South Atlantic Central Water Intrusion Research Articles

South Atlantic Central Water Intrusion in the Southeast Brazil Bight and its impacts

The responses of biogeochemical cycles and ecosystems are influenced by a variety of physical conditions, including but not limited to temperature, moisture, and nutrient availability. The ecological significance of cold and nutrient-rich water inflow into coastal embayments is well-established and is often attributed to the presence of an upwelling system in the adjacent area. The Southeast Brazil Bight (SBB) provides a sheltered environment for both the Cabo Frio upwelling system and Guanabara Bay. The water mass from the Cabo Frio upwelling system is known as the South Atlantic Central Water (SACW), and cold water events related to it have a particular impact on Guanabara Bay. To understand the ecosystem’s response to cold water events in the SBB during the 2014–2015 austral summer, a compilation of datasets including temperature time series, satellite chlorophyll-a data, meteorological station data, tidal gauge readings, and biogeochemical model results were used. The results revealed an east-to-west cold water flow, with a 43-hour delay in the response observed in Guanabara Bay after it was observed in Cabo Frio. There was no indication of a second upwelling system near Guanabara Bay. The biogeochemical analysis indicated that each area presents a different limitant nutrient, resulting in unique and distinct trophic webs in both areas.

Temperature stratification in the Guanabara Bay and its relationship with wind-induced coastal upwelling off Cabo Frio, Brazil

This study investigates the mechanisms that control the occurrence of cold water intrusion events observed in Guanabara Bay, Rio de Janeiro, Brazil. To achieve this, we used multiple time-series data comprising in-situ temperatures recorded at depths of 5 m and 20 m in three sites in the Guanabara Bay region, wind velocities from three meteorological stations along the Brazilian coast, and sea level data from a tidal gauge located in the interior of Guanabara Bay. We found that from October 2014 to May 2016, near-bed temperatures in Guanabara Bay were dominated by subinertial variability. We identified subinertial cold water events, defined by temperature values below 18 °C, that were associated with South Atlantic Central Water intrusions. We obtained a total of 21 cold water events during the 18-month (545-day) sampling period, corresponding to 92.5 days or 17% of the time. Additionally, we found statistically significant lag-adjusted correlation coefficients (0.37-0.95) between near-bed (20 m) temperatures in Guanabara Bay and alongshore winds from the Arraial do Cabo meteorological station, which act in the well-known Cabo Frio upwelling system along the coast. These upwelling-favorable winds led near-bed temperature values in Guanabara Bay by 12 to 56 h and provided the remote forcing mechanism for the cold water intrusions in Guanabara Bay. Locally, the shelf-bay intrusion velocity values ranged from 0.04 to 0.50 m⋅s−1, consistent with a gravity current enhanced by baroclinic pressure gradient, the local forcing mechanism. The cold water intrusions caused an increase in the vertical temperature stratification within Guanabara Bay, as verified for all analyzed events. We also identified subinertial oscillations due to Coastal Trapped Waves (CTWs) propagation from our sea level data records and evaluated their impacts on cold water events.

A 3-year study of the seasonal variability of abundance, biomass and reproductive traits of Oncaea venusta (Copepoda, Oncaeidae) in a subtropical coastal area

Abstract This study reports on the abundance, biomass and reproductive traits of Oncaea venusta, a common pelagic copepod in tropical oceans, in a coastal area influenced by bottom intrusions of the cold and nutrient-rich South Atlantic Central Water (SACW). In addition to O. venusta abundance and biomass, we estimated egg production and weight-specific fecundity rates by applying the egg-ratio method and incubation experiments, at monthly intervals over 3 years (July 2005–June 2008), to understand potential SACW influences on reproductive patterns. Mean abundance and biomass were low, and no differences were detected with and without SACW intrusions. Mean clutch size (±SD) was 65.1 ± 28.9 eggs female−1 and egg production rates averaged 7.6 ± 12.4 and 9.3 ± 5.2 eggs female−1 day−1 (viability >80%), for the egg-ratio method and incubation experiments, respectively, corresponding to a female weight-specific fecundity from 0.03 ± 0.05 to 0.04 ± 0.06 day−1. Oncaea venusta abundance and biomass were positively correlated with temperature, but no SACW influence on the reproductive rates was recorded. The egg production rate and the female weight-specific fecundity were influenced by the chlorophyll a concentration and the sex ratio. Although egg production and female weight-specific fecundity rates had peak values during intrusion events, no immediate impact on population abundance was observed within the same time interval.

