How does regional geography control the intensity of microplastic pollution and impact plant life in floodplain-adjacent ecosystems?

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Abstract

Microplastics are tiny particles of plastic produced by a wide variety of manufacturing and weathering processes involving plastic materials. They are defined as plastic pieces less than 5 millimetres in maximum length. Environments polluted by these particles face a variety of problems, including the impacts of harmful organic chemicals absorbed onto their surface and long-term consequences on soil organisms along with soil health. To date, most research on the microplastics issue has focused on coastal environments, but floodplain-adjacent environments such as mangrove patches, salt marshes and alluvial plains may also be at particular risk of microplastic contamination. Such environments act as a sink for microplastics and include biological mechanisms that lead to particularly high rates of microplastic retention. Indeed, it has been shown that freshwater outlets to the ocean with large flood catchment areas output significantly less microplastic particles to the ocean. This study examines the impact of the retention of microplastics on environments and ecosystems established in these kinds of floodplain environments. A literature review as well as a novel compilation of satellite data cross-referenced against existing studies on microplastic concentrations are employed. It is shown that there is an indiscernible effect of elevation on microplastic concentration and variable microplastic concentrations in urban areas. Rural areas have less microplastic concentration, and areas with less microplastic concentration had more plant growth, while areas with higher microplastic concentrations had less plant growth.

Introduction

Microplastics are a byproduct of human development defined as pieces of plastic less than 5 millimeters in length, which as a result, can be found throughout the world1 because they pass through water filtration systems easily. Microplastics are formed through breaking apart larger plastics already in the ocean whereas others are intentionally designed to be smaller. Microplastics are found in household products such as facial cleansers and toothpaste. The principal danger posed by microplastics is the contaminants and potentially harmful organic chemicals contained within them, which can cause significant harm when ingested or inhaled2 and destroy soil organisms along with their health and function3 . The effects of microplastics may be more potent in some areas as compared to others. In a study done by Ivar do Sul titled “Plastic debris retention and exportation by a mangrove forest patch”, it is discovered that the mangrove forests have high plastic retention in their ecosystems, causing them to experience the effects of concentrated microplastics more as they damage the ecosystem4 . However, microplastics are not only found in plants like mangrove trees, but they are also found throughout the world in the soil and in the plants around us. In a year study done by Markus Rolf, researchers concluded that floodplains may act as a sink for microplastics5 . However, the effect of excessive microplastic bioaccumulation in mangroves has on the plant life in surrounding floodplain ecosystems has yet been determined.

Materials and Methods

Data were taken from both research papers and a database with given microplastic concentrations from available satellite data. Research papers were taken directly from the Google Scholar web engine and web of science searches. Satellite data was gathered from the ArcGIS satellite imaging database using the Landsat 8-9 Views map overlay6 . Landsat 8-9 was used since it is the most used for vegetation analysis. A dark and intense red would represent a lot of plant growth and it would be more vibrant because of the chlorophyll. Lighter shades of red will still represent vegetation but are not necessarily as healthy as the more intense reds. The area was assigned a qualitative data value and noted in a data table. Once selected, the processing template titled Color Infrared with DRA was chosen and pictures were taken as reference from the satellite. For data points taken outside of research papers, the website globalfloodmaps.org7 was used to pinpoint floodplain locations on the map, which were then cross referenced with another database with given data points8 and the microplastic concentrations of those areas. When there was a data point matching the area, it was added to the list of data. After taking this, google maps was used to roughly determine the land use and the type of environment. Some of the lands where already pre-classified by the research paper data was taken from and those data points were used9 . For the River Irwell data points, data was taken from the Hurley research paper8. Rural land use was averaged between the points from Irwell Springs (1), Love Clough (3), and Ramsbottom (5). Suburban land use was taken from Bacup (2), Rawtenstall (4), Bury (6), Clifton (19), and Pendlebury (20). Brownfield land use was taken from Irwell-Croal Confluence (12). Urban land use was taken from Manchester (21). For areas that were offshore of the beach area and around the ocean, the data point was taken with respect to the closest city to that beach and the land use. For all data points, if the city had multiple environments, the majority environment was taken based off an estimation given the environmental characteristics and written down. All elevations were found with the coordinates given by the database and an elevation finder at those exact coordinates10 . The average color of the areas was written in the side column for comparison to one another for relative vegetation in that area.

