Environmental Engineering Reference
In-Depth Information
oil platform, John Gray (Gray et al.
1990
) showed that, although the area of
reduced diversity extends only for about 250m out from the platform, more
subtle ecological changes were detectable across an area of sea bed with a
diameter of around 5 km. Subsequently, Olsgard and Gray (
1995
) carried out
the same multivariate statistical analyses on data from a number of other oil
and gas fields in the Norwegian sector of the North Sea. They showed that,
when environmental conditions were heterogeneous, the only adverse effect
that could be detected around oil and gas platforms in the North Sea was a
small area where diversity was reduced. However, when the environment was
homogeneous, a much larger area of altered faunal composition could be
detected. At more heterogeneous sites, presumably the oil-based drilling muds
were causing similar changes to the fauna outside of the area of reduced
diversity, but these effects were not indistinguishable from differences pro-
duced by the environmental heterogeneity. Other studies of oil and gas plat-
forms have failed to demonstrate subtle effects on community composition as a
result of environmental heterogeneity (e.g., Terlizzi et al.
2008
). Changes in
community composition not associated with reduced diversity have been repli-
cated in an experimental mesocosm study, in which sediment cores were
amended with a 3-mm layer of drill cuttings (Schaanning et al.
2008
). After
3months, there were no significant differences in diversity between treatments,
but community composition did differ between clean and cuttings treated
cores. However, the authors suggested that the differences in community com-
position were due to altered sediment characteristics, rather than toxicity,
as the cuttings contained low toxicity water- and olefin-based drilling muds.
There is also evidence for this hypothesised pattern in biofilms in rivers
subject to herbicide runoff from vineyards (Dorigo et al.
2007
). These authors
found significant differences in the community structure of both eukaryotic
and prokaryotic micro-organisms between pristine and contaminated sites, but
no associated differences in diversity. Evidence that the differences were due to
herbicides rather than other environmental differences between sites came
from the fact that the sensitivity of photosynthesis to diuron exposure in
laboratory conditions was lower in contaminated sites than it was in pristine
sites. And in a study of herbivorous insects along an air pollution gradient in
California, Jones and Paine (
2006
) found that particular groups of insects were
associated with the pollution gradient, although there were no marked rela-
tionships between pollutant concentrations and insect abundance, species
richness or diversity.
Is a change in community composition a deleterious effect?
So a number of studies have been able to identify sites where community
composition is altered by exposure to pollutants without diversity being
reduced. Whether this alteration in community composition represents a