Frequent upwelling intrusions and rainfall events drive shifts in plankton community in a highly eutrophic estuary

Seasonal upwelling and rainfall events influence estuarine communities, but their interaction is not well understood, particularly in estuaries impacted by coastal eutrophication. Here we evaluated the responses of the plankton community to multiple drivers in Guanabara Bay, a eutrophic estuary in the western Atlantic influenced by coastal upwelling. Plankton taxonomy and biomass were determined through weekly sampling over 1-year. The presence of the South Atlantic Central Water (SACW) signature was detected in the bay in half of our samples during the wet season. Importantly, SACW intrusions derived from coastal upwelling had a significant impact on plankton community composition. For microplankton, taxonomic shifts reflected in different cell sizes, with larger mixotrophic and heterotrophic taxa characterizing SACW intrusions. Larger species of copepods, including indicators of SACW, were also associated to the upwelling intrusion. Overall, plankton biomass was not explained by the SACW presence or absence alone; instead, the interaction between rainfall and upwelling was key to determine temporal variations in biomass. Our findings suggest that SACW intrusions can impact food web functioning through shifts in community composition and size depending on the relative intensity and frequency of upwelling events and rainfall. Frequent SACW intrusions might also dilute the highly eutrophic waters, with the potential to shift the metabolism of an estuary heavily impacted by eutrophication.

Spatio-temporal sublittoral macrobenthic distribution and dominant species in Guanabara Bay, Rio de Janeiro, Brazil.

Soft-bottom macrobenthic invertebrates are sensitive to natural or anthropogenic changes in aquatic ecosystems. The distribution patterns of sublittoral macrobenthic species in Guanabara Bay were studied from 2005 to 2007. Samples were collected at ten stations during six surveys throughout the rainfall regime (dry, early and late rainy). Ten replicates were collected at each station by Gravity corer or skin diving. Van Dorn bottles (bottom water) and by Ekman sediment sampler (granulometry) provided material for abiotic data. Stations were grouped into sectors (Entrance, Intermediary and Inner) based on abiotic data and location. The Redundancy Analysis (RDA) and Parsimonious RDA for all years and each annual cycle showed indicator taxa with high dominance in each sector. PERMANOVA indicated a regular seasonality between the surveys for the first annual cycle (p <0.05), and an atypical pattern for the second (p> 0.05), possibly due the low rainfall observed during this period. The mosaic of soft-bottom substrates infers structural variables, and patterns of temporal distribution were basically influenced by parameters those indicating pollution and the SACW (South Atlantic Central Water) intrusion, as well as ecological attributes among species, such as: predation, competition. The Ervilia concentrica and Cypridinidae could be used as indicators for anthropic and natural impacts in the Guanabara Bay for the Entrance sector, while Cyprideis salebrosa and Cyprideis sp. for the Intermediary sector and Heleobia australis for the Inner sector.

Nutrients and particulate organic matter dynamics in the outer-shelf of the South Brazil Bight: Two distinct scenarios during summer 2013

The pool of particulate organic matter driven by primary production in western boundary continental margins is known to be smaller than the highly eutrophic upwelling systems of eastern margins. Its production and export, however, can be high compared to productive ocean environments due to inherent specific physical dynamics such as mesoscale instabilities driven by the boundary current. To investigate how such physical process influence nutrients, chlorophyll-a and particulate matter on the outer shelf of the South Brazil Bight, two oceanographic cruises were conducted three months apart (November 2012 and January 2013), both under wind conditions that favor intrusions of South Atlantic Central Water (SACW). Our results showed two distinct hydrographic scenarios derived by the relative positions of oceanographic fronts in the outer shelf and shelf break region, associated with the Brazil Current (BC). In November 2012, the oligotrophic Tropical Water occupied a larger area of the continental shelf. Low nutrients and chlorophyll-a concentrations were found, typical of a regenerated production system. In contrast, maximum concentration of particulate matter and particulate inorganic carbon occurred at deeper layers in this period, together with low POC:PON ratio — probably from sediment resuspension. In January 2013, a strong shelf break upwelling was detected that enhanced SACW intrusion, notably increasing nutrient availability in the euphotic zone and the chlorophyll-a concentration at deeper layers. These deep chlorophyll maximum layers contribute to the production and accumulation of particulate organic carbon and nitrogen under these conditions. BC mesoscale instabilities appear to act as a particulate matter transport and export agent to the deep waters throughout the South Brazil Bight outer shelf.

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.

