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Oil spills in mangroves: a conceptual model based on long-term field observations
Ecological Modelling, 52 (1990) 53-59
53
Elsevier Science Publishers B.V., Amsterdam
Oil spills in mangroves" a conceptual model based on long-term field observations Claudia Mafia Jacobi Departamento Zoologia, Instituto Bioci~ncias, Unioersity of Sro Paulo, C.P. 11461, 05499 S~o Paulo, S.P., Brazil
and Yara Schaeffer-Novelli Instituto Oceanogrrfico, Unioerstiy of S6o Paulo, C.P. 9075, 01051 S~o Paulo, S.P., Brazil
ABSTRACT Jacobi, C.M. and Schaeffer-Novelli, Y., 1990. Oil spills in mangroves: a conceptual model based on long-term field observations. Ecol. Modelling, 5 2 : 5 3 - 5 9 A conceptual model is proposed for evaluating residence time of oil in mangrove environments. It assumes that, after oil has spread over a mangrove coastline, it remains in the environment by retention in the sediment. Removal is mainly in association with seaward particle export. Since detritus export depends on tidal flush, the area affected by an oil spill can be divided into sections parallel to the coastline having different removal rates increasing seaward (under little river flush and regular topography).
INTRODUCTION
Oil pollution in the marine and coastal environments is a common feature in our days, on account of the wide use of this product, its production and transport. The impact of an oil spill depends on the type and amount of oil, on climatic factors, on abiotic site characteristics and on the sensitivity of local species (Cintr6n et al., 1981). Damages caused to marine organisms include direct mortality by coating or absorption of soluble toxic fractions, reduced tolerance to infections, predators and other stressors, and incorporation of mutagenic and carcinogenic agents into the food web (Blumer, 1971). Mangroves characterize one of the most vulnerable types of coast. These highly productive environments are most sensitive to oil spills because they 0304-3800/90/$03.50
© 1990 - Elsevier Science Publishers B.V.
54
C M . J A C O B I A N D Y. S C H A E F F E R - N O V E L L I
usually develop in calm, accretive shores, where removal of pollutants can be extremely reduced (Gundlach, 1987). Furthermore, the risk of impact by toxic substances increases in protected coastlines since these concentrate navigational and operational activities, as well as storage and refining facilities (Schaeffer-Novelli et al., 1988). It is estimated that around 60-70% of the tropical coastlines (25 ° N - 2 5 ° S) are lined with mangrove swamp forests. In Brazil, mangrove communities are found between latitudes 04°20'N and 28°30'S, covering an area estimated between 10 000 and 25 000 km 2. The productivity of mangrove forests derives mostly from the high biomass of vascular plants. The fallen leaves (leaf litter) are the primary source of detritus in the system, of which about half is exported to adjacent aquatic regions. A great percentage of the world's fisheries - estimated in some studies to be as high as two-thirds (Lai, 1984) - depends on detritus supplies from mangrove zones. Oil as a stressor affects all the elements of a mangrove community. However, due to the importance of the vascular plants as energetic sources, its effects related to this c o m p a r t m e n t have received more attention. Also, damage is somehow easier to visualize in trees than in small animals when assessing field impact. The initial response to the acute effects of oil pollution is an abnormally high defoliation rate. Responses to chronic effects include reduced n u m b e r of new leaves a n d / o r abnormal elements. Sapling survival is also affected. The ensemble of anomalies in the tree stand causes depletion of the canopy and reduced detritus production, resulting in drastic reduction of organicmatter export to adjacent environments (Schaeffer-Novelli et al., 1988). Populations of decomposers living in the soil may be also altered. Studies on the bioavailability and residence time of the different classes of petroleum hydrocarbons in Brazilian mangrove forests are necessary to relate toxicant concentrations to effects on the biota and to estimate the system's recovery time. At present, however, the use of current toxic-substance models in the case of oil spills in mangroves is hindered by problems such as lack of empirical data for detailed models (in Brazil), lack of data on the combined effects -on the biota- of the various oil fractions, incomplete data on spilled sediment behaviour, and need to adapt some chemical parameters to the salinities of the marine environment. Specific models for dispersion and degradation of oil in intertidal regions are still in the early stages compared to those of surface (continental) waters. A m o n g the first ones are Gundlach's simulation model (1987), and those mentioned by Siripong (1988) on circulation and dispersion in S.E. Asia. Also, data on degradation and dispersion are available, based on observations of experimental and natural oil spills in different shore types including
OIL SPILLS IN MANGROVES
55
