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Quantitative evaluation of beach-stranded tar balls by means of air photographs
Volume 18/Number 6/June 1987
conditions of sedimentation in each sampling station seem to be responsible for the observed local differences in hydrocarbon concentration. Differently from other highly petroleum polluted sites, no biodegradation processes are observed in the sediments, probably due to inhibitory effects produced by the chemicals present in the waters, as a result of the industrial activity developed in the river shores.
Albaigfs, J. (1980). Fingerprinting petroleum pollutants in the Mediterranean Sea. In Analytical Techniques in Environmental Chemistry (J. Albaigds, ed), pp. 69-81. Pergamon Press, Oxford. Albaigds, J. & Albrecht, P. (1979). Fingerprinting marine pollutant hydrocarbons by computerized gas chromatography-mass spectrometry. In Recent Advances in Environmental Analysis (R. W. Frei, ed.), pp. 261-280. Gordon and Breach, London. Albaigds, J., Algaba, J., Bayona, J. M. & Grimalt, J. (1984). New perspectives in the evaluation of anthropogenic inputs of hydrocarbons in the Western Mediterranean Coast. I.C.S.E.M. Reports 1982, 199-206. Atlas, R. M. (1984). Petroleum Microbiology. McMillan, New York. Blumer, M., Mullin, M. M. & Thomas, D. W. (1963). Pristane in zooplankton. Science 140, 974. Boehm, P. D. & Requejo, A. G. (1986). Overview of the recent sediment hydrocarbon geochemistry of Atlantic and Gulf coast outer continental sheff environments. Est. Coast. ShelfSci. 23, 29-58. Boehm, P. D., Fiest, D. L., Kaplan, I., Mankiewicz, P. & Lewbel, G. S. 11983). A natural resources damage assessment study: The Ixtoc I blowout. In Proceedings of the 1983 Oil Spill Conference, pp. 507516. American Petroleum Institute, Washington, D.C. Botello, A. V. & Macko, S. A. (1982). Oil pollution and the carbon isotope ratio in organisms and recent sediments of coastal lagoons in the Gulf of Mexico. Oceanol. Acta SP, 55-62. DastiUung, M. (1976). Lipides de s~diments recents, Ph.D. Thesis Louis Pasteur University, Strasbourg.
Eglinton, G. & Hamilton, R. J. (1967). Leaf epicuticular waxes. Science 156, 1322-1335. Farrington, J. W. & Tripp, B. W. (1977). Hydrocarbons in western North Atlantic surface sediments. Geochim. Cosmochim. Acta 41, 1627-1641. Gearing, P., Gearing, J. N., Lytle, T. F. & Lytle, J. S. (1976). Hydrocarbons in 60 northeast Gulf of Mexico shelf sediments: a preliminary study. Geochim. Cosmochim. Acta 47, 2115-2119. Goossens, H., de Leeuw, J. W., Schenck, P. A. & Brassell, S. C. (1984). Tocopherols as likely precursors of pristane in ancient sediments and crude oils. Nature 312,440-442. Grimalt, J. (1983). Organic geochemistry of deltaic systems, Ph.D. Thesis, Authonomous University of Barcelona, Bellaterra. Grimalt, J., Bayona, J. M. & Albaig~s, J. (1986). Chemical markers for the characterization of pollutant inputs in the coastal zones. I.C.S.E.M. Reports 1984, 533-543. Grimalt, J., Marfil, C. & Albaigds, J. (1984). Analysis of hydrocarbons in aquatic sediments. Internat. J. Environ. Anal Chem. 18, 183-194. Jernelov, A. & Linden, O. (1981). Ixtoc. I: A case study of the world's largest oil spill, Ambio 10,299-306. Jones, D. M., Rowland, S. J. & Douglas, G. (1986). Steranes as indicators of petroleum-like hydrocarbons in marine surface sediments. Mar. Pollut. Bull. 17, 24-27. Keizer, P. D., Dale, J. & Gordon, D. C. Jr. (1978). Hydrocarbons in surficial sediments from the Scotian shelf. Geochim. Cosmochim. Acta 42,165-172. Paez-Osuna, F., Botello, A. V. & Villaneuva, S. (1986). Heavy metals in Coatzacoalcos estuary and Ostion lagoon, Mexico. Mar. Pollut. Bull. 17,516-519. Reed, W. E. (1977). Molecular composition of weathered petroleum and comparison with its possible source. Geochim. Cosmochim. Acta 41,237-247. Solanas, A. M., Pards, C., Marfil, C. & Albaigds, J. (1982). A comparative study of chemical and microbiological monitoring of pollutant hydrocarbons in urban aquatic environments. J. Environ. Anal, Chem. 12, 141-151. Venkatesan, M. I., Brenner, S., Ruth, E., Bonilla, J. & Kaplan, I. R. (1980). Hydrocarbons in age-dated sediment from two basins in the Southern California Bight. Geochim. Cosmochim. Acta 44, 789802. Wakeham, S. G. & Carpenter, R. (1976). Aliphatic hydrocarbons in sediments of lake Washington. Limnol. Oceanogr. 21, 711-723.
