Effect of Neustonic Microplastic Debris on the Pelagic Insect, Halobates sericeus
Welcome to our plastic world. From cups to bags, bottles to CDs, clips to pipes, a nifty collection of polymers is here and thriving. Annually, the world utilizes 245 million tons of plastic, most of which is polypropylene (Andrady and Neal 2009). Plastic has many advantages: it is lightweight, has a high strength to weight ratio and is easy to mass produce (Andrady and Neal 2009, Thompson et al. 2009). But, unfortunately, it is also persistent, long-lasting and slow to degrade, especially in the marine environment. Though the degradation rate of plastic in the open ocean is highly variable, depending upon the composition of the polymer and environmental conditions, multiple researchers have concluded that the rate of degradation is in the order of hundreds to thousands of years (Andrady and Neal 2009, Barnes et al. 2009).
Plastic marine pollution is not a recent occurrence. In 1972, Carpenter and Smith found an average of 3,500 pieces of plastic per square kilometer on the surface of the North Atlantic Gyre. Recently, Law et al. (2010), after analyzing 22 years of sea surface samples taken in the North Atlantic Ocean and the Caribbean Sea, reported an average of 1,534 pieces of plastic per square kilometer in the North Atlantic Gyre. While these studies counted and described plastic debris on the open ocean's surface, the mere presence of plastic did not indicate its consequences. The biggest and most daunting question is that of the effect. How does the presence of plastic impact the lives of marine organisms? The answer to this question is multifaceted and difficult to answer.
Barnes et al. (2009) studied plastic as fouling grounds for organisms, including hydroids and bryozoans, while Gregory (2009) investigated the employment of plastic as a vector for foreign invasive species. More research is still needed on a wider variety of organisms to conclusively determine the effects of plastic on the marine ecosystem.
Though one could find many types and sizes of plastic debris in the open ocean, most plastic pieces found in the ocean are very small particulates called microplastics (Barnes et al. 2009). Microplastics are defined by the National Oceanic and Atmospheric Administration as plastics that are less than 5 millimeters across (NOAA Marine Debris Program 2010). Oceanic microplastic abundance has increased in recent decades and is becoming a subject of increasing concern (Barnes et al. 2009).
In the research that addresses the effects of plastic debris on the marine ecosystem, most of the organisms studied are large vertebrates (Derraik 2002). As Sheavly and Register (2007) explain, a much less understood question is the "impact of marine debris on the species at the base of the food chain," like plankton and algae. This is a significant gap in the literature, especially because the organisms at the base of the marine food chain are widespread and present in massive numbers across the oceans (Campbell and Reece 2005). Given the lack of literature in this area, my research focuses on the effect of plastic debris on the unique marine insect Halobates sericeus (Figure 1).
Halobates is a genus of insects belonging to the Gerridae family (common name: water striders). By the most recent count, there are 46 Halobates species, of which only five maintain a completely oceanic habitat (Andersen and Cheng 2004). These five species, including H. sericeus, are the only known fully pelagic insects (Cheng 1985). H. sericeus has many reasons to be affected by the presence of microplastic debris in the North Pacific Central Gyre. Halobates, and specifically H. sericeus, are known to interact with floating matter in the ocean, using this matter as a substrate for laying eggs (Cheng and Pittman 2002). Furthermore, since these fully pelagic water striders live solely at the sea-air interface, their life is played out completely on the two-dimensional surface of the ocean (Cheng 1985). Because the majority of plastic is also concentrated at the surface of the sea, this makes H. sericeus the perfect planktonic organism to study when trying to determine the effect of plastic debris on the lives of marine invertebrates.
My hypotheses were as follows:
Question 1: What is the difference in the abundance of H. sericeus and plastic in the NPCG between the 1970s and today?
H : There is no change in the abundance of plastic and/or H. sericeus from the 1970s to today.
H1: There is a positive change in the abundance of plastic and/or H. sericeus from the 1970s to today.
H2: There is a negative change in the abundance of plastic and/or H. sericeus from the 1970s to today.
Question 2: What is the relationship between H. sericeus and plastic in the NPCG?
H : There is no relationship between neustonic plastic abundance and H. sericeus abundance.
H1: H. sericeus abundance is positively correlated with abundance of neustonic plastic.
H2: H. sericeus abundance is negatively correlated with abundance of neustonic plastic.
MATERIALS AND METHODS
Figure 2: This image is a GoogleEarth depiction of the locations of the samples that I analyzed. The cruises are separated by pin color: SEAPLEX (yellow, 2009), SOUTHTOW 13 (red, 1973), 7205 (green, 1972), 7210 (orange, 1972)
Figure 3: This is my standard workspace, which includes a plastic tray (this tray has been filled with a portion of the sample), a wooden teasing needle, forceps, and two glass trays for removal ofH. sericeus and/or plastic debris.
Figure 4: an Adult female H. sericeus (abdomen facing up, head on the right of the image).
