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

The Baltic Sea is the largest brackish water area of the world.On the basis of the data from 16 cruises, we show the seasonaland vertical distribution patterns of the appendicularians Fritillariaborealis, Oikopleura dioica and the cyclopoid copepod Oithonasimilis, in the highly stratified Bornholm Basin. These specieslive at least temporarily below the permanent halocline anduse different life strategies to cope with the brackish environment.The cold-water species F. borealis is abundant in the upperlayers of the water column before the thermocline develops.With the formation of the thermocline abundance decreases andthe specimens outlast higher temperatures below the halocline.Distribution and strategy suggest that F. borealis might bea glacial relict species in the Baltic Sea. Although Oikopleuradioica is only abundant during summer, O. similis is presentall year round. Both species have in common that their verticaldistribution 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 ecophysiologicalconstraints and life history traits. These species share anomnivorous feeding behaviour and the capability to utilise aspectra of small particles as food. As phytoplankton concentrationis negligible below the halocline, we suggest that these speciesfeed on organic material and heterotrophic organisms that accumulatein the density gradient of the halocline. Therefore, the deephaline waters in the Baltic Sea represent a habitat providingshelter from predation and food supply for adapted species thatallows them to gather sufficient resources and to maintain populations.