The Spatiotemporal Distribution Of Marine Plastic Debris And Microplastics In Xiamen Bay And Their Response To Human Activities

Plastic,a unique material of the Anthropocene Epoch,provides an important base for the progress and development of modern human civilization,but at the same time,it has also produced increasingly serious environmental problems.Research on the spatiotemporal distribution of marine plastic debris(MPDs)and microplastics(MPs)in Xiamen Bay and their response to human activities improves understandings of transport and fate of plastics in the coastal setting,regulating human activities,and reducing the harm of plastics to the marine environment.This dissertation focuses on the distribution of MPs in Xiamen Bay and Xiamen urban wastewater treatment plant(WWTPs),the historical records of MPs in marine sediment and their environmental implication.The main results are detailed as follows.The surface marine debris(MDs)in Xiamen Bay is unevenly distributed,which is typically controlled by precipitation,tidal currents,salinity,wind direction and the level of municipal solid waste(MSW)management surrounding land area.The highest abundance of MDs is 70751 items/km~2,which is nearly 20 times more than the lowest one(3556 items/km~2).They are mainly concentrated in the Jiulong River Estuary and the adjacent sea area,accounting for 59.5%of MDs in Xiamen Bay.About 76.67%MDs are anthropogenically generated in the basin and surrounding land,and only 23.33%are produced by marine time activities such as fishing and aquaculture.MDs are highly correlated with precipitation(R~2=0.75),particularly the extreme weather such as typhoon and continuous rainstorm,followed by a large amount of land-based debris into the sea,resulting in a significant increase in the surface MDs.For instance,the average abundance of MDs caused by typhoon Meranti is 7-11 times higher than that under normal weather conditions in the same period of the previous year.The salinity front of Jiulong River Estuary where the saltwater and freshwater intersect,the areas with weak hydrodynamic conditions and the downwind direction of dominant wind,as well as the areas with beaches and islands barrier,are more likely to accumulate MDs.With the decrease in size,the abundance of MDs increases exponentially,and the proportion of plastic increases logarithmically,indicating that the miniaturization trend of MPDs is obvious.The distribution of MDs with 3600-53143 items/km~2 in the seafloor is similar to that in the sea surface of Xiamen Bay,but slightly disparate due to the difference in bottom flow and salinity front.Especially in estuary area,the MDs depositional center moves upwards with the salinity front.The abundance of MDs is approximately an“inverted V”shape in the seafloor,and the proportion of plastic decreases slightly with the decrease in size.Plastic films/bags and ropes are the two important types of MPDs in all size in the sea-surface and seafloor.The types of small and mega MPDs are relatively single,while the types of meso and macro MPDs are more diverse.With the size decreasing,the identifiability of the MPD types is significantly reduced.The MDs abundance in the beach is 5.25 times and 5.61 times higher than that in the sea-surface and seafloor,respectively,and its distribution patterns can reflect the source and sink to some certain extent.Some of the debris from the river is blocked by the salinity front and coastal vegetation in the estuary,and accumulates in the near beaches of Jiulong River Estuary under the action of wave energy,forming the"Relay station"for the river debris;the debris is redistributed under the action of typhoon,rainstorm or high tide water level,and then re-enters the Xiamen Bay,playing a"source"role.Under the influence of tidal currents and wave energy,MDs is gradually collected into the southeastern beaches of Xiamen Island,making it one of the important"sinks"of sea-surface MDs.In addition to the mega-debris,the proportion of other debris in the beaches of Jiulong River Estuary decreases with decreasing size,and the proportion of MPDs is a similar pattern.However,in the southeastern beaches of Xiamen Island,the size distribution of debris is in an“inverted V”shape with the meso-debris as the peak,and the proportion of MPDs in a“V”shape.In the beach,the average types of macro and meso MPDs are 28 and 24,respectively,which is 3-4 times higher than that in the sea-surface and seafloor.Polystyrene foam is one of the main types of MPDs in the beach and should be given special attention.The distribution of MPs in Xiamen Bay is quite different from that of MPDs;especially the mechanism affected by precipitation is obviously distinct.The average abundance of MPs in the sea-surface is 135269 items/km~2,which is 5.5 times than that of MPDs.The proportion of MPs with which size is less than 2.5 mm is 78.1%.The main types of MPs are Polyethylene(31%),Polyamide(22.3%)and Polyvinyl Chloride(14.5%).Due to the shear velocity is positively correlated with the size,the blocking