. These distributions were compared with the literary information based on field observations of the hypoxia cases in the same area and with the changes in the vertical DO, Chl, temperature, salinity distribution obtained by coastal expeditions in October-November 2010 and February-March 2011. Significant interrelations within 95% confidence level between the satellite Chl, FLH values calculated at the MUMM atmospheric correction and in situ Chl values obtained in the autumn of 2010 were reached separately for the cases with winds of northern and southern directions with the correlation coefficients of 0.71, 0.48 and 0.49, 0.71, respectively. Significant dependences of Chl on SST and Chl on wind speed explained by the influence of continental runoff and water ventilation were obtained. Therefore, the changes of Chl reflect the changes of hypoxic conditions in the near-bottom water. In Amursky Bay the onset of hypoxia was at the Chl and SST values equal to 4 mg m−3 and 13 °C (↑ – at increasing SST); near Furugelm Island it was at 1.6 mg m−3 and 25 °C (↑), 1 mg m−3 and 21 °C (↓). The difference in the Chl values was reflected in the hypoxia onset timings that were the beginning of June (2011), August (2013), and September (2014), respectively. The water flow from the eastern coast of Amursky Bay in early August of 2013 recorded from the OC and SST satellite imagers appeared in an additional hypoxic zone. Decreased OC characteristics in the runoff of the Razdolnaya River in August-September of 2014 were a sign of hypoxia at its mouth. Near Furugelm Island the hypoxia destruction (increase in the DO level from 1 to 4.5 ml L−1) was observed at the Chl of 0.9 mg m−3 and SST = 18 °C (↓). At the autumn maximum of Chl equal to 1.7 mg m−3 and SST = 4 °C (↓) in mid-November the DO level here increased to 8 ml L−1. In Amursky Bay, short-term destructions/weakening of hypoxia manifested themselves in sharp increases of Chl. At that, the ratio between the Chl value and the approximation level was equal to 2 and higher for SST equal to 22–25 °C (↑), to 0.9 and higher for SST equal to 5–13 °C (↓). With the water stratification destruction in temperature and the noticeable weakening of the stratification in salinity (mid-November), the hypoxia destructed (the DO level increased from 2 to 6 ml L−1). In this case, Chl and SST were about 3 mg m−3 and 5 °C (↓).

Marine lakes as biogeographical islands: a physical model for ecological dynamics in an insular marine lake, Palau

Marine lakes are emerging ecological and evolutionary natural experimental systems, with genetically isolated resident populations that exhibit extreme population dynamics and rapid phenotypic change. Marine lakes are posited to be marine islands, however, unlike terrestrial islands for which rich models have been developed over the past half-century, we know little of the mechanisms driving changes in marine lakes. This is a critical knowledge gap in efforts to reconcile theory on, or distinguish differences among, island and island-like systems. To reduce this critical knowledge gap, we present a mathematical model describing marine lakes based on a case study of Jellyfish Lake (Ongeim’l Tketau, Mecherchar: OTM), Palau. Empirical data show that marine lakes exhibit delayed and reduced tidal motions, suggesting exchange of a limited amount of water with the neighboring (‘mainland’) ocean. Our model tracks changes in lake level, allowing determination of an exchange rate that is a physical null model for biological colonization and a proxy for colonization distance in island biogeography theory. In addition, we track horizontally averaged in-lake quantities such as salinity and temperature (i.e., marine weather, climate) and stratification (i.e., habitat) — that are known to influence resident species’ distributions and population dynamics — by solving an advection-diffusion equation. We find that weather, ocean conditions, groundwater, and exchanges through tunnels determine the abiotic environment in OTM. By comparing simulations and data, we estimate the difficult-to-measure properties of the surrounding groundwater — the ‘matrix’ in the vernacular of habitat islands — and give a range of realistic values for the effective diffusion coefficient. This coefficient is found to increase in a tropical storm, suggesting that other drivers can be important during perturbations.

