Subtropical Shelf Water Research Articles

High-resolution UK’37 sea surface temperature reconstruction in the southwestern Atlantic Ocean during the Holocene

Modern sea surface temperatures (SST) were assessed using the alkenone calibration index (UK′37; SST-UK′37) values from twenty-one core-top sediment samples collected along the Southwestern Atlantic Ocean and validated by comparison with satellite-derived SSTs. The SST-UK′37 values revealed an annual signal between 28 and 30°S and a winter signal (83% of the region samples) between 30 and 34° S, with deviations falling within the UK′37 calibration error range. The paleotemperature (SST-UK′37) reconstruction through the Holocene was performed using two high-resolution marine sediment cores from 33 to 34°S latitude band. Millennial-scale variations dominate most of the SST-UK′37 results with changes up to ∼6 °C. Considering the modern ocean circulation dynamics, the SST-UK’37 in our records likely stems from the combined influence of the Subtropical Shelf Front (STSF) position and the interaction between the warm, salty, and nutrient-poor Subtropical Shelf Water (STSW) and the cold, fresh, and nutrient-rich Subantarctic Shelf Water (SASW). Our SST-UK’37 results also suggest that the northernmost position limit of the STSF reached around 33°S during the mid-to-late Holocene, while after ∼5500 cal yr BP the Plata Plume Water became the dominant influence on the regional SST-UK’37 signal. Spectral analyses revealed statistically significant oscillations of ∼8600 and ∼ 2250 years in record MDBT 557, and ∼ 5300 and ∼ 2600 years in record MDBT 561 which suggests the influence of Southern Westerly Winds (SWW) on the SST-UK’37, and therefore, on the STSF dynamic through the Holocene.

Tracing last millennium cycles of Río de la Plata Plume Water input into the Southwestern Atlantic Ocean

Recent studies established a strong connection between instrumental hydroclimatic changes in the Río de la Plata (RdlP) watershed and the influx of continental terrigenous material and productivity changes. This correspondence was further validated for the last millennium. Here, we identified centennial, multi-decadal, and interannual changes in diatom composition, as a proxy for La Plata Plume Water (PPW) discharge into the inner shelf. Core GeoB13813-4 diatom data were revisited, and grouped into freshwater, marine, and brackish categories for multivariate, timeseries, and correlation analyses. Such record exhibits a noteworthy resemblance to paleoclimatic and paleoceanographic variations throughout the past millennium. A Detrended Correspondence Analysis (DCA) on the dominant taxa, and Axis-1 revealed a salinity gradient over the past millennium. DCA1-scores exhibited significant correlation with reconstructions of El Niño Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), and the Atlantic Multidecadal Oscillation (AMO) over the same period.By plotting DCA1-scores, we inferred time intervals with varying levels of PPW influence. During ∼936–1500 CE, the PPW influence was weak, as the record was dominated by marine taxa, indicating lateral transport from the Brazilian upwelled water and a stronger influence of the Subtropical Shelf Water. After 1500 CE, a well-developed plume was inferred, with the highest influence occurring after 1800 CE, as evidenced by an increased presence of freshwater/brackish taxa. This increased PPW influence after 1500 CE can be attributed to the onset of more humid conditions and associated increased river discharge, particularly under El Niño conditions, with further ENSO intensification after 1800 CE. We identified significant centennial (250 years) and multidecadal cycles (25–50 years) in diatom community composition. These cycles are related to solar forcing, PDO, and AMO. During the 20th Century, cycles of 2 to 7 years were associated with interannual ENSO variability, while 40-year cycles were linked to AMO and PDO variability.

Hydrographic structure of the continental shelf in Santos Basin and its causes: The SANAGU and SANSED campaigns (2019)

This study describes the hydrography and water masses of the Santos Basin Continental Shelf (SBCS) during two hydrographic campaigns (SANAGU, SANSED) in 2019. Coastal Water (CW) is the dominant water mass in the southern portion of the SBCS, with relatively low salinity values (S<35.5 g kg–1), and satellite data show that local precipitation and river discharge could not account for the low salinity observed during the cruises in the southern region of the domain. The low salinity observed is explained by the transport from the south influenced by Subtropical Shelf Water (STSW), which was influenced by the La Plata River discharge. In the northern region of the SBCS, the South Atlantic Central Water (SACW) dominates the bottom layers of the water column, with the wind playing a major role in the uplift of this water mass, as evidenced by a wind impulse analysis. In this part of the shelf, Tropical Water (TW) was the second water mass in volume and occupied the surface layers and offshore the shelf. CW is restricted to a thin surface layer, which reaches distances of a few kilometers from the coast. Our analysis show the differences in the hydrographic structure of the SBCS and suggests that the SBCS can be divided in two regions with distinct characteristics: 1) the area southwest of São Sebastião, where the hydrographic parameters were modulated by the presence of the Subtropical Shelf Water (STSW); 2) the area northeast of São Sebastião, where the uplifting of SACW was the dominant process.

