Microplastic Pollution in the Water Column and Sediment of Cochin Estuary, India: Abundance, Characteristics, and Potential Ecological Risk Assessment

Microplastics (MPs) pollution in water bodies has become a serious environmental issue impacting the ecosystems and public health. Studies on their occurrence and impacts are abundantly available on the marine and freshwater systems, while reports on the estuarine systems are scarce; focusing on this research gap, the current study was conducted in Cochin Estuary (CE), which is a part of Vembanad Lake (VL)—a Ramsar site—in Kerala, India. Cochin Estuary (CE) is in the northern part of Vembanad Lake (VL), which is the largest lake in Kerala; VL spreads into 1,51,250 ha covering almost three districts of Kerala: Ernakulam, Kottayam, and Alappuzha. This lake receives discharges from six rivers, besides many canals and streams that empty their contents into it. The artificial barrier, Thannermukkam bund (TM bund), separates VL into a saltwater-dominant northern part and a freshwater- dominant southern part. The north part of VL consists of the Cochin Estuary (CE), which is located near Kochi, the largest and densely populated metropolitan city in Kerala. Water and sediment samples were collected from eight selected locations of CE using a depth water sampler (Hydro-Bios) and a sediment grab sampler (hydro-Bios), respectively. In water samples, sub-surface (SSW) and bottom water (BW) samples were collected from the selected locations. The extraction and analysis of MPs from the samples were done as per the protocol prescribed in the NOAA manual. The findings of this study indicate the ubiquitous distribution of MPs in Cochin Estuary, with the mean abundance of 35.34±7.68 particles per L, 26.87±3.36 particles per L, and 1367.5±403.75 particles per kg (dry weight) in sub-surface water (SSW), bottom water (BW), and sediment samples, respectively. Five morphotypes of MPs were identified: fiber, fragment, film, foam, and beads; among these, fragments were predominant in both SSW and BW. At the same time, fiber was the major morphotype in sediment samples. The shape inventory of the MPs indicates that they originate from pre-existing plastics (secondary microplastics category) through the breakdown of larger plastic debris. Polyamide (PA), Polypropylene (PP), Low-Density Polyethylene (LDPE), Polyethylene terephthalate (PET), Polyvinyl chloride (PVC), Polystyrene (PS), and High-Density Polyethylene (HDPE) were the polymers identified through Raman spectroscopy (WiTech, Germany) analysis. Polyamide (PA) and Polypropylene (PP) were dominant in SSW, while PVC and HDPE were dominant in BW and HDPE; PE was dominant in sediment samples. More than 50% of MPs were in the size range of 100–500 µm in both SSW and BW samples, while in the sediment samples, about 46% of MPs were in this size range. These small particles are more bioavailable to the aquatic organisms and pose a risk of entering the food chain, web of the estuary ecosystem, and being transferred to the higher trophic levels. The accidental or intentional ingestion of these MPs by aquatic organisms may lead to injuries and physiological effects like reduced growth rate, false satiation, reproductive failure, enzyme inhibition, and gradual death of the organisms. The transfer of MPs into humans is also possible through the consumption of contaminated seafood items such as fish, clams, prawns, and crabs. The MPs’ pollution level in the CE was analyzed using the pollution load index (PLI).