Oil has been spilled from an anchored vessel MV Maritime Wisdom on 23 March 2005 at around 1:30 am, Lat 15° 24'N; Long 73° 43'E. Initial position of the spill is shown in the figure below. A barge carrying iron ore collided with the vessel resulting in development of a 3x6" wide hole. The hole was plugged at about 04:00 am and until then about 110 tons of bunker oil has been spilled. Coast guard is in operation to monitor using aircraft. They are also working on control of spread of the spill by using booms and dispersants.

What is a bunker oil?

It is a heavyweight material that is difficult to pump and requires preheating for use. This fuel oil may be heavier than water, is not likely to dissolve, is difficult or impossible to disperse, and is likely to form tar balls, lumps, and emulsions. It has a low volatility and moderate flash point.

Additional information can be found at:

• Oil and Nature: from US Fish & Wildlife Service
• The Status of Bunker C Fuel Oil
• Fact Sheet: No. 6 Fuel Oil (Bunker C) Spills
• Bunker Fuels

The fate of oil spills at sea

Once the oil has been spilt at sea, different and sometimes competitive processes will become active. Evaporation and dissolving of the components leave a residue with properties differing from original oil. Chemical biological degradation may set in with similar results while the formation of water-in-oil emulsions gives a substance with quite different properties. These changes in the oil properties influence the transport and especially the spreading of oil.

On the other hand the size and thickness of a slick have their influence on the rate of above mentioned processes. So the combination of different factors (size of the spill, properties of the oil, meteorological and oceanographic conditions determine the fate of the oil.

Weathering of oil

Once the oil has been spilt on the surface of the sea, evaporation and later dissolving of some components tend to change its physical properties. There is considerable increase of evaporation of volatile components with increasing wind speed. Also, the thickness of oil layer and thus the spreading determine the rate of evaporation and of dissolving. . The final residue after a period of weathering (sometimes lasting a few days) may amount to 90%- 50% of the original mass, depending on the type of the oil. Under the influence of sunlight and high temperature, larger hydrocarbon molecules are formed from smaller ones over long time spans. The weathered oils have higher densities and viscosities, which means that their movement at the surface will be different. The viscosity of weathered oil may be 10 times or more that of the original oil. Density and viscosity of no.6 oil apparently are only slightly affected by weathering.

Chemical degradation

Chemical decomposition of oil under sunlight due to wavelengths 300-350 u appears to be slow but could be considerable after a long exposure. The rate of chemical decomposition is dependant on the type of oil. Highly paraffinic and low sulphur crude oils oxidize more rapidly than less paraffinic and highly sulphurous crude oils. Presence of nutrients like inorganic nitrogen and phosphorus compounds favour rapid removal.

Emulsification of water in oil.

The formation of water in oil emulsion have physical properties different from those of pure oil which influences the spreading and transport as well as the removal by natural or artificial causes. Emulsfication mainly occurs if the oil layer has sufficient thickness and it is promoted by strong agitation of the sea surface. Rainfall also promotes water in oil emulsions. Time requires could vary from hours to days and the stability of these emulsions could also vary.

Tar balls or lumps

After sometime weathered oil or emulsion may fall apart into lumps with diameters up to 30 cm. Residues of oil formed after weathering process may attain densities that are close to or even higher than that of water. A further increase of density may result from the inclusion of sand or other particulate matter of sufficient density.

Effects

The effect of the oil spilled on water depends on the movement due to surface currents and wind at the site of spill. Less important is the internal spread of the slick. The damage would involve contamination of coastal and inland shorelines. Oil tends to wash ashore in patches and streaks. Prediction or even estimation of the degree of contamination by oil is subjective and can be difficult depending primarily on the type of oil and beach substrate and the existing tidal and wave action. The most obvious effects of any spillage are mechanical. The heavier oil may clog or blanket surfaces and fine structures inhibiting movement and therefore respiration and feeding of animals. Sedentary forms are at the mercy of stranded oil although barnacles apparently continue their activities even when quite thick layer covers the rocks to which they are attached. Winkles and other small mobile animals may become overweighted by such clinging oil even to the point of grazing it away. Thinner or nonemulsified oils usually fail to adhere to living surfaces which are covered by a mucous film in marine animals or mucilaginous slime in seaweeds. They may nevertheless stick to the horny covering of shells, penetrate feathers of fur and cling to some top-shore seaweeds or more terrestrial plants. Hydrocarbons dissolved or dispersed in water can easily reach unprotected surfaces such as the delicate skin covering fish gills. Aromatics in particular often irritate these surfaces stimulating the copious secretion of thick mucus and the surfactants cause erosion of both gill and gut tissues. These effects are important because such areas are involved in the control of water and salt exchange between body fluids and the medium in which the animal is living as well as respiratory exchange. Crude oil could also interfere with the activity of enzyme systems and other proteins from a variety of animals and plants. In addition to these effects on the immediate health of individual organisms, whether exerted mechanically or their outer structures or interfering with their most basic biochemical mechanisms, spilt oil may affect the whole structure of natural communites by tilting the balances which have developed in response to long established environmental pressures.