Results

Literature Review

Microplastics (plastics less than 5mm in size) are regarded as a large problem and there have been plenty of research papers written on these topics. One of the articles used for this paper discusses how microplastic contamination is significantly reduced by catchment-wide flooding. It states that the movement of water may contribute 64 to 90% of plastic debris in oceans8. Furthermore, it discusses the possible effects of microplastics in these environments, especially those organisms that live in fine-grained fluvial bed sediments. In summary, the article showed an average decrease in microplastic contamination in urban rivers in post-flood conditions. Water movement may also be a factor in the microplastic concentration in floodplain areas since they are so often flooded. In another study involving flood events versus dry periods, it is stated that there is a scarcity of freshwater microfiber and microplastic studies11. Thus, much of the effect of microplastics on freshwater plants and organisms is left widely unknown and needs further research and more data in comparison to saltwater environments. The paper goes on to state how in freshwater environments like a lake, concentration was found to be more downstream. Some floodplains connect rivers directly to oceans and plant life may be significantly affected there by the larger amount of microplastics that are concentrated there. It was found in the study that concentrations are in the same order of magnitude regardless of the hydrological conditions like after a flood. Plants may also retain microplastics. However, there needs to be further development on the effect of concentrations on plants. In a study involving mangrove forests, it is revealed that mangrove forests retain plastic marine debris for long periods, and on higher grounds, there is significantly more plastic retention in comparison to the river and tidal creek conditions. It goes on to conclude that vegetation is important for trapping debris. In this study, areas with water movement relative to the areas without had less microplastic retention, explaining how water movement may increase or decrease the microplastic concentrations.

Satellite imagery

Satellite data was gathered from the ArcGIS satellite database using the Landsat 8-9 overlay and the processing template titled Color Infrared with DRA.

Greater Manchester, United Kingdom

Catchment areas of River Irwell and River Tame, Greater Manchester, UK. Referenced from the Hurley research paper and ArcGIS satellite imaging. In the UK results tested, the elevation of the River Irwell and River Mersey had lots of variation. The environment for both was Riverbed and the average microplastic concentration was 0.87 kg/km^{2} for the river Irwell and 0.22 kg/km^{2} for the river Mersey. The land use for these areas is urban.

Figure 1 | Image of all the lakes located in the central area of England, indicated by the top right box with the black outline. In the data, only letter A, river Irwell (traced in light green), and letter G, river Mersey (traced in yellow)were used.

A map (Figure 1) shows the River Irwell and River Mersey. Located at letter A on the text is the river Irwell, with an average microplastic concentration of 0.87 kg/km^{2}. Located at letter G of the text is River Mersey, with an average microplastic concentration of 0.22 kg/km^{2}

Figure 2 | Map of the River Irwell location and overlaying satellite imagery of the area. River Irwell is located near the top half and the middle (traced in light green), and River Mersey is located near the direct middle of the satellite imagery (traced in yellow). The line in the middle is due to the stitching together of the different satellite images. Both rivers have a large population.

Satellite imaging of the area (Figure 2) around the river Irwell shows results of darker reds, gray, and green areas of plant growth. The area around the river Mersey shows both darker reds, gray colors, and green.
The United Kingdom has a temperate mid-latitude climate and is mostly warmed by the Gulf Stream Ocean Current. Due to its positioning between the Ferrel and Polar cells, it will also have fluctuating weather, including a larger likelihood of precipitation. Also due to the position, there is a higher humidity in the UK12.

Lake Setúbal, Argentina

Catchment area of Lake Setúbal in Argentina. Referenced from Blettler research paper13 and ArcGIS satellite imaging.
In the Lake Setúbal results, the elevation was around 11 meters. The environment was mostly floodplain in the zone tested. The average microplastic concentration of this area was 7.493 kg/km^{2}. The land use for this area is urban.

Figure 3 | Image showing the overall general sampling location with given coordinates and a scale for reference. Only the area within the white box on the left of the map was used.

A map showing the Lake Setúbal sampling location13. The catchment site is located next to a lake with movement of water. It has an elevation of 11 meters.

Figure 4 | Image showing both the sampling location and the satellite image overlay of the area in Lake Setúbal. Data collected from the zone had a variety of colors including blue and red. On the sampling location, it is located to the left side of the map, on the far right of the light blue patch. The area next to the sampling site is highly populated.

Satellite imaging shows most of the sampling site is relatively pink with a large blue area located near it.
Argentina is mostly subtropical, experiencing little rainfall throughout the year14. The Santa Fe province has humid conditions and abundant rainfall.