Marine Pollution Bulletin, Volume 18, No. 6, pp. 289-293, 1987. Printed in Great Britain.
0025-326X/87 $3.1)0+11.1111 O 1987 Pergamon Journals Ltd.
Quantitative Evaluation of Beach-stranded Tar Balls by means of Air Photographs A. GOLIK* and N. ROSENBERG?
*National Institute of Oceanography, P.O. Box 8030, Haifa 31080, Israel t Faculty of Engineering, Tel Aviv University, Ramat Aviv, Israel
Relative quantities of beach stranded tar were determined by means of image processing techniques of air photographs which were taken on six different dates between 1975 and 1985 of a section of the Israeli coastline. The results show a drastic reduction in the tar quantity during this period. This reduction is attributed to reduction of marine oil transport during this period as a result of the 1979 oil crisis, to enforcement of regulations against oil pollution, and to systematic cleaning of the Israeli beaches.
Beach stranded tar is formed as a result of natural or accidental release of hydrocarbon compounds into seawater. The light fractions of these compounds evaporate, and the viscosity of the remaining material increases until it becomes what is known as tar lumps or tar balls. Being less dense than sea water, the tar floats on the water and is carried by winds and currents until it lands on a beach. There, it accumulates along successive lines, from the water line to the back of the beach, which were formed at various stands of sea level 289
Marine Pollution Bulletin
due to tides or storms. Tar stains people who come to the beach for bathing and recreation. It is therefore a nuisance which causes serious damage to the tourist industry, especially in countries which offer coastal tourism. A quantitative estimation of the tar which contaminates the beach is important for combating this phenomenon and for the determination of trends in its development. Quantitative measurements of tar on beaches were carried out as early as the late 1950s (Dennis, 1959). In the 1970s and early 1980s, the number of reports, coming from all over the world, on quantities of tar on the beaches has greatly increased, and it is impossible to quote all of them here. These measurements were made by collecting and weighing tar balls from a unit area of beach, or a unit of frontal length of beach. Measurement of tar in the field is a difficult and time-consuming procedure. One cannot, therefore, measure tar on lengthy beaches at a high frequency. It was noticed, however, that in air photographs which were taken at low altitude flights above the beach, tar is clearly seen. This raised the question whether modern image processing techniques could not be employed to analyze air photographs and determine tar quantities on the beach. The purpose of this study was to investigate this possibility. Method
The original approach to the problem was to indicate on the beach several strips 1 m wide from the waterline to the back of the beach, to photograph them from an airplane, and to determine the quantity of tar in each strip by the conventional method of collecting the tar balls and weighing them. Then, determination of the size of the black area in each of the strips was planned to be done in the laboratory, by image processing procedures. Comparison of the tar quantity found in the field with the black area determined in the laboratory should have provided a calibration curve by means of which tar quantity could be determined from air photographs. However, during the period of this study (1985-1986), the beaches of Israel, where this study was conducted, were clean of tar and this approach could not be used. As an alternative, it was decided to test whether a similar approach, only in relative rather than absolute terms, could be employed to determine changes in tar quantity with time on the basis of old air photographs. A search in the air photographs archives of the Survey of Israel yielded six air photograph sorties that were conducted along the Mediterranean beaches of Israel at low altitude so that stranded tar could be seen on them clearly. These were from the following dates: 18 July and 11 October 1975, 19 August 1978, 28 August 1982, 27 August 1983 and 16 November 1985. From each of these, a section of close to 2 km beach near Belt Yanay, some 50 km north of Tel-Aviv, was selected for this study. It was selected because there was minimum human activity on it in all the six photographs. From the original negatives of the photographs, diapositives at a scale of 1:375 were produced. From 290
this beach, six beach sections, each 25 m long, were selected for the analysis. Each of the six beach sections was the same one for all of the six dates. The transparency aerial photographs were framed with black paper to define a length of 25 m and a width cut out to include all the beach and to exclude the builtup regions above the high-water mark. Digital image processing is done by scanning a scene or a photograph with a photometer and digitizing the photometer reading on a rectangular grid of points equally spaced over the image. The light intensity values typically range from 0 (black) to 255 (white) and the digital data can then be processed in a computer. In our tests, each transparency-converted aerial photographic sample (as described above) was placed on a light table and image-procesed to