Thermohaline Front Research Articles

Temporal changes of the Oyashio water distribution east of Japan under the changing climate: development of an objective evaluation method and its application

AbstractThe Oyashio, a southern part of the western boundary current in the North Pacific subarctic gyre, carries cold and fresh seawater with abundant nutrients southward from the high-latitude, influencing regional climate in the East Asia and marine environment in the western mid-latitude North Pacific. Previously, a distribution of the Oyashio water has been evaluated by empirical temperature thresholds; for example, in spring (March–May) when the Oyashio intrudes southward into the east of Japan, the Oyashio water is defined at 100-m depth as ≤ 5 °C. However, this method is not necessarily adequate under the changing climate because upper ocean temperature may change over time due to some causes unrelated to cold water transport by the Oyashio (e.g., surface heat fluxes). In this study, we developed an objective method to evaluate the Oyashio water distribution applicable under the changing climate with a focus on a thermohaline front located at the warm- and salty-side boundary of the Oyashio water. We identified isohalines at 100-m depth best corresponding to the thermohaline front in each month and used them as the Oyashio water threshold. Using the developed method, we further investigated the springtime Oyashio water distribution east of Japan (in the North Pacific south of 43°N, 141–148°E). The area of the Oyashio water shows inter-annual variation and significant long-term decrease. It was suggested that these temporal variation and change reflect changes in a distribution of anti-cyclonic meso-scale eddies off Hokkaido, which block the southward Oyashio intrusion into the east of Japan.

Protistan community structure and the influence of a branch of Kuroshio in the northeastern East China Sea during the late spring

The northeastern East China Sea is an ecologically important marine ecosystem influenced by warm water derived from the Kuroshio Current. However, relatively little is known about the spatial variation of protist communities and their regulating factors from this region’s ecosystem during the spring season. Here, we investigated protistan community structures using a complementary approach combining 18S rRNA gene amplicon sequencing and light microscopy cell counts at nine stations from the northeastern East China Sea to the west of Jeju Island. The vertical profiles of physicochemical properties revealed that the Jeju Warm Current water mass, flowing from the southeast towards the northwest, created a thermohaline front dividing the region in two. These two regions had similar planktonic biomass, but the protistan communities differed significantly: dinoflagellates accounted for higher proportions of the protistan communities in the warm and saline waters, particularly at stations E35, E44, and E45, while the relative abundances of diatoms and picochlorophytes were higher in the low-density water of the western stations (E32 and E42). Furthermore, higher species richness and Shannon Diversity Index values in the warm and saline waters suggests that the Jeju Warm Current, a branch of the Kuroshio, increases protistan taxonomic diversity in the northeastern East China Sea during the late spring. Seed populations of harmful algal bloom-causing species were discovered in the warm and saline water originating from the Kuroshio, which is particularly important as it indicates that these waters could introduce harmful species that may spread to the Yellow Sea and Korea Strait. Taken together, the study suggests that potential changes to the current systems in the region could dramatically alter the structure of its protistan community.

Southern coastal system of the San Jorge Gulf during spring

The San Jorge Gulf was declared an area of national economic, commercial, conservation and diversity interest. To contribute to its study, the southern coastal sector of the gulf was characterised in physical and chemical parameters in spring campaigns (November 2016 and 2017). Samples were taken on board the oceanographic vessel Puerto Deseado in three perpendicular to the coast transects and another one to the southeast of the gulf, which crosses a thermohaline front. The perpendicular to the coast transects showed a similar pattern of distribution of physical and chemical properties in the water column. At nearshore stations, the water column was homogeneous and the nutrient concentrations between the surface and bottom layers were similar. At offshore stations, the water column was stratified and the nutrient concentrations were higher near the bottom than at surface. In 2016, a wind induced upwelling front was observed in the southwest of the gulf. Both sides of the thermohaline front did not show significant chemical differences. Throughout the study area, silicic acid and nitrate limited primary production. Satellite chlorophyll-a data and fluorescence profiles showed that the southern sector of the gulf has a high biomass of phytoplankton during November. We conclude that the southern sector of the SJG has different features depending on the external forcing caused by the front. The presence of fronts stimulates the growth of phytoplankton biomass through the injection of nutrients from the homogeneous part to the stratified sector. This would affect phytoplankton community and, thus, the productivity of the system.

