Fate and effects of oil and dispersed oil
on mangrove ecosystems in Australia

FIELD EXPERIMENT
IMPACT AND RECOVERY OF FAUNA

In the field study, we also evaluated the impact of oil and dispersed oil treatments on key mangrove fauna. Oil treatments caused mass mortality of both epifauna and infauna within two days of oiling.

There were no significant differences between results for oil and dispersed-oil treatments Dead macro-fauna were collected from the sediment surface shortly after oiling. We measured species diversity, density and biomass. The dominant fauna included Alpheid pistol shrimp, Thalassinid mud lobsters, and Grapsid crabs. There were no significant differences between results for oil and dispersed-oil treatments. Densities of dead Thalassinids collected at the time of oiling however, correlated with tree mortality which occurred after oiling.
Measures of leaf removal by Grapsid crabs were used as an index of their presence and recovery from thetime of oiling to 22 months afterwards. Crabs returned to treated plots after one month of oiling, however, their activity remained around 50% of control levels until the end of monitoring at 22 months post oiling. There were no significant differences between oil and dispersed-oil treatments on initial mortality or recovery. Infauna were sampled specifically from sediments to a depth of 22 cm in treated and control plots at 3 and 22 months after oiling.
There was no change in density between collection dates, indicating the lack of recovery in infauna in oiled plots The dominant in fauna were Sipunculid peanut worms. Sipunculids were absent from oiled plots and present in one of three dispersed-oil plots. There was no change in density between collection dates, indicating the lack of recovery in infauna in oiled plots. Herbivory of green leaves (folivory) by small caterpillars was 3-4 times higher than normal in the entire study area, however, there was no correlation between herbivory and tree mortality in treated plots. Folivory levels after oiling were not correlated with oil concentrations in sediments.
Higher densities of Thalassinds were correlated with the relatively coarse grain sediments, but Sipuculids were found in an inverse relationship, with numbers higher in finer sediments.
Plate 9a, b, c, d, e, f.During field trials, infauna were monitored for the impacts of oil treatments.
A.Both oiled and control plots were subject to high levels of caterpillar grazing, around 30-40% on average. The impact of oil was greater in plots with more grazing.
B.Grapsid crabs along with other infauna were killed in oiled plots within two days of oiling.
C.Most dead animals were collected from the plots. Most were covered in thick layers of oil, like this mud lobster, thalassinid crustacean, approximately 12 cm long. Collected animals were identified, weighed and measured for determination of infaunal biomass in oiled plots.
D.Two months after oiling some fauna had returned to plots but normal levels of leaf removal by grapsid crabs were not achieved two years after oiling. Experimental estimates of leaf removal were made by offering leaves tethered to convenient branches and checking their condition after approximately three hours.
E.Fallen leaves in oiled plots were left uneaten on the sediment surface while those in control plots were removed.
F.Sediment cores were taken to sample for sipunculid peanut worms.

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April 7, 2010