mangroves (Feng, 1984; Little, 1987; Little and Scales, 1987). This background suggests that an initial approach to the problem of oil pollution in Brazilian mangroves be simple and emphasize aspects of residence-time modelling (i.e. not considering oil bioavailability). This could be done by considering that oil behaves as one toxicant, and starting with only the main forcing functions affecting residence time. In the case of mangroves, residence time is an important item to be estimated, since it can be measured in years or even decades. The question is critical if considered that it is a basic feature in assessing long-term (chronic) effects of environmental impacts caused by petroleum. MAIN OIL-WEATHERING PROCESSES Petroleum is a complex mixture of compounds, with a predominance of hydrocarbons. The weathering of oil after its introduction in the marine environment comprises mechanical (e.g. spreading, dispersion and sedimentation) and chemical transformations (e.g. dissolution, oxidation and biodegradation). The importance of each process varies with time (TOVALOP, 1986). Also, the contribution of each of these processes to oil removal depends on the type of oil and on environmental conditions. In mangrove swamps, some weathering processes are negligible while others are enhanced.
Spreading.
Depends on weight and viscosity of the oil, and on currents, tidal streams and wind speed. The present conceptual model focuses on the fate of oil after this phase.
Dispersion. The formation of droplets (oil-in-water) in mangrove environments is negligible (unless dispersants are used): it depends mainly on waves and turbulence of water.
Emulsification.
Water-in-oil emulsions are also negligible in mangrove en-
vironments.
Evaporation.
Depends on the volatility of the oil and on weather conditions. Though the lower-boiling-point components of oil can persist for months in certain intertidal environments, around 50% of the total mass can evaporate from mangroves in less than 48 hours, on account of the wide surface involved and the high temperatures, and in spite of the high relative humidity and the lack of winds (Feng, 1984).
Dissolution.
Depends on dispersion, temperature and the composition of the oil. Light compounds which can suffer dissolution will evaporate well
56
C . M . J A C O B 1 A N D Y. S C H A E F F E R - N O V E L L I
before they dissolve (10-100 times faster), so it can be assumed that this process is not important in mangrove environments on a long-term scale.
Oxidation (usually photo-oxidation). Depends on sunlight, transparency of the water and oxygen level. Thus, in mangrove environments this process is of minor importance on account of the well-developed canopy, the a m o u n t of suspended particles in the water column and the little oxygen in the sediment. Oil adhered to tree structures can become tarry through evaporation and oxidation, rendering further weathering difficult. Sedimentation. The most important weathering process in intertidal regions. In mangrove environments, infiltration of oil will occur in spite of the fine sediment being more difficult to penetrate than coarse ones. This is favoured by the shallow waters, the low hydrodynamics, the great a m o u n t of suspended particles in the water column and the high percentage of organic matter in the sediment. Also, increasing salinity enhances absorption of hydrocarbons (Feng, 1984).
Biodegradation. Depends on oxygen, nutrients (usually N and P) and temperature, and is done by bacteria, yeasts and fungi. The rate of biodegradation will be reduced if oil is in the sediment, as in this case, or inhibited if the concentration of oil in water is high.
Uptake by higher organisms. Not really a weathering process, this can reduce the organisms.
amount of oil in the area. Done by plants and migrating
A TENTATIVE CONCEPTUAL MODEL (Fig. 1) After an oil spill occurs in a mangrove swamp, its removal will d e p e n d mainly on a few weathering processes, namely evaporation and seaward transport in connection with particulate matter export due to tidal flush. Since evaporation is measured in days or weeks, it could be considered as an instantaneous process in the model, compared to removal from the sediment. It is also assumed that there is no significant river flushing in the area. In this model the oil spill is 'instantaneous', and the whole load is spread over a certain area of the mangrove. This area can be estimated empirically. Otherwise, calculations on the area affected can be done through a coastline/oil interaction model including length of coastline affected, slope, tidal amplitudes after spill, and amount of oil spilled. Oil spread in the mangrove will evaporate very quickly on account of the (usually) great area involved and the high temperatures (Feng, 1984). The
OIL SPILLS IN MANGROVES
57
I I I
I
I I I
-I
I t
I
_1
Fig. 1. Conceptual diagram of a model for oil removal from mangrove environments. Removal is done basically through detritus export and tidal flush. Detritus export rates are taken from an energetic model (discontinuous line). Qi = oil in each mangrove segment.
