Reproduction Dynamics in Copepods Following Exposure to Chemically and Mechanically Dispersed Crude Oil Bjørn Henrik Hansen,* ,† Iurgi Salaberria, † Anders J. Olsen, ‡ Kari Ella Read, † Ida Beathe Øverjordet, † Karen M. Hammer, † Dag Altin, § and Trond Nordtug † † Environmental Technology, SINTEF Materials and Chemistry, 7465 Trondheim, Norway ‡ Department of Biology, Norwegian University of Science and Technology, 7491 Trondheim, Norway § Biotrix, 7022 Trondheim, Norway * S Supporting Information ABSTRACT: Conflicting reports on the contribution of chemical dispersants on crude oil dispersion toxicity have been published. This can partly be ascribed to the influence of dispersants on the physical properties of the oil in different experimental conditions. In the present study the potential contribution of dispersants to the reproductive effects of dispersed crude oil in the marine copepod Calanus f inmarchicus (Gunnerus) was isolated by keeping the oil concentrations and oil droplet size distributions comparable between parallel chemically dispersed (CD, dispersant:oil ratio 1:25) and mechanically dispersed oil (MD, no dispersant) exposures. Female copepods were exposed for 96 h to CD or MD in oil concentration range of 0.2-5.5 mg·L -1 (THC, C 5 -C 36 ) after which they were subjected to a 25-day recovery period where production of eggs and nauplii were compared between treatments. The two highest concentrations, both in the upper range of dispersed oil concentrations reported during spills, caused a lower initial production of eggs/nauplii for both MD and CD exposures. However, copepods exposed to mechanically dispersed oil exhibited compensatory reproduction during the last 10 days of the recovery period, reaching control level of cumulative egg and nauplii production whereas females exposed to a mixture of oil and dispersant did not. ■ INTRODUCTION Seaborne crude oil is dispersed naturally by turbulence. 1 As an oil spill response (OSR) action, chemical dispersants can be applied to accelerate dispersion of oil into the water column and subsequently enhance oil weathering through dissolution, spreading, and biodegradation. 1 These dispersants may elicit direct toxic effects, 2,3 and usage increases oil exposure to pelagic organisms. 1 Hence, chemically dispersed oil (CD) may be more toxic than mechanically dispersed oil (MD). 4-6 Traditionally, water-accommodated fractions (WAFs) of oil are used for toxicity testing of dispersions in static or static- renewal exposure systems. 7 To simulate environmentally relevant exposure scenarios, WAFs are usually prepared by low-energy stirring (LE-WAF), which leads to limited droplet formation, high-energy stirring (HE-WAF), and/or the addition of chemical dispersants (chemically enhanced WAF (CE- WAF)). These WAF preparations are used to simulate differences in weather (degree of wave action causing dispersion) and dispersant application. However, to determine the contribution of chemical dispersant to the toxicity of oil dispersions, these conventional and widely used methods are inadequate. The degree of oil droplet generation using such methods will be highly affected by oil viscosity, oil dispersibility, and dispersant efficiency, and oil dispersions (i.e., WAFs vs CE- WAFs) will contain different oil concentrations and droplet size ranges. WAFs consist of a particulate phase (dispersed fraction) and a water phase (dissolved fraction), the latter considered being the most bioavailable and toxic fraction. 8-11 Moreover, these two phases are not stable over time in WAFs due to droplet surfacing velocity, being related to the diameter of the droplets and the density of the oil, which will differ between the two treatments. This causes shifts in the equilibrium between the two phases, thereby further impairing direct comparison between WAF and CE-WAF. To determine the contribution of a chemical dispersant to the toxicity of oil dispersions, the effects of mechanically and chemically dispersed oil differing only in the presence of dispersant should be compared, while all other exposure parameters are maintained unaltered. Therefore, a standardized flow-through method for oil-in- water dispersion generation is to be preferred. 12 It allows for in- line and continuous generation of comparable MD and CD in Received: October 8, 2014 Revised: January 29, 2015 Accepted: February 6, 2015 Published: February 6, 2015 Article pubs.acs.org/est © 2015 American Chemical Society 3822 DOI: 10.1021/es504903k Environ. Sci. Technol. 2015, 49, 3822-3829