Descripción: Both the biomass of autotrophic dinoflagellates and its contribution to total chlorophyll were found to increase significantly with seawater temperature and the level of stratification in southern Patagonian waters during spring and winter. The highest peak of biomass corresponded to a single species, Prorocentrum minimum (Pavillard) Schiller, and was detected in middle shelf waters, coinciding with the primary productivity and CO 2 uptake maxima reported for the area under spring conditions. © The Author 2011. Published by Oxford University Press. All rights reserved.

Descripción: Although there have been many ecological field studies on the key components of planktonic food webs, there are still gaps in knowledge on some groups, environments and seasons. This is the first report on the spatial distribution of the density and biomass of almost all the taxonomic groups and size fractions of protozooplankton across a cold-temperate shelf during winter. Twenty-eight stations (two or three depths) were sampled on four cross-shelf transects in Patagonian waters (south-western Atlantic; 47-55°S, 60-69°W) during September 2006. Loricate ciliates, radiolarians and foraminiferans showed the lowest densities, and were distributed mainly in coastal, slope or the whole shelf waters, respectively. The density and biomass of aloricate ciliates and heterotrophic nanoflagellates and dinoflagellates were low and homogeneous both vertically and across the shelf south of 51°S, but peaked in the upper 40 m in offshore waters at 47°S. Microplanktonic aloricate ciliates, which represented 53% of the total protozooplankton biomass, reached values as high as 16 μg CL-1 on the last transect. Consequently, both protozooplankton biomass and its ratio to chlorophyll a concentration were significantly higher in the northern offshore waters. These trends were linked to higher subsurface temperature and chlorophyll a concentration, and lower copepod nauplii biomass. Our results probably reflect changes in both the availability of food resources and predators and the physical structure of the water column, which are a consequence of the different environmental conditions that coexist over the large latitudinal and longitudinal gradients covered during late winter. © The Author 2010.

Descripción: Little attention has been paid to small copepods and other zooplankton inhabiting pelagic ecosystems of the Southwestern Atlantic Ocean under the influence of the Antarctic Circumpolar Current circulation, despite their important role in the trophic chain and fisheries. This study gives a synoptic view (January 2001) of the micro and mesoplankton size fractions and normalized biomass size spectra (NBSS) in upper waters of five different ecological domains (shelf and oceanic Subantarctic and Antarctic waters) including the Brazil-Malvinas confluence (BMC). Copepods were always the main component of the zooplankton; the <300-μm fraction represented between 70 and 99% in terms of numbers and from 20 to 88% in terms of biomass. Other zooplankton contributed with <40% to total zooplankton densities, though showing some biomass peaks (>50%). Chlorophyll a, sea surface temperature and salinity were the environmental variables that best explained the distribution trend of zooplankton, mainly that of the <300-μm fraction. For all the domains, NBSS revealed flat slopes (-0.6 to-1), suggesting a higher proportion of large organisms than expected at equilibrium. A dome-shape feature was detected in the BMC. Total biomass and trophic levels of the system were related to the composition of the community and the hydrological conditions of the domains covered. © 2013 The Author.

Descripción: This is the first study on diatom spatial patterns in relation to major oceanographic features along a megascale transect in the Southwestern Atlantic Ocean and provides a comparison with diatom distribution in surface sediments. Absolute abundances of diatoms, silicoflagellates and dinoflagellates (>10-μm fraction) were assessed in 80 bottle samples from 5 to 50 m, retrieved in November 1993 at 20 stations (30-61°S) along 53°W. Siliceous phytoplankton were scarce in the northern half of the transect and in the south of 57°S (100-150 cells L-1), with a strong peak in the vicinity of the Polar Front (∼200 000 cells L-1), whereas dinoflagellates were more abundant at the northern stations (up to 24 000 cells L-1). In the south of 50°S phytoplanktonic cell densities were loosely (but significantly, r = 0.54, P < 0.01) associated with chlorophyll a, whereas in the north of this latitude, this relationship disappeared (r = 0.018, P > 0.1). In total, 191 diatoms and 4 silicoflagellates were recorded. Changes in diatom assemblage compositions along the transect allowed identification of five discrete areas: Subtropical (29°S), Northern Transitional (34-41°S), Southern Transitional (43-48°S), Subantarctic (49-54°S) and Antarctic (55-59°S), each characterized by a set of typical species. Diversity changed little with latitude, but numbers of species were higher in the north of 40°S. Comparison of diatom assemblage makeup in the plankton and in the surface sediments shows very strong disagreements, whereby cold water species are very significantly over-represented in the sedimentary record, suggesting enhanced preservation and strong subsurface equatorward advection of the cold water taxa. © The Author 2006. Published by Oxford University Press. All rights reserved.

Descripción: Several satellite models classify phytoplankton functional types (PFT) based on cell size. In this study we used field data from the Argentine Sea on both the photosynthetic and the bio-optical properties of phytoplankton to distinguish photosynthetic and bio-optical phytoplankton types (PBPT). Cluster analyses were run using data from 70 stations sampled during 3 periods to distinguish different PBPT, and principal component analysis was used to describe them. We examined the main taxonomic composition and percentage of chl a in the <5 μm size fraction found within the PBPT. The distribution of PBPT in relation to hourly primary production and environmental conditions was also investigated. The results showed a high degree of variability in biooptical and photosynthetic properties, e.g. the specific absorption coefficient of phytoplankton, aB ph(440), varied between 0.015 and 0.067 m2 (mg chl a)-1, and the maximum production at light saturation, PB m, varied between 0.68 and 10.05 mg C (mg chl a)-1 h-1. This resulted in the discrimination of 11 PBPT. Some had similar average cell sizes but differed in their bio-optical or photosynthetic characteristics, e.g. PBPT1 (with diatoms <5 μm and Emiliania huxleyi 2-5 μm) and PBPT6 (with diatoms <5 μm and coccal cells ∼2 μm) had markedly different PB m values (PBPT1: 1.20 mg C (mg chl a)-1 h-1 and PBPT6: 6.71 mg C (mg chl a)-1 h-1). This variability in the bio-optical and physiological properties is most likely the result of adaptation by phytoplankton communities to the high heterogeneity in environmental conditions in this region. These results indicate that satellite models describing the distribution of PFT based on cell size alone will not provide a realistic representation of the phytoplankton composition in this highly productive and heterogeneous area. © 2013 The authors.