percentage lost (wet weight) is determined by its composition. A small amount of oil may be degraded in the water column or on the sediment surface. Also, adherence to tree trunks and other structures can occur. The remaining oil is deposited in the sediment, according to a m a x i m u m concentration of oil in the sediment, or loading capacity (see Gundlach, 1987). If this value is exceeded, oil will be transported to adjacent areas. The loading capacity is assumed to be the same along the mangrove area (i.e. the composition of the sediment doesn't vary). However, tidal flush and, consequently, detritus export can vary in each mangrove region. Usually, in normal topographic conditions - a gentle slope - removal will increase seaward on account of the tidal flush. This is verified by field observations: mangrove recovery in the seaward portions is faster. Therefore, the total area affected could be divided into bands parallel to the coastline and their width determined by the local tidal regime. In this example, the spill area was arbitrarily divided into three segments: a region washed only by high neap or spring tides, another reached by neap tides, and a third one washed even by the lowest tides. The intial mass of oil in the most inward segment is the initial concentration (which is the same for all segments) times a certain area (related to the tidal amplitude after the spill). Though oil can flow into this region during
58
C.M. JACOBI A N D Y. S C H A E F F E R - N O V E L L I
incoming tides, we will consider that the main removal direction is seaward. Oil is removed from the sediment basically as a function of detritus export. The rate of detritus export depends on tidal flush and is highly independent of detritus production, under normal conditions of health. The inflow of oil to the subsequent segments is the oil removed from the previous segment. The amount of oil in each compartment is a function of the original amount deposited in the segment, the amount of oil removed from it, and the amount removed from the previous (inward) segment. DISCUSSION
Oil spilled in mangrove swamps with little river flush can be treated as a bulk, considering already its depletion of lower-boiling-point components, and assuming that outflow of oil from the system is made mainly through removal via sediment export and tidal flush. Naturally, though detritus export depends only slightly upon productivity under normal conditions of health in a developed mangrove (Lugo et al., 1976), oil in the system will affect these values. Also, the weight of the particles will tend to increase due to oil binding, probably diminishing export rates. Evaporation was considered to be instantaneous, taking days for the lower-boiling-point components to be removed. However, under some environmental situations, even these components m a y take weeks or months to leave the system. In these cases, evaporation has to be incorporated into the model in another way, to account for the uptake and accumulation of low-molecular-weight aromatics by organisms. The division of the total spilled area into segments allows the overcoming of situations of more complicated topography than a regular mild slope. In these cases, removal will still depend on tidal flush but not as a direct function of tidal amplitude. Though m a n y studies have been made on Brazilian mangroves, still the unpredictability of oil spills sometimes makes it impossible to use even the simplest models to evaluate damage or recovery time. Data usually available in Brazil related to this kind of study are temperature, salinity, solar radiation, rainfall, tidal regime, sediment characteristics and leaf litter. Normally in the case of a spill, the load and area covered can be well estimated, and the composition of the oil analysed. However, more sophisticated data such as loading capacities, sediment transport, detritus export in each region and certain rates still have to be taken from similar studies. In this aspect, the model may be useful in stimulating research concerning missing data. For example, studies on the toxic levels of oil in sediment are a necessary feedback to calibrate the model. As stated before,
OIL SPILLS IN M A N G R O V E S
59