For this project I analyzed 90 samples from four cruises: SEAPLEX (45 samples, 2009), SOUTHTOW 13 (14 samples, 1973), 7210 (21 samples, 1972) and 7205 (10 samples, 1972) (Figure 2). SOUTHTOW 13, 7205 and 7210 will be collectively known as the "historical cruises," and their samples will be referred to as the "historical samples." The SEAPLEX samples were gathered using a manta net-1.0 by 0.2 meters, 333 micrometers mesh size (Brown and Cheng 1981). The nets were towed, as prescribed by the CalCOFI method, for 15 minutes at a 20° wire angle at approximately 1.5 knots (Southwest Fisheries Science Center 2007). The SOUTHTOW samples were captured with a neuston net-1.0 by 0.5 meters, 505 micrometers mesh size-towed at four knots for varying lengths of time, from two to three hours. The 7205 and 7210 samples were also collected with a neuston net, towed as prescribed by the CalCOFI method. The samples were then preserved in 5% Formalin in glass jars.
In the lab, my mentor removed the contents of each Formalin-preserved sample and placed them into filtered seawater. Each sample was then analyzed under a dissecting microscope (60x to 120x magnification) in small aliquots using a square plastic tray, forceps and a wooden teasing needle (Figure 3).
I sorted through the plastic tray very thoroughly. Any H. sericeus and/or plastic debris that was found was counted, classified and removed for further analysis. The H. sericeus were sorted into five groups: juvenile, adult male, adult female, newly molted and molts of organisms. Juveniles are noticeably lighter in color (tan/brown) and smaller than adults. Adult males and females are similar sizes and black, but their genitalia are different, allowing for clear separation and identification (Figure 4). Newly molted organisms are very light in color. The molts of H. sericeus exhibited a clear slit in the back, with no body present inside the shed exoskeleton. In addition, when H. sericeus eggs were found, they were counted and their position noted. H. sericeus eggs are a creamy pastel yellow and are relatively large (about 5 millimeters).
For all of the SEAPLEX samples, I separated the H. sericeus, placing them into vials and preserving them in 95% ethanol. My advisor and the laboratory technician used a large, scanner-like piece of equipment, called the ZooScan, to count and measure the high number of plastic particles found in each sample (Gillifillan et al. 2009).
Because I wanted to see the change in the abundance of plastic and H. sericeus over time, I needed access to historical neustonic samples from the NPCG. At the Scripps Institution of Oceanography, scientists have formed the Pelagic Invertebrate Collection, an extensive collection of samples taken on cruises going back more than 100 years. From this collection I was able to gather and count archived samples, a key component of my research.
The sorting method was mostly the same for the historical samples. However, since plastic abundance was significantly lower in all of the historical samples, I counted the plastic particles under a microscope.
Figure 5: in 2009, more tows hadH. sericeus present than in 1972-1973 (p-value calculated by R).
Figure 6: Tows from 2009 contained significantly more plastic than tows from 1972-1973 (p-value calculated by R).
Figure 7: There is no relationship between the density of plastic debris and the density of H. sericeus in the NPCG in the early 1970s (R2 and p values calculated in Numbers and Mathematica).
Figure 8: There is a significant positive correlation between the density of plastic debris and the density of H. sericeus in the NPCG in 2009 (R2 and p values calculated in Numbers and Mathematica).
An H. sericeus egg
After counts of both H. sericeus and the plastic particles were completed, the data was graphically and statistically analyzed. The first question of my research involved the change in abundance of H. sericeus and plastic from the 1970s to today.
The second main question of my research was whether there was a correlation between the abundance of H. sericeus and of plastic debris in the NPCG. To answer this question, I performed linear regressions.
Plastic debris has been documented throughout the world's oceans but has never been linked to changes in zooplankton populations. This study represents the first research investigating the impacts of plastic on a marine invertebrate.
Question 1: What is the difference in abundance of H. sericeus and plastic in the NPCG between the 1970s and today?
Halobates sericeus abundance: I did detect a significant increase in the abundance of H. sericeus in the NPCG between the 1970s and today (Figure 5). Thus I can accept my first hypothesis: There is a positive change in the abundance of H. sericeus from the 1970s to today.
Halobates and plastic could be positively related because the increase in high-plastic areas accessible by H. sericeus would augment the potential surface area available for egg laying, thus potentially increasing the population of H. sericeus. Data from this research does show an increase in the abundance of plastic (Figure 6), though similar studies (Law et al. 2010) did not find an increase in the abundance of plastic in the North Atlantic Gyre over time.
However, plastic could have no effect on H. sericeus, and the increase in the abundance of H. sericeus could be the result of natural fluctuations in population. Since plastic and H. sericeus share the same habitat in the NPCG, it is unlikely that there is a lack of interaction between them (Cheng 1985). However, the fact that plastic and H. sericeus co-occur within the NPCG cannot prove that one affects the other.