a digital output image of 512 x 512 pixels (about 5 x 5 cm per pixel). The intensity values ranged from about 100-230 in the beach area with a noise level of + 1 unit. Inspection of the intensity distribution (histogram) did not disclose a break in the distribution which would clearly separate sand from 'tar', so an interactive 'expert system' was used, relying on the interpretive capability of the operator. This also permitted the operator to distinguish and reject dark areas caused by footprints and vehicle tracks, which would be difficult to programme automatically. In this method, the image was first scanned to define and separate the beach from the (black) frame. It was then contrast-stretched to provide a scene which ranged from near-black in the darkest areas to near-white in the lightest areas. This scene was displayed on one quadrant of a TV monitor (Fig. la). Three copies were displayed on the other quadrants of the screen. These were thresholded to different digital values displaying sand as white and tar as black with no grey (Fig. 1 b-d). The operator selected one of the three frames (Fig. 1 b-d) as most correctly separating tar from sand. This was taken as the 'central' image (Fig. lc), and the other two frames were adjusted to lower (Fig. lb) and higher (Fig. ld) thresholds. The process was repeated, narrowing the differences in threshold until the operator could no longer define a significant difference among the three images. The programme then counted black pixels in each image, converting to area in m 2, and defined the tar-covered area as that measured in the central scene (Fig. i c) with an uncertainty (error) represented by the difference in areas of the two surrounding scenes (Fig. 1 b,d). Results
Altogether 35 images of beach sections were analyzed (one was ruined due to a mishap). In order to evaluate this technique, all the images were analyzed twice. A correlation test between the results of the two analyses yielded a correlation factor r ---- 0.915 with a significance level of p < 0 . 0 0 1 , indicating a high repeatability of results using this analysis. The results in terms of area covered by tar per 25 m of beach length are presented in Fig. 2 which provides the mean and the standard deviation for each date. This
Volume 1 8 / N u m b e r 6/June 1987
Fig. 1 Four quadrant scene of one sample area used for determination of the tar covered area. (a) image after contrast stretching; (b-d) same image using three threshold values--(b) low, (c) middle and (d) high. In b - d images, no grey exists, only black or white. Black pixels are counted by the computer to provide the area covered by tar. I
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Fig. 2 Mean and standard deviation of the area covered by tar per 25 m beach length in the central coast of Israel between 1975 and 1985.
figure clearly Shows a drastic and continuous reduction from a means of 28.1 m 2 25 m -j in July 1975 to zero in November 1985. This trend can be seen clearly in Fig. 3 which shows pictures of the same beach section on the six dates tested.
Discussion The use of image processing techniques for quantitative evaluation of stranded tar on the beach faces several difficulties. The technique is based on the assumption that black areas in the photograph represent tar, but other objects on the beach appear
black in air photographs, such as charred wood, black plastic refuse, metal scraps, vegetation, and the sea itself. Another problem is that of shadows which, again, are black. Even on a smooth sandy beach, footprints and vehicle tracks will create black shadows. The spread of shades of black that tar exhibits is wide enough so that it cannot be distinguished from other black objects. Another problem is that there is a certain element of human intervention in the decision of the shade intensity which is used for the analysis of the picture. Results of analyses carried out by two individuals on the same set of pictures may differ. Therefore, this method cannot be used for exact measurement of tar quantities but rather for evaluation and estimation. However, it should be emphasized that in this type of study, there is no need for great accuracy in determination of tar quantity. In most of the cases, the important information is of a relative nature, and the questions to be answered are: has there been a distinguishable change in tar contamination with time? or is there a difference between beaches? The case which was brought here is a demonstration of the efficacy of the method. During the period covered by this study, two investigations on the quantity of tar on the Israeli beaches were carried out. The first, by Golik (1982), was conducted in 1975-1976 and was based on six stations along the Israeli coast that were sampled at a bi-weekly frequency. The mean tar content on the beach at that time was 3625 gm -~ of beach front. The other study (Golik, in preparation) was restricted to the beaches of Haifa (about 3 km length) where daily samplings were done from 29 stations between 11 July and 16 September 1984. The mean tar content at that time was 12.9 gm -~ of beach front. The sharp reduction in tar quantity which is noticeable in the air photos is therefore supported by findings in the field. Reduction of beach stranded tar during the last decade or so