Hydrological controls on base metal precipitation and zoning at the porphyry-epithermal transition constrained by numerical modeling

Ore precipitation in porphyry copper systems is generally characterized by metal zoning (Cu–Mo to Zn–Pb–Ag), which is suggested to be variably related to solubility decreases during fluid cooling, fluid-rock interactions, partitioning during fluid phase separation and mixing with external fluids. Here, we present new advances of a numerical process model by considering published constraints on the temperature- and salinity-dependent solubility of Cu, Pb and Zn in the ore fluid. We quantitatively investigate the roles of vapor-brine separation, halite saturation, initial metal contents, fluid mixing and remobilization as first-order controls of the physical hydrology on ore formation. The results show that the magmatic vapor and brine phases ascend with different residence times but as miscible fluid mixtures, with salinity increases generating metal-undersaturated bulk fluids. The release rates of magmatic fluids affect the location of the thermohaline fronts, leading to contrasting mechanisms for ore precipitation: higher rates result in halite saturation without significant metal zoning, lower rates produce zoned ore shells due to mixing with meteoric water. Varying metal contents can affect the order of the final metal precipitation sequence. Redissolution of precipitated metals results in zoned ore shell patterns in more peripheral locations and also decouples halite saturation from ore precipitation.

Thermodynamic characteristics of marine atmospheric boundary layer across frontal regions of the Indian Ocean Sector of the Southern Ocean based on three field campaigns

The Southern Ocean (SO) marine atmospheric boundary layer (MABL), regulated by continuous air-sea exchanges, plays a key role in transporting exchanges between tropics and poles. Among SO sectors, the Indian Ocean sector of SO (ISSO) remained least explored in terms of MABL characterization and is examined in this study. In ISSO, occurrence of sharp oceanic thermohaline fronts regulates the vertical thermodynamic structure of MABL, clouds, and inversions. In this study, these properties of ISSO MABL are investigated over three oceanic domains (Sub-Tropical Indian Ocean (STIO), ISSO, and High-Latitude SO (HLSO)). To achieve this, near-sea surface air-sea exchanges along with profiles of meteorological parameters generated between 25°S and 68°S and 57°E to78°E during three field campaigns conducted in the austral summers of 2017, 2018, and 2020 were utilized. Results showed strong SST-Tair variability across the study region. In STIO, positive SST-Tair indicated Low-Level Cold Air Advection (LLCAA)-induced destabilized and coupled MABLs capped by multiple inversions (>3INV's of strength∼0.35 K m−1 above∼1200 m) and thin mid/high-altitudes clouds (cloud-base∼830 m, cloud-top∼2309 m, and cloud-thickness∼758 m). Over ISSO and northern HLSO, weak and negative SST-Tair indicated Low-Level Warm Air Advection (LLWAA)-induced stratified and decoupled MABLs. Aided by advective mixing of multiple air-masses, low-level thick multilayered clouds (>2 layers, average cloud-base∼604 m, cloud-top∼2288 m, and cloud-thickness∼1314 m) and multiple strong high-level inversions (>2INV's, strength>0.4 K m−1,∼1836 m) was observed. In HLSO, weakly positive SST-Tair indicated LLCAA-induced weakly destabilized MABLs capped by mid-altitude inversions (strength∼0.22 K m−1,∼1632 m) and mid-altitude clouds (cloud-base∼979 m, cloud-top∼2465 m, and cloud-thickness∼1263 m) supported by sublimation leading to virga conditions.