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.

Nutrient and carbon dynamics under the water mass seasonality on the continental shelf at the South Brazil Bight

Abstract The enrichment of the euphotic zone and the production of organic matter (OM) are induced by wind-driven processes that change the water mass distribution in the central inner and mid continental shelf of Santa Catarina State (27°18′–27°37′S), South Brazil Bight (23–28.5°S). During austral summer, the Ekman transport by north-eastern winds is balanced by onshore bottom intrusions of the nutrient-rich South Atlantic Central Water (SACW), while during austral winter, the Plata Plume Water (PPW) reaches the region, which is pushed northward by south-eastern winds. Between 2014 and 2016, six oceanographic campaigns were conducted at the isobaths of 30 and 50 m, one in the early fall of 2014, others in the summers of 2015 and 2016, and in each winter of the stated years. The aim was to investigate the spatio-temporal variability of nutrients (nitrate + nitrite, ammonium, phosphate and silicate), and composition of the particulate organic matter (POM), i.e. particulate organic carbon and nitrogen, in relation to the water masses dynamics. In summer of 2015 and 2016, the SACW reached all over the bottom layers of the studied area, while in 2014, it only reached the outer stations. The SACW intrusion was reflected by relatively high phosphate (0.61 ± 0.29 μM), silicate (9.51 ± 4.34 μM), and notably high nitrate (4.21 ± 2.83 μM) concentrations. The POM composition revealed the deep chlorophyll-a maximum at intermediate and bottom layers, indicating new primary production associated with the nutrient-rich SACW. The denitrification was identified in bottom water by nitrogen loss (up to 9.66 μM) associated with the low dissolved oxygen saturation (DO%) (53.07 ± 7.8%). In winter, the PPW reached the region in the surface, especially in 2015 and 2016, with high silicate concentrations (8.36 ± 2.46 μM). Nitrate (1.69 ± 0.92 μM) and phosphate (0.46 ± 0.23 μM) were mainly related to regeneration processes in the bottom layers associated with low DO% (78.25 ± 7.55%). In summer, the SACW presented higher potential for biological production against Subtropical Shelf Water (STSW), while in winter, the STSW was higher than the PPW. Besides the new nutrient supply associated with wind-driven water mass dynamics, the nutrients regeneration contributed to the food chain structure via a regenerated primary production.

Bacterioplankton abundance and biomass stimulated by water masses intrusions over the Southern Brazilian Shelf (between 25°57'S and 29°24'S)

In order to understand the distribution of bacterioplankton along the continental shelf off the Santa Catarina coast (25°57'S and 29°24'S) and whether water masses were affecting this distribution, we measured abundance, biovolume, and biomass of bacterioplankton, and the physico-chemical characteristics of the water masses in three transects in two distinct seasons: summer and winter of 2010. During the austral summer, high continental runoffs from the Itajaí River and Babitonga Bay mix over the shelf with Tropical Water (TW) and originate the summer Subtropical Shelf Water (STSW). Bacterial biomass reached an average of 2.5–5.0 μgC L−1 in summer, with values observed in the southernmost transect (Araranguá) being one order of magnitude higher when compared to the others, where nutrient-rich South Atlantic Central Water (SACW) intrusions are often near the coast. On the contrary, in the austral winter, the Plata Plume Water (PPW) spread on the first 40 m of water positively influenced bacterial abundance and biomass, which were on average ten times higher than those observed in the summer (abundance of up to 2.5 × 109 cells L−1, and biomass up to 60 μgC L−1). As a conclusion, PPW, indicated by the colder waters, high silicate and Kd values, and high concentrations of particulate organic carbon have a positive effect on the abundance and biomass of bacterioplankton, whereas the intrusions of SACW, and local freshwater runoffs have secondary effects on bacterial parameters. This is the first description of the influence of PPW intrusions over the dynamics of bacterioplankton, considering two contrasting environmental periods, and, consequently, to carbon budget studies in the Southern Brazilian Shelf (SBS). Taking into account both abundance and biomass of bacterioplankton and their association with water masses in the SBS, our results provide fundamental information for further biogeochemical studies.

