Drake passage-a bottleneck where spatial distribution of the planktonic Ostracoda reflects the dynamic hydrography well

Across the Drake Passage, planktonic ostracods are a significant component of mesozooplankton communities. Thus, the aim of this study was to investigate their composition structure and distribution within the upper 300-m layer in summer 2010. Our study revealed a complex pattern in their assemblages along the south–north transect, which strongly coincides with three distinct hydrographic regimes the transect crossed: the Antarctic Zone (AZ), the Polar Frontal Zone (PFZ), and the Subantarctic Zone (SAZ). Superimposed on this hydrographic zonation was a northward gradient in increasing ostracod abundance. The overall species number was low—just 12 species—but the composition of ostracod assemblages reflected the origin of the water masses well, since the numerically dominant species in the three hydrographic zones were: Alacia hettacra , in the AZ; Discoconchoecia elegans , in the PFZ; and Pseudoconchoecia serrulata , in the SAZ. Although the Passage acts as a physical bottleneck, diverting typically Antarctic waters further north than in other sectors of the Southern Ocean (SO), the distributional ranges of Conchoecia magna , Obtusoecia antarctica , and P. serrulata were observed further south than had previously been reported. A possible explanation for this shift is that the Polar Front position is located further south than in the other Atlantic part of the SO. It is also likely that, in such a narrow passage containing pelagic fauna of various hydrographical zones, even subtle shifts in distribution, including changes in climate, will be observed at first; therefore, our data might constitute important reference material for tracking further zooplankton biogeography in Antarctic waters.