Search

The mesoscale activity of western boundary currents can induce changes in the oligotrophic conditions of the oceanic subtropical gyres. Off SE Brazil, recurrent meanders in the Brazil Current can modulate the pelagic ecosystem through eddy‐induced upwelling, as well as drive coastal upwelling and cross‐shelf exchanges. Despite their relevance, direct measurements of such meanders are scarce. At the end of the Spring of 2019, we acquired measurements of both physical and biogeochemical properties along a transect through one of these features, with 5–10 km spacing between stations. Displacements of the isopycnals at the meander edges arose from submesoscale filaments along the meander's rim, associated with the signature of shelf water masses. Chlorophyll distributions along the transect indicated phytoplankton responses to the different dynamic regimes in the meander: eddy stirring in the surface mixed layers versus eddy pumping in the subsurface waters, and mesoscale dynamics in the meander center versus submesoscale dynamics in its periphery. Subsurface phytoplankton biomass was enhanced in the meander center, likely due to eddy pumping raising the nutricline into the euphotic zone. Eddy pumping also appears to have contributed to a shift in the phytoplankton community composition to larger organisms. Our results indicate that the physical dynamics create different habitats across the meander, leading to spatial patchiness in the chlorophyll distributions, with different types of organisms in distinct parts of the meander. Like rivers, ocean currents can meander along their paths. Off southeastern Brazil, the Brazil Current flows southward and meanders as it flows past capes in the region. These meanders are hundreds of kilometers wide (mesoscale) and can generate smaller features that are tens of kilometers wide (submesoscale). These multiscale processes impact the planktonic ecosystem by inducing nutrient inputs to the nutrient‐poor open ocean and consequently enhancing primary productivity. However, direct measurements of these features are scarce. We had the opportunity to survey one of these meanders in 2019, collecting physical and biogeochemical data along a single line through the meander from southwest to northeast. We found small‐scale distortions that indicated the presence of submesoscale features swirling around the meander's rim. Different parts of the meander showed distinct phytoplankton responses, with more phytoplankton in the center, below the surface, likely growing on nutrients pumped upward by the meander. This pumping process might have been correlated with a phytoplankton community shift from small to larger organisms. Overall, our study shows that mesoscale and submesoscale motions create a variety of habitats, leading to distinct chlorophyll distribution patterns in different parts of the meander. We present the first physical‐biological measurements of a Brazil Current cyclonic meander drawing high chlorophyll shelf waters offshoreEddy pumping raised nutricline to the euphotic zone driving subsurface phytoplankton enhancement at eddy center and invisible to satellitesDistinct hydrodynamic regimes modulated the phytoplankton response in the center, rim, surface, and subsurface of the eddy We present the first physical‐biological measurements of a Brazil Current cyclonic meander drawing high chlorophyll shelf waters offshore Eddy pumping raised nutricline to the euphotic zone driving subsurface phytoplankton enhancement at eddy center and invisible to satellites Distinct hydrodynamic regimes modulated the phytoplankton response in the center, rim, surface, and subsurface of the eddy

Symbiotic octocorals are important reef inhabitants, from shallow to mesophotic depths. However, information regarding their nutritional ecology along the depth gradient, and in particular the changes in the autotrophic and heterotrophic acquisition of nutrients, is limited, despite the fact that nutrient acquisition is a fundamental process in explaining the distribution of reef organisms. Here, the abundance of three lipid biomarkers, specific for autotrophy and heterotrophy, was investigated in the tissue of four octocoral species and one scleractinian sampled in shallow and upper mesophotic reefs of the oligotrophic Northern Red Sea. Our findings show functional mixotrophy for all dinoflagellate‐bearing species, with a significant input of heterotrophic feeding on herbivorous crustacean zooplankton at both depths. The relatively high level of the heterotrophic marker in all symbiotic species in shallow conditions does not corroborate the common idea that corals rely mostly on autotrophy in shallow waters. The increase in heterotrophic capacity with depth was however species‐specific, likely related to physiological and morphological characteristics. In addition, octocorals maintained similar concentrations of autotrophic markers with increasing depth, while the scleractinian species exhibited a decrease in these markers with depth. These results are in agreement with previous measurements of photosynthetically‐fixed carbon acquisition, suggesting that autotrophy remains stable in Red sea soft corals, whereas it decreases in scleractinian corals. This study highlights the importance of heterotrophy across the euphotic‐upper mesophotic depth gradient and brings relevant advances on our understanding of the ecological significance of feeding for reef corals.

Symbioses between eukaryotic algae and nitrogen-fixing cyanobacteria have been recognized in recent years as a key source of new nitrogen in the oceans. We investigated the composition of the small photosynthetic eukaryote communities associated with nitrogen-fixing cyanobacteria in the Brazilian South Atlantic Bight using a combination of flow cytometry sorting and high throughput sequencing of two genes: the V4 region of 18S rRNA and nifH. Two distinct eukaryotic communities were often encountered, one dominated by the Mamiellophyceae Bathycoccusand Ostreococcus, and one dominated by a prymnesiophyte known to live in symbiosis with the UCYN-A1 nitrogen-fixing cyanobacterium. Among nifHsequences, those from UCYN-A1 were most abundant but three other UCYN-A clades (A2, A3, A4) were also found. Network analysis confirmed the relation between A1 and A2 clades and their hypothesized hosts and pointed out to the potential association between novel clade A4 with Braarudosphaera bigelowii, previously hypothesized to host A2.

Symbioses between eukaryotic algae and nitrogen-fixing cyanobacteria have been recognized in recent years as a key source of new nitrogen in the oceans. We investigated the composition of the small photosynthetic eukaryote communities associated with nitrogen-fixing cyanobacteria in the Brazilian South Atlantic Bight using a combination of flow cytometry sorting and high throughput sequencing of two genes: the V4 region of 18S rRNA and nifH. Two distinct eukaryotic communities were often encountered, one dominated by the Mamiellophyceae Bathycoccusand Ostreococcus, and one dominated by a prymnesiophyte known to live in symbiosis with the UCYN-A1 nitrogen-fixing cyanobacterium. Among nifHsequences, those from UCYN-A1 were most abundant but three other UCYN-A clades (A2, A3, A4) were also found. Network analysis confirmed the relation between A1 and A2 clades and their hypothesized hosts and pointed out to the potential association between novel clade A4 with Braarudosphaera bigelowii, previously hypothesized to host A2.

The potential seeding impact of sea ice microbial communities was studied during late austral winter early spring 1988 in the Weddell Sea, Antarctica. Experiments were performed in seawater aquariums with natural seawater and seawater enriched with crushed ice. Algal, protozoan and bacterial cell numbers were followed, as well as nutrients and DOC levels. The results showed a potential seeding effect of sea ice communities to the water column. However, the type of ice communities differed greatly from each other and the effect of such seeding will be patchy. In our experiments seeding of seawater by ice rich in algae, flagellates and/or particulate organic carbon lead to the development of communities dominated either by diatoms or bacteria.

The physical and chemical environment, and the phytoplankton primary production of southeastern Brazil were studied in relation to the general oceanographic structure during two research cruises (winter and summer). In each cruise, a total of 91 stations were occupied. Data were collected on the spatial distribution of nutrients, phytoplankton biomass and photosynthetic capacity over the coastal, shelf and oceanic areas off São Paulo, Paraná and Santa Catarina States.