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The effects of oil spills on mangroves

Suprayogi, Bambang (1996) The effects of oil spills on mangroves. Masters by Research thesis, Murdoch University.

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Mangrove communities are vulnerable to oil spills due to their location close to harbours, onshore and offshore oil production facilities, oil exploration facilities and tanker routes. Many oil spill accidents have been reported and the literature on these accidents has been examined with particular emphasis on the effects of oil on mangroves and other organisms. Most of the published studies have resulted from research after oil spill accidents occurred. However, there are very few detail studies on oil dose-response (plant symptomatology, growth, physical and chemical action) relationships in plants and sediments. An understanding of the effects of oil on mangroves may assist in predicting the interactions between oil exposure factors, species sensitivity and environmental factors. The study was designed to determine the concentrations, the time dependencies, and the lethal and subtle effects of spilled oils on different ages of mangroves, with particular focus on mangrove seedlings. The effects of volatile hydrocarbons and the interactions of oil with anoxia (lack of oxygen) on plants and sediments were also studied. Moreover, the aims of the experiments were to characterise the toxic fractions by recording chemical action of two oil types on plant leaves and sediments. The results were expected to determine the resistant index of mangroves to oil spills (time and dose) and to clarify the chemical compounds and concentrations which were toxic to mangroves.

Kuwait Crude Oil (KCO) and North West Shelf Condensate (NWSC) were chosen for use as common heavy and light grade oils, respectively. Different age levels (seeds, seedlings and saplings) of Avicennia marina, Ceriops tagal, Rhizophora stylosa and Rhizophora mncronata were chosen for experiments due to their differences in morphological features, physiological processes and sizes. The research was divided into two main exposure conditions, laboratory and field conditions. Each species was exposed to different selected doses of oil, ranging from very low (27.5 g m-2) to very high (1100 g m-2), applied to the sediment surface only, or to the sediment surface and shoots.

Very low (27.5 g m-2), low (275 g m-2) and medium (1375 g m-2) doses of KCO did not permanently affect the total metabolic processes for plant survival. In certain case, these doses stimulated growth. However, application of the same doses of NWSC produced chronic effects. Exposure to higher doses (2750, 5500 and 11000 g m-2) of both oils significantly increased injury symptoms and decreased plant growth. The interactive effects between oil treatment and duration of treatment were mostly antagonistic at medium high and high doses of oil and became synergistics at very high doses of oil. Application of oil to the sediment and shoots had more acute impacts than application to the sediment surface only, as indicated by a higher symptom index, leaf abscission and mortality, decreased plant growth and reduced biomass. There were variable effects on leaf area and biomass accumulation as responses of any species were affected more by individual plant-size than by oil treatments. The greater tolerance of biomass responses to oil treatments may be because of its slower response to the stress as it follows physiological and biological changes. In certain cases, the effects were more complicated due interactions of response to oil with other environmental stresses.

Although the effects of NWSC and KCO on mangroves were variable, A. marina was more sensitive to both types of oil than the three other species. The differences in morphological features and physiological processes may play an important role in sensitivities of different species. Plant stress in Avicennia mangroves was exhibited as primary effects in response to the toxicity of high concentrations of hydrocarbons and other toxic fractions in plant tissues; while, the stress in Rhizophora mangroves was caused by secondary effects such as physical and chemical changes in sediments which affected nutrient deficiencies and metabolic disruptions. Dose-response relationships for individual oil types were different in each species, and were variable under different conditions of experiment. Different species origin, culturing system, sediment characteristics and environmental factors may cause different sensitivities. Furthermore, differences in the capacity of metabolism, and different ages of mangroves resulted in different sensitivities when the same type and doses of oil were applied. The most sensitive age was seed germination, followed by seedlings and saplings, respectively.

NWSC as a light oil was more toxic than KCO (a heavy oil) in all species and all age levels of mangroves. The chemical compositions of hydrocarbons in plants was more important than concentration in producing lethal and sublethal impacts than in KCO. The higher increased content of aromatic fractions in NWSC may confer the considerably degree of toxicity to plants. However, different doses of oil caused different responses in each species.

While both oils were greatly degraded with time under laboratory and field conditions, the degradation of NWSC was faster than KCO in sediments. The degradation processes may also be influenced by rainfall, tidal flushing, weathering processes (evaporation), biological factors (bacteria, fungi and other micro-organisms) and environmental factors (temperature, oxygen, nutrients, salinity and pressure).

In conclusion, different types and doses of oil, and duration of exposures produced different responses in each species of mangroves. Depending on amount of oil applied, the responses developed from growth stimulation to chronic and acute impacts. However, the mechanism of damage appeared to be similar in all species. The responses included foliar injury (leaf chlorosis and necrosis), leaf abscission, stem deformation, reduced number of new leaves, reduced plant growth and biomass accumulation, and mortality.

Item Type: Thesis (Masters by Research)
Murdoch Affiliation: School of Biological and Environmental Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Murray, Frank
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