Seasonal and event-driven changes in the phytoplankton communities in the Araçá Bay and adjacent waters

Biomass and composition of phytoplankton are reported for the Araçá Bay (AB) and adjacent waters in the São Sebastião Channel (SSC) in relation to physical-chemical variables. The AB has unequivocal ecological and socio-economic importance but it is under current anthropic pressures that are expected to increase. Needed future management programs require establishing the natural scales of variability of the phytoplankton communities and main drivers. In this study, samplings were conducted every 5–6 weeks from September 2013 to August 2014 to evaluate the extent to which AB and SSC differ in time. The approach was: 1) to compare nutrient concentration and phytoplankton abundance and composition found at AB and adjacent SSC waters; 2) to associate significant changes in nutrient concentration and in phytoplankton composition with time and main meteo-oceanographic processes, such as changes in wind direction or intrusions of upwelled South Atlantic Central Water (SACW). Sampling design included two parallel sections of oceanographic stations (inside and outside AB) that were repeated in the morning and afternoon for 3 consecutive days. Temperature, salinity and chlorophyll-a fluorescence were measured vertically, and discrete water samples were collected for analyses of inorganic nutrients, chlorophyll (Chla) and enumeration of selected groups of phytoplankton. SSC water masses characteristics showed clear seasonality but also a significant hourly to daily variability driven by changes in wind speed and direction. Nutrients and Chla varied mainly on a scale of days, as did phytoplankton abundances and compositions and all variables compared were consistently higher at AB. Samplings were characterized in four groups: 1) warm and stratified waters, 2) cold and homogenous waters, 3) low salinity waters, and 4) an intermediary condition between groups 1 and 3. Diatoms were numerically important, including the potential bloom forming Pseudo-nitzschia spp, which were more frequent during warm and stratified conditions, when SACW intrusions were observed. Occurrences of Asterionellopsis spp and Trichodesmium spp were associated with changes in wind direction. Future monitoring programs in the region should combine short-time coastal observations and near-real-time regional hydrodynamic models. The results raise the awareness for the increasing urban expansion, inefficient sewage treatment and changes in the hydrodynamics of AB.

Seasonal variation of nutrients and hydrological conditions in the State Marine Park of Laje de Santos (SMPLS) and adjacent continental shelf areas (South Atlantic Ocean - Brazil)

Abstract Marine parks constitute important areas for the conservation of marine life and the genetic heritage around the world. The creation of such marine parks must be accompanied by careful measures to guarantee the coexistence of natural biota and human activities in these systems. The State Marine Park of Laje de Santos (SMPLS) is so close to an industrial pole and urban area that its creation and maintenance is an example for humanity. However, no program has yet been installed for the monitoring of its biotic and abiotic water parameters. Thus, the objective of this study is to provide hydrological and hydrochemical parameters with emphasis on dissolved nutrients to establish a starting point for the monitoring of these waters. The presence of the South Atlantic Central Water (SACW) in the marine park during the spring and summer sampling periods was evidenced by the observation of low temperatures (&lt; 17ºC) associated with salinity around 35 at the bottom of the water column, indicating a thermohaline feature. The dissolved oxygen presented high values, with few data below 4 mL L-1 occurring at the bottom of the water column. The pH was always in accordance with marine values (~8). Dissolved silicate and phosphate presented high values associated with inputs from the SACW intrusion, continental runoff and bottom resuspension processes, reaching maximum values of 15.86 and 1.23 µmol L-1, respectively. Nitrogen compounds were also associated with a natural fertilization process by the presence of the SACW at the bottom of the water column, as evidenced by the high nitrate concentrations (&gt; 7.00 µmol L-1), while the concentration of N-ammonium (maximum 9.86 µmol L-1) demonstrated a rapid regeneration of the organic matter, mainly in the euphotic zone. Analysis of the data from summer periods revealed an annual difference, showing January 2014 to be drier than January 2015, which influenced the availability of some nutrients and the standard distribution of hydrochemical parameters in this region. The results of the distribution of hydrochemical parameters in the marine park confirms the preserved conditions of the seawater around the Laje de Santos, demonstrated by the excellent water quality, concluding the need to implant monitoring actions based on these reference data to preserve this important reserve of marine life.

