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PLASTICS IN MARINE ENVIRONMENTS By: Theophilos Collins MEM Duke University [email protected] Objective: Comprehension: Students will understand that plastic exists in the ocean in tiny fragments, and that scientists are studying these plastic fragments. Application: Students apply this knowledge by recreating the procedures researchers use to collect and quantify ocean plastic debris while evaluating those methods. Synthesis: Students formulate their own questions and design an experiment to address those questions. Grade: 9-10 Time: One 50 minute class Setting: In classroom Key Terms: Marine Debris, North Atlantic Subtropical Gyre, Neuston, Ocean Plastic, Hypothesis, Error, Bias Materials Needed: Cut up pieces of plastic, varying colors. Lots of small bits of white and colored paper to make the different tow samples. At least five tweezers to grab plastic pieces. Large map of North Atlantic Ocean, Sargasso Sea, and Eastern U.S. This could be projected on a smart board or some other electronic display as well. Red, orange, yellow, and blue markers to mark on the map. Or a way to mark up the electronic map with red, orange, yellow, and blue marks. Lecture Activity 1 Activity 2 • Clip introduces students to marine plastics as a problem • How do we quantify and measure this problem? Show another video clip. Review and outline collection methods as a class • Instructor leads students through an example before students split into groups to work more independently • Students in groups perform experiment, answer questions on the process, make calculations • As a class, create a map showing the results of their work. Students make a hypothesis about what would happen if they extended their work and did it many more times. • Compare class hypothesis to the 22 year dataset. • Pose some follow-up questions and investigations to this research. Instructor helps class think through an example. • Students break into groups again and design an experiment to address a follow-up question Lecture: Measuring Plastics in the Ocean This part is meant to show students how researchers study plastics in ocean environments. Introduce ocean plastic debris through a clip: https://www.youtube.com/watch?v=lA_CM4txdx k There is a lot of plastic in the oceans. We have to study its effects, and how it has been changing over time. But it would also be a good idea to know how big the problem is. How much is out there, and where? Are some places worse than others? The most straightforward way to find out how much plastic is in the ocean is to directly measure and count pieces. That’s what these scientists are doing: filtering water from different places in the ocean and counting all the pieces of plastic they find. Introduce collection and measurement methods https://www.youtube.com/watch?v=kZGJTdbdvO 4 Review and outline the process as a class 1. Need a boat with a net. Remember the woman in the video said their net was 1 m wide. The mesh size on the net is 0.33 mm, so it catches anything larger than 0.33 mm. 2. Drop net in the water, and tow alongside the boat. They sail at 2 nautical miles/hr for 0.5 hours. What is the tow distance? 3. Strain the water out of the jar at the end of the net by filtering through another 0.33 mm filter. Now we only have the solid items we caught in our net. What would be the smallest size item we would find? 4. Pick through the items found in the tow, identify pieces of plastic, and set aside in a labeled jar. Calculation Example Done Together Place a dark red marker at 300N, 400W on the map. Here, these scientists caught 1069 pieces of plastic! 1 Lead the class through calculating area concentration of plastic (# pieces/km2). The process is shown on the answer key sheet. 1 group has 19 pieces of plastic, an example of a sample from SE of New England. 1 group has 37 pieces, an example of a sample taken from about 300N, 650W (in the Sargasso Sea) 1 group has 27 pieces, another example from the Sargasso Sea/North Atlantic Subtropical Gyre. Class Review After no more than 20 minutes, start discussing as a class. Quickly review calculations. Answer key is attached. Thought questions meant to get students thinking about the scientific method, to view this experiment and method with a critical eye and think about differences between error and bias. On the large map, place markers where each sample came from, and have the markers color-coded by concentration. Blue for North Carolina and Gulf of Maine samples with only a few samples, yellow for the intermediate New England sample, orange and red for the two Sargasso Sea samples. o Based on our limited sampling, where does it look like most of this microplastic debris is concentrated? This is our hypothesis. We know wind makes waves on the sea. The waves can mix up the pieces of plastic so they’re not all at the surface, they’ll get down to a few meters below the surface. However, this is VERY dependent