is reported from other parts of the world as well. Demetropoulos (in UNEP, 1980) reports that in 1976-1978 the mean tar quantity in Paphos, Cyprus was 268 gm -2. In 1983, Demetropoulos (1985) found in that beach a mean of only 67 gm -2. Another case of tar reduction is that of the beaches of Bermuda. Three tar studies were conducted on these beaches: in 19711972 by Butler et al., (1973), in 1978-1979 by Knap et aL, (1980) and in 1982-1983 by Robertson Smith & Knap (1985). The results of these studies show that between 1971-1972 and 1978-1979, there was an increase of tar content, but a statistical test was not conducted because different beaches were sampled each time. A comparison between 1978-1979 and 1982-1983 (for the same beaches) showed a decrease of 59 and 78% in the arithmetic mean and 79 and 87% in the geometric mean. The reduction in tar on the beaches of Israel, and probably in other places as well, is a result of several processes that took place during the last decade. In 1978, the 1969 amendment to the International Convention O I L P O L 54 entered into force. This amendment permits release of oil from tankers only in 291
Marine Pollution Bulletin
restricted areas and even there only at certain rates and quantities. At about the same time, the oil crisis of 1979 caused an increase in oil prices and a reduction in oil transport, and therefore encouraged tanker owners to reduce to a minimum the loss of oil through spillage or otherwise. The continuing grim economic condition of oil transportation facilities, high oil prices, the adoption of the Mediterranean Sea as a special area (into which no oil release with a concentration higher than 15 ppm is permitted) in the MARPOL 73/78 convention, and the enforcement of this convention caused a tighter control on oil pollution as well as the development of innovative techniques and procedures aimed at
preventing the waste of oil into the sea. Cohen (in press) reports that during recent years, facilities for reception of oil and oily compounds were installed in all harhours, marinas, and anchorages off the Israeli coast. Also, any oil spills along this coast from either ship, oil terminal, or land (and at times the quantities of these are hundreds of tonnes) is immediately handled and cleaned. In addition to all these, systematic cleaning of long (scores of km) beach sections of all kinds of refuse, including tar, has been conducted in Israel since 1984 twice a year by the Israeli Environmental Protection Service (Y. Cohen, pers. comm.). The rate of tar reduction on the beach (Fig. 2) corresponds fairly well in the
Fig. 3 A series of air photographs of the same 25 m beach section at different dates showing the change in tar concentration between 1975 and 1985.
292
Volume 18/Number 6/June 1987 Demetropoulos, A. (1985). Long term programme for pollution monitoring and research in the Mediterranea Sea. MED POL Phase II, 2nd Ann. Rep. Dennis, J. V. (1959). Oil pollution survey of the U.S. Atlantic coast with special reference to southeast Florida coast conditions. Amer. Petrol. Inst., Div. Transport., (4054), AI-KI. Golik, A. (1982). The distribution and behaviour of tar bails along the Israeli eoast. Estua~ Coast Shelf Sci. 15,267-276. Knap, A. H., Iliffe, T. M. & Butler, J. N. (1980). Has the amount of tar on the open ocean changed in the past decade? Ma~ Pollut. Bull. 11, 161-164. Robertson Smith, S. & Knap, A. H. (1985). Significant decrease in the amount of tar stranding on Bermuda. Mar. PoUut. Bull. 16, 19-21. UNEP. (1980). Summary reports on the scientific results of MED POL Part I. UNEP/IG 18/INF 3.
dates of the above mentioned developments, which enhances the feeling that these are indeed the causes for the tar reduction. This study was supported by IOC/UNESCO grant SC 2176705. R. Zcltser assisted in the image processing analyses.
Butler, J. N., Morris, B. F. & Sass, J. (1973). Pelagic tar from Bermuda and Sargasso Sea. Bermuda Biol. Sta. Res. Spec. Publ. 10. Cohen, Y. (in press). Marine and coastal pollution. In Ann. Rept. 13 on Environmental Quality in Israel 1985/86. Environment Protection Service, Israel, Ministry of the Interior (in Hebrew).
0025-326X/87 $3.00+0.00 1987 PergamonJournals Ltd.
.~4arinePollution Bulletin, Volume18. No. 6, p. 293. 1987.
Printedin GreatBritain.
Northern Gannet Starvation After Swallowing Styrofoam During November and December 1985 one of us (RGG) salvaged birds found dead on Gardiners Island, located off the tip of Long Island, in Suffolk County, New York. Included were Common Loons (Gavia immer), Northern Gannets (Sula bassanus) and a Redbreasted Merganser (Mergus serrator). Most were heavily encrusted with oil and were preserved as skeletons in the anatomical collections of the American Museum of Natural History. One Northern Gannet in first year plumage was somewhat emaciated, and was only lightly oiled. It was skinned, washed, and prepared (by RWD) as a study
0
1
2
3
4
specimen for the collection. Upon dissection, the digestive tract was found to be empty of food, but the lower end of the stomach was occluded by a large piece of styrofoam from the pointed end of a lobster-pot buoy (Fig. 1). The cracked surface nearest the centimetre ruler in the photograph is dull orange in colour, not dissimilar to the colour obtained by some fish or perhaps dead crabs.