The Partitioning of Carbon Biomass among the Pico- and Nano-plankton Community in the South Brazilian Bight during a Strong Summer Intrusion of South Atlantic Central Water

To investigate how pico- and nanoplankton respond to oceanographic conditions in the Southwestern Atlantic Ocean, we assessed the influence of a summer intrusion of the South Atlantic Central Water (SACW) on the spatial and vertical dynamics of planktonic abundance and carbon biomass across environmental gradients. Seawater samples were collected from six depths within the euphotic zone at nine oceanographic stations in a transect on the Brazilian continental shelf in January 2013. The abundance of pico- and nanoplankton populations was determined by flow cytometry, and carbon biomass was calculated based on conversion factors from the literature. The autotrophic Synechococcus spp., picoeukaryotes and nanoeukaryotes were more abundant in the surface layers of the innermost stations influenced by Coastal Water (maximum of 1.19x105, 1.5x104, and 8.61x103 cell·mL−1, respectively), whereas Prochlorococcus spp. dominated (max. of 6.57x104 cell·mL−1) at the outermost stations influenced by Tropical Water and in the uplifting layers of the SACW around a depth of 100 m. Numerically, heterotrophic bacterial populations were predominant, with maximum concentrations (2.11x106 cell·mL−1) recorded in the surface layers of the inner and mid shelves in Coastal Water and the upper limits of the SACW. Nutrient-rich (high silicate and phosphate) and relatively less saline waters enhanced the picoeukaryotic biomass, while Synechococcus and heterotrophic bacteria were linked to higher temperatures, lower salinities, and higher inputs of ammonia and dissolved organic carbon. The relative importance of each group to carbon biomass partitioning under upwelling conditions is led by heterotrophic bacteria, followed by picoeukaryotes, Synechococcus and Prochlorococcus, and when the SACW is not as influential, the relative contribution of each phytoplanktonic group is more evenly distributed. In addition to habitat preferences, the physical structure of oligotrophic waters has a large impact on the vertical and spatial distribution patterns of picoplankton, reflecting the strong effect of the SACW intrusion.

Distribution of loliginid squids in a eutrophicated tropical coastal bay

ABSTRACTGuanabara Bay is one of the most polluted bays in Brazil, although it supports some fisheries resources, among which are three species of loliginid squids: Lolliguncula brevis, Doryteuthis plei, and Doryteuthis sanpaulensis. This study aimed to assess the temporal and spatial variations of squids in the bay, relating their abundance variation to local environmental factors. We also aimed to characterize the population structure of squids, in order to establish their degree of occupancy in this estuarine system. Ten fortnightly hauls were carried out (two per area) from July 2005 to June 2007. Squids were identified, measured, weighed, sexed and classified according to maturity stage. The spatial distribution of the three species was not uniform; most individuals were found near the mouth of the bay. Abundances were significantly different among seasons due to the presence of SACW (South Atlantic Central Water) during the spring–summer season. Most of the populations of the three species consist of juveniles, and most adult Doryteuthis were also immature, which reinforces the role of Guanabara Bay as a nursery ground for loliginid squids. Concerning the degree of occupancy of squids in Guanabara Bay, L. brevis is considered an estuarine resident, D. plei uses the estuarine system as a nursery area for feeding and growth, and juveniles of D. sanpaulensis enter the bay with the flood tide to feed, generally following the SACW intrusion. This is the first study on squids in this estuarine system, which despite the high degree of environmental impact is shown to be an important area for the loliginid species.

Surface-to-subsurface temperature variations during the last century in a western boundary upwelling system (Southeastern, Brazil)

The upper thermal gradient of a western boundary upwelling system was reconstructed for the last 100 years. The reconstruction was based on oxygen isotopic composition (δ18O) of four planktonic foraminifera species derived from two boxcores (BCCF10-01 and BCCF10-04) located in the southeastern Brazilian shelf. Calcification depths of the four planktonic foraminifera species were estimated in order to understand which layer of the water column was assessed. Changes in the upper thermal gradient were evaluated by using the δ18O difference from the surface-dwelling species Globigerinoides ruber (pink) and the deep-dwelling Neogloboquadrina dutertrei. The relative abundance of cold-water species and the δ13C of G. ruber (pink) were also used to evaluate changes on the surface layer. Our results demonstrate a trend to reduction on the temperature difference (∆T) between the surface and the thermocline layer towards the present for both cores, together with an increase on the relative abundance of cold-water species for the mid-shelf core (BCCF10-04) and a decrease in δ13C values of G. ruber (pink) for both cores. Despite the observable trend on the proxies, only the relative abundance of Turborotalita quinqueloba and the δ13C of G. ruber (pink) for the mid-shelf core presented statistically significant trends. These results were related to an increase in South Atlantic Central Water (SACW) intrusions in the sub-surface layer, especially on the middle shelf region. The SACW intrusions would lower the sea surface temperature and would bring a depleted δ13C of the dissolved inorganic carbon signature to the surface, which would be in agreement with our findings. Moreover, these mid-shelf SACW intrusions in the region were attributed to the Ekman pumping (wind stress curl-driven), which was previously reported to be an important mechanism in this upwelling system. The major outcomes to this western boundary current ecosystem from an intensification of the mid-shelf upwelling were discussed.

