Toxicity evaluation of ballast water discharged at The Onne Port complex using the microtox assay

Ballast water from ninety-six (96) International vessels calling the Onne Port complex were analyzed for physicochemical and acute toxicity test using the bioluminescence bacteria (Vibrio fisheri) from October 2015 to September 2016. The result of the microtox assay showed that the ballast water were generally toxic in the months under review. The Tanker vessels were generally more toxic than the RoRo/Container carriers. For the RoRo/Container carriers, the EC50 values for the 5 minutes and 15 minutes microtox assay ranged from 7.99± 2.45% in November, 2015 to 44.04±1.70% in June 2016 and 8.97±1.07% in November 2015 to 44.98±11.13% in June 2016. Similarly, for the tanker vessels, the EC50 values ranged from 5.63±1.97% in June 2016 to 28.17±1.71% in January 2016. In comparison to the reference chemical (Zinc sulphate) for the 5 minutes and 15 minutes microtox assay test, the EC50 values ranged from 1.00 ± 0.24% to 3.49±0.61% and 2.03±1.15% to 5.16±2.99% respectively. The seawater which served as controls were generally not toxic as the EC50 values was 100% from October 2015 to September 2016. The discharge of these ballast water poses a major environmental threat to the water quality and Port infrastructures at the Onne Port complex as contaminants may find their way into the food chain/food web and bioaccumulate in the tissues of indigenous biota (microorganisms, crabs, mangrove oysters and fin-fishes). This research work flags off the need for the regulators of Maritime Safety and Administration in Nigeria to monitor closely International vessels calling at major ports in Nigeria and the need to begin to domesticate some of the IMO guidelines on Ballast water management and ensure strict compliance.Key words: Ballast water, Toxicity, Vibrio fischeri, International Maritime Organization