The effect of crude oil and chemical dispersant on sinking rates of Gulf of Mexico diatoms

In the open and coastal ocean, primary productivity is derived largely from phytoplankton. Diatoms are a major group of phytoplankton that account for almost half of all oceanic primary production. Sinking is a fundamental aspect of diatom ecology usually linked to loss process but is important in vertical migration for nutrient uptake, avoidance of predators, and completion of lifecycle events. Sinking is tightly linked to both physiological state and size of diatom cells. Oil spills are one of the many stressors now evident in the marine environment. Since diatom physiology is generally adversely affected by crude oil in the form of growth inhibition, reduced photosynthesis, and cell death, it was hypothesized that diatom sinking would be adversely affected by the addition of crude oil, and that increasing concentration, time, and the addition of chemical dispersant would magnify this effect. There has been no previous study attempting to quantify how diatom sinking is affected by the presence of crude oil and chemical dispersant. In this study, laboratory cultures of diatom species were experimentally treated with crude oil, dispersant, and a mixture that was filmed at three timepoints over the course of a week. Images were processed with ImageJ to quantify individual trajectories. There were 20-50 cells examined per treatment, with a goal of 50 observed cells. Killed cells showed higher average sinking rates (C. wailesii=93.7 ± 32.9 m day-1, H. cuneformis= 22.9 ± 5.37 m day-1) than the highest observed treatment (C. wailesii=65.9 ± 26.9 m day-1, H. cuneformis=20 ± 5.66 m day-1). There was no clear trend of increased mean sinking rate with respect to treatment or time. Clear doseresponse curves for sinking were not evident. Skewness and kurtosis was calculated for each treatment to examine changes in the frequency distributions of sinking rate histograms, and compared to the controls to observe any patterns of change in central tendency or kurtosis. There was no trend with respect to skewness or kurtosis, although the data suggests there may be species-specific differences in response in H. cuneformis and Skeletonema spp. treatments, where one side of the skewness axis was favored. The data suggests that exposure to crude oil and chemical dispersant does not elicit a clear increase in sinking rate. If these results can be generalized to the field, then diatom population changes after an oil spill is likely not due to major changes in sinking losses.