Effect on Phytoplankton

Diatoms and dinoflagellates could develop some abnormalities as they float very near the surface and have little external protection. However, the rapidity with which phytoplankton proliferate could overcome the effect of any local or temporary reduction in their numbers.

Zooplankton

Beneath a thick oil slick, the penetration of light may be reduced thus impairing the photosynthesis in phytoplankton and may also interfere with the daily vertical migration of zooplankton which is regulated by light intensity. Many of the larger members of zooplankton and small fish take their food by visual selection rather than by unselective filtration, so that overshadowing by oil in large quantities may exert an indirect effect on the nutrition and behaviour of planktonic organisms of all sizes in addition to its direct chemical or mechanical effects.

The larvae of benthic animals ie the meroplankton are much more sensitive than the holoplankton even though they are in turn often, much more sensitive than their adult forms.

Fish

Since oil in large quantities is found generally floating on the surface of the sea or stranded on its shores, and since fish occur mostly in mid waters or on the bottom, it is probable that adult fish suffer massive contamination only in tide pools or enclosed waters during a catastrophic spill. The outer surfaces, mouth and gill chambers of fish are coated with a slimy mucus which fresh oil cannot wet. If the oil has been emulsified either by vigorous agitation at sea or particularly after spraying with surfactants it is more likely to cling. Most fish manage to avoid localized spill . The larger food fish occupy a position high in marine food chains, they may accumulate considerable quantity of oil or its components when feeding. True or bony fishes need to swallow water constantly in order to avoid osmotic dessication a means by which they may also accumulate the soluble constituents of dispersed oil. Even if such components do not cause any obvious harm to the fish they often contribute an unpleasant taste to its flesh and thus render the catch unsaleable.

Benthos

Benthic organisms are those that spend the major part of the lives at the sea bottom, whether on the shore or below tide marks. Many spend their larval phase in the plankton a swim freely again during a short breeding season, but some become permanently cemented to rocks or piles and most move only slowly over short distances when adult. . They are thus likely to get smothered when massive oil spillages. Forms inhabiting the inter-tidal zone suffer the greatest risk of oil especially on exposed shores or at high levels. This oil is most likely to be weathered and therefore chemically harmless, but may well be re-mobilised by potentially toxic emulsifiers or removed by other damaging methods.

Shore crabs are certain to become contaminated on a heavily oiled shore as they occupy shallow crevices or burrows and because their scavenging habits they are likely to feed on animals affected by the oil. Among the smaller crustaceans shrimps live in shallow water or tide pools, while sand hoppers and sea slaters occupy the upper part of the beach which are most frequently polluted.

Otto, L. (1973): Reports on Marine Science Affairs Report No.9 . Environmental Factors in operations to combat oil spills. WMO No.359

Nelson-Smith, A., (19 ) Oil Pollution and Marine Ecology: Some biological published data on these waters : (Compiled by M.Gauns)

Off Mandovi mouth

Phytoplankton diversity

Diatoms

Chetoceros, Planktoniella, navicula, Melosira, Cyclotella, Biddulphia, Skeletonema, Triceratium, bacteriastrum, ditylum, Amphora, Aasterionella, Rhizosolenia, Streptotheca, Corethron, Pleurosigma, Hemidscus are common

Dinoflagellates

Ceratium, Peridinium, Dinophysis, Prorocentrum

Presence of non-toxic bloom : Trichodesmium erythraeum (ca. 38640 filaments/ml) are predominant

Zooplankton

Copepods, decapod larvae, chaetognaths, appendicularians, salps, polychaets Lamellibranches gastropods, siphonophores, medusae, fish eggs etc.

Canthocalanus pauper, Eucalanus monachus, Paracalanus sp.,, Acrocalanus gibber, Centropages furcatus, Temora turbinate, Labidocera minuta, Acartia erythraea, A. spinicauda, A. southwelli, Microsetella sp . are some of the common species of copepods (Goswami and Padmavati,1996)

Cladoceran (Evadne tergestina and Penilia avirostris) follows trichodesmium bloom Devassy et al., 1978

References

Bhattathiri, PMA ; Devassy, VP ; Bhargava, RMS . Production at different trophic levels in the estuarine system of Goa. Indian J. Mar. Sci. Vol. 5, no. 1, pp. 83-86. 1976

Devassy,V.P. ; Bhattathiri,P.M.A. ; Qasim,S.Z Trichodesmium phenomenon Indian. J. Mar. Sci., 7(3), 168-186, (1978)

Devassy, VP ; Bhargava, RMS Diel changes in phytoplankton population in the Mandovi and Zuri esturies of Goa.Mahasagar – Bulletin of the National Institute of Oceanography, 11 (3&4), 1978, 195-199.

Goswami, SC Zooplankton Standing Stock And Composition In Coastal Waters Of Goa, West Coast Of India. Indian Journal Of Marine Sciences. New Delhi [Indian J. Mar. Sci.], Vol. 14, No. 4, Pp. 177-180, 1985