on wind speeds. o In average wind conditions at sea, about 54% of plastic pieces CAN (but not necessarily WILL) remain below the surface of the water where our net collected.3 So even those 1069 pieces could have been only 46% of the entire amount of plastic that was really there at that sample site! o How much plastic was there actually? (1069/0.46, which is 2324 pieces) o Recalculate concentration. o But since now we’re looking at a depth dimension, let’s now calculate #pieces/volume (L). Assume that at our wind speeds all of the plastic at our site is in the first 0.4 m below the surface. Activity 1: Recreate collecting methods in groups Students recreate the experiment in groups Organize students in groups. They perform experiment Split into groups of 4-5. Explain how you did the first three steps for them and are now handing each group a sample containing a bunch of items, some of which are little bits of plastic. Explain that each sample came from a different place and reflects actual data collected by the same scientists we watched earlier.2 Have some variety of size of plastic, and a variety of colors. Have students fill out their worksheet. Each worksheet has specific latitude and longitude coordinates for the instructor to match with their homemade samples. 1 group has 3 pieces of plastic in their sample. This is an example of samples taken from North Carolina waters. 1 group has 4 pieces in their sample, and example of samples taken from the Gulf of Maine Lucky for us we have a net with a mouth 1 m wide and 0.4 m tall! Calculations done on instructor’s answer sheet. Are the data from your samples enough to determine a pattern in the North Atlantic? How could you make a stronger case for our hypothesis about a pattern in the North Atlantic? Need more samples!!! These scientists have been doing exactly what you did, but they’ve been doing it since 1986! So they have a lot of samples by now, and their map looks like Figure 1. o Does their map reinforce our hypothesis about where most of plastic is? There are lots of ways using math to “color in” the rest of the areas we didn’t sample. A process called interpolating. o Example of interpolating: Two data points a certain distance apart. You can say that the point directly in the middle of them would be the average of the two. One way of interpolating yields Figure 2. Note that this is an example of a mathematical model. o Does this image agree with our hypothesis? What do you notice about this picture? It turns out that the wind driven surface currents in the Atlantic form a ring around the basin, and all of those currents converge in the center of the basin. This area of convergence is called the North Atlantic Subtropical Gyre. (Show Figure 3. If wanted, can briefly explain the basic takeaways of the image.) The same features exist in the South Atlantic and in the North and South Pacific and Indian Ocean. So if that’s where all the water is converging, that means it will also be where all the plastic will converge! Activity 2: Follow-Up Investigations As a class, come up with follow-up questions to this study and design experiments to address those questions. Class analyzes a follow-up question together Let’s look at some basic questions and think them through. o A piece of plastic will have a given total surface area and volume. What will happen to the total measurable surface area once a piece of plastic starts to break apart. Lead students through creating Figure 4, but there’s no need to show this figure. A brief conceptual overview is below. As each particle breaks into more and more particles, see an exponential increase in number of fragments. With this increase in fragments comes an exponential increase in TOTAL surface area. Note that the surface area of each piece shrinks, but the total combined surface area increases. Five key takeaways 1. Most of the plastic in the ocean are little bits the size of a fingernail and smaller. 2. There’s plastic everywhere in the ocean, like confetti, with some patches/regions in higher concentration than other regions. 3. Ocean currents transport plastics, so the areas of higher concentrations are associated with the center of subtropical ocean gyres, where currents converge. 4. Both field research and mathematical models of ocean circulation back up these conclusions. 5. Researchers are using direct ways to measure quantities of plastic in the ocean. 6. Direct methods don’t necessarily mean easy, or without error or bias. Assignment: Students come up with their questions and design experiments Other questions might come up. A homework assignment can be to devote time to creating their own experiments to all test the same question, or have each group work on different ones. Examples of questions are below. Supplementary information in appendix will address these questions for the instructor to help them guide students. 1. What animals will eat plastic? 