the importance in estimating residence time of a toxicant in areas like mangroves is related to the assessment of chronic effects in cases of environmental impact. The model has to be improved with data of the concentration of oil in the field over time, for better estimations of removal rates and related factors, and with calibration of detritus export in each segment, since these affect removal rates. Usually, in energetic models, only a general export rate is considered. ACKNOWLEDGEMENTS
This paper is based on working notes discussed by CMJ at a workshop on Mathematical Ecology held at the International Centre for Theoretical Physics (Italy), and benefitted from the comments of many of its participants. REFERENCES Blumer, M., 1971. Scientific aspects of the oil spill problem. Environ. Aff., 1: 54-73. Cintr6n, G., Lugo, A.E., Martinez, R., Cintr6n, B. and Encarnaci6n, L., 1981. Impact of oil in the tropical environment. Department of Natural Resources, Puerto Rico, 29 pp. Feng, M.C., 1984. Weathering of chemically-dispersed and undispersed oil on mangrove sediment. In: H.C. Lai and M.C. Feng (Editors), Fate and Effect of Oil in the Mangrove Environment. Univ. Sains Malaysia, Penang, pp. 177-200. Gundlach, E.R., 1987. Oil-holding capacities and removal coefficients for different shoreline types to computer simulate spills in coastal waters. In: Proc. 1987 Oil Spill Conf., American Petroleum Institute, Washington, DC, pp. 451-457. Lai, H.C., 1984. A review of oil spills with special reference to mangrove environment. In: H.C. Lai and M.C. Feng (Editors), Fate and Effects of Oil in the Mangrove Environment, Univ. Sains Malaysia, Penang, pp. 5-19. Little, D.I., 1987. Oiled sediments in the Humber estuary following the Sivand incident. In: Proc. 1987 Oil Spill Conf., American Petroleum Institute, Washington, DC, pp. 419-425. Little, D.I. and Scales, D.L., 1987. The persistence of oil stranded on sediment shorelines. In: Proc. 1987 Oil Spill Conf., American Petroleum Institute, Washington, DC, pp. 433-438. Lugo, A.E., Sell, M. and Snedaker, S.C., 1976. Mangrove ecosystem analysis. In: B.C. Patten (Editor), Systems analysis and simulation in ecology. Academic Press, New York, pp. 113-145. Schaeffer-Novelli, Y., Rodrigues, F.O. and Contr6n-Molero, G., 1988. Mangroves: a methodology for oil pollution impact assessment. Abstr. JOA Mexico 88 (288.57), p. 99. Siripong, A., 1988. Hydrodynamic and oil-spill modelling for the East Asian Seas Region. Ambio, 17: 183-185. TOVALOP, 1986. Fate of marine oil spills. Tech. Inf. Pap. 11, International Tanker Owners Pollution Federation Ltd., London, 8 pp.
53
Elsevier Science Publishers B.V., Amsterdam
Oil spills in mangroves" a conceptual model based on long-term field observations Claudia Mafia Jacobi Departamento Zoologia, Instituto Bioci~ncias, Unioersity of Sro Paulo, C.P. 11461, 05499 S~o Paulo, S.P., Brazil
and Yara Schaeffer-Novelli Instituto Oceanogrrfico, Unioerstiy of S6o Paulo, C.P. 9075, 01051 S~o Paulo, S.P., Brazil
ABSTRACT Jacobi, C.M. and Schaeffer-Novelli, Y., 1990. Oil spills in mangroves: a conceptual model based on long-term field observations. Ecol. Modelling, 5 2 : 5 3 - 5 9 A conceptual model is proposed for evaluating residence time of oil in mangrove environments. It assumes that, after oil has spread over a mangrove coastline, it remains in the environment by retention in the sediment. Removal is mainly in association with seaward particle export. Since detritus export depends on tidal flush, the area affected by an oil spill can be divided into sections parallel to the coastline having different removal rates increasing seaward (under little river flush and regular topography).
INTRODUCTION
Oil pollution in the marine and coastal environments is a common feature in our days, on account of the wide use of this product, its production and transport. The impact of an oil spill depends on the type and amount of oil, on climatic factors, on abiotic site characteristics and on the sensitivity of local species (Cintr6n et al., 1981). Damages caused to marine organisms include direct mortality by coating or absorption of soluble toxic fractions, reduced tolerance to infections, predators and other stressors, and incorporation of mutagenic and carcinogenic agents into the food web (Blumer, 1971). Mangroves characterize one of the most vulnerable types of coast. These highly productive environments are most sensitive to oil spills because they 0304-3800/90/$03.50
© 1990 - Elsevier Science Publishers B.V.