Mauna Loa, Hawaii

Referenced from Hurley research paper supplemental data and satellite imaging from ArcGIS.
The data point for Mauna Loa was taken in the ocean and elevation was written as 0 meters. The environment is mostly ocean in the area tested. The average microplastic concentration for this area is 0.326 kg/km^{2}. The land use for this area is rural.

Figure 5 |Image showing the Satellite imagery of the Hawaiian Islands. Data was taken from offshore of the medium-sized island above the large one and close to the middle of the image. The data point was in the sea slightly above the island. The area has a lower population. 

Satellite imaging of this area shows an intense, dark red.

Crystal River, Florida, USA

Referenced from Hurley research paper supplemental data and satellite imaging from ArcGIS. In the Crystal River sampling site, the elevation was 42 meters. The majority environment of Crystal River, Florida is Salt Marsh and Estuarine. It has a microplastic concentration of 11.296 kg/km^{2}, and the land use is urban.

Figure 6 | Image of the Crystal River Florida Location. Data was taken directly from the area in the center of the image. The area is mostly light red. It is relatively close to the coastline. The area is very populated.

The Crystal River sampling area of Florida was mostly a pink color.

Atmore City, Alabama, USA

Referenced from Hurley research paper supplemental data and satellite imaging from ArcGIS.

The Atmore City location in Alabama has an elevation of 83 meters. The environment is mostly plains. It has a microplastic concentration of 1.417 kg/km^{2}, with urban land use.

Figure 7 | Image of the Atmore City location. Data was taken from approximately the lower half middle of the image. It is an intense dark red. It is located relatively close to a coastline. The sampling area does not have a very large population, but it does have surrounding towns.

Using Satellite imaging, the color at the sampling site is mostly dark red.

Solona Beach, California, USA

Referenced from Hurley research paper supplemental data and satellite imaging from ArcGIS.

The point taken at Solona Beach was in the ocean, so elevation was taken as 0. The environment is ocean, and it has a microplastic concentration of 2.250 kg/km^{2}. The land use is undetermined.

Figure 8 | Image of the Solona Beach location data was gathered from. Data was taken from off the coast of the image near the center of the image. It is a relatively deep colored red. The line in the middle of the image is due to the stitching together of satellite images. The nearby city is populated in some areas.

The satellite imaging shows a dark red color with bits of gray too.

Cutchogue, New York state, USA

Referenced from Hurley research paper supplemental data and satellite imaging from ArcGIS.

In Cutchogue, the sampling site had an elevation of 9 meters. The environment was floodplain, and the microplastic concentration was 13.014 kg/km^{2}.The land use was urban.

Figure 9 | Image of the sampling location at Cutchogue. The area is surrounded by water and highly populated. Data was taken from relatively close to the center of the image. It is colored a dark red in some areas, however, in the location the sample was taken at the location is lighter red.

The area where the sample was taken was a light red.

Cross-comparative database

The data collected involved elevation, environment, microplastic concentration, land use, and plant growth. The elevation and land use were described to see if the elevation of the data points had a significant effect on the microplastic concentration. Determining the environment helped with determining the type of plant growth normally expected in that area. The data from Table 1 compiles all the data gathered from the database and papers. With so much variability in the elevation with River Irwell, River Mersey, Atmore, and Solona Beach, they still had a dark red plant growth value, meaning it couldn’t be reasonably concluded that elevation had a significant effect on microplastic concentrations. Data values for elevation from the table with the lighter values like Manua Loa, Crystal River, Cutchogue, and Lake Setubal further demonstrate the elevation variability. The land use showed that areas with rural land had smaller microplastic concentrations relative to the rest, and areas with urban land use would vary.

In Table 1, urban land use meant a larger population and more city-like conditions. The rural land use meant more countryside like living with a smaller population. In this table, when looking at the maps, it was determined by the number of buildings seen on the maps. Plant growth was determined based off what the color meant. A darker red means more vegetation in the area, while a lighter red would mean less vegetation.  

Table 1 | Table showing microplastic concentrations in different flood plain environments, as well as the use of the land where the reading was taken and the category of plant growth. For points marked with *, they are points that were taken from the database which had the microplastic concentration written in mg/m3. This meant that to convert to kg/m2, we would need to convert from mg to kg and then from m3 to km2. To do that, we multiplied each value by a factor of 1, meaning they would only need to have the units converted to have the same comparison. Similarly, Lake Setubal was originally in g/cm2, which when converted to kg/m2 would need to be multiplied by a factor of 1000.

