BfR-Wissenschaft
27
• Is it possible to mitigate the damage of oil pollution by using chemical dispersants
as (part of) the response measures?
(Hypothesis: If there is an increased degradation of oil and a decreased occurrence of oil
slicks on the water surface, the ecological damage will be lower.)
The coastal water of this area shows a wide range of salinity, turbidity and energy characteristics:
With increasing distance from the coast, depending on the tide especially in estuarine
areas, the salinity changes from less than 5 %o to more than 30 %o. The gradient of turbidity is
reverse but it is interrupted by high differences of about 80 to far more than 1000 ppm. The
heterogeneity of wave energy on a small scale (some 100 m), caused by changing wind
conditions, water depth and current speed in estuaries, tidal channels and creeks also decreases
with distance from the coastline while the heterogeneity and number of sensitive
tidal and subtidal habitats increases. Briefly: the effectiveness as well as the controlled application
of dispersants may be greater the further offshore they are applied (wave energy and
salinity) but is very different on a small scale in nearshore areas (up to 25km wide), depending
on currents and water depth - the danger to fundamental systems functions is greater if
they are used nearshore (high adsorption rate of oil droplets to particles leads to increased
microbial degradation and potentially detrimental to the oxygen balance).
Additionally there are several nearshore phenomena of a high sensitivity to oil slicks: e.g.
mussel beds, shell mounds, sea grass meadows, salt marshes and stocks of resting and
moulting birds. For most of these phenomena an evaluation about the “good or bad” effects
of chemical dispersant use also depends on the individual conditions of an accident. However,
a special case exists with regard to moulting and resting birds. In particular, moulting bird
stocks are clearly much more vulnerable to untreated oil slicks in comparison to chemical
dispersions. During one to two months in summer these birds are not able to fly because of
changing their feathers. In distinct areas these stocks can reach far more than 100.000 individuals;
just swimming or drifting on the water they are helpless in the face of being contaminated
by oil slick residues. Their stock sizes and population dynamics are very well known
and steadily monitored so that the degree of uncertainty in estimating damages to the population
level is comparably low. Although there may be no danger to the survival of their population
and role in systems functions, nevertheless a rough decrease of their local stock size
has to be avoided for reasons of natural resource protection.
Extracting the formulation of “endpoints” out of these roughly summarized conditions to develop
special strategies of oil pollution response, the resulting simplified scenarios would
correspond as examples.
Example 1:
Moulting birds
A drift model shows the probability of a high damage to moulting birds. As endpoint the decision
is: natural protection of local stocks is given higher priority than possible ecological
consequences. Resulting from this statement, dispersants must be used in such a way that
success is guaranteed as far as possible. Any half-hearted and ineffectual use will lead to
damages in both directions and means a defeat of the strategy involved.
Example 2 (Figure 5.2):
A drift model shows: the oil remains in the „Jade“, a big tidal channel leading to the Jadebasin.
The subtidal and adjacent intertidal of this channel is of minor sensitivity (Figure 5.2).
Additionally the subtidal and the flats are accessible by foot and suitable vehicles. Effective
cleaning is possible. As an endpoint may hold: because removal of oil is possible without
greater harm to sub- and intertidal habitats and with focus on the sedimentation problems