Characterization of fronts in the Western Mediterranean with a special focus on the North Balearic Front

We focus on the characterization of thermohaline fronts in the Western Mediterranean, with a particular focus on the North Balearic Front (NBF), separating the Atlantic Waters that spread into the Algerian Basin from the saltier and colder waters of the Liguro-Provençal area. We use a simple gradient-based method of front detection applied to a 20-year (from June 1993 to June 2013) reanalysis of the Mediterranean. Statistics of daily frontal indices are used to identify areas of recurrent fronts, i.e., frontal zones. Comparisons with data from glider transects and remotely sensed sea surface temperature and altimetry data have been used to validate our approach from daily to seasonal and interannual time scales. Our method yielded two co-existing frontal zones in the area of the NBF. One with an almost permanent haline surface frontal zone extending southeastward from the Balearic Islands to Sardinia, representing the northern limit of the fresher Atlantic Water that spreads via instabilities of the Algerian Current and associated Algerian Eddies. Between years, its position shifts by about 1° of latitude, possibly due to processes associated with both the deep water formation in the Provençal Basin and the spreading of southern Algerian Eddies. The second frontal zone is seasonal and thermally driven extending off the northeast of Menorca to the northwest of Corsica. The Pyrenees Front, a sharp thermal front off Cap de Creus which marks the boundary between the warm surface waters of the Balearic Sea and the cooler waters in the Gulf of Lion in late summer, appears to facilitate the formation of the aforementioned seasonal thermal front through the northeastward advection of its waters toward the West Corsican Current. The divergent eastward extensions of the two frontal zones, and their differences in nature, structure, and spatio-temporal variability, call for a revisit of the NBF appellation and a clarification of its dynamics.

Исследование формирования и распространения речного плюма Дуная на основе численного моделирования

The relevance of the studied circulation caused by the water river runoff deals with the anthropological impact on the ecological state of the shelf. River waters, entering into the sea, form mesoscale structures in the delta’s area, characterized by low salinity with a high level of suspended matter and dissolved organic matter. Such structures are called “plume” in modern literature. In this case, the inertial motion of the plume is free to form a rounded area or “bulge” like anticyclonic circulation type of flow. The purpose of this work is to investigate the propagation of freshened waters, formed by the river inflow, the formation of the waters hydrological structure regularities, dynamics of the buoyancy current, and thermohaline front formation on the base of numerical modeling. Numerical modeling is used to study the formation of a river plume and downward propagation of buoyancy current on the shelf without taking into account the tides forcing. A three-dimensional σ-coordinate numerical model was used, adapted for the shelf and estuaries. The calculations were carried out for a rectangular box area. The influence of changes in the main parameters of the runoff, the mouth geometry and wind forcing are considered to development of the plume and the alongshore propagation of buoyancy current. The obtained modeling results for the conditions of the Northwestern Black Sea shelf and the Danube discharge are consistent with the estimates of plume characteristics based on the archival hydrological observations data of water temperature and salinity. The results of this work can be used for further study of hydrological processes in the region of river mouths, the peculiarities of the plumes formation and evolution, assessment of the suspended matter, biogenic elements and microplastics transport in the sea and ocean coastal zones.

Microplastics on the sea surface of the semi-closed Tokyo Bay

Microplastics (MPs) pollution surveys were conducted in Tokyo Bay using neuston nets (May 2019 and January 2020). Although the pollution level in Tokyo Bay was high (3.98 pcs/m3, May), it was lower than reported in other semi-closed bays because of differences in the Enclosed Index. It was found that polyethylene fragments dominated the retrieved MPs; the mode of MPs size was 800 μm. As MPs abundance in rivers had the same seasonality as found in the inner bay, rivers were considered the main source of MPs. The seawater residence time is shorter than the time required for the density of MPs to become greater than that of seawater; therefore, it was considered that MPs are transported out of the bay instead of sinking. MPs were aggregated into a convergence zone by residual currents (the thermohaline front) in May (January). These findings will improve understanding of MPs pollution in other bays.