effecting by salinity front in the estuary on the MPs are weaker than that of MPDs,and MPs are easily transported to open sea farther away from the“source”,making their distribution different from MPDs.The abundance of MPs is highest in drought period(121276 items/km~2),middle in wet period(73925 items/km~2),and lowest in flat water period(24908 items/km~2),which is only 20.5%of that in drought period.The low precipitation and the longtime of solar radiation in drought period are conducive to the degradation and fragmentation of large plastics to form"secondary"MPs,which significantly increases the abundance of MPs in the sea-surface.The distribution patterns of MPs in the beach can also indicate the"source and sink",which is similar to that of MPDs.An electromagnetic flowmeter combining with a fast digital camera were used to improve the MPs sampling method in WWTPs.The improved sampling method whose measurement error is about±1-2 mL/s,effectively solves the problem of large difference in sampling volume between influent and effluent of WWTPs,and improves the comparability of the results.The influent MPs concentration is 1.57-13.69 items/L,and it is reduced to 0.20-1.73 items/L in the effluent,indicating that 79.3-97.8%MPs is removed.Based on the daily effluent discharge and MPs removal rate,it is estimated that~6.5×108 MPs are released from the seven WWTPs into the Xiamen Bay each day.The light microscopic and micro-Raman spectroscopic analysis indicates that~62.68%of particles are plastic polymers,including polypropylene(31.6%),polyethylene(21.9%),polystyrene(10.1%),propylene/ethylene copolymer(9.2%),and polyethylene terephthalate(7.5%).The color of MPs is mainly composed of white(27.3%)and clears(25.8%).Our results show that granules(41.1%)are the dominant shape of MPs,followed by fragments(31.3%),fibers(23.7%),and pellet(3.9%).The characteristics of MPs such as size,shapes,and densities affect the MPs removal in WWTPs.Our findings show that MPs concentration in the influent is positively correlated with the suspended solids(SS),however,in the effluent,it is associated with the WWTPs operating load,as reflected by obviously higher MP abundance in overloaded ones.The abundance and characteristics of MPs in GY sediment cores is fluctuating.The historical records of MPs first appeared in the late 1940s and early 1950s,which has a good indication of the early development history of plastics industry in the world and China.After the MP abundance increased slowly in 1950s,it showed a downward trend in 1960s.While,with the rapid development of plastics production and consumption after China's reform and opening up in 1978,the MP abundance peaked in the mid-1980s(189241 Items/kg·dw),and then fell to the low value of the mid-1990s.Since the mid-to-late 1990s,the MP abundance has gradually increased.The main types of MPs in GY sediment cores are PES and Acrylic,with an average proportion of 41.07%and 23.12%,respectively.The proportion of PE,PP,PE+PP and PA are 11.47%,6.52%,5.63%and3.84%,respectively.The MP?100?m account for 82.20%,which mainly concentrate between 20-40?m.The MP size are varies in different years,especially in 1972-1984,when the size distribution of MPs is more concentrated,and PES with higher density is dominant,accounting for 64.5%on average.The historical records of MPs in the GY sediment cores are an effective response to multiple environmental pressures.Before 1988,the plastic production was the main factor controlling the distribution of historical MP abundance in GY sediment cores,that is,the historical MP abundance increased with the increase of plastic production,and the impact of world plastic production on it was more obvious(R~2=0.85).After 1988,the historical distribution of MPs abundance in GY sediment core was influenced and controlled by multiple factors.In the middle and late 1980s,the collection and disposal rate of land-based plastic debris was gradually improved,and the MDs cleaning policy started in 1996further reduced the flux of land-based plastic debris into Xiamen Bay.At the same time,the centralized treatment rate of municipal wastewater gradually increased after 1989,which made the MP discharge from municipal wastewater show a downward trend.MARPOL Annex V,which came into force in 1989,directly and effectively reduces the discharge of plastic debris from ships.These factors jointly control the inputs of MPs directly or indirectly.The historical records of MP abundance,types,and morphology in GY sediment core are the result of the interaction of MP source input,MP physical and chemical properties and sedimentary environment,and are effective responses to multiple environmental pressures.The temporal distribution of MPs in sedimentary environment can better retrieve the history of human plastic industry,the changes in the consumption structure of plastic products,and the history of regional important human activities,which demonstrate the potential of important indicative significance of MPs as a stratigraphic marker of the"Anthropocene".