54
C M . J A C O B I A N D Y. S C H A E F F E R - N O V E L L I
usually develop in calm, accretive shores, where removal of pollutants can be extremely reduced (Gundlach, 1987). Furthermore, the risk of impact by toxic substances increases in protected coastlines since these concentrate navigational and operational activities, as well as storage and refining facilities (Schaeffer-Novelli et al., 1988). It is estimated that around 60-70% of the tropical coastlines (25 ° N - 2 5 ° S) are lined with mangrove swamp forests. In Brazil, mangrove communities are found between latitudes 04°20'N and 28°30'S, covering an area estimated between 10 000 and 25 000 km 2. The productivity of mangrove forests derives mostly from the high biomass of vascular plants. The fallen leaves (leaf litter) are the primary source of detritus in the system, of which about half is exported to adjacent aquatic regions. A great percentage of the world's fisheries - estimated in some studies to be as high as two-thirds (Lai, 1984) - depends on detritus supplies from mangrove zones. Oil as a stressor affects all the elements of a mangrove community. However, due to the importance of the vascular plants as energetic sources, its effects related to this c o m p a r t m e n t have received more attention. Also, damage is somehow easier to visualize in trees than in small animals when assessing field impact. The initial response to the acute effects of oil pollution is an abnormally high defoliation rate. Responses to chronic effects include reduced n u m b e r of new leaves a n d / o r abnormal elements. Sapling survival is also affected. The ensemble of anomalies in the tree stand causes depletion of the canopy and reduced detritus production, resulting in drastic reduction of organicmatter export to adjacent environments (Schaeffer-Novelli et al., 1988). Populations of decomposers living in the soil may be also altered. Studies on the bioavailability and residence time of the different classes of petroleum hydrocarbons in Brazilian mangrove forests are necessary to relate toxicant concentrations to effects on the biota and to estimate the system's recovery time. At present, however, the use of current toxic-substance models in the case of oil spills in mangroves is hindered by problems such as lack of empirical data for detailed models (in Brazil), lack of data on the combined effects -on the biota- of the various oil fractions, incomplete data on spilled sediment behaviour, and need to adapt some chemical parameters to the salinities of the marine environment. Specific models for dispersion and degradation of oil in intertidal regions are still in the early stages compared to those of surface (continental) waters. A m o n g the first ones are Gundlach's simulation model (1987), and those mentioned by Siripong (1988) on circulation and dispersion in S.E. Asia. Also, data on degradation and dispersion are available, based on observations of experimental and natural oil spills in different shore types including
OIL SPILLS IN MANGROVES
55
mangroves (Feng, 1984; Little, 1987; Little and Scales, 1987). This background suggests that an initial approach to the problem of oil pollution in Brazilian mangroves be simple and emphasize aspects of residence-time modelling (i.e. not considering oil bioavailability). This could be done by considering that oil behaves as one toxicant, and starting with only the main forcing functions affecting residence time. In the case of mangroves, residence time is an important item to be estimated, since it can be measured in years or even decades. The question is critical if considered that it is a basic feature in assessing long-term (chronic) effects of environmental impacts caused by petroleum. MAIN OIL-WEATHERING PROCESSES Petroleum is a complex mixture of compounds, with a predominance of hydrocarbons. The weathering of oil after its introduction in the marine environment comprises mechanical (e.g. spreading, dispersion and sedimentation) and chemical transformations (e.g. dissolution, oxidation and biodegradation). The importance of each process varies with time (TOVALOP, 1986). Also, the contribution of each of these processes to oil removal depends on the type of oil and on environmental conditions. In mangrove swamps, some weathering processes are negligible while others are enhanced.
Spreading.
Depends on weight and viscosity of the oil, and on currents, tidal streams and wind speed. The present conceptual model focuses on the fate of oil after this phase.
Dispersion. The formation of droplets (oil-in-water) in mangrove environments is negligible (unless dispersants are used): it depends mainly on waves and turbulence of water.
Emulsification.
Water-in-oil emulsions are also negligible in mangrove en-
vironments.
Evaporation.
Depends on the volatility of the oil and on weather conditions. Though the lower-boiling-point components of oil can persist for months in certain intertidal environments, around 50% of the total mass can evaporate from mangroves in less than 48 hours, on account of the wide surface involved and the high temperatures, and in spite of the high relative humidity and the lack of winds (Feng, 1984).
Dissolution.