Using a biophysical model to investigate connectivity between spawning grounds and nursery areas of King George whiting (Sillaginodes punctatus: Perciformes) in South Australia’s gulfs

AbstractMany demersal marine fish species depend on a dispersive larval stage that connects geographically discrete sub‐populations. Understanding connectivity between these sub‐populations is necessary to determine stock structure, which identifies the appropriate spatial scale for fishery management. Such connectivity is poorly understood for King George whiting (Sillaginodes punctatus; Perciformes) in South Australia's gulf system, even though spawning grounds and nursery areas are adequately defined. In response to declines in commercial catches and estimated biomass, this study aimed to determine the most important spawning grounds and nursery areas to recruitment, and the connectivity between them. A biophysical model was seeded with particles according to the distribution and density of eggs throughout the spawning area in 2017 and 2018. Despite inter‐annual differences in the origins of particles, dispersal pathways and predicted settlement areas remained consistent between years. Predicted settlement was generally highest to nursery areas only short distances from regional spawning grounds, consistent with previous hydrodynamic models. However, the model also predicted that spawning in one region could contribute to recruitment in an adjacent region later in the spawning season, which aligned with the breakdown of thermohaline fronts at the entrance of each gulf. The connectivity between spawning grounds and nursery areas predicted by the model is supported by spatio‐temporal patterns in the otolith chemistry of pre‐flexion larvae and settled juveniles. Consequently, the most parsimonious explanation is that the populations of King George whiting in South Australia's gulf system constitute a single, panmictic stock, which has implications for fishery management.

Observations of Turbulence at a Near‐Surface Temperature Front in the Arctic Ocean

AbstractHigh‐resolution ocean temperature, salinity, current, and turbulence data were collected at an Arctic thermohaline front in the Nansen Basin. The front was close to the sea ice edge and separated the cold and fresh surface melt water from the warm and saline mixed layer. Measurements were made on 18 September 2018, in the upper 100 m, from a research vessel and an autonomous underwater vehicle. Destabilizing surface buoyancy fluxes from a combination of heat loss to the atmosphere and cross‐front Ekman transport by down‐front winds reduced the potential vorticity in the upper ocean. Turbulence structure in the mixed layer was generally consistent with turbulence production through convection by heat loss to atmosphere and mechanical forcing by moderate winds. Conditions at the front were favorable for forced symmetric instability, a mechanism drawing energy from the frontal geostrophic current. A clear signature of increased dissipation from symmetric instability could not be identified; however, this instability could potentially account for the increased dissipation rates at the front location down to 40 m depth that could not be explained by the atmospheric forcing. This turbulence was associated with turbulent heat fluxes of up to 10 W m−2, eroding the warm and cold intrusions observed between 30 and 60 m depth. A Seaglider sampled across a similar frontal structure in the same region 10 days after our survey. The submesoscale‐to‐turbulence‐scale transitions and resulting mixing can be widespread and important in the Atlantic sector of the Arctic Ocean.

Observations of the warm-tongue circulation in the northern East China Sea

A subsurface thermohaline front semi-permanently formed in association with near-bottom cyclonic circulation in the northern East China Sea was newly found from detailed hydrographic data collected during two cruises in February 2017 (winter) and April 2018 (spring) along with supplementary satellite remote sensing and historical hydrographic data. An alternate intruding frontal structure in water properties was observed across the cyclonic circulation in both seasons as formed by two contrasting water masses—low-temperature and low-salinity (i.e., low spiciness) water transported by the East China Sea Current and high-temperature and high-salinity (i.e., high spiciness) water transported by the Tsushima Warm Current. Consistent structures were confirmed from current observations during the two cruises, historical hydrographic observations, and satellite altimetry-derived sea surface height and surface frontal structure, indicative of retroflection of the Cheju Warm Current as deemed by the seasonal development of thermal stratification in spring. Our results reveal significant heat and material exchanges between the open Pacific and the broad shelf, particularly via diapycnal mixing and cross-front transports associated with across-front flow and cyclonic circulation, in the northern East China Sea.

Oceanic versus continental influences over the last 7 kyrs from a mid-shelf record in the northern Bay of Biscay (NE Atlantic)