ROBERT W. DICKERMAN ROBERT G. GOELET American Museum of Natural History, New York, 10024, USA
5
5
?
B
t.0
Fig. 1
293
conditions of sedimentation in each sampling station seem to be responsible for the observed local differences in hydrocarbon concentration. Differently from other highly petroleum polluted sites, no biodegradation processes are observed in the sediments, probably due to inhibitory effects produced by the chemicals present in the waters, as a result of the industrial activity developed in the river shores.
Albaigfs, J. (1980). Fingerprinting petroleum pollutants in the Mediterranean Sea. In Analytical Techniques in Environmental Chemistry (J. Albaigds, ed), pp. 69-81. Pergamon Press, Oxford. Albaigds, J. & Albrecht, P. (1979). Fingerprinting marine pollutant hydrocarbons by computerized gas chromatography-mass spectrometry. In Recent Advances in Environmental Analysis (R. W. Frei, ed.), pp. 261-280. Gordon and Breach, London. Albaigds, J., Algaba, J., Bayona, J. M. & Grimalt, J. (1984). New perspectives in the evaluation of anthropogenic inputs of hydrocarbons in the Western Mediterranean Coast. I.C.S.E.M. Reports 1982, 199-206. Atlas, R. M. (1984). Petroleum Microbiology. McMillan, New York. Blumer, M., Mullin, M. M. & Thomas, D. W. (1963). Pristane in zooplankton. Science 140, 974. Boehm, P. D. & Requejo, A. G. (1986). Overview of the recent sediment hydrocarbon geochemistry of Atlantic and Gulf coast outer continental sheff environments. Est. Coast. ShelfSci. 23, 29-58. Boehm, P. D., Fiest, D. L., Kaplan, I., Mankiewicz, P. & Lewbel, G. S. 11983). A natural resources damage assessment study: The Ixtoc I blowout. In Proceedings of the 1983 Oil Spill Conference, pp. 507516. American Petroleum Institute, Washington, D.C. Botello, A. V. & Macko, S. A. (1982). Oil pollution and the carbon isotope ratio in organisms and recent sediments of coastal lagoons in the Gulf of Mexico. Oceanol. Acta SP, 55-62. DastiUung, M. (1976). Lipides de s~diments recents, Ph.D. Thesis Louis Pasteur University, Strasbourg.
Eglinton, G. & Hamilton, R. J. (1967). Leaf epicuticular waxes. Science 156, 1322-1335. Farrington, J. W. & Tripp, B. W. (1977). Hydrocarbons in western North Atlantic surface sediments. Geochim. Cosmochim. Acta 41, 1627-1641. Gearing, P., Gearing, J. N., Lytle, T. F. & Lytle, J. S. (1976). Hydrocarbons in 60 northeast Gulf of Mexico shelf sediments: a preliminary study. Geochim. Cosmochim. Acta 47, 2115-2119. Goossens, H., de Leeuw, J. W., Schenck, P. A. & Brassell, S. C. (1984). Tocopherols as likely precursors of pristane in ancient sediments and crude oils. Nature 312,440-442. Grimalt, J. (1983). Organic geochemistry of deltaic systems, Ph.D. Thesis, Authonomous University of Barcelona, Bellaterra. Grimalt, J., Bayona, J. M. & Albaig~s, J. (1986). Chemical markers for the characterization of pollutant inputs in the coastal zones. I.C.S.E.M. Reports 1984, 533-543. Grimalt, J., Marfil, C. & Albaigds, J. (1984). Analysis of hydrocarbons in aquatic sediments. Internat. J. Environ. Anal Chem. 18, 183-194. Jernelov, A. & Linden, O. (1981). Ixtoc. I: A case study of the world's largest oil spill, Ambio 10,299-306. Jones, D. M., Rowland, S. J. & Douglas, G. (1986). Steranes as indicators of petroleum-like hydrocarbons in marine surface sediments. Mar. Pollut. Bull. 17, 24-27. Keizer, P. D., Dale, J. & Gordon, D. C. Jr. (1978). Hydrocarbons in surficial sediments from the Scotian shelf. Geochim. Cosmochim. Acta 42,165-172. Paez-Osuna, F., Botello, A. V. & Villaneuva, S. (1986). Heavy metals in Coatzacoalcos estuary and Ostion lagoon, Mexico. Mar. Pollut. Bull. 17,516-519. Reed, W. E. (1977). Molecular composition of weathered petroleum and comparison with its possible source. Geochim. Cosmochim. Acta 41,237-247. Solanas, A. M., Pards, C., Marfil, C. & Albaigds, J. (1982). A comparative study of chemical and microbiological monitoring of pollutant hydrocarbons in urban aquatic environments. J. Environ. Anal, Chem. 12, 141-151. Venkatesan, M. I., Brenner, S., Ruth, E., Bonilla, J. & Kaplan, I. R. (1980). Hydrocarbons in age-dated sediment from two basins in the Southern California Bight. Geochim. Cosmochim. Acta 44, 789802. Wakeham, S. G. & Carpenter, R. (1976). Aliphatic hydrocarbons in sediments of lake Washington. Limnol. Oceanogr. 21, 711-723.
