Marine Pollution

(Adapted from the Duke University's Connected Curriculum Project)

Background

Tributyltin (TBT) is a chemical that is acutely toxic to many species of marine animals. For this reason, it effectively inhibits the growth of marine organisms when used in marine paints (e.g., on boat hulls) and in other "antifoulants." However, because of concern about the adverse effects on marine life of TBT leaching from treated surfaces, strict controls on the use of TBT have been implemented in France, the United Kingdom, the US, and other jurisdictions.

Maine, a state with a low population density, is perceived as having a relatively unpolluted coastline, but there are activities that do pose pollution threats. Activities such as shipyards, pleasure craft, commercial ship traffic, and wood and fishing net preserving are all potential sources of TBT. Because of this, members of the Bowdoin College Hydrocarbon Research Center and the Marine Research Laboratory conducted a study to learn about the extent of TBT inputs in Maine locations; learn about TBT in mussels from various field locations; and relate TBT in mussel tissues to an overall indicator of stress. Mussels were used in this study because they are an effective organism for detection of marine pollutants.

Data

Collections of blue mussels, Mytilus edulis, were sampled from 15 coastal Maine intertidal locations (plus one location in England) at various times between October 1987 and November 1989. Six of the samples in 1987 formed three pairs, with one member of each pair in an area of high marine activity and the other in the same general vicinity but more remote from such activity. As part of this study, chemistry professor David Page and undergraduate student Tamara Dassanayake analyzed the relationship between mussel shell thickness (as an indicator of stress) and the concentration of TBT found in the cell tissues of the organisms.

Shell thickness index (TI) was determined on the shells of 12 to 25 animals collected from each site. TI is a dimensionless quantity defined as 100 times shell depth (i.e., "thickness") divided by shell length. The concentration of TBT in the cell tissues (abbreviated below as TBT) was measured in micrograms (mg) per gram of tissue. (The measurement of this quantity required a complicated analytical procedure whose description we omit.) The results are given in the following table.

Results of TBT and Thickness Index analyses in tissue samples of mussels

TBT (mg/g)	TI	TBT (mg/g)	TI
0.04	0.40	1.66	0.50
0.11	0.42	3.56	0.56
0.50	0.43	0.27	0.48
0.60	0.43	0.45	0.45
0.73	0.46	0.74	0.44
0.77	0.46	1.75	0.48
1.75	0.48	0.04	0.42
4.31	0.53	0.04	0.39

Problems

1. Plot the data on TBT concentration and thickness index. What sort of relationship seems to exist between the concentration of TBT in the cells of the mussels and the thickness of their shells?

2. Compute the regression line for the data. How good is the fit between the line and the data?

3. Plot the residuals for your regression line. What does this plot tell you about your linear model?

4. Give physical interpretations for the slope and intercept of the line.

5. What conclusions can you draw from your results? Is there reason to be concerned about TBT pollution (i.e. who cares about thick-shelled mussels, anyway)?