2. How long does it take for pieces to get to the gyre? 3. Can we trace where certain plastics came from? 4. How small do the plastic pieces get, and how fast do they break apart? 5. What are some consequences of many smaller pieces of plastic? 6. If plastic trash disposal is theoretically increasing, do we see an increase in the amounts of ocean plastic over the past 30 years? 7. Does plastic ever disappear? Does anything break it down completely? The groups design their own experiment to address their research question. They should hand in a written product with answers to the following questions: 1. What will your basic “experiment” or process look like and what is it testing? Is it testing a hypothesis or is it exploratory/descriptive? 2. What will your methods be? 3. What is your hypothesis? Part of your hypothesis should include a graph showing some sort of relationship along with an explanation for this proposed relationship (Like we just did in class). 4. What would a result supporting your hypothesis look like? 5. What would a result that doesn’t back up your hypothesis look like? 6. Could something else be observed? 7. What could be a source of error in your methods? 8. What could be a source of bias? Sea Education Association’s set of data showing concentration of plastics at every sample site of theirs since 1986. The black stars indicate locations with measured concentrations greater than 200,000/km2 while the concentration at the green star measured 26 million pieces/km2 4 Figure 1 Figure 2 Interpolated surface from datapoints above. The black line shows the approximate location of the 2cm/s surface current, a general boundary of the North Atlantic Subtropical Gyre.5 Image of currents in the North Atlantic, including the Subtropical Gyre bounded by the Gulf Stream, North Atlantic Drift, Canary Current, North Equatorial Current, and Antilles Current. 67 Figure 3 Image above shows a PURELY HYPOTHETICAL breakdown over time of different plastic items8 Figure 4 Collectors: Date: Time: Latitude: 300N Longitude: 650W Tow Distance : 1 nautical mile (6,076 ft). Convert to meters. Plastic Count: Plastic concentration (#pieces/km2) Thought Questions 1. If we sampled once and didn’t find any plastic, does that mean it isn’t there? Justify your answer in at least one complete sentence 2. What other information would be good to have for your sample site before putting the net in the water? 3. Describe your success in identifying and picking out pieces? Were any pieces easier to identify than others? Why or why not? 4. Can you identify any sources of error in this process? How well did it work? What challenges did you have? 5. Bias: a) What are some sources of bias? b) Does something get systematically cut out of our analysis that could be important? c) Could our methods actually interfere with or influence what results we get? Collectors: Date: Time: Latitude: 290N Longitude: 600W Tow Distance : 1 nautical mile (6,076 ft). Convert to meters. Plastic Count: Plastic concentration (#pieces/km2) Thought Questions 1) If we sampled once and didn’t find any plastic, does that mean it isn’t there? Justify your answer in at least one complete sentence. 2) What other information would be good to have for your sample site before putting the net in the water? 3) Describe your success in identifying and picking out pieces? Were any pieces easier to identify than others? Why or why not? 4) Can you identify any sources of error in this process? How well did it work? What challenges did you have? 5) Bias: a) What are some sources of bias? b) Does something get systematically cut out of our analysis that could be important? c) Could our methods actually interfere with or influence what results we get? Collectors: Date: Time: Latitude: 420N Longitude: 650W Tow Distance : 1 nautical mile (6,076 ft). Convert to meters. Plastic Count: Plastic concentration (#pieces/km2) Thought Questions 1. If we sampled once and didn’t find any plastic, does that mean it isn’t there? Justify your answer in at least one complete sentence 2. What other information would be good to have for your sample site before putting the net in the water? 3. Describe your success in identifying and picking out pieces? Were any pieces easier to identify than others? Why or why not? 4. Can you identify any sources of error in this process? How well did it work? What challenges did you have? 5. Bias: a) What are some sources of bias? b) Does something get systematically cut out of our analysis that could be important? c) Could our methods actually interfere with or influence what results we get? Collectors: Date: Time: Latitude: 320N Longitude: 780W Tow Distance : 1 nautical mile (6,076 ft). Convert to meters. Plastic Count: Plastic concentration (#pieces/km2) Thought Questions 1. If we sampled once and didn’t find any plastic, does that mean it isn’t there? Justify your answer in at least one complete sentence. 2. What other information would be good to have for your sample site before putting the net in the water? 