Phyto- and protozooplankton assemblages and hydrographic variability during an early winter survey in the Southern Brazilian Continental Shelf

Abstract The distribution of phytoplankton and protozooplankton assemblages in relation to hydrographic variability across the southern Brazilian continental shelf were studied based on data collected during early winter (June/2012), complemented with MODIS-Aqua satellite imagery. The in situ data set was comprised by water column structure properties, dissolved inorganic nutrients (ammonium, nitrite, nitrate, phosphate and silicate), phytoplankton and protozooplankton composition and biomass [chlorophyll a (Chl a) concentration and carbon content]. Phytoplankton assemblages were assessed by both microscopy and HPLC-CHEMTAX approaches. A canonical correspondence analysis associating physical, chemical and phytoplankton composition data at surface revealed a tight coupling between the phytoplankton community and hydrographic conditions, with strong environmental gradients across three different domains: the pelagic, outer shelf Tropical Water (TW); the mid shelf domain under influence of Subtropical Shelf Water (STSW); and the inner shelf domain, mainly under influence of riverine outflow of the La Plata River Plume Water (PPW). Results showed that diatoms dominated the phytoplankton assemblages throughout the study region, but other groups were also important and varied according to the different water masses at surface. The low salinity and nutrient-rich PPW stimulated a high phytoplankton biomass and diversity of diatoms and dinoflagellates within the inner shelf region, with enhanced Chl a levels (>1.3 mg m−3) and a great abundance of large and medium-sized diatoms (some of these appeared exclusively in PPW), as well as ciliates, dinoflagellates, raphidophyceans and cryptophytes. Conversely, smaller diatoms (e.g. Rhizosolenia clevei), picoplanktonic species (prochlorophytes and cyanobacteria) and a noticeable contribution of dinoflagellates and other flagellates, associated with lower Chl a levels (

Phytoplankton community structure in relation to hydrographic features along a coast-to-offshore transect on the SW Atlantic Continental Shelf

The continental shelf in Southern Brazil is characterized by high biological productivity associated with horizontal and vertical density gradients due to the mixing of distinct water masses. Phytoplankton biomass and composition were evaluated in summer 2013 along an on-offshore transect off the mouth of the Patos Lagoon (Lat. 32°12S). Photosynthetic active radiation, temperature, salinity and fluorescence vertical profiles were carried out and Brünt-Väisäla frequency was estimated. Three water bodies were identified: the Subtropical Shelf Water along the entire transect, the Plata Plume Water on the middle shelf surface and the Tropical Water farther offshore. The water was sampled (N = 40) for the analyses of dissolved inorganic nutrients, phytoplankton cell density and composition. Phytoplankton present in the water was identified and quantified by the classical microscope sedimentation technique, complemented with CHEMTAX analysis of high-performance liquid chromatography (HPLC) pigment data. From the results obtained, chlorophyll a concentration was higher at both coastal stations (1.6–2.0mgm−3) where the water column was homogeneous and diatoms dominated the stations. This group was replaced by dinoflagellates in stratified conditions on the shelf and farther offshore. Along the onshore-offshore gradient, two types of dinoflagellates were found: the peridinin-containing dinoflagellates Prorocentrum and Scrippsiella with a small contribution at the coastal stations, and the fucoxantin-containing small Gymnodiniales cells (< 15µm) with more than 50% of the total chlorophyll a at the stations on the continental shelf, especially associated with the chlorophyll maximum at the base of the euphotic zone. The positive (negative) relationship between the biomass of dinoflagellates (diatoms) with the Brünt-Väisäla frequency, respectively, support the hypothesis that stratification is the most important environmental factor that determines the biomass of phytoplankton communities and distribution on the shelf and in coastal waters off Southern Brazil in summer. Picoplankton cells (Prochlorococcus and Synechococcus), recorded for the first time in the region under study, were predominant in the nutrient-poor and well-lit surface layers along the transect, indicating the importance of their low sedimentation rates (small size) and photo-adaptive strategies to survive on the upper layers of the water column.

