Geoscience Reference
In-Depth Information
Oiling had a significant impact on the mangrove communities over the 5 years to 1991.
On the open coast, area of mangrove was reduced by 13% and root density was reduced by
24%. In channel and lagoon sites, the area of fringing mangroves reduced by 23% (root density
decreased by 20%), and in drainage streams, mangrove area reduced by 56.5% (root density
declined by 16%) (Levings & Garrity 1994). Release of dissolved and suspended oil residues
from mangrove sediments appears to be a relatively frequent occurrence and has led to ongo-
ing, pulsed phases of recontamination long after the initial spill. This secondary oiling occurs
following erosion of mangrove substrates and is most likely related to storm activity, periods
of increased rainfall leading to sediment washout, or to the cutting and degradation of areas of
dead mangrove. This pulsed release of oil from the sediments is evidenced by the occurrence
of localized oil slicks (which are most common adjacent to those sites most heavily impacted
after the oil spill) and by the re-oiling of artificial marker stakes within the mangroves (Case
Fig. 9.4b). This release of oil demonstrates the potential for mangrove sediments to act as
long-term storage sites of oil and to periodically release it into the adjacent environments,
thus prolonging the time-scales over which oil has an impact on nearshore environments.
Time-scales of toxin persistence in mangrove sediments after heavy oiling are estimated to be
at least 20 years (Burns et al. 1993).
Relevant reading
Burns, K.A., Garrity, S.D. & Levings, S.C. (1993) How many years until mangrove ecosystems recover from
catastrophic oil spills?
Marine Pollution Bulletin
26
, 239 - 48.
Burns, K.A., Garrity, S.D., Jorissen, D., et al. (1994) The Galeta oil spill: II. Unexpected persistence of oil
trapped in mangrove sediments.
Estuarine, Coastal and Shelf Science
38
, 349 - 64.
Guzmán, H.M., Jackson, J.B.C. & Weil, E. (1991) Short-term ecological consequences of a major oil spill on
Panamanian subtidal corals.
Coral Reefs
10
, 1-12.
Guzmán, H.M., Burns, K.A. & Jackson, J.B.C. (1994) Injury, regeneration and growth of Caribbean reef corals
after a major oil spill in Panama.
Marine Ecology Progress Series
105
, 231- 41.
Jackson, J.B.C., Cubit, J.D., Keller, B.D., et al. (1989) Ecological effects of a major oil spill on Panamanian
coastal marine communities.
Science
243
, 37- 44.
Levings, S.C. & Garrity, S.D. (1994) Effects of oil spills on fringing red mangroves (
Rhizophora mangle
): losses
of mobile species associated with submerged prop roots.
Bulletin of Marine Science
54
, 782-94.
carbonate production. At many sites, reductions
in live coral cover and associated increases in
macroalgal abundance are more realistically
discussed in the context of prolonged and varied
disturbances. This is seen particularly clearly on
the reefs of north Jamaica, which have under-
gone major changes in coral community struc-
ture over the past 25 years (Liddell & Ohlhorst
1993). Coral cover in shallow reef environ-
ments has declined from around 60% to 2-3%
at present, and this has been accompanied by
a marked increase in the cover of non-calcifying
macroalgae. The causes of this decline have been
attributed to hurricane disturbance, coral disease
and coral bleaching, eutrophication, mortality
of the key herbivore
Diadema antillarum
, and
a prolonged history of extensive overfishing.
The last two factors, in particular, have resulted
in the removal of most herbivorous species,
and as a result algal species effectively now out-
compete corals. Although the impacts on net
rates of carbonate production have not yet been
quantified (but are likely to be significant), shifts
in carbonate sediment production patterns have
occurred (Perry 1996).
Similar levels of reef decline to those in
Jamaica have also been recorded along the west
coast of Barbados, and again reflect a varied