3. Describe your success in identifying and picking out pieces? Were any pieces easier to identify than others? Why or why not? 4. Can you identify any sources of error in this process? How well did it work? What challenges did you have? 5. Bias a) What are some sources of bias? b) Does something get systematically cut out of our analysis that could be important? c) Could our methods actually interfere with or influence what results we get? Collectors: Date: Time: Latitude: 390N Longitude: 680W Tow Distance : 1 nautical mile (6,076 ft). Convert to meters. Plastic Count: Plastic concentration (#pieces/km2) Thought Questions 1. If we sampled once and didn’t find any plastic, does that mean it isn’t there? Justify your answer in at least one complete sentence. 2. What other information would be good to have for your sample site before putting the net in the water? 3. Describe your success in identifying and picking out pieces. Were any pieces easier to identify than others? Why or why not? 4. Can you identify any sources of error in this process? How well did it work? What challenges did you have? 5. Bias a) What are some sources of bias? b) Does something get systematically cut out of our analysis that could be important? c) Could our methods actually interfere with or influence what results we get? ANSWER KEY: Collectors: Date: Time: Positions: The ones at 300N (sample NASG-1) and 290N (NASG-2) are in the North Atlantic Subtropical Gyre. The one at 320N is off the coast of NC (sample NC). Sample at 420N is from the Gulf of Maine (GOM). The sample from 390N is just SE of New England on the fishing grounds (NEF). Tow Distance : 1 nautical mile (6,076 ft). Convert to meters. 1852 m Plastic Count: NASG-1: 37 pieces. NASG-2: 30 pieces. NC: 3 pieces. GOM: 4 pieces. NEF: 19 pieces Plastic concentration (#pieces/km2): Example with 1069 pieces to do together with class: We’re calculating # of pieces/tow area in km2. The tow area is the length of the tow in meters multiplied by the width of the net. 1069 𝑝𝑖𝑒𝑐𝑒𝑠 1069 𝑝𝑖𝑒𝑐𝑒𝑠 1069 𝑝𝑖𝑒𝑐𝑒𝑠 577,213 𝑝𝑖𝑒𝑐𝑒𝑠 = = = 2 1,852 𝑚 ∗ 1 𝑚 1.852 𝑘𝑚 ∗ 0.001 𝑘𝑚 0.001852 𝑘𝑚 1 𝑘𝑚2 Thought Questions 1. If we sampled once and didn’t find any plastic, does that mean it isn’t there? No. We could have just not picked up any of the pieces of plastic. Wind could have mixed it below the sea surface. 2. What other information would be good to have for your sample site before putting the net in the water? Environmental conditions like wind, sea state (wave height). 3. Describe your success in identifying and picking out pieces. Were any easier to identify than others? Why or why not? More pieces take more time and are harder! Colors are easier to see, larger pieces easier to see. 4. Can you identify any sources of error in this method? How well did it work? What challenges did you have? We could have missed a piece. Harder to count at night or in different light conditions. 5. Bias a) What are some sources of bias? Our own eyes. It’s easier to see certain colors and sizes. b) Do certain sizes of plastic pieces get systematically cut out of our analysis that could be important? We don’t catch plastic pieces smaller than 0.33mm, but that doesn’t mean they’re not there. What’s the smallest size we can see with our naked eye? c) Could our methods actually interfere with or influence what results we get? Were we “good” at looking for plastic only because we were told to look for it- has it been in the oceans in large concentrations for a long time but we’re just now getting good at looking for it so it seems like there’s more? One way to calculate #pieces/volume (L) on the example given in class. 2324 𝑝𝑖𝑒𝑐𝑒𝑠 2324 𝑝𝑖𝑒𝑐𝑒𝑠 3,137,149,028 1𝑥10−12 𝑘𝑚3 = = ∗ ( ) 1.852 𝑘𝑚 ∗ 0.001 𝑘𝑚 ∗ 0.0004 𝑘𝑚 7.408𝑥10−7 𝑘𝑚3 1 𝑘𝑚3 1𝐿 0.003 𝑝𝑖𝑒𝑐𝑒𝑠 3 𝑝𝑖𝑒𝑐𝑒𝑠 = = 𝐿 1000 𝐿 The volume of water we towed through in km3 was the tow distance (1.852 km) multiplied by the width of the net (1 m = 0.001 km) and the depth of our net (0.4 m = 0.0004 km). We find out how many pieces this amounts to per 1 km3, and then convert from km3 to L. SUPPLEMENTARY INFORMATION FOR INSTRUCTORS Learn About Marine Debris “The Great Pacific Garbage Patch”? The idea of a huge island of trash “twice the size of Texas” swirling around in the middle of the Pacific Ocean in some sort of current vortex isn’t correct. It doesn’t accurately represent the issue of marine debris. There are certainly large pieces of plastic that float around. But most plastic at sea is the size of your fingernails and smaller. One usually can’t see them from the deck of a ship, and we certainly can’t see these “patches” from space. Think of it as confetti mixed in the water, and in some places this “confetti soup” is more concentrated than in other places What is Marine Debris? Solid, man-made material Persists in the marine environment. Lasts for some amount of time. What are some examples of Marine Debris? Trash bags, soda cans, food cans, wood, paper products, glass bottles, plastic bottles. Plastic is the big one! Scientists estimate that about 60%-80% of all marine debris is plastic! Image at right of pile of trash. 