Regional modeling of the water masses and circulation annual variability at the Southern Brazilian Continental Shelf

The Southern Brazilian Continental Shelf (SBCS) is one of the more productive areas for fisheries in Brazilian waters. The water masses and the dynamical processes of the region present a very seasonal behavior that imprint strong effects in the ecosystem and the weather of the area and its vicinity. This paper makes use of the Regional Ocean Modeling System (ROMS) for studying the water mass distribution and circulation variability in the SBCS during the year of 2012. Model outputs were compared to in situ, historical observations and to satellite data. The model was able to reproduce the main thermohaline characteristics of the waters dominating the SBCS and the adjacent region. The mixing between the Subantarctic Shelf Water and the Subtropical Shelf Water, known as the Subtropical Shelf Front (STSF), presented a clear seasonal change in volume. As a consequence of the mixing and of the seasonal oscillation of the STSF position, the stability of the water column inside the SBCS also changes seasonally. Current velocities and associated transports estimated for the Brazil Current (BC) and for the Brazilian Coastal Current (BCC) agree with previous measurements and estimates, stressing the fact that the opposite flow of the BCC occurring during winter in the study region is about 2 orders of magnitude smaller than that of the BC. Seasonal maps of simulated Mean Kinetic Energy and Eddy Kinetic Energy demonstrate the known behavior of the BC and stressed the importance of the mean coastal flow off Argentina throughout the year.

Living benthic foraminiferal species as indicators of cold-warm water masses interaction and upwelling areas

The western South Atlantic continental margin, between 27° and 37°S, is dominated by four main water masses: cold-fresh Subantarctic Shelf Water (SASW), warm-salty Subtropical Shelf Water (STSW), cold upwelled South Atlantic Central Water (SACW), and fresh Plata Plume Water (PPW). Despite the large seasonal variability of PPW extension along the shelf, an intense and relatively stable temperature-salinity gradient separates the SASW and the STSW forming the Subtropical Shelf Front (STSF) around 32°S. The two dominant shelf water masses (SASW and STSW) arise from the process of mixing of oceanic waters. The SASW originates from the dilution of Subantarctic Water due to excess precipitation and continental runoff, and the STSW consists of modified warm tropical waters and South Atlantic Central Water (SACW) diluted below PPW. A previous article demonstrates distribution of Bulimina marginata, a shelf environment and deep-sea species of benthic foraminifera, is influenced by the front location and it can be used as a proxy of the STSF in sediment core analysis. Here we show three other infaunal living species inhabiting at the Continental margin: Buccella peruviana, Globocassidulina subglobosa and Uvigerina peregrina and their distribution limits show the interaction of Subantartic Shelf Water, Subtropical Shelf Water, and upwelling of SACW, in the bottom sediment of coastal studied areas.

TRACING THERMOHALINE PROPERTIES AND PRODUCTIVITY OF SHELF-WATER MASSES USING THE STABLE ISOTOPIC COMPOSITION OF BENTHIC FORAMINIFERA

We explored the applicability of stable isotopic ratios of nine foraminiferal species, four epifaunal (Pseudononion atlanticum, Cibicides fletcheri, Hoeglundina elegans, Hanzawaia boueana) and five infaunal (Angulogerina angulosa, Uvigerina peregrina, Buccella peruviana, Cassidulina subglobosa, Bulimina marginata), as environmental tracers on the southwest Atlantic continental shelf from samples collected along the Argentinean-Uruguayan-Brazilian Atlantic coast during austral winter 2003 and summer 2004. Linear regressions show that the stable isotopic composition of living foraminifera is associated with geographical and environmental variables (latitude, water depth, temperature, salinity, and dissolved silica). Multiple linear regressions show that the relevant variables in this study are latitude, salinity, and biogenic silica. There is a general trend of higher δ 18 O and lower δ 13 C values with increasing latitude, reflecting the influence of cooler water masses toward the south. Specifically, the δ 18 O data from U. peregrina, H. elegans, and P. atlanticum follow meridional temperature gradients with lower δ 18 O values in relatively warm Subtropical Shelf Water at the northernmost sites (27°S) and higher values in colder Sub-Antarctic Shelf Water at the southern sites (to 37°S). Angulogerina angulosa and P. atlanticum δ 18 O values correlate more strongly with salinity than temperature. Positive correlation of δ 13 C and water depth observed for some infaunal species indicates greater input of organic matter to the deeper-water benthos. In summer samples, however, this correlation between δ 13 C and water depth weakens, reflecting a less stratified water mass associated with upwelling. Results from this study support the application of these foraminifera to reconstructions of paleoenvironmental changes from sediment cores.

