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JPR Advance Access originally published online on August 18, 2007
Journal of Plankton Research 2007 29(10):881-894; doi:10.1093/plankt/fbm066
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© The Author 2007. Published by Oxford University Press. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org

Living below the halocline: strategies of deep-living species in the highly stratified brackish Bornholm Basin (central Baltic Basin)

Jan Schulz* and Hans-Jürgen Hirche

Alfred-Wegener-Institute for Polar and Marine Research, Am alten Hafen 26, 27568 Bremerhaven, Germany

* Corresponding Author: jan.schulz{at}awi.de

Received on January 19, 2007; accepted on August 13, 2007


  Abstract

The Baltic Sea is the largest brackish water area of the world. On the basis of the data from 16 cruises, we show the seasonal and vertical distribution patterns of the appendicularians Fritillaria borealis, Oikopleura dioica and the cyclopoid copepod Oithona similis, in the highly stratified Bornholm Basin. These species live at least temporarily below the permanent halocline and use different life strategies to cope with the brackish environment. The cold-water species F. borealis is abundant in the upper layers of the water column before the thermocline develops. With the formation of the thermocline abundance decreases and the specimens outlast higher temperatures below the halocline. Distribution and strategy suggest that F. borealis might be a glacial relict species in the Baltic Sea. Although Oikopleura dioica is only abundant during summer, O. similis is present all year round. Both species have in common that their vertical distribution is restricted to the waters below the halocline, most likely due to their requirements of higher salinities. We argue that the observed strategies are determined by ecophysiological constraints and life history traits. These species share an omnivorous feeding behaviour and the capability to utilise a spectra of small particles as food. As phytoplankton concentration is negligible below the halocline, we suggest that these species feed on organic material and heterotrophic organisms that accumulate in the density gradient of the halocline. Therefore, the deep haline waters in the Baltic Sea represent a habitat providing shelter from predation and food supply for adapted species that allows them to gather sufficient resources and to maintain populations.

Communicating editor: K.J. Flynn


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