9 What Does Marine Debris Look Like? People think that marine debris looks like this picture at right. And near land, yes it can look like this. But in the ocean, plastic debris looks like this lower picture. All of these little pieces of plastic are called “microplastics”. Image at lower right of little plastic pieces.10 Some of these pieces are smooth round pieces called “nurdules” which are what plastic looks like before they get melted and reshaped into products that we all use. Others are fragments have broken down from other larger pieces. Image directly below of model. 11 Where Are These Microplastics Coming From? What are the sources? LAND: Litter and waste On beaches, in rivers, from street/drainage outflows and runoff Catastrophic Events Hurricanes and Floods OCEAN: Ships and at-sea work platforms (like oil rigs, etc) Accidental or intentional dumping, lost cargo, fishing gear, recreational equipment, oceanographic research tools It is likely that the majority of plastic pollution in the ocean comes from land. Where Does It All Go? We know that these pieces of plastic can get transported from land to the ocean. And we know that once they’re in the ocean, they can break into even tinier pieces. But here’s another question. Once plastic debris gets to the ocean, will it stay in the same place in the ocean? If you dropped a bunch of bits of plastic in the ocean from a beach in North Carolina, would they stay near that same beach? Important to communicate that the ocean is always moving. Waves go in and out, the wind blows things everywhere, and many times the water goes where the wind blows. So since the water is carrying that plastic, we can hypothesize that those pieces of plastic entering the ocean from North Carolina beaches will get transported wherever the ocean water goes. Let’s assume we did a really terrible thing, and dropped thousands of microplastic debris off of the NC coast. Imagine we could track each little piece. After one month, where do you think those little pieces will be? Will they all be in the same place? Picture shows the timeline results of a mathematical model a bunch of scientists developed to track plastic debris in the ocean. They have computer models that simulate how ocean water moves, and they can place imaginary pieces of plastic into the system and see how it responds. The areas of higher concentrations are in green, orange, and red. Find the east coast of the U.S, and the Atlantic Ocean. Where does it look like our pieces ended up? In the green and yellow patch. This is only a prediction. We need data to back this up. That’s what SEA scientists are doing. Global Ocean Surface Currents We have many models which show microplastics transportation is influenced strongly by global ocean currents. This section reviews global surface currents, how they develop and why they form “gyres”. Remember two basic concepts driving this whole thing: The way the sun shines on the Earth to provide energy and the way the Earth turns dictate the formation of ocean surface currents and of ocean gyres. 1. The Sun shines on the Earth. But a ray of light hitting the Equator heats a little surface very efficiently while a ray of light hitting an area near the north pole hits a much larger surface. So the equatorial regions should heat up more efficiently and be warmer than the poles. We know this- most of the hottest places in the world are closest to the equator. 2. Air and water distribute heat. We also know that air (and water) at the equator is much warmer than air and water at the poles. But they are not going stay in one place! The warm air will spread out from equator to poles, trying to warm the cold part of the world. The first thing this hot air at the equator will do is rise and move away from the equator. 3. Development of pressure regions causes wind. Hot air rising causes a generally lower pressure system around the equator. But as air rises and moves away from the equator, it cools which causes it to get denser and sink! Results in a high pressure system around 30N and 30S. This is what’s called a Hadley Cell. Some of this air escapes the Hadley Cell, then flows along the surface of the Earth north (trying to get towards the poles still), and another similar cell forms between 30N and 60N. Called a Ferrell Cell. Some air escapes the Ferrell Cell and forms another circulation unit called a Polar Cell. Look back at the Hadley Cell. See the H and L pressure systems. Imagine the high pressure as a mound of air and the low pressure as a small valley Which way will air want to flow-“up the hill” from L to H or “down the hill” from H to L? Air will want to flow from H to L pressure, “downhill” from the mid-latitudes to the equator. This is wind! It’s the result of air flow between areas of H pressure and L pressure, the result of imbalance of the heating of the Earth! Image above of Circulation Cells and prevailing surface winds affected by Coriolis. 