Temporal changes in pelagic copepod assemblages off Ubatuba, Brazil

AbstractThe inner shelf waters off Southeastern Brazil are periodically enriched by bottom intrusions of the cold and nutrient‐rich South Atlantic Central Water (SACW), which is transported offshore by the Brazil Current. This study examined the temporal contrasts in abundance and structure of pelagic copepod assemblages in a neritic station off Ubatuba, in relation to hydrography and phytoplankton biomass, to investigate the effects of SACW bottom intrusions on copepod population dynamics during three consecutive years. The water‐column characteristics shifted from a well‐mixed, more turbid and phytoplankton‐poor scenario during subsidence conditions to a stratified, less turbid and high Chl‐a concentration scenario during SACW bottom intrusions, leading to increased copepod diversity, abundance, and biomass. The rise in copepod diversity during SACW intrusions was related to the contribution of oceanic species in addition to coastal water species. The copepod community was numerically dominated by small‐sized species, such as Oncaea waldemari, Oithona plumifera, and clausocalanid and paracalanid copepodids, regardless of seasonality and SACW intrusions. Some large calanoid species contributed considerably to the total copepod biomass during intrusions. In addition to confirming that SACW seasonal intrusions play a key role in pelagic processes off Southeast Brazil, this study showed that the multiannual variability of SACW seasonal intrusions is important in regulating the structure and dynamics of copepod communities in this subtropical area.

Seasonal patterns of cephalopod paralarvae distribution in relation to oceanographic features off South Brazil Bight (SBB)

Seasonal patterns of cephalopod paralarvae distribution in relation to oceanographic features off South Brazil Bight (SBB)

Seasonal and interannual variability of zooplankton vertical distribution and biomass size spectra off Ubatuba, Brazil

The analysis of plankton size distributions in response to physical and biological forcing is relevant to the understanding of marine ecological processes. In this study, we evaluated the influence of seasonal intrusions of the South Atlantic Central Water (SACW) on zooplankton vertical distribution and normalized biomass size spectra (NBSS) on the inner shelf off subtropical Brazil. Monthly sampling was performed at a fixed station for 5 years and zooplankton samples were analyzed with a ZooScan system. SACW intrusions led to increases in nitrate, chlorophyll and zooplankton biomass in different depth strata. NBSS slopes and intercepts varied between SACW intrusion and retraction phases and stratification was a key factor explaining seasonal fluctuations in NBSS parameters. The zooplankton interannual variability was related to the changing strength of SACW intrusions during the time series, as indicated by surfacing 18 and 208C isotherms in certain years and intrusion phases, indicative of transient upwelling. We conclude that the temporal components of zooplankton variability in this subtropical coastal ecosystem were governed by SACW intrusions, affecting the vertical distribution of zooplankton biomass and size structure.

Distribution of planktonic cnidarians in response to South Atlantic Central Water intrusion in the South Brazilian Bight

Five oceanographic cruises were made between November 2005 and June 2006, sampling a cross-shelf transect off the South Brazilian Bight (SBB; 26°46′S) to follow the seasonal development of the South Atlantic Central Water (SACW) intrusion over the shelf and its influence on the assemblage of planktonic cnidarians. An onshore wind-driven bottom intrusion of the SACW was clearly perceptible, reaching the coast in January. From March onward, the SACW influence was gradually displaced seaward due to wind and tidal mixing. By late June the SACW influence was offshore and the inshore was dominated by low-salinity waters (<34.5). The abundance, distribution, and general taxonomic composition of both medusae and siphonophores were strongly influenced by the onshore intrusion of the SACW. An inshore–offshore gradient was clear. The Canonical Correspondence Analysis suggested that coastal species – dominated by Liriope tetraphylla, actinula larvae and Muggiaea kochi – were mostly related to food availability and a vertically mixed environment inshore, and their abundance and extent were reduced during intrusion periods. In contrast, species with offshore affinities tended to increase their abundance and distribution during intrusion periods, and were mostly related to the presence of thermal stratification and a deep chlorophyll maximum layer. Most of these offshore species, such as Aglaura hemistoma, Rhopalonema velatum and many calycophorans, are associated with the warm upper layer. However, high concentrations of large (>20mm in diameter) Solmaris corona were observed exclusively in cold waters, suggesting this medusa is a SACW indicator.

