Macrofauna and hydrocarbons in the marine enviroment

In my work in Port Valdez, Alaska, we sample around the marine oil terminal in Valdez to determine the biological and chemical characteristics in support of the NPDES permit for discharges of treated water from the oil terminal.  Some sites have been sampled since 1989 and some since 1971.   Our work serves as a indicator for potential changes in the sediment environment that could signal a disturbance event.
Responses of macrofauna (animals living within the sediments) to hydrocarbons in the marine environment are varied and can be difficult to identify.  Animals living in sediments can persist in the presence of fairly high hydrocarbon concentrations after oil spills.  Dauvin (1982) found that while adults persisted, some young invertebrates didn't appear to recruit to contaminated sediments.  Crustaceans such as amphipods are particularly sensitive to hydrocarbons and marine worms can be harmed as well. Others, the opportunists (largely tolerant polychaetes) increase in abundance so following changes in abundance of the macrofauna is a very suitable tool for measuring effects from disturbance.

Sediment quality criteria are a means to evaluate the risk of effects on biota by considering a single value.  It is desirable to use one value as an index of risk for effects on fauna.  A popular choice is the Effects-Range Low (ERL).  For aromatic hydrocarbons (the part that animals are most sensitive to) the ERL value is about 4,ooo ng per gram.  A question lingers though, at what levels do animals really respond?

Our research in Port Valdez has found animals responding to sediment hydrocarbon values between 100 to 300 ng per gram, much less than the ERL.  The figure below is a space-time model of the changes in hydrocarbons in sediments (PAH) and the abundance of two worms.  The vertical (Y) axis in each plot is distance along a transect that crosses the point of discharge at the marine oil terminal in Valdez.  The horizontal (X) axis is time.  The first plot shows the areas of highest risk (red) for effects from PAH.  PAH concentrations have declined over time due to improvements in the ballast water treatment plat in Valdez.  My interpretation of the other plots is that as PAH concentrations declined to around 100 ng per gram, the abundance of the two sensitive worms increased as juveniles began to recruit in.  

 

A conclusion from this work is that researchers in our field may be understating the effects of hydrocarbons in the marine environment.  Effects are occurring at levels lower than the commonly used ER-L criterion and that values is becoming widely used.  Thus, we should be careful in drawing conclusions of no effect from hydrocarbons in the marine environment when concentrations are slightly elevated.  

References:

Blanchard, A. L., H. M. Feder and D.G. Shaw. In press. Associations between macrofauna and sediment hydrocarbons from treated ballast water effluent at a marine oil terminal in Port Valdez, Alaska. Environmental Monitoring and Assessment.

Dauvin, J. C. (1982). Impact of Amoco Cadiz oil spill on the muddy fine sand Abra alba and Melinna palmata community from the Bay of Morliax. Estuarine, Coastal and Shelf Science, 14, 517-531.