Marine Pollution Bulletin, Volume 18, No. 6, pp. 289-293, 1987. Printed in Great Britain.
0025-326X/87 $3.1)0+11.1111 O 1987 Pergamon Journals Ltd.
Quantitative Evaluation of Beach-stranded Tar Balls by means of Air Photographs A. GOLIK* and N. ROSENBERG?
*National Institute of Oceanography, P.O. Box 8030, Haifa 31080, Israel t Faculty of Engineering, Tel Aviv University, Ramat Aviv, Israel
Relative quantities of beach stranded tar were determined by means of image processing techniques of air photographs which were taken on six different dates between 1975 and 1985 of a section of the Israeli coastline. The results show a drastic reduction in the tar quantity during this period. This reduction is attributed to reduction of marine oil transport during this period as a result of the 1979 oil crisis, to enforcement of regulations against oil pollution, and to systematic cleaning of the Israeli beaches.
Beach stranded tar is formed as a result of natural or accidental release of hydrocarbon compounds into seawater. The light fractions of these compounds evaporate, and the viscosity of the remaining material increases until it becomes what is known as tar lumps or tar balls. Being less dense than sea water, the tar floats on the water and is carried by winds and currents until it lands on a beach. There, it accumulates along successive lines, from the water line to the back of the beach, which were formed at various stands of sea level 289
Marine Pollution Bulletin
due to tides or storms. Tar stains people who come to the beach for bathing and recreation. It is therefore a nuisance which causes serious damage to the tourist industry, especially in countries which offer coastal tourism. A quantitative estimation of the tar which contaminates the beach is important for combating this phenomenon and for the determination of trends in its development. Quantitative measurements of tar on beaches were carried out as early as the late 1950s (Dennis, 1959). In the 1970s and early 1980s, the number of reports, coming from all over the world, on quantities of tar on the beaches has greatly increased, and it is impossible to quote all of them here. These measurements were made by collecting and weighing tar balls from a unit area of beach, or a unit of frontal length of beach. Measurement of tar in the field is a difficult and time-consuming procedure. One cannot, therefore, measure tar on lengthy beaches at a high frequency. It was noticed, however, that in air photographs which were taken at low altitude flights above the beach, tar is clearly seen. This raised the question whether modern image processing techniques could not be employed to analyze air photographs and determine tar quantities on the beach. The purpose of this study was to investigate this possibility. Method
The original approach to the problem was to indicate on the beach several strips 1 m wide from the waterline to the back of the beach, to photograph them from an airplane, and to determine the quantity of tar in each strip by the conventional method of collecting the tar balls and weighing them. Then, determination of the size of the black area in each of the strips was planned to be done in the laboratory, by image processing procedures. Comparison of the tar quantity found in the field with the black area determined in the laboratory should have provided a calibration curve by means of which tar quantity could be determined from air photographs. However, during the period of this study (1985-1986), the beaches of Israel, where this study was conducted, were clean of tar and this approach could not be used. As an alternative, it was decided to test whether a similar approach, only in relative rather than absolute terms, could be employed to determine changes in tar quantity with time on the basis of old air photographs. A search in the air photographs archives of the Survey of Israel yielded six air photograph sorties that were conducted along the Mediterranean beaches of Israel at low altitude so that stranded tar could be seen on them clearly. These were from the following dates: 18 July and 11 October 1975, 19 August 1978, 28 August 1982, 27 August 1983 and 16 November 1985. From each of these, a section of close to 2 km beach near Belt Yanay, some 50 km north of Tel-Aviv, was selected for this study. It was selected because there was minimum human activity on it in all the six photographs. From the original negatives of the photographs, diapositives at a scale of 1:375 were produced. From 290
this beach, six beach sections, each 25 m long, were selected for the analysis. Each of the six beach sections was the same one for all of the six dates. The transparency aerial photographs were framed with black paper to define a length of 25 m and a width cut out to include all the beach and to exclude the builtup regions above the high-water mark. Digital image processing is done by scanning a scene or a photograph with a photometer and digitizing the photometer reading on a rectangular grid of points equally spaced over the image. The light intensity values typically range from 0 (black) to 255 (white) and the digital data can then be processed in a computer. In our tests, each transparency-converted aerial photographic sample (as described above) was placed on a light table and image-procesed to a digital output image of 512 x 512 pixels (about 5 x 5 cm per pixel). The