Elevation had an indiscernible effect on microplastic concentration as areas like Atmore had a very high elevation of 83 meters with a microplastic concentration of 1.417, while Cutchogue had an elevation of 9 meters with a microplastic concentration of 13.014. With the points involving darker red colors, meaning more vegetation, the average microplastic concentration of River Irwell, River Mersey, Atmore, and Solona Beach was 1.189 kg/km^{2}. The points involving the lighter red colors, meaning less vegetation, the average microplastic concentration of Lake Setúbal, Mauna Loa, Crystal River, and Cutchogue was 8.032 kg/km^{2}. There was significantly more vegetation growth with the areas of less microplastic concentration in comparison with the areas of higher microplastic concentration.

Discussion

The data shows that there is a correlation between the microplastic concentration in floodplain adjacent ecosystems and plant growth. River Irwell, River Mersey, Atmore, and Solona Beach had a dark red color, meaning that they had significantly more plant growth or vegetation as compared to other areas. When comparing these areas to the other data points, their microplastic concentration is significantly lower, with the average microplastic concentration in the dark red areas being 1.189 kg/km^{2}, while the areas with the lighter red color had an average microplastic concentration of 8.032kg/km^{2}. This may be due to the water movement in these areas. For example, the River Irwell and the River Mersey have constant flowing water between them which may result in the larger amount of vegetation located near there since microplastics won’t affect plant growth as much as areas with larger amounts of microplastic concentration. However, one of the areas with less vegetation and less healthy plants was Lake Setubal. It has a constant moving stream of water next to it, meaning with prior reasoning it should have more vegetation, but the microplastic concentration suggests otherwise. The microplastic concentration is significantly higher than the darkest red area. This may be due to its location, which is extremely close to an urban area. It may be argued that in some other cities with more vegetation (dark red), they have highly urban areas. It may be since we are taking a rough area of microplastic concentration, and it is a larger range, rather than Lake Setubal which focuses on a specific area located next to a highly urban area. Another argument could have to do with the types of plant growth endemic to the region, such as grasses and bushes. However, the locations chosen were mostly floodplains in roughly the same climate zones, which should indicate relatively similar plant species, being the same biome. Also, relative to the elevation, there were no significant data points compared to it. So, there were inconclusive results with the elevation. This may have been due to a variety of factors including water movement and the location and environment of the area in general.

Conclusions

Microplastics have a negative impact on plant growth, with a higher concentration of microplastics leading to a lower amount of plant growth and vice versa. Plant growth was determined with satellite imaging and a labelling of the color that was found on the graph. The dark red represented more plant growth while the light red represented less plant growth. When compared along with the different microplastic concentrations of the specific areas, the average for dark red was 1.189 kg/km^{2} while the average for light red was 8.032kg/km^{2}, indicating high concentrations mean lower plant growth and low concentrations mean higher plant growth. Using the differing levels of elevation in comparison with the concentration of microplastics. There was no significant effect of the elevation on the amount of microplastics within this study. Elevation with microplastics of relatively low concentration and a dark red overlay on the graph ranged from 0 to 83, while the elevation with microplastics of relatively higher concentration ranged from 0 to 42. The overlap leads me to conclude that elevation does not have a significant enough impact on the microplastic concentration. The other aspect is the type of land usage, with urban having larger values on average when compared with the rural data point. Overall, these results show that areas with a high microplastic concentration and in an urban land area may have lower levels of plant growth when compared to areas of lower microplastic concentration in a rural land environment. To confirm these results, laboratory testing should be done by isolating just the water from each region and growing the plants inside. Further testing could also analyze the water composition to see what other factors are working in combination to create the outcome of diminished plant growth. There should be more precaution taken in urban land areas as they have a larger microplastic concentration, people should also be interested in plastic removal efforts throughout the world like the Ocean Cleanup, which could help deal with the microplastic problem causing harm to plant growth. Issues with plant growth may ruin food webs and ecosystems as plants are the basis for photosynthesis, where plants harness energy from the sun to feed the ecosystem. People in urbanized areas should be wary of microplastics because their high-density results in more trash and waste being thrown out, meaning more microplastics released into their area. Future work could focus on developing plans to create better incentives for plastic recycling and options for removing microplastics that are currently in ecosystems. Efforts should not only include microplastic prevention, meaning possibly a transition from plastic into more environmentally friendly materials, but also removal of the current microplastics out of ecosystems where it has caused major harm and creating storage places for them.

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