Living Bulimina marginata in the SW Atlantic continental margin: Effect of the Subtropical Shelf Front and South Atlantic Central Water

The western South Atlantic continental margin, between 27° and 37°S, is dominated by three main water masses: cold-fresh Subantarctic Shelf Water (SASW), warm-salty Subtropical Shelf Water (STSW) and Plata Plume Water (PPW). Despite the large seasonal variability of PPW extension along the shelf, an intense and relatively stable temperature-salinity gradient separates the SASW and the STSW forming the Subtropical Shelf Front (STSF) around 32°S. This article demonstrates that the distribution of Bulimina marginata, (shelf environment and deep-sea) a species of benthic foraminifera is influenced by the front location and it can be used as a proxy of the STSF in sediment core analysis.

Shifts of the Subtropical Shelf Front controlled by atmospheric variations

In the western South Atlantic, off the coast of South America, a band of cold, fresh, nutrient‐rich Sub‐Antarctic Shelf Water (SASW) meets warm, salty, nutrient‐poor Subtropical Shelf Water (STSW) to form the Subtropical Shelf Front (STSF). This front is the shallow‐water expression of the major Brazil‐Malvinas Confluence Zone and has moved northward and southward during the Holocene (the past ~12,000 years). Bender et al. reconstruct the latitudinal shifts of the STSF over the past 11,000 calendar‐equivalent years using records of oxygen and carbon stable isotope compositions of benthic foraminifera and total organic carbon and calcium carbonate content from a sediment record collected off Uruguay. These measurements serve as proxies for ocean water temperature and nutrient content, which can be used to distinguish the SASW and STSW.

Holocene shifts of the Subtropical Shelf Front off southeastern South America controlled by high and low latitude atmospheric forcings

Over the Uruguayan shelf and uppermost slope, the coalescence of northward flowing Subantarctic Shelf Water and southward flowing Subtropical Shelf Water forms a distinct thermohaline front termed the Subtropical Shelf Front (STSF). Running in a SW direction diagonally across the shelf from the coastal waters at 32°S toward the shelf break at ca. 36°S, the STSF represents the shelfward extension of the Brazil‐Malvinas Confluence zone. This study reconstructs latitudinal STSF shifts during the Holocene based on benthic foraminifera δ18O and δ13C, total organic carbon, carbonate contents, Ti/Ca, and grain size distribution from a high‐accumulation sedimentary record located at an uppermost continental‐slope terrace. Our data provide direct evidence for: (1) a southern STSF position (to the South of the core site) at the beginning of the early Holocene (&gt;9.4 cal ka BP) linked to a more southerly position of the Southern Westerly Winds in combination with restricted shelf circulation intensity due to lower sea level; (2) a gradual STSF northward migration (bypassing the core site toward the North) primarily forced by the northward migration of the Southern Westerly Winds from 9.4 cal ka BP onward; (3) a relatively stable position of the front in the interval between 7.2 and 4.0 cal ka BP; (4) millennial‐scale latitudinal oscillations close to 36°S of the STSF after 4.0 cal ka BP probably linked to the intensification in El Niño Southern Oscillation; and (5) a southward migration of the STSF during the last 200 years possibly linked to anthropogenic influences on the atmosphere.

Remarks on the morphology and distribution of some rare centric diatoms in southern Brazilian continental shelf and slope waters

This paper provides information on the morphology and occurrence of extant diatoms of the family Asterolampraceae and Azpeitia species of the southernmost Brazilian continental shelf and slope waters in the Southwestern Atlantic Ocean (28 -34ºS), under the influence of Tropical, Subtropical, Subantarctic Waters, terrestrial discharges from La Plata River and Lagoa dos Patos lagoon. Plankton was sampled (20 µm net; vertical hauls) at the same 27 stations in winter 2005 and summer 2007. Among Asterolampraceae, Asteromphalus flabellatus was the most frequent species (57% of samples) observed mainly in winter samples (92%) in Subtropical Shelf Water but also under the influence of La Plata River plume (salinity &lt;35). Lower frequencies were observed for Asterolampra marylandica, Asteromphalus elegans, Asteromphalus heptactis and Spatangidium arachne. Four species of Azpeitia were observed: A. barronii and A. neocrenulata for the first time in the South Atlantic Ocean while A. africana and A. nodulifer had already been registered in equatorial and southern areas of Brazil. All Azpeitia species were rare (19%) in offshore samples (100-200 m depth. Remarks on the morphology based on light and scanning electron microcopy observations are provided along with their distribution in the study area.