It is also possible that there are simultaneous positive and negative effects observed in H. sericeus as a result of its interaction with plastic. In January 2010, my advisor and I conducted a pilot study that investigated the effects of plastic on the lifespan of adult H. sericeus. Prior to this experiment, I organized and prepared three different conditions that the H. sericeus were subjected to. These conditions were a Nalgene glass container (the control environment), an unseasoned plastic Gladware container and a seasoned plastic Gladware container that had been soaked in ocean water for three weeks. The seasoned container was used as a representation of the plastic that ends up in the NPCG-plastic that has floated in the ocean for a while. Because the seasoned container had ample time to lose its inherently toxic properties (e.g., plasticizers), it was expected that the H. sericeus in the seasoned container would survive about as long as the H. sericeus in the control container. Experiments showed that adult female H. sericeus died 7.5 hours after placement in the unseasoned container, while adult female H. sericeus died 22 hours after placement in the seasoned container, and adult female H. sericeus died 6.5 days after placement in the control container. Since the H. sericeus in the seasoned plastic only survived 14 hours longer than the H. sericeus in the unseasoned plastic, it is possible that even pre-leached plastic poses a toxicological threat to H. sericeus. At the same time, H. sericeus do use floating matter as a substrate to lay eggs (Figure 9), so plastic might provide an increased surface area for gravid adult female organisms (Cheng and Pittman 2002).
Plastic abundance: The abundance of plastic debris in the NPCG significantly increased between the 1970s and today (Figure 6). I accept my first hypothesis: There is a positive change in the abundance of plastic from the 1970s to today. The significant increase in plastic density has a variety of causes. Human population is increasing, and with that comes an increase in people along the coast (Population Reference Bureau 2010). Thus, there is more opportunity for debris to directly enter the marine ecosystem. In addition, urban litter is increasing, specifically with an increase in thermoplastic disposal (Andrady and Neal 2009). This provides many opportunities for litter mismanagement, with the end result being an increase in plastic litter.
Question 2: What is the relationship between plastic and H. sericeus in the NPCG?
Historical abundance: I did not detect a relationship between plastic and H. sericeus abundance in the 1970s (Figure 7). I cannot reject my null hypothesis. There is no relationship between neustonic plastic abundance and H. sericeus abundance. There was such a low density of plastic in all of the historical samples that there were very few data points that had the potential to show a relationship. This is seen in the fact that many of the samples' plastic densities had a range of less than 0.05 plastic pieces per square meter. Furthermore, the historical samples were not as evenly distributed as those from 2009, which could have caused a bias. However, the NPCG is mostly homogeneous and the samples cover a large area, so the sampling process should not have strongly affected the results (McGowan and Walker 1979).
Modern abundance: Figure 8 shows a significant positive correlation between the densities of H. sericeus and plastic debris in the NPCG today (2009). I accept my first hypothesis: H. sericeus abundance is positively correlated with abundance of neustonic plastic.
The NPCG is the site of the Subtropical Convergence Zone. Due to this convergence zone and other small-scale currents (e.g., eddies and fronts), a high density of both plastic and H. sericeus could occur in the NPCG without the formation of a true interactive relationship between the debris and the insect. In this way, plastic and H. sericeus could be in the same place at the same time without a biologically significant reason.
However, H. sericeus could also actively move to areas of high plastic density in order to increase the available surface area for egg-laying purposes. This is a significant possibility because, as Cheng and Pittman (2002) note, "There may be a shortage of suitable oviposition substrates in the open ocean." Cheng and Pittman also collected a plastic gallon milk jug to which approximately 70,000 Halobates eggs were attached; more than 7,000 females had utilized the jug, according to the authors' calculations.
My research demonstrated a strong positive relationship between H. sericeus and plastic debris in the NPCG in 2009. However, this relationship was not observed in an analysis of historical samples, possibly due to a very low occurrence of plastic debris. I also showed a large increase in the abundance of plastic in the NPCG over the last forty years, with a similar increase in the abundance of H. sericeus. While my results strongly support a relationship between the abundance of Halobates and of plastic debris, the scope of my project prevented me from exploring the potential reasons for this relationship. Because the physical oceanography was not analyzed, I cannot claim that there exists a causative relationship between plastic and H. sericeus. It is possible that the physical oceanography caused the samples to be biased toward a high density of both plastic and H. sericeus, without a biological reason.
FUTURE RESEARCH DIRECTIONS
In the future, there are two experiments I wish to pursue to solidify my findings. First, I would like to perform an extensive and complete field study to determine the toxicological impact of plastic debris-filled versus nonplastic debris-filled habitats on the lifespan of H. sericeus. I would model this experiment after the preliminary field study I had previously designed. Second, I would pursue an analysis of the physical oceanography during the times of the four cruises. Both of these experiments would be key in determining if there exists a relationship between the abundance of H. sericeus and of plastic debris.
I have discovered a strong relationship between plastic and Halobates sericeus, a unique insect. I do not know the facets of this relationship, or whether it is ecologically significant. I am happy these questions linger because it means I have more to pursue, to answer and to learn.
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