We discuss paleoenvironments of north-western France over the last 7 kyrs in terms of: i) long-term changes (relative sea-level rise and boreal summer insolation), ii) rapid climate changes (millennial-scale Bond events and multi-decadal regimes of the North Atlantic Oscillation: NAO) and iii) growing human impacts in watersheds. Our study focuses on the CBT-CS11 core, retrieved in the northern Bay of Biscay, with new high-resolution (70 years) palynological data (dinoflagellate cysts and pollen), combined with sedimentological and oxygen stable isotopic records. This multiproxy approach enabled us to gain a better understanding of the influence of varying Middle to Late Holocene climate regimes on marine, coastal and terrestrial ecosystems as well as on human coastal societies. We especially show that the slowdown of the relative sea-level rise, starting at around 5.9 ka BP, led to the stabilization of tidal flats in estuarine environments. Subsequently, increasing river flows to the ocean resulted in a progressive seasonal stratification of the shelf under increasing winter precipitations and establishment of the modern winter thermohaline front at 3.3 ka BP. In addition, within the 4–2 ka BP interval, palynological and sedimentological evidence suggests increasing river discharges in north-western France, in a context of a weakened Atlantic subpolar gyre and recurrent negative “NAO-like” conditions. Finally, we identified a major transition at around 1.2 ka BP (Early Middle-Ages) that appears to be linked to a period of maximal anthropogenic landscape opening and soil erosion, implying stronger primary productivity in coastal surface waters of the studied region.

Engraulis encrasicolus larvae from two different environmental spawning areas of the Central Mediterranean Sea: first data on amino acid profiles and biochemical evaluations

Early life stages of marine fish populations may be strongly affected by environmental factors. Changes in the physical environment or the availability of food resources could lead to stress-related physiological responses affecting larval fitness, growth and survival. In the present study, we determined, for the first time, amino acid composition (AAC), lipid, and carbohydrate content, as well as alkaline phosphatase and peroxidase activities in larvae from the European anchovy Engraulis encrasicolus. Fishes were caught in two different spawning areas of the Strait of Sicily, characterized by different environmental conditions, including a coastal upwelling with a lower temperature (Adventure Bank; 20.22 ± 0.38°C) and a thermohaline front with a higher temperature (Maltese Bank 23.10 ± 0.25°C). The results showed that the two groups of larvae, in their early life, had similar nutritional status. However, compared with the samples from the Maltese Bank, the specimens collected in the Adventure Bank area exhibited higher alkaline phosphatase activity, lower concentrations of aspartate plus asparagine, threonine, and arginine but a higher concentration of leucine, highlighting different patterns of amino acid metabolism. Collectively, these results indicated that AAC analysis could represent an additional valid tool to evaluate the link between physiological responses and environmental conditions at early life stages.

Bottom Boundary Layer and Particle Dynamics in an Upwelling Affected Continental Margin (NW Iberia)

AbstractThe dynamics of the bottom boundary layer and their effects on the development of nepheloid layers (NLs) on a high‐energy and upwelling‐affected margin, the NW Iberian continental shelf, were studied by means of ADP currents, wave time series, and across‐shelf hydrographic surveys covering an entire annual cycle. The bottom boundary layer hydrodynamics showed that high levels of bottom shear stress over the inner shelf occurred mainly during downwelling seasons, when there was a coupling between storm waves and intense currents. This wave‐current coupling promotes strong resuspension events that favored the generation of bottom NLs (BNLs). BNLs were well developed on the inner continental shelf inshore of a thermohaline front generated by the interaction of the Iberian Poleward Current and the West Iberian Buoyant Plume. The existence of this front limited the extent of offshore export of resuspended particles during the downwelling season. In contrast, during the upwelling season, only thin BNLs were developed, and surface NLs, principally composed by biogenic particulate material, were advected offshore in the Ekman layer, often as part of the development of upwelling filaments. Our study confirmed that wave orbital velocity under stormy conditions, in combination with the hydrography, could explain the shoreward boundary of the Galician mud depocenter.

Wintertime Large Temperature Inversions in the Yellow Sea Associated With the Cheju and Yellow Sea Warm Currents

