Efficacy of open-ocean ballast water exchange: a review

A literature search was conducted to summarize studies that quantified the efficacy of ballast water exchange. Here, a number of parameters were collated from 68 studies to provide a meta-analysis of the efficacy of exchange. The percent removal of ballast water, organisms, and various physical parameters was investigated to quantify exchange efficacy in terms of volumetric, biological, and water quality measurements, respectively. The two types of exchanges implemented—empty-refill and flow-through—demonstrated 66 to >99% efficiencies of volumetric exchange when this factor was measured using rhodamine dye as a tracer. Of the reports and papers reviewed, zooplankton were the most common biological parameter studied, and nearly all data showed a decrease in concentrations following exchange, although the changes in community structure varied. Additional biological parameters reported were the concentrations of protists, bacteria, and virus-like particles (VLPs). Results of protist studies were highly variable—ranging from a decrease in cell concentrations to an increase following exchange— while bacteria and VLPs concentrations showed no change in most cases. Of course, the diversity and physiological condition of organisms in ballast water can cause differences in responses that may lead to inconsistencies when determining if organisms were removed during exchange. Additionally, organisms present in sediment may also repopulate ballast water following exchange. In contrast to the biological results, some water quality parameters, such as chromophoric dissolved organic matter (CDOM) and trace elements (e.g., barium and manganese), were consistent indicators of exchange. In conclusion, although the volumetric efficacy of exchange showed, in most studies, that the majority of the water was removed from the tanks, biological and most water quality parameters did not show a consistent response