Summer/winter stratification variability in the central part of the South Brazil Bight

Analysis of the hydrographic data collected during seven consecutive high resolution summer/winter cruises in the central part of the South Brazil Bight off the coastal city of Ubatuba confirmed an observable summer/winter stratification variability of the shelf waters. The maximum bulk stratification occurred at mean distances of 85.6km and 39.1km from the coast in the austral winter and summer cruises, respectively. Estimates of the equivalent mixing power of the physical processes that increase or decrease the stratification in the inner and middle shelves showed that both shelf regions would be vertically well mixed were it not for buoyancy advection. In the inner shelf, buoyancy advection was associated with the along-shelf transport of low salinity waters originating from river runoff. In the middle shelf, buoyancy advection was due to the oceanic South Atlantic Central Water intrusions toward the coast.

Hydrographic climatology of South Brazil Bight shelf waters between Sao Sebastiao (24°S) and Cabo Sao Tome (22°S)

A hydrographic climatology was created with the goal of establishing the mean hydrographic conditions and oceanic waters intrusion mechanisms in the central and northern parts of the South Brazil Bight on a seasonal timescale. The climatology was constructed by applying objective analysis to a historical temperature and salinity database, by season and depth, considering the anisotropy and coastal and oceanic boundary conditions. The dominance of South Atlantic Central Water (SACW) in the bottom layer, the presence of Tropical Water in the surface layer, the seasonal onshore–offshore displacement of those water masses, and the along-shore remote southward advection of SACW from Cabo Frio (23°S) are well-defined features in the analyzed and derived fields. The inshore displacement increases the volume of SACW in the shelf by approximately 780km3 from winter to spring; the offshore displacement decreases the volume of SACW by approximately 840km3 from summer to autumn. The remote advection is linked to southwestward, along-shelf transport of SACW with temperatures lower than 15°C from Cabo Frio during summer, winter, and spring. In association with the remote advection, a primary intrusion pathway of SACW is identified off Cabo Frio during those same three seasons. In spring, a secondary SACW intrusion pathway is also detected off Ubatuba (23.5°S). The existence of those two SACW intrusion pathways is promoted by the combined effect of three factors: outer shelf deepening, upwelling-favorable winds, and Brazil Current mesoscale variability. Minimum volumes of SACW and maximum volumes of Tropical Water occur in autumn, when wind activity is minimal and the minimum bottom temperature on the shelf is approximately 1°C warmer than in the other seasons (colder than 16°C).

Reproductive traits of Euterpina acutifrons in a coastal area of Southeastern Brazil

AbstractWe investigated the reproductive biology of the planktonic harpacticoid copepod Euterpina acutifrons, including morphometric data, egg production rates (EPR) and viability, and weight‐specific egg production. Experiments were carried out during 1 year in an inner‐shelf area off Ubatuba (SE Brazil), a site seasonally influenced by bottom intrusions of the relatively cold and nutrient‐rich South Atlantic Central Water (SACW). We hypothesized that E. acutifrons attain higher reproductive rates when SACW penetrates in this region. Live females were incubated individually in cell culture plates during two periods of 24 h each, under controlled temperature and light conditions. Euterpina acutifrons carried on average 16.9 ± 6.9 eggs·sac−1, ranging between 10.8 ± 5.7 and 30.8 ± 7.4 eggs·sac−1. Estimated EPRs ranged from 6.3 ± 3.4 to 13.6 ± 4.2 eggs·female−1·day−1, with mean weight‐specific egg production rates of 0.06 ± 0.04 and 0.17 ± 0.08 per day. Euterpina acutifrons was not directly influenced by SACW intrusions, but body length and clutch size were positively related to temperature and chlorophyll content. Egg hatching time was clearly dependent on water temperature, as a 2 °C increase resulted in a decrease of 15 h in egg hatching time. This shows that even a small variation in temperature may considerably affect E. acutifrons population dynamics. Reproductive traits of this pelagic harpacticoid seem, therefore, to be controlled by the trade‐offs between increased food supply and the metabolic demands at low temperatures associated with SACW bottom intrusions toward this coastal area.