Severe Toxic Effects on Pelagic Copepods from Maritime Exhaust Gas Scrubber Effluents

To reduce sulfur emission from global shipping, exhaust gas cleaning systems are increasingly being installed on board commercial ships. These so-called scrubbers extract SO by spraying water into the exhaust gas. An effluent is created which is either released directly to the sea (open-loop system) or treated to remove harmful substances before release (closed-loop system). We found severe toxic effects in the ubiquitous planktonic copepod Calanus helgolandicus of exposure to effluents from two closed-loop systems and one open-loop system on North Sea ships. The effluents contained high concentrations of heavy metals and polycyclic aromatic hydrocarbons (PAHs), including alkylated PAHs. We observed significantly elevated mortality rates and impaired molting already in the lowest tested concentrations of each effluent: 0.04 and 0.1% closed-loop effluents and 1% open-loop effluent. These concentrations correspond to total hydrocarbon concentrations of 2.8, 2.0, and 3.8 μg L^-1, respectively, and compared to previous studies on oil toxicity in copepods, scrubber effluents appear more toxic than, for example, crude oil. None of the individual PAHs or heavy metals analyzed in the effluents occurred in concentrations which could explain the high toxicity. The effluents showed unexpected alkylated PAH profiles, and we hypothesize that scrubbers act as witch's cauldrons where undesired toxic compounds form so that the high toxicity stems from compounds we know very little about.