AbstractLarge temperature inversions in the Yellow Sea during winter were investigated by analyzing conductivity‐temperature‐depth data collected in January and February 1986, April 1996, and February 1997. Large inversions with temperature differences across inversion layers greater than 2 °C were observed at three places: southwest of Korea, southeast of the Shantung Peninsula, and on the northeastern flank of the Changjiang Banks. Pronounced thermohaline fronts were commonly established where warm offshore waters with high salinity were found below cold coastal waters with low salinity. In the southwestern area of Korea, where the front develops in an east‐to‐west direction, the Cheju Warm Current water flows eastward to the south of the front, while north of the front, cold Korean coastal water flows westward. In the southeastern area of Shantung Peninsula, where the front forms in northeast to southwest, the Yellow Sea Warm Current flows northeastward on the offshore side, while cold Chinese coastal water flows southwestward. Moderate temperature inversions were observed midwest of Korea and on Changjiang Banks when warm offshore waters extended into cold coastal areas. In the inversion areas, warm waters are heavier than cold waters, reflecting a stronger contribution of salinity to density than that of temperature. Consequently, isotherms and isohalines in the inversion areas commonly decline downward toward cold water areas. We concluded that the occurrence of large inversions is closely related to the advection of the Cheju Warm Current and Yellow Sea Warm Current waters to the thermohaline frontal zones where the warm offshore waters underlay lighter cold coastal waters.

Spatial variability of phytoplankton primary production characteristics in the North Atlantic in summer 2013

Spatial variability of phytoplankton primary production characteristics has been studied along the transects between Shetland Islands and Iceland (transect I) and along 59.5° N (transect II) from 30 June to 16 July 2013. It has been shown that surface chlorophyll a concentration (Chl0) varied more than two order of magnitude from 0.07 to 6.67 mg/m3 (transect I) and from 0.02 to 3.63 mg/m3 (transect II). Water column integrated primary production (IPP) changed by a factor of 3.8 from 273 to 1040 and by a factor of 5.6 from 68 to 379 mgC/m2 per day along transects I and II, respectively. It has been established that spatial variability of Chl0 and IPP was consistent with distribution of the main surface flows and thermohaline fronts. That conclusion was made on the basis of reliable positive correlation between Chl0 and the zonal potential temperature gradient (R = 0.43, p < 0.01, N = 65). Phytoplankton assimilation activity along transect II depends on nutrients concentration. That conclusion was confirmed by reliable positive correlation between optimum chlorophyll specific carbon fixation rate (Pbopt) and phosphate concentration (R = 0.58, p < 0.05, N = 76) and between Pbopt and dissolved silica (R = 0.51, p < 0.05, N = 76).

Impact of a dipole on the phytoplankton community in a semi-enclosed basin of the southern Gulf of California, Mexico

Summary The present study assesses the impact of a dipole on the abundance and distribution of phytoplankton groups as well as the chlorophyll-a concentration in the Bay of La Paz, Gulf of California, Mexico. Based on in situ observations obtained in a multidisciplinary research cruise during the summer of 2008, a mesoscale dipole (cyclone-anticyclone) was observed; the cyclone had ∼25 km diameter and tangential speed of ∼45 cm s−1, while the anticyclone had ∼15 km diameter and tangential speed of ∼40 cm s−1. Strong gradients in conservative temperature and density were observed between both structures, suggesting the presence of an oceanic thermohaline front. Differences in phytoplankton distribution showed minimum abundance of diatoms in the southern bay and close to Roca Partida Island, and maximum in the periphery in the northern cold core. The maximum abundance of dinoflagellates and silicoflagellates occurred at the frontal zone. The chlorophyll-a concentration was high in the region associated with the frontal zone. Although mesoscale eddies are ubiquitous processes in the Bay of La Paz, this study represents the first observational report of the impacts of a dipole on the phytoplankton structure and chlorophyll-a in the region. The observations presented here indicate the existence of a strong association between the mesoscale processes and the phytoplankton community in the study area. This study highlights the value of efforts to improve projections of physical forcing and its influence on the planktonic ecosystem.