Depends on dispersion, temperature and the composition of the oil. Light compounds which can suffer dissolution will evaporate well
56
C . M . J A C O B 1 A N D Y. S C H A E F F E R - N O V E L L I
before they dissolve (10-100 times faster), so it can be assumed that this process is not important in mangrove environments on a long-term scale.
Oxidation (usually photo-oxidation). Depends on sunlight, transparency of the water and oxygen level. Thus, in mangrove environments this process is of minor importance on account of the well-developed canopy, the a m o u n t of suspended particles in the water column and the little oxygen in the sediment. Oil adhered to tree structures can become tarry through evaporation and oxidation, rendering further weathering difficult. Sedimentation. The most important weathering process in intertidal regions. In mangrove environments, infiltration of oil will occur in spite of the fine sediment being more difficult to penetrate than coarse ones. This is favoured by the shallow waters, the low hydrodynamics, the great a m o u n t of suspended particles in the water column and the high percentage of organic matter in the sediment. Also, increasing salinity enhances absorption of hydrocarbons (Feng, 1984).
Biodegradation. Depends on oxygen, nutrients (usually N and P) and temperature, and is done by bacteria, yeasts and fungi. The rate of biodegradation will be reduced if oil is in the sediment, as in this case, or inhibited if the concentration of oil in water is high.
Uptake by higher organisms. Not really a weathering process, this can reduce the organisms.
amount of oil in the area. Done by plants and migrating
A TENTATIVE CONCEPTUAL MODEL (Fig. 1) After an oil spill occurs in a mangrove swamp, its removal will d e p e n d mainly on a few weathering processes, namely evaporation and seaward transport in connection with particulate matter export due to tidal flush. Since evaporation is measured in days or weeks, it could be considered as an instantaneous process in the model, compared to removal from the sediment. It is also assumed that there is no significant river flushing in the area. In this model the oil spill is 'instantaneous', and the whole load is spread over a certain area of the mangrove. This area can be estimated empirically. Otherwise, calculations on the area affected can be done through a coastline/oil interaction model including length of coastline affected, slope, tidal amplitudes after spill, and amount of oil spilled. Oil spread in the mangrove will evaporate very quickly on account of the (usually) great area involved and the high temperatures (Feng, 1984). The
OIL SPILLS IN MANGROVES
57
I I I
I
I I I
-I
I t
I
_1
Fig. 1. Conceptual diagram of a model for oil removal from mangrove environments. Removal is done basically through detritus export and tidal flush. Detritus export rates are taken from an energetic model (discontinuous line). Qi = oil in each mangrove segment.
percentage lost (wet weight) is determined by its composition. A small amount of oil may be degraded in the water column or on the sediment surface. Also, adherence to tree trunks and other structures can occur. The remaining oil is deposited in the sediment, according to a m a x i m u m concentration of oil in the sediment, or loading capacity (see Gundlach, 1987). If this value is exceeded, oil will be transported to adjacent areas. The loading capacity is assumed to be the same along the mangrove area (i.e. the composition of the sediment doesn't vary). However, tidal flush and, consequently, detritus export can vary in each mangrove region. Usually, in normal topographic conditions - a gentle slope - removal will increase seaward on account of the tidal flush. This is verified by field observations: mangrove recovery in the seaward portions is faster. Therefore, the total area affected could be divided into bands parallel to the coastline and their width determined by the local tidal regime. In this example, the spill area was arbitrarily divided into three segments: a region washed only by high neap or spring tides, another reached by neap tides, and a third one washed even by the lowest tides. The intial mass of oil in the most inward segment is the initial concentration (which is the same for all segments) times a certain area (related to the tidal amplitude after the spill). Though oil can flow into this region during
58
C.M. JACOBI A N D Y. S C H A E F F E R - N O V E L L I
incoming tides, we will consider that the main removal direction is seaward. Oil is removed from the sediment basically as a function of detritus export. The rate of detritus export depends on tidal flush and is highly independent of detritus production, under normal conditions of health. The inflow of oil to the subsequent segments is the oil removed from the previous segment. The amount of oil in each compartment is a function of the original amount deposited in the segment, the amount of oil removed from it, and the amount removed from the previous (inward) segment. DISCUSSION
