Rubén M. Negri

Zooplankton assemblages and hydrography in the spawning area of anchovy (Engraulis anchoita) off Río de la Plata estuary (Argentina-Uruguay)

The massive spawning of anchovy (Engraulis anchoita) in Argentine waters takes place in spring in the coastal sector off the Buenos Aires Province, with the highest concentrations of spawners occurring in front of the Rio de la Plata mouth. In October 1995, the size structure, composition and abundance of the zooplankton community was analyzed and related to the hydrography and the abundance of anchovy eggs. Oceanographic conditions were different from the recorded means for the season. A higher proportion of the colder and more saline shelf waters were present in the coastal area and strong haloclines were not observed. High values of plankton biomass were observed in the estuarine frontal area and in the weakly stratified waters of the median shelf. The cluster analysis of the stations gave rise to the formation of coastal (CA), transitional (TA) and shelf (SA) faunistic areas. Mesozooplankton smaller than 1 mm total length (TL) dominated the whole area. The copepods, Paracalanus spp. and Oithona similis, and the cladocerans, Evadne nordmanni and Podon spp., constituted the bulk of this mesozooplankton fraction. Oithona similis was the dominant species in the SA. The copepod, Acartia tonsa (1–2 mm TL), was extremely abundant in the estuarine front (more than 17 600 individuals m–3) followed by appendicularians, members of Pseudocalanidae, anchovy eggs and decapod larvae. Species of Calanidae (Calanoides carinatus and Calanus simillimus), the amphipod hyperiid, Themisto gaudichaudii, and the euphausiid, Euphausia lucens, largely dominated the macrozooplankton (>2 mm TL) in the TA and SA. In the CA, significant positive correlations between the abundance of small copepods (size ranges <1 and 1–2 mm) and anchovy eggs were found. Trophic implications of these findings for first-feeding anchovy larvae were discussed. The adequacy of the TA and SA as feeding ground for adults of anchovy was evaluated.

Seasonal succession of zooplankton in coastal waters of the Argentine Sea (Southwest Atlantic Ocean): prevalence of classical or microbial food webs

Abstract The demographic characteristics of marine zooplankton make it especially suitable for examining the variability of marine ecosystems. The zooplankton annual succession was studied at a permanent coastal station in the Argentine Sea (38°28′S, 57°41′W) in relation to physical conditions and phytoplankton size fractions. Small copepods (<1 mm total length), mainly represented by Oithona nana (Cyclopoida) and adults and copepodites of Calanoida, numerically dominated the metazooplankton throughout the year. In summer, small copepods also exceeded large copepods in biomass. Larvaceans (mostly <1 mm total length) were the second most important metazooplankton group, with strong dominance of Oikopleura dioica. The zooplankton succession exhibited two main periods throughout the year: (1) a cold winter–spring period characterized by a dominant classical herbivore food web in which the large copepod Calanoides carinatus and lamellibranch larvae were associated with the lowest temperatures and highest Chl-a and microphytoplankton, and (2) a warm summer period dominated by a microbial food web in which microbial filter-feeders such as Oithona nana, Paracalanus spp., Oikopleura dioica and Penilia avirostris predominated and the highest density of picophytoplankton and lowest concentrations of Chl-a were recorded. The implications of the present findings for the growth and survival of fish larvae distributed in the study area are discussed.

Azadinium (Amphidomataceae, Dinophyceae) in the Southwest Atlantic: In situ and satellite observations

Azaspiracid toxins (AZAs) have been identified in marine invertebrates and are of wide geographical distribution, but have not been reported in Argentina or ...

The First Molecular Characterization of Picocyanobacteria from the Argentine Sea

Picocyanobacteria are abundant throughout the world’s oceans. Particularly, it has been reported that Synechococcus strains have a wide latitudinal distribution, from polar to tropical waters. However, their molecular characterization in the Southwest Atlantic Ocean is still missing. We analyzed Synechococcus genetic diversity in a sector of the Argentine Sea, one of the richest biological areas of the world oceans. 16S rRNA amplicons obtained after PCR amplification of environmental DNA extracted from water samples of this area were used for DGGE and sequenced. Only Synechococcus sequences could be retrieved. On the other hand, we isolated two Synechococcus strains from the environment. Our analyses revealed that the clade I group was widespread from latitude 38°S to 48°S and that can coexist with clade IV strains in shelf waters. The cooccurrence of these two clades may be related to an adaptation to high-nutrient/low-temperature waters. Our data are the first report on Synechococcus ecotypes that would be important contributors to phytoplankton biomass in the Argentine Sea, one of the richest biological areas of the world oceans.