Even though compelling evidences indicate that marine microbes show biogeographic patterns, very little is known on the mechanisms driving those patterns in aquatic ecosystems. In the present study, bacterial community structure was examined in epipelagic waters of a highly hydrodynamic area of the Southern Ocean to gain insight into the role that biogeochemical factors and water mass mixing (a proxy of dispersal) have on microbial biogeography. Four water masses that converge and mix around the South Shetland Islands (northern tip of the Antarctic Peninsula) were investigated. Bacterioplankton communities were water-mass specific, and were best explained by dispersal rather than by biogeochemical factors, which is attributed to the relatively reduced environmental gradients found in these cold and nutrient rich waters. These results support the notion that currents and water mixing may have a considerable effect in connecting and transforming different water bodies, and consequently, in shaping communities of microorganisms. Considering the multidimensional and dynamic nature of the ocean, analysis of water mass mixing is a more suitable approach to investigate the role of dispersal on the biogeography of planktonic microorganisms rather than geographical distance.

The spatial distribution of meroplankton in Bransfield Strait, Antarctica, and its relationships with hydrographical conditions are described. Biological sampling was carried out with BIONESS sampling gear at 19 stations and at 5 depths between 10 and 300 m. The main hydrographic features were a shallow hydrographic front in the southern part of the strait that separates Transitional Zonal Water with Bellingshausen influence (TBW) from Transitional Zonal Water with Weddell influence (TWW) and a slope front, the so-called Bransfield front, along the South Shetland Islands Slope (SSI). A northeastward baroclinic jet known as the Bransfield Current (BC) originates from this slope front. The meroplankton community was very diverse and included 12 types of larvae, dominated by polychaete and echinoderm larvae. The meroplankton were more abundant closer to the SSI in the BC and decreased in number towards the Antarctic Peninsula. Polychaete larvae were found close to both shores but mainly close to the SSI in TBW; they were very sparse in the central basin, appeared again below 100 m depth in TWW, and increased in abundance in upper layers at stations between the hydrographic front and the Antarctic Peninsula, in TWW. By contrast, echinoderm larvae mainly occupied the central basin and were always associated with the upper 100 m in TBW, just above the polychaete larvae; they were almost absent from TWW. In summary, meroplanktonic larval exhibit a strong dependence on water masses, depth, and fronts.

Polar marine ecosystems have global ecological and economic importance because of their unique biodiversity and their major role in climate processes and commercial fisheries, among others. Portugal and Spain have been highly active in a wide range of disciplines in marine biology of the Antarctic and the Arctic. The main aim of this paper is to provide a synopsis of some of the results and initiatives undertaken by Portuguese and Spanish polar teams within the field of marine sciences, particularly on benthic and pelagic biodiversity (species diversity and abundance, including microbial, molecular, physiological and chemical mechanisms Journal of Sea Research 83 (2013) 9-29 ☆ Given his role as Guest Editor, Joao-Carlos Marques had no involvement in the peer-review of this article and has no access to information regarding its peer-review. Full responsibility for the editorial process for this article was delegated to Angel Borja.⁎

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The central part of the Bellingshausen Sea has been poorly studied, partly because of the presence of ice during most of the year. The main aim of this study was to analyse the abundance and distribution of meroplankton, and the influence of oceanographic properties were investigated in the Bellingshausen Sea (West Antarctica) during the BENTART-06 cruise carried out in January-February 2006. Zooplankton samples were collected with a 80-μm mesh plankton net hauled vertically from a depth of 200 m to the surface at fifteen stations across the Bellingshausen Sea. Fifteen types of larval benthic invertebrates were found, with echinospira and nudibranch veligers being the most abundant. Hierarchical analysis and non-metric multidimensional scaling revealed a high degree of spatial variability in both larval abundance and larval types across the Bellingshausen Sea. The variability was significantly correlated with total chlorophyll-a and the contribution of large (>5 μm) phytoplankton to total chlorophyll, indicating the availability of food as an important factor determining the larval distribution observed. Nudibranch veligers, nemertean pilidia, echinoderm and planula larvae were more abundant at stations in the central Bellingshausen Sea, which was characterized by low phytoplankton biomass and production. Higher abundances of echinospira veligers and polychaete larvae were found at the more productive stations close to Peter I Island and the Antarctic Peninsula. The abundance and diversity of larval types found in the Bellingshausen Sea during the BENTART-06 cruise support the hypothesis that indirect development through larval swimming stages plays a key role in benthic recruitment in polar areas.