Millennial-scale Holocene hydrological changes in the northeast Atlantic: New insights from ‘La Grande Vasière’ mid-shelf mud belt

A mid- to late-Holocene paleohydrological reconstruction from the northeast Atlantic is proposed through the study of a high-resolution sedimentary record from the northern continental shelf of the Bay of Biscay (BoB). Three foraminiferal species dominate the assemblages with Rosalina globularis showing an overall decrease in absolute and relative abundances from ~7 to 0.4 cal. ka BP, whereas the opposite trend is observed for Cibicides refulgens and Lobatula. These long-term patterns are interpreted as a response to the overall cooling trend and/or the progressive deepening of the water column because of the relative sea-level (RSL) rise. Foraminiferal δ18O and grain-size analyses show a significant shift around 3.5–2.5 cal. ka BP toward a heavier isotopic signature and finer sediments. We mainly link this change to enhanced contribution of continental freshwaters and fine sediments after the near-stabilization of the RSL rise. By reducing coastal accommodation spaces, this led to a better channelization of river outflows and probably to the formation of the modern winter thermohaline front. Superimposed on these long-term patterns, our data highlight strong millennial-scale variability (1250-year peak). Such cyclicity is consistent with several records tracing changes in rainfall and storminess regimes in northern Europe, and the dynamics of the subpolar gyre (SPG). We suggest a millennial time-scale control of a NAO-like (North Atlantic Oscillation) climatic process modulating continental humidity (and the associated river discharges) and SPG dynamics through wind stress. Spectral analyses reveal an additional 500-year frequency peak implying a possible solar forcing.

Front‐Eddy Influence on Water Column Properties, Phytoplankton Community Structure, and Cross‐Shelf Exchange of Diatom Taxa in the Shelf‐Slope Area off Concepción (∼36–37°S)

AbstractIn eastern boundary current systems (EBCSs), submesoscale to mesocale variability contributes to cross‐shore exchanges of water properties, nutrients, and plankton. Data from a short‐term summer survey and satellite time series (January–February 2014) were used to characterize submesoscale variability in oceanographic conditions and phytoplankton distribution across the coastal upwelling and coastal transition zones north of Punta Lavapié, and to explore cross‐shelf exchanges of diatom taxa. A thermohaline front (FRN‐1) flanked by a mesoscale anticyclonic intrathermocline eddy (ITE‐1), or mode‐water eddy, persisted during the time series and the survey was undertaken during a wind relaxation event. At the survey time, ITE‐1 contributed to an onshore intrusion of warm oceanic waters (southern section) and an offshore advection of cold coastal waters (northern section), with the latter forming a cold, high chlorophyll‐a filament. In situ phytoplankton and diatom biomasses were highest at the surface in FRN‐1 and at the subsurface in ITE‐1, whereas values in the coastal zone were lower and dominated by smaller cells. Diatom species typical of the coastal zone and species dominant in oceanic waters were both found in the FRN‐1 and ITE‐1 interaction area, suggesting that this mixture was the result of both offshore and onshore advection. Overall, front‐eddy interactions in EBCSs could enhance cross‐shelf exchanges of coastal and oceanic plankton, as well as sustain phytoplankton growth in the slope area through localized upward injections of nutrients in the frontal zone, combined with ITE‐induced advection and vertical nutrient inputs to the surface layer.

Factors Influencing the Distribution of Chl-a along Coastal Waters of East Peninsular Malaysia

Determination of chlorophyll-a (Chl-a) distribution in the coastal waters is important to understand the coastal environmental conditions. This study was conducted to understand the spatial and temporal distribution of Chl-a along coastal waters of east Peninsular Malaysia and factors influencing its variability using Chl-a data derived from Aqua MODIS satellite (1 km spatial resolution) from January 2006 to December 2012. Chl-a variability was described using empirical orthogonal function (EOF) analysis. In-situ data (temperature, salinity, density and nitrate) and rainfall data from the Department of Meteorology were analyzed using spatial interpolation to determine factors influencing the distribution of Chl-a. The seasonal progressions of Chl-a showed high value during northeast monsoon along the coast. This variability was described by four modes of the EOF analysis. The first mode (72.08% of total variance) indicated seasonal cycle with high variability along the coast. Second mode (17.03% of variance) explained the northeast monsoon with high variability from river mouth to the south. Third mode (2.39% of variance) indicated variability during southwest monsoon along the coast and much higher to the south. Mode 4 (1.93% of variance) explained the inter-monsoon period observed along the northern and southern coastline. Concentration and distribution of Chl-a were related to availability of nutrient influenced by rainfall. The thermohaline front was also observed to play an important role in accumulation of phytoplankton biomass during northeast and southwest monsoon.