Oil spilled in mangrove swamps with little river flush can be treated as a bulk, considering already its depletion of lower-boiling-point components, and assuming that outflow of oil from the system is made mainly through removal via sediment export and tidal flush. Naturally, though detritus export depends only slightly upon productivity under normal conditions of health in a developed mangrove (Lugo et al., 1976), oil in the system will affect these values. Also, the weight of the particles will tend to increase due to oil binding, probably diminishing export rates. Evaporation was considered to be instantaneous, taking days for the lower-boiling-point components to be removed. However, under some environmental situations, even these components m a y take weeks or months to leave the system. In these cases, evaporation has to be incorporated into the model in another way, to account for the uptake and accumulation of low-molecular-weight aromatics by organisms. The division of the total spilled area into segments allows the overcoming of situations of more complicated topography than a regular mild slope. In these cases, removal will still depend on tidal flush but not as a direct function of tidal amplitude. Though m a n y studies have been made on Brazilian mangroves, still the unpredictability of oil spills sometimes makes it impossible to use even the simplest models to evaluate damage or recovery time. Data usually available in Brazil related to this kind of study are temperature, salinity, solar radiation, rainfall, tidal regime, sediment characteristics and leaf litter. Normally in the case of a spill, the load and area covered can be well estimated, and the composition of the oil analysed. However, more sophisticated data such as loading capacities, sediment transport, detritus export in each region and certain rates still have to be taken from similar studies. In this aspect, the model may be useful in stimulating research concerning missing data. For example, studies on the toxic levels of oil in sediment are a necessary feedback to calibrate the model. As stated before,
OIL SPILLS IN M A N G R O V E S
59
the importance in estimating residence time of a toxicant in areas like mangroves is related to the assessment of chronic effects in cases of environmental impact. The model has to be improved with data of the concentration of oil in the field over time, for better estimations of removal rates and related factors, and with calibration of detritus export in each segment, since these affect removal rates. Usually, in energetic models, only a general export rate is considered. ACKNOWLEDGEMENTS
This paper is based on working notes discussed by CMJ at a workshop on Mathematical Ecology held at the International Centre for Theoretical Physics (Italy), and benefitted from the comments of many of its participants. REFERENCES Blumer, M., 1971. Scientific aspects of the oil spill problem. Environ. Aff., 1: 54-73. Cintr6n, G., Lugo, A.E., Martinez, R., Cintr6n, B. and Encarnaci6n, L., 1981. Impact of oil in the tropical environment. Department of Natural Resources, Puerto Rico, 29 pp. Feng, M.C., 1984. Weathering of chemically-dispersed and undispersed oil on mangrove sediment. In: H.C. Lai and M.C. Feng (Editors), Fate and Effect of Oil in the Mangrove Environment. Univ. Sains Malaysia, Penang, pp. 177-200. Gundlach, E.R., 1987. Oil-holding capacities and removal coefficients for different shoreline types to computer simulate spills in coastal waters. In: Proc. 1987 Oil Spill Conf., American Petroleum Institute, Washington, DC, pp. 451-457. Lai, H.C., 1984. A review of oil spills with special reference to mangrove environment. In: H.C. Lai and M.C. Feng (Editors), Fate and Effects of Oil in the Mangrove Environment, Univ. Sains Malaysia, Penang, pp. 5-19. Little, D.I., 1987. Oiled sediments in the Humber estuary following the Sivand incident. In: Proc. 1987 Oil Spill Conf., American Petroleum Institute, Washington, DC, pp. 419-425. Little, D.I. and Scales, D.L., 1987. The persistence of oil stranded on sediment shorelines. In: Proc. 1987 Oil Spill Conf., American Petroleum Institute, Washington, DC, pp. 433-438. Lugo, A.E., Sell, M. and Snedaker, S.C., 1976. Mangrove ecosystem analysis. In: B.C. Patten (Editor), Systems analysis and simulation in ecology. Academic Press, New York, pp. 113-145. Schaeffer-Novelli, Y., Rodrigues, F.O. and Contr6n-Molero, G., 1988. Mangroves: a methodology for oil pollution impact assessment. Abstr. JOA Mexico 88 (288.57), p. 99. Siripong, A., 1988. Hydrodynamic and oil-spill modelling for the East Asian Seas Region. Ambio, 17: 183-185. TOVALOP, 1986. Fate of marine oil spills. Tech. Inf. Pap. 11, International Tanker Owners Pollution Federation Ltd., London, 8 pp.