Net efficacy of open ocean ballast water exchange on plankton communities

Abstract

We evaluated the efficacy of two ballast water exchange (BWE) methods during two transoceanic voyages of a bulk carrier in September 1999 and October 2000 between Rotterdam (The Netherlands) and Sept-les (Canada). The experimental design accounted for the uptake of new species during exchanges by considering only those taxa observed prior to BWE (initial taxa). To account for natural decreases due to mortality in the tanks, communities in exchanged ballast tanks were compared with those in control tanks, thus allowing the 'net BWE efficacy' of the procedures to be determined. The efficacy of the removal of organisms varied among BWE methods, plankton communities (microplankton and zooplankton) and taxonomic groups. BWE efficacy was greater for zooplankton (72-90%) than microplankton (49-80%). When the fairly high natural mortality observed in control tanks was considered in the calculation of BWE efficacy (net BWE efficacy), much lower efficacy was observed (microplankton: 29-40%; zooplankton: 23-54%). The 300% flow-through method (FT) is the most effective BWE method (net efficacy) for removing initial microplankton taxa (1999 and 2000), whereas the FT was either similarly (1999) or less than (2000) effective compared with the procedure normally carried out on board this vessel (NORM method) for the zooplankton community. However, BWE was more efficient in removing microplankton than zooplankton in 1999 while the opposite pattern occurred in 2000. The seasonal timing of voyages and the BWE site influenced the density and composition of species introduced to tanks during BWE. © 2011 The Author. Published by Oxford University Press. All rights reserved.