Unintended “biological cargo” of ships entering the River Odra estuary: assemblages of organisms in ballast tanks

Water and sediment in ships’ ballast tanks provide habitats for various organisms, and thus facilitate alien species introductions. Ballast tank water and sediment of 19 ships docked in the GRYFIA Szczecin Ship Repair Yard (Szczecin, Poland) located in an area connected with the River Odra estuary (Southern Baltic), were sampled in 2009–2011 to find out if the ships could be vectors of species introductions to the estuary, already known for the presence of non-indigenous taxa. This study showed the ballast water of the ships examined to house rotifers, copepods, cladocerans, and bivalve and cirriped larvae – common constituents of zooplankton assemblages in coastal waters. The ballast tank sediment supported meiobenthic foraminiferans, nematodes, harpacticoid copepods, turbellarians, bivalves, polychaetes, and chironomid and cirriped larvae. It is not possible at this stage to judge what meiofaunal taxa constitute an alien component in the estuary biota. Macrobenthos in the ships’ ballast tank sediment examined was represented mainly by nereid polychaetes. Although the unintended “biological cargo” examined proved quite diverse and abundant, it contained few identified alien taxa. It does not seem likely than any of them could pose a threat of a biological invasion in the River Odra estuary. However, numerous species remained unidentified, and therefore assessment of the risk of alien species introduction and invasion contains a large measure of uncertainty. On the other hand, the risk as such remains, since the density of ballast water-borne organisms in all ships exceeded the allowed limits.