12 4. The Earth turns, which affects how air and water move on the surface of the Earth. Take a large ball. Have a marked point at the “equator” and one near a “pole”. Rotate the globe once counterclockwise- this is one day. Which point “traveled” farther-the one at the equator or the one at the pole? The one near the equator. It covered a longer distance in the same amount of time. This means that an object thrown at the equator will have a faster Eastward velocity (to the right, facing north) than an object at a point near the poles (or even the mid-latitudes). So an object moving from the equator towards the pole with that Eastward velocity will not appear to move in a straight line. It will seem to bend to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Note that this motion isn’t the result of an actual force pushing or pulling it. This is because there is no actual force acting on it-the object is moving in a straight line. It is only from inside our rotational frame of reference that we can see this “bend”. This is the Coriolis Effect. Why is this important? Because when air masses flow from the equator to the poles, the Coriolis Effect affects them. We “see” this in the “bottom” of the Hadley, Ferrel, and Polar cells. Air will consistently move from ~30N towards the equator, and it will, predicted by the Coriolis Effect, bend right. Hence the NE tradewinds, some of the most consistent, predictable prevailing winds on Earth Air moves water. People have shown experimentally and can predict very well with mathematical models that when air blows in a certain direction along water for great distances, water will flow 90 degrees to the right of the wind direction. This is called Ekman transport. 5. Ocean Surface Currents Form as a result of Coriolis Force, Ekman Transport, and Pressure Gradients. They form Gyres! Ekman transport of water resulting from the NE trades and prevailing westerlies causes water to pile up in the center of the ocean basin. Water pressure is higher in center at top of pile, and lower at the edges. Water will fall down the pile going from H to L pressure, and then get deflected to the right in the Northern Hemisphere by Coriolis “Force”. Surface currents develop as a result, the balance of coriolis force and pressure gradients. Examples in North Atlantic are the Gulf Stream, North Atlantic Current, Canary Current, and North Atlantic Equatorial Current. Inside these currents is that pile of water in the area of water convergence called the North Atlantic Subtropical Gyre. Image at left of ocean gyre forming as a result of surface winds13 Image above of gyres in the major world’s oceans. 14 Image of how coriolis deflection and pressure gradients affect water flow around a gyre. 15 REFERENCES/ENDNOTES Lavender-Law, K., Moret-Ferguson, S., Maximenko, N.A., Proskurowski, G., Peacock, E., Hafner, J., Reddy, C. M., “Plastic Accumulation in the North Atlantic Subtropical Gyre,” 329 Science p. 1186. 2 Lavender-Law, K., Moret-Ferguson, S., Maximenko, N.A., Proskurowski, G., Peacock, E., Hafner, J., Reddy, C. M., “Plastic Accumulation in the North Atlantic Subtropical Gyre,” 329 Science p. 1186. 3 Kulkulka, T., Proskurowski, G., Moret-Ferguson, S., Meyer, D.W., Lavender Law, K., “The effect of wind mixing on the vertical distribution of buoyant plastic debris” 39 L07601 Geophysical Research Letters, (2012), 4 Lavender-Law, K., Moret-Ferguson, S., Maximenko, N.A., Proskurowski, G., Peacock, E., Hafner, J., Reddy, C. M., “Plastic Accumulation in the North Atlantic Subtropical Gyre,” 329 Science p. 1186. 5 Lavender-Law, K., Moret-Ferguson, S., Maximenko, N.A., Proskurowski, G., Peacock, E., Hafner, J., Reddy, C. M., “Plastic Accumulation in the North Atlantic Subtropical Gyre,” 329 Science p. 1186. 6 http://en.wikipedia.org/wiki/North_Atlantic_Gyre#mediaviewer/File:North_Atlantic_Gyre.png 7 http://en.wikipedia.org/wiki/North_Atlantic_Gyre#mediaviewer/File:North_Atlantic_Gyre.png 8 Figure courtesy of R. Thompson 9 juiceonline.com 10 Image courtesy of Sea Education Association 11 International Pacific Research Center. (2008). Newsletter of the International Pacific Research Center. Tracking Ocean Debris. Retrieved from http://iprc.soest.hawaii.edu/newsletters/iprc_climate_vol8_no2.pdf. p.16 12 Townsend, D. (2012) Oceanography and Marine Biology. An Introduction to Marine Science. Sunderland, MA: Sinauer Associates. p. 169. 13 Townsend, D. (2012) Oceanography and Marine Biology. An Introduction to Marine Science. Sunderland, MA: Sinauer Associates. p. 181. 14 Townsend, D. (2012) Oceanography and Marine Biology. An Introduction to Marine Science. Sunderland, MA: Sinauer Associates. p. 182. 15 Pinet, P.R. (1992). Oceanography. An Introduction to Planet Oceanus. St. Paul: West Publishing Company. p.172. 1