Tracing latitudinal gradient, river discharge and water masses along the subtropical South American coast using benthic Foraminifera assemblages

More than 30% of Buccella peruviana (D'Orbigny), Globocassidulina crassa porrecta (Earland & Heron-Allen), Cibicides mackannai (Galloway & Wissler) and C. refulgens (Montfort) indicate the presence of cold Sub Antarctic Shelf Water in winter, from 33.5 to 38.3º S, deeper than 100 m, in the southern part of the study area. In summer, the abundance of this association decreases to less than 15% around 37.5-38.9º S where two species (Globocassidulina subglobosa (Brady), Uvigerina peregrina (Cushman) take over. G. subglobosa, U. peregrina, and Hanzawaia boueana (D'Orbigny) are found at 27-33º S in both seasons in less than 55 m deep in the northern part, and are linked with warm Subtropical Shelf Water and Tropical Water. Freshwater influence was signalized by high silicate concentration and by the presence of Pseudononion atlanticum (Cushman), Bolivina striatula (Cushman), Buliminella elegantissima (D'Orbigny), Bulimina elongata (D'Orbigny), Elphidium excavatum (Terquem), E. poeyanum (D'Orbigny), Ammobaculites exiguus (Cushman & Brönnimann), Arenoparrella mexicana (Kornfeld), Gaudryina exillis (Cushman & Brönnimann), Textularia earlandi (Parker) and thecamoebians in four sectors of the shelf. The presence of Bulimina marginata (D'Orbigny) between 34.1-32.8º S in the winter and 34.2-32.7º S in the summer indicates that the influence of the Subtropical Shelf Front on the sediment does not change seasonally, otherwise, the presence of Angulogerina angulosa (Williamson) in the winter, only in Mar del Plata (38.9º S), show that Malvinas currents are not influencing the sediment in the summer.

Ichthyoplankton of arvoredo biological marine reserve, Santa Catarina, Brazil

Arvoredo Island, located in Santa Catarina state - south Brazil, and its surrounding area were defined as a Conservation Unit (CU) in the category of Biological Reserve since 1990. This research aimed to analyze the inter-annual and seasonal (winter and summer) variations of ichthyoplankton densities at Arvoredo Biology Marine Reserve (ABMR), and their relationship with environmental variables in 1997/1998 (Campaign 1), 2007/2008 (Campaign 2) and 2008/2009 (Campaign 3). Fish eggs and larvae were sampled using a WP-2 net with 200 µm mesh size. The study area was influenced by three water masses, (i) Coastal Water throughout the whole year, (ii) Subtropical Shelf Water during the winter, and (iii) South Atlantic Central Water mainly in summer. A total of 4,891 eggs were collected and classified as Engraulidae and Sardinella brasiliensis (Clupeidae). The total number of larvae was 467 belonging to 5 orders, 19 families, and 21 species. Taxonomic composition demonstrated a seasonal pattern among periods, with the highest densities of Engraulidae occurring in winter and the families Carangidae, Clupeidae and Gerreidae in summer. The high number of families and abundance of ichthyoplankton observed in ABMR may be important in supplying the adjacent coastal areas impacted by fishing.

Composition of Trichiuridae and Gempylidae larvae (Teleostei) and their association with water masses in the Southwest Atlantic Ocean

Abstract In this paper the relationship between temporal changes in the occurrence of water masses and Trichiuridae and Gempylidae larvae composition and distribution in the Southwest Atlantic ocean were analysed between 25° and 40° S. Ichthyoplankton was collected during the three expeditions of the Subtropical Convergence Project: Winter and Spring 1977, Autumn 1978 and Summer 1981, realized in the Southwest Atlantic waters. Oblique tows were conducted using a Hensen net with 250 μm mesh size. Steep salinity and temperature gradient were found, where the river outflows from La Plata river (Argentina) and Patos Lagoon (Brazil) met the Tropical Water over the continental shelf between 32 and 36° S. We examined 524 Hensen-net samples that contained about 283 larvae from five species of Trichiuridae and Gempylidae. The most abundant and frequent specie were Trichiurus lepturus Linnaeus 1758 and Diplospinus multistriatus Maul 1948. The mesopelagics species D. multistriatus, Nealotus tripes Johnson 1865 and Lepidopus altifrons Parin&amp;Collette, 1993 were associated with Tropical Water. The benthopelagic T. lepturus and the epipelagic Thyrsitops lepidopoides (Cuvier 1832) were associated mainly with Coastal Water and Subtropical Shelf Water.