Coastal Upwelling Events, Salinity Stratification, and Barrier Layer Observed Along the Southwestern Coast of Sumatra

AbstractCoastal upwelling along the southwestern coast of Sumatra is a seasonal upwelling that occurs in areas of high sea surface temperature and abundant precipitation at the southeastern edge of the Indian Ocean warm pool. Based on observations from two Argo floats that drifted and stayed around Sumatra, we investigated ocean temperature and salinity variations during several coastal upwelling events observed in 2013–2017. The Argo floats observed the vertical structure of temperature and salinity every 10 days within 100 km from the southwestern coast of Sumatra. The observation data show intraseasonal‐scale subsurface temperature cooling events with significant upward displacements of the thermocline and high‐salinity water, led by anomalous local southesterly winds and equatorial easterly winds. During the coastal upwelling events, salinity stratification and a thick barrier layer related to local precipitation were also observed. Surface mixed layer temperature cooling were relatively small in contrast to the significant subsurface anomalies. It was found that during the coastal upwelling events, subsurface cold‐water upwelling signals did not necessarily reach the mixed layer when salinity stratification and a thick barrier layer were present. The implications of these observational results for understanding the local atmosphere ocean interaction, and hence the occurrence of the Indian Ocean Dipole, are discussed.

Impact of Remote Equatorial Winds and Local Mesoscale Eddies on the Existence of “River in the Sea” Along the East Coast of India Inferred From Satellite SMAP

AbstractThe Northern Bay of Bengal (NBOB) exhibits extreme salinity stratification caused by intense summer monsoon rainfall and associated river discharge. This accumulated fresh water plume in NBOB is believed to be transported along the East coast of India (ECI) by East India Coastal Current (EICC), known as River in the Sea (RIS). Though in situ sea Surface Salinity (SSS) measurements are sparse, the advancement of satellite monitoring system has provided a unique opportunity to unravel salient features of RIS. In the present study, in situ SSS at five selected regions around the Indian coast and open ocean are utilized to validate the Soil Moisture Active Passive (SMAP) (L3, v4.3) products. Further, this new SMAP data is used to examine year‐to‐year variability of RIS during 2015–2019 and the driving forces responsible for its existence. This SMAP SSS exhibited 1.52, 0.46, 0.38, 1.45 RMSD and 0.97, 0.82, 0.85, 0.86 correlation in NBOB1, NBOB2, Central BOB (CBOB) and Western BOB (WBOB), respectively with in situ measurements. This data also depicted the presence of “RIS” all along the ECI only during November in consistent with the magnitude of Kelvin wave induced EICC. The southward extension of RIS along the ECI is primarily based on the magnitude of remote equatorial wind induced forces. The mesoscale eddies in south‐western BOB (SWBOB) alters the shape of RIS and even inhibit its southward propagation completely.

Climate change overtakes coastal engineering as the dominant driver of hydrological change in a large shallow lagoon

