Latitudinal patterns and interannual variations of spring phytoplankton in relation to hydrographic conditions of the southwestern Atlantic Ocean (34°–62°S)

Héctor F Olguín Salinas,Frederico Brandini,Demetrio Boltovskoy

doi:10.1007/s10152-015-0427-6

Abstract

In the present study, the distribution and abundance of microphytoplankton were assessed in the southwestern Atlantic Ocean from subtropical to polar waters (34°–62°S) in two austral springs with contrasting hydrographic conditions. Vertical profiles of open-ocean (ca. 54°W) samples (≥ 18 L filtered water) were performed in 20 stations (0–100 m) in 1994 and in 17 stations (0–50 m) in 1995. The latitudinal patterns of microphytoplankton groups, chlorophyll a, and diatom and silicoflagellate species were analyzed and compared with our previous study carried out in spring 1993 (30°–61°S). In the three springs, 197 diatom species, 173 of which showed mean relative density lower than 1 %, were identified. A consistent diatom biogeographic pattern emerged in the three consecutive springs, defining the Transitional (corresponding to the Brazil–Malvinas Current Confluence), Subantarctic and Antarctic Zones; and the Subantarctic and Polar Fronts, as their boundaries. This zonation reflects persistent features of this hydrographically heterogeneous region. The Polar Front was a weaker biogeographic boundary for diatom species than the Subantarctic Front. Interannual changes in diatoms (mainly in species richness, dominant species and degree of dominance of species with different ecological affinities) were detected. The Transitional Zone, which is one of the most hydrographically variable regions of the world ocean, showed the highest changes in the diatom assemblage, reflecting a colder spring in 1995 and a warmer spring in 1994 than normal (1993). These changes agreed with differences reported for 1993–1995 in large-scale hydrographic conditions, including a widespread diversity decrease in 1995 due to a weaker influence of subtropical waters.

Highlights

Strong persistent currents along the western boundaries of the world’s major ocean basins are some of the most prominent features of ocean circulation (Imawaki et al 2013).The southwestern Atlantic Ocean (SWA) encompasses portions of the eastern shelf of South America and the deep ocean of one of the most energetic and biologically productive oceanic regions of the southern hemisphere (Combes and Matano 2014a)
A consistent diatom biogeographic pattern emerged in the three consecutive springs, defining the Transitional, Subantarctic and Antarctic Zones; and the Subantarctic and Polar Fronts, as their boundaries
Between ca. 308 and 46°S, these two currents come in contact with each other, defining the Brazil–Malvinas Confluence Zone (BMCZ), a permanent feature characterized by the interleaving and mixture of subantarctic and subtropical waters, as well as strong mesoscale features, such as thermohaline fronts, rings, filaments and eddies (Olson et al 1988; Peterson and Stramma 1991; Peterson 1992; Willson and Rees 2000)

Summary

Introduction

Strong persistent currents along the western boundaries of the world’s major ocean basins are some of the most prominent features of ocean circulation (Imawaki et al 2013).The southwestern Atlantic Ocean (SWA) encompasses portions of the eastern shelf of South America and the deep ocean of one of the most energetic and biologically productive oceanic regions of the southern hemisphere (Combes and Matano 2014a). 308 and 46°S, these two currents come in contact with each other, defining the Brazil–Malvinas Confluence Zone (BMCZ), a permanent feature characterized by the interleaving and mixture of subantarctic and subtropical waters, as well as strong mesoscale features, such as thermohaline fronts, rings, filaments and eddies (Olson et al 1988; Peterson and Stramma 1991; Peterson 1992; Willson and Rees 2000). This zone of complex hydrodynamics and rich spatial structure (d’Ovidio et al 2010) is identified as a hot spot of enhanced phytoplankton diversity (Follows et al 2007). Both zones are of great importance due to their ecological and biogeochemical features (Schloss et al 2007; d’Ovidio et al 2010) and recognized among the richest chlorophyll a areas, according to ocean color satellite images (Saraceno et al 2005; Rivas et al 2006)

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