Oil and Water Don’t Mix

Review of the journal article: Toxicity of crude oil to the mayfly, Hexagenia bilineata
Authored by Mark P. Ort, Susan E. Finger and John R. Jones, published in Elsevier Science Limited, 1995. DOI: 10.1016/0269-7491(94)00085-r

The April 20, 2010 tragedy involving the Deepwater Horizon oil rig and subsequent 3 month long oil leak has raised questions concerning how the oil will affect the Gulf coast. Given the timeliness of the topic, we review a scientific article published in 1995 that examined the toxicity of oil using a series of laboratory experiments. The research was conducted in response to oil spills that occurred in Missouri; a 1988 spill that contaminated the Gasconade River and a 1990 spill near the Chariton River. While field studies following these and other spills indicate a substantial effect on aquatic insect populations (which are analogous to the canary in the coal mine), the response of insects to oil spills was poorly studied and not well understood.

The Oil

Crude oil breaks down into three main components in the aquatic environment. The volatile fraction evaporates off of the water surface and thus poses no long term threat to aquatic life. The water soluble fraction remains in the water, where it goes through chemical changes over time. Because crude oils differ in their initial chemical makeup, and the environments in which spills occur vary (temperature, dissolved oxygen, etc.), the rate of breakdown and the chemistry of by-products produced during breakdown are not easily predicted. The third component is the insoluble residue that tends to settle out of the water column, contaminates the sediment and poses a long term source of exposure.

Two crude oils were used in these experiments, a sweet crude similar to that involved in the Gasconade spill and the intermediate weight oil spilled into the Chariton River. In lab, the oils were processed to isolate the water soluble fractions and the insoluble residue.

Hexagenia (mayfly) naiad, or nymph

The Test Subjects

Hexagenia bilineata, a common burrowing mayfly, was chosen as the test subject for the experiments. This mayfly is abundant in a variety of aquatic environments, making results applicable to a wide geographic range. This species is also ecologically important to aquatic systems as a food item for fish and is universally considered as representing healthy aquatic environs. The experiments involved immature or nymph stages of the mayfly that live in and feed near the sediments. Hexagenia species generally have a life span of one year (two in colder climates), which is substantially longer than any of the conducted experiments.

Experiment 1

Short term experiments were conducted to determine if mortality would occur during a 4 day exposure to high levels of the water soluble fraction of crude oil. The goal of these experiments was to see if the mayfly could survive the initial high concentrations of the water soluble fraction of crude oil associated with a spill. Separate experiments were conducted for each type of oil, and treatments within the experiments ranged from 6.25 - 100% pollutant.

Significant mortality did not occur, with survival rates that were 90% or greater in all treatments. This finding indicates that the mayflies were able to withstand the high levels of pollution that would occur initially after an oil spill. The limited exposure (4 days) to the water soluble fraction was a reasonable time period, given the oil spills in Missouri that prompted these experiments occurred in river systems. The constant flow of water would translate to limited exposure to water soluble fractions of oil for most aquatic insects in the river (the exception being in backwater areas where the water does not constantly flush).

Experiment 2

A 24 hour behavioral test was conducted to see if mayflies actively avoided burrowing into residue contaminated sediment. The goal of this experiment was to determine if the mayflies would recognize the presence of oil residue and avoid contact with contaminated sediment. The treatments ranged from sediments that were 0.005% residue to .08% residue. Visual inspections to determine if the mayflies were burrowing or actively avoiding the sediments were made eight times during the 24 hour period.

The experiment found more mayflies burrowed into uncontaminated sediment (92-96% of individuals) than contaminated sediment (81-90% of individuals), but the differences were not statistically significant. Individuals that did not burrow tended to remain on the sediment surface and did not exhibit active avoidance through swimming. These results suggest that the mayflies did not recognize the contamination, and by remaining in contact with residue tainted sediment the mayflies would be susceptible to long term exposure.

Adult Hexagenia (mayfly)

Experiment 3

Long term (21 days) exposure to contaminated sediment at fairly low concentrations of residue was conducted to investigate how extended exposure would affect mortality. The goal of this experiment was to address the toxicity of long-term exposure and investigate if 6 weeks of “weathering would reduce the toxicity of residue contaminated sediment. This experiment was conducted using residue from the oil involved in the Chariton River spill at different levels of sediment contamination (0.01 - 0.14% residue). A separate experiment was conducted using sediment collected from the Chariton River six weeks after the spill had occurred. Contaminated sediment was collected 35 miles below the spill site, as well as 6 miles above the spill site (used as uncontaminated “control” treatment). The level of residue contamination in the downstream sediment was measured at 0.19% of dry weight.

The study found limited mortality after the first week in all of the treatments. After week two there was some mortality in treatments with contaminated sediment, but it was not statistically significant. By the end of the study there was significant mortality (45-60% of individuals) in the three treatments with the highest levels of residue. Similar results were found in the experiment using sediment from the Chariton River; mortality was significant after 3 weeks in the treatment containing the contaminated sediment relative to the upstream, uncontaminated sediment. Combined, these two experiments indicate that long-term exposure can be a problem for the mayfly, and that the problem exists even after the residue has been in the environment for 6 weeks. Given that mortality increased with each week of the experiment, it is possible that longer exposures would lead to even greater mortality.

Relevance to the Gulf Oil Leak

It is difficult to draw directly from this work to predict outcomes in the Gulf due to differences in both the type of oil and the environment in which the spill occurred. The most applicable findings might be that even if initial exposure to water soluble fractions of the oil do not cause harm to aquatic life, extended exposure to residue contaminated sediment might. It should be noted that the exposure to water soluble fractions only lasted 4 days in these experiments, a time period that seems reasonable in the case of the Gasconade and Chariton river spills. Given the duration of the Deep Water Horizon oil leak, exposures to water soluble fractions of oil probably lasted longer. Also, the enormous scale of this disaster (estimated 4.9 million barrels of oil leaked into the Gulf) means that a quick and thorough clean-up of all contaminated areas is virtually impossible. It is hard to imagine that residue contaminated sediment will not have some long-term effects on aquatic life in the Gulf.