Abstract. Ecosystems in shallow micro-tidal lagoons are particularly sensitive to hydrologic changes. Lagoons are complex transitional ecosystems between land and sea, and the signals of direct human disturbance can be confounded by variability of the climate system, but from an effective estuary management perspective, the effects of climate versus direct human engineering interventions need to be identified separately. This study developed a 3D finite-volume hydrodynamic model to assess changes in hydrodynamics of the Peel–Harvey Estuary, a large shallow lagoon with restricted connection with ocean; this was done by considering how attributes such as water retention time, salinity and stratification have responded to a range of factors, focusing on the drying climate trend and the opening of a large artificial channel over the period from 1970 to 2016, and how they will evolve under current climate projections. The results show that the introduction of the artificial channel has fundamentally modified the flushing and mixing within the lagoon, and the drying climate has changed the hydrology by comparable magnitudes to that of the opening of the artificial channel. The results also highlight the complexity of their interacting impacts. Firstly, the artificial channel successfully improved the estuary flushing by reducing average water ages by 20–110 d, while in contrast the reduced precipitation and catchment inflow had a gradual opposite effect on the water ages; during the wet season this has almost counteracted the reduction brought about by the channel. Secondly, the drying climate caused an increase in the salinity of the lagoon by 10–30 PSU (Practical Salinity Unit); whilst the artificial channel increased the salinity during the wet season, it has reduced the likelihood of hypersalinity (&gt;40 PSU) during the dry season in some areas. The opening of the artificial channel was also shown to increase the seawater fluxes and salinity stratification, while the drying climate acted to reduce the salinity stratification in the main body of the estuary. The impacts also varied spatially in this large lagoon. The southern estuary, which has the least connection with the ocean through the natural channel, is the most sensitive to climate change and the opening of the artificial channel. The projected future drying climate is shown to slightly increase the retention time and salinity in the lagoon and increase the hypersalinity risk in the rivers. The significance of these changes for nutrient retention and estuary ecology are discussed, highlighting the importance of these factors when setting up monitoring programmes, environmental flow strategies and nutrient load reduction targets.

Harmful algal blooms of Heterosigma akashiwo and environmental features regulate Mesodinium cf. rubrum abundance in eutrophic conditions

Functional drivers of phytoplankton that can potentially form harmful algal blooms (HABs) are important to understand given the increased prevalence of anthropogenic modification and pressure on coastal habitats. However, teasing these drivers apart from other influences is problematic in natural systems, while laboratory assessments often fail to replicate relevant natural conditions. One such potential bloom-forming species complex highlighted globally is Mesodinium cf. rubrum, a planktonic ciliate. This species occurs persistently in the Sundays Estuary in South Africa yet has never been observed to “bloom” (> 1,000 cell.ml−1). Modified by artificial nutrient-rich baseflow conditions, the Sundays Estuary provides a unique Southern Hemisphere case study to identify the autecological drivers of this ciliate due to artificial seasonally “controlled” abiotic environmental conditions. This study utilised a three-year monitoring dataset (899 samples) to assess the drivers of M. cf. rubrum using a generalised modelling approach. Key abiotic variables that influenced population abundance were season and salinity, with M. cf. rubrum populations peaking in summer and spring and preferring polyhaline salinity regions (>18) with pronounced water column salinity stratification, especially in warmer months. This was reflected in the diel vertical migration (DVM) behaviour of this species, demonstrating its ability to utilise the optimal daylight photosynthetic surface conditions and high-nutrient bottom waters at night. The only phytoplankton groups clearly associated with M. cf. rubrum were Raphidophyceae and Cryptophyceae. Although M. cf. rubrum reflects a niche overlap with the dominant HAB-forming phytoplankton in the estuary (the raphidophyte, Heterosigma akashiwo), its reduced competitive abilities restrict its abundance. In contrast, the mixotrophic foraging behaviour of M. cf. rubrum exerts a top-down control on cryptophyte prey abundance, yet, the limited availability of these prey resources (mean < 300 cells ml−1) seemingly inhibits the formation of red-water accumulations. Hydrodynamic variability is necessary to ensure that no single phytoplankton HAB-forming taxa outcompetes the rest. These results confirm aspects of the autecology of M. cf. rubrum related to salinity associations and DVM behaviour and contribute to a global understanding of managing HABs in estuaries.

Life and death in the Chicxulub impact crater: a record of the Paleocene–Eocene Thermal Maximum