intensity values ranged from about 100-230 in the beach area with a noise level of + 1 unit. Inspection of the intensity distribution (histogram) did not disclose a break in the distribution which would clearly separate sand from 'tar', so an interactive 'expert system' was used, relying on the interpretive capability of the operator. This also permitted the operator to distinguish and reject dark areas caused by footprints and vehicle tracks, which would be difficult to programme automatically. In this method, the image was first scanned to define and separate the beach from the (black) frame. It was then contrast-stretched to provide a scene which ranged from near-black in the darkest areas to near-white in the lightest areas. This scene was displayed on one quadrant of a TV monitor (Fig. la). Three copies were displayed on the other quadrants of the screen. These were thresholded to different digital values displaying sand as white and tar as black with no grey (Fig. 1 b-d). The operator selected one of the three frames (Fig. 1 b-d) as most correctly separating tar from sand. This was taken as the 'central' image (Fig. lc), and the other two frames were adjusted to lower (Fig. lb) and higher (Fig. ld) thresholds. The process was repeated, narrowing the differences in threshold until the operator could no longer define a significant difference among the three images. The programme then counted black pixels in each image, converting to area in m 2, and defined the tar-covered area as that measured in the central scene (Fig. i c) with an uncertainty (error) represented by the difference in areas of the two surrounding scenes (Fig. 1 b,d). Results
Altogether 35 images of beach sections were analyzed (one was ruined due to a mishap). In order to evaluate this technique, all the images were analyzed twice. A correlation test between the results of the two analyses yielded a correlation factor r ---- 0.915 with a significance level of p < 0 . 0 0 1 , indicating a high repeatability of results using this analysis. The results in terms of area covered by tar per 25 m of beach length are presented in Fig. 2 which provides the mean and the standard deviation for each date. This
Volume 1 8 / N u m b e r 6/June 1987
Fig. 1 Four quadrant scene of one sample area used for determination of the tar covered area. (a) image after contrast stretching; (b-d) same image using three threshold values--(b) low, (c) middle and (d) high. In b - d images, no grey exists, only black or white. Black pixels are counted by the computer to provide the area covered by tar. I
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Fig. 2 Mean and standard deviation of the area covered by tar per 25 m beach length in the central coast of Israel between 1975 and 1985.
figure clearly Shows a drastic and continuous reduction from a means of 28.1 m 2 25 m -j in July 1975 to zero in November 1985. This trend can be seen clearly in Fig. 3 which shows pictures of the same beach section on the six dates tested.
Discussion The use of image processing techniques for quantitative evaluation of stranded tar on the beach faces several difficulties. The technique is based on the assumption that black areas in the photograph represent tar, but other objects on the beach appear
black in air photographs, such as charred wood, black plastic refuse, metal scraps, vegetation, and the sea itself. Another problem is that of shadows which, again, are black. Even on a smooth sandy beach, footprints and vehicle tracks will create black shadows. The spread of shades of black that tar exhibits is wide enough so that it cannot be distinguished from other black objects. Another problem is that there is a certain element of human intervention in the decision of the shade intensity which is used for the analysis of the picture. Results of analyses carried out by two individuals on the same set of pictures may differ. Therefore, this method cannot be used for exact measurement of tar quantities but rather for evaluation and estimation. However, it should be emphasized that in this type of study, there is no need for great accuracy in determination of tar quantity. In most of the cases, the important information is of a relative nature, and the questions to be answered are: has there been a distinguishable change in tar contamination with time? or is there a difference between beaches? The case which was brought here is a demonstration of the efficacy of the method. During the period covered by this study, two investigations on the quantity of tar on the Israeli beaches were carried out. The first, by Golik (1982), was conducted in 1975-1976 and was based on six stations along the Israeli coast that were sampled at a bi-weekly frequency. The mean tar content on the beach at that time was 3625 gm -~ of beach front. The other study (Golik, in preparation) was restricted to the beaches of Haifa (about 3 km length) where daily samplings were done from 29 stations between 11 July and 16 September 1984. The mean tar content at that time was 12.9 gm -~ of beach front. The sharp reduction in tar quantity which is noticeable in the air photos is therefore supported by findings in the field. Reduction of beach stranded tar during the last decade or so is reported from other parts of the world as well. Demetropoulos (in UNEP, 1980) reports that in 1976-1978 the mean tar quantity in Paphos, Cyprus was 268 gm -2. In 1983, Demetropoulos (1985) found in that beach a mean of only 67 gm -2. Another case of tar reduction is that of the beaches of Bermuda. Three tar studies were conducted on these beaches: in 19711972 by Butler et al., (1973), in 1978-1979 by Knap et aL, (1980) and in 1982-1983 by Robertson Smith & Knap (1985). The results of these studies show that between 1971-1972 and 1978-1979, there was an increase of tar content, but a statistical test was not conducted because different beaches were sampled each time. A comparison between 1978-1979 and 1982-1983 (for the same beaches) showed a decrease of 59 and 78% in the arithmetic mean and 79 and 87% in the geometric mean. The reduction in tar on the beaches of Israel, and probably in other places as well, is a result of several processes that took place during the last decade. In 1978, the 1969 amendment to the International Convention O I L P O L 54 entered into force. This amendment permits release of oil from tankers only in 291
Marine Pollution Bulletin
restricted areas and even there only at certain rates and quantities. At about the same time, the oil crisis of 1979 caused an increase in oil prices and a reduction in oil transport, and therefore encouraged tanker owners to reduce to a minimum the loss of oil through spillage or otherwise. The continuing grim economic condition of oil transportation facilities, high oil prices, the adoption of the Mediterranean Sea as a special area (into which no oil release with a concentration higher than 15 ppm is permitted) in the MARPOL 73/78 convention, and the enforcement of this convention caused a tighter control on oil pollution as well as the development of innovative techniques and procedures aimed at
preventing the waste of oil into the sea. Cohen (in press) reports that during recent years, facilities for reception of oil and oily compounds were installed in all harhours, marinas, and anchorages off the Israeli coast. Also, any oil spills along this coast from either ship, oil terminal, or land (and at times the quantities of these are hundreds of tonnes) is immediately handled and cleaned. In addition to all these, systematic cleaning of long (scores of km) beach sections of all kinds of refuse, including tar, has been conducted in Israel since 1984 twice a year by the Israeli Environmental Protection Service (Y. Cohen, pers. comm.). The rate of tar reduction on the beach (Fig. 2) corresponds fairly well in the
Fig. 3 A series of air photographs of the same 25 m beach section at different dates showing the change in tar concentration between 1975 and 1985.
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Volume 18/Number 6/June 1987 Demetropoulos, A. (1985). Long term programme for pollution monitoring and research in the Mediterranea Sea. MED POL Phase II, 2nd Ann. Rep. Dennis, J. V. (1959). Oil pollution survey of the U.S. Atlantic coast with special reference to southeast Florida coast conditions. Amer. Petrol. Inst., Div. Transport., (4054), AI-KI. Golik, A. (1982). The distribution and behaviour of tar bails along the Israeli eoast. Estua~ Coast Shelf Sci. 15,267-276. Knap, A. H., Iliffe, T. M. & Butler, J. N. (1980). Has the amount of tar on the open ocean changed in the past decade? Ma~ Pollut. Bull. 11, 161-164. Robertson Smith, S. & Knap, A. H. (1985). Significant decrease in the amount of tar stranding on Bermuda. Mar. PoUut. Bull. 16, 19-21. UNEP. (1980). Summary reports on the scientific results of MED POL Part I. UNEP/IG 18/INF 3.
dates of the above mentioned developments, which enhances the feeling that these are indeed the causes for the tar reduction. This study was supported by IOC/UNESCO grant SC 2176705. R. Zcltser assisted in the image processing analyses.
Butler, J. N., Morris, B. F. & Sass, J. (1973). Pelagic tar from Bermuda and Sargasso Sea. Bermuda Biol. Sta. Res. Spec. Publ. 10. Cohen, Y. (in press). Marine and coastal pollution. In Ann. Rept. 13 on Environmental Quality in Israel 1985/86. Environment Protection Service, Israel, Ministry of the Interior (in Hebrew).
0025-326X/87 $3.00+0.00 1987 PergamonJournals Ltd.
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Printedin GreatBritain.
Northern Gannet Starvation After Swallowing Styrofoam During November and December 1985 one of us (RGG) salvaged birds found dead on Gardiners Island, located off the tip of Long Island, in Suffolk County, New York. Included were Common Loons (Gavia immer), Northern Gannets (Sula bassanus) and a Redbreasted Merganser (Mergus serrator). Most were heavily encrusted with oil and were preserved as skeletons in the anatomical collections of the American Museum of Natural History. One Northern Gannet in first year plumage was somewhat emaciated, and was only lightly oiled. It was skinned, washed, and prepared (by RWD) as a study
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specimen for the collection. Upon dissection, the digestive tract was found to be empty of food, but the lower end of the stomach was occluded by a large piece of styrofoam from the pointed end of a lobster-pot buoy (Fig. 1). The cracked surface nearest the centimetre ruler in the photograph is dull orange in colour, not dissimilar to the colour obtained by some fish or perhaps dead crabs.
ROBERT W. DICKERMAN ROBERT G. GOELET American Museum of Natural History, New York, 10024, USA
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