Variability of the subtropical shelf front off eastern South America: Winter 2003 and summer 2004

Hydrographic data collected during surveys carried out in austral winter 2003 and summer 2004 are used to analyze the distributions of temperature ( T) and salinity ( S) over the continental shelf and slope of eastern South America between 27°S and 39°S. The water mass structure and the characteristics of the transition between subantarctic and subtropical shelf water (STSW), referred to as the subtropical shelf front (STSF), as revealed by the vertical structure of temperature and salinity are discussed. During both surveys, the front intensifies downward and extends southwestward from the near coastal zone at 33°S to the shelf break at 36°S. In austral winter subantarctic shelf water (SASW), derived from the northern Patagonia shelf, forms a vertically coherent cold wedge of low salinity waters that locally separate the outer shelf STSW from the fresher inner shelf Plata Plume Water (PPW) derived from the Río de la Plata. Winter T– S diagrams and cross-shelf T and S distributions indicate that mixtures of PPW and tropical water only occur beyond the northernmost extent of pure SASW, and form STSW and an inverted thermocline characteristic of this region. In summer 2004, dilution of Tropical water (TW) occurs at two distinct levels: a warm near surface layer, associated to PPW–TW mixtures, similar to but significantly warmer than winter STSW, and a colder ( T∼16 °C) salinity minimum layer at 40–50 m depth, created by SASW–STSW mixtures across the STSF. In winter, the salinity distribution controls the density structure creating a cross-shore density gradient, which prevents isopycnal mixing across the STSF. Temperature stratification in summer induces a sharp pycnocline providing cross-shelf isopycnal connections across the STSF. Cooling and freshening of the upper layer observed at stations collected along the western edge of the Brazil Current suggest offshore export of shelf waters. Low T and S filaments, evident along the shelf break in the winter data, suggest that submesoscale eddies may enhance the property exchange across the shelf break. These observations suggest that as the subsurface shelf waters converge at the STSF, they flow southward along the front and are expelled offshore, primarily along the front axis.

Space–time variability of the Plata plume inferred from ocean color

Satellite ocean color and surface salinity data are used to characterize the space–time variability of the Río de la Plata plume. River outflow and satellite wind data are also used to assess their combined effect on the plume spreading over the Southwestern South Atlantic continental shelf. Over the continental shelf satellite-derived surface chlorophyll- a (CSAT) estimated by the OC4v4 SeaWiFS retrieval algorithm is a good indicator of surface salinity. The log (CSAT) distribution over the shelf presents three distinct modes, each associated to: Subantarctic Shelf Water, Subtropical Shelf Water and Plata Plume water. The log (CSAT) 0.4–0.8 range is associated with a sharp surface salinity transition across the offshore edge of the Plata plume from 28.5 to 32.5. Waters of surface salinity <31, derived from mixtures of Plata waters with continental shelf waters, are associated to log (CSAT)>0.5. In austral winter CSAT maxima extend northeastward from the Plata estuary beyond 30°S. In summer the high CSAT waters along the southern Brazil shelf retreat to 32°S and extend south of the estuary to about 37.5°S, only exceeding this latitude during extraordinary events. The seasonal CSAT variations northeast of the estuary are primarily controlled by reversals of the along-shore wind stress and surface currents. Along-shore wind stress and CSAT variations in the inner and mid-shelves are in phase north of the estuary and 180° out of phase south of the estuary. At interannual time scales northernmost Plata plume penetrations in winter (∼1200 km from the estuary) are associated with more intense and persistent northeastward wind stress, which in the period 2000–2003, prevailed over the shelf south of 26°S. In contrast, in winter 1999, 2004 and 2005, characterized by weaker northeastward wind stress, the plume only reached between 650 and 900 km. Intense southwestward plume extensions beyond 38°S are dominated by interannual time scales and appear to be related to the magnitude of the river outflow. The plume response to large river outflow fluctuations observed at interannual time scales is moderate, except offshore from the estuary mouth, where outflow variations lead CSAT variations by about 2 months.