Abstract. Thermal stress on the biosphere during the extreme warmth of the Paleocene–Eocene Thermal Maximum (PETM) was most severe at low latitudes, with sea surface temperatures at some localities exceeding the 35 ∘C at which marine organisms experience heat stress. Relatively few equivalent terrestrial sections have been identified, and the response of land plants to this extreme heat is still poorly understood. Here, we present a new record of the PETM from the peak ring of the Chicxulub impact crater that has been identified based on nannofossil biostratigraphy, an acme of the dinoflagellate genus Apectodinium, and a negative carbon isotope excursion. Geochemical and microfossil proxies show that the PETM is marked by elevated TEX86H-based sea surface temperatures (SSTs) averaging ∼37.8 ∘C, an increase in terrestrial input and surface productivity, salinity stratification, and bottom water anoxia, with biomarkers for green and purple sulfur bacteria indicative of photic zone euxinia in the early part of the event. Pollen and plants spores in this core provide the first PETM floral assemblage described from Mexico, Central America, and the northern Caribbean. The source area was a diverse coastal shrubby tropical forest with a remarkably high abundance of fungal spores, indicating humid conditions. Thus, while seafloor anoxia devastated the benthic marine biota and dinoflagellate assemblages were heat-stressed, the terrestrial plant ecosystem thrived.

On the control of subantarctic stratification by the ocean circulation

A shallow mixed layer depth bias in Austral winter in the Subantarctic Zone is a common feature of Coupled Model Intercomparison Project (CMIP5) models, including the Community Earth System Model (CESM). The bias is related to other deficiencies in the model solution, including too-weak Subantarctic Mode water formation and excessive leakage of Agulhas waters into the Atlantic instead of into the Indian Ocean and Subantarctic Frontal Zone. This work investigates the hypothesis that the shallow bias is due to errors in the simulated ocean circulation. Results from a model with low resolution ocean component (1° grid) are compared against: (i) an experiment with an eddy resolving (0.1°) grid and (ii) experiments using the 1° grid and employing an adiabatic nudging of the ocean pressure field to observations that is referred to as a semi-prognostic method. For both the higher resolution and semi-prognostic experiments, improved horizontal advection of warm and salty water near the surface leads to a more realistic sea surface temperature (SST) and salinity front associated with the Agulhas Return Current and Antarctic Circumpolar Current. The warmer surface waters in the high-resolution model and semi-prognostic model lead to stronger air–sea heat loss, with a response of about 40 Wm−2 °C−1 in winter in the area of deep mixed layers. Budgets of the temperature and salinity stratification show that the deeper mixed layers in the high-resolution experiment are primarily a result of the increased surface heat loss, whereas in the semi-prognostic case salinity advection is the main factor leading to destabilization of the water column. Both results indicate that ocean circulation is a key factor in wintertime deep Southern Ocean mixing, associated with advection of water masses and air–sea feedbacks.

Anticyclonic Eddies Enhance the Winter Barrier Layer and Surface Cooling in the Bay of Bengal

AbstractOceanic barrier layer impedes the vertical exchanges of heat and biogeochemical substances between the mixed layer and deep ocean, playing essential roles in modulating air‐sea heat flux and oceanic biogeochemical cycles. Recent studies revealed that mesoscale eddies can modify the vertical motions of the mixed layer. But their influence on the barrier layer has not been determined. By analyzing more than a decade of in situ, satellite, and reanalysis data, this study investigated the influence of eddies on barrier layer in the Bay of Bengal. The results show that the barrier layer in anticyclonic eddies is significantly thickened in winter (by 50% on average). This thickening consists of the deepening of isothermal layer by eddy downwelling and the shoaling of mixed layer by near‐surface salinity restratification. The thickening of the barrier layer enlarges with latitude, in consistent with the reinforcement in salinity stratification and surface cooling. The thickened barrier layer impedes the upward supply of heat into the mixed layer from the below, promoting the winter surface cooling (by 0.7°C on average) and subsurface temperature inversion (by 0.6°C on average). The feature is contrary to the intuitive expectation that anticyclonic eddies possess high sea surface temperature in eddy interiors. As a consequence, both cyclonic and anticyclonic eddies result in negative sea surface temperature anomalies, indicative of a net decrease in the local sea surface temperature. These effects of eddies may be a common feature in salinity‐stratified oceans in winter and have potential implications for the local biogeochemistry and air‐sea coupled models.