Indonesian Tropical Seagrass ecosystem under the Stress of plastics pollution

The tropical seagrass ecosystem is highly important to Indonesia’s marine. The ecosystem has major role in supporting Indonesian biodiversity and providing vital ecosystem services. The extensive distribution (30,000 km²) of Indonesian seagrass is known to be part of the world’s natural heritage. Seagrass meadow significantly supports fisheries productivity which directly links to the food security and the livelihoods of all Indonesian communities. Many commercial fish and invertebrate species spend their early stage of life in the seagrass meadow.  The meadow also acts as pollutant filter by purifying the water and reducing the pathogen that causes the disease to coral. Moreover, seagrass ecosystem also contributes to climate change mitigation by capturing and storing carbon in their leaves and below-ground sediment. It is reported that Indonesian seagrass can store carbon up to 5.62 – 8.40 tons C ha^-1 year^-1 (Wahyudi et al., 2020). This refers that Indonesian seagrass is not just an important national asset, but also a crucial world asset.  Unfortunately, tropical seagrass ecosystem continues to decline from time to time due to multiple stressors, including anthropogenic disturbances.

Marine plastic pollution is one of the anthropogenic stressors threatening the tropical seagrass ecosystem. As a synthetic material, plastic can not be naturally degraded in the environment, allowing it to be persistent and broken down its sizes into smaller pieces called microplastics. Generally, there are three types of marine plastic waste, macroplastics (>25 mm), mesoplastics (5 – 25 mm), and microplastics (< 5 mm). Despite all sizes, all forms of marine plastic waste can cause detrimental impacts on the seagrass beds.

Indonesia ranks 2^nd in emitting plastic waste to the global ocean. Hence, the plastic waste issue in the Indonesian marine environment is critical, especially in the seagrass ecosystem. Indonesian researchers have reported that marine plastics in all forms are polluted Indonesian seagrass meadows, such as in Pramuka and Tidung in Thousand Islands, and Kodingareng Lompo Island in Makassar. Marine plastic waste (macro-and-micro-plastics) can be trapped in the intertidal and subtidal seagrass beds vegetation. Navarrete-Fernández et al., (2022) highlighted seagrass meadow ecosystem services as marine plastic trap. He mentions that the outer edge of the seagrass meadow margin is mostly trapping macroplastic, whereas microplastic predominantly get trapped in the inner margin. Ironically, the ecosystem service seagrass provides may also cause a seagrass vegetation loss.  

Marine macroplastic waste impact to seagrass vegetation

Various types of macroplastics have been reported to trap in the vegetated seagrass meadow, including plastic bags, food wrappers, cups, bottles, containers, straws, etc. The common size of macroplastics detected is ranging from 0.5 cm to 10 cm (Cozzolino et al., 2020). The origin source of marine macroplastics is from the land. They enter the marine environment in many ways, such as from waterways, direct littering, and natural disaster (e.g., flood, storms, and tsunami). As ocean is a dynamic system, many plastics debris float and distribute to all marine habitats. The debris also can get washed to the shorelines or stranded on the coastal vegetated habitat, including in seagrass beds.

Although plastic debris is not the only cause of the decline of seagrass vegetation, its occurrence critically contributes to the vegetation decline.  The impact of marine macroplastics on the tropical seagrass habitat is still largely unknown. However, there are several studies from other regions showing the potential impact of macroplastic on seagrass ecosystem. It includes:

- Interfering the nutrient dynamics and cycling of seagrass habitat by decelerating the decomposition rate of seagrass that directly slows down the nitrogen liberation;

- Interfering the photosynthesis process of seagrass due to sunlight blockage;

- Altering the sediment geochemistry as plastic is persistent in the sediment and potentially leaking hazardous chemicals;

- Increasing the vulnerability of seagrass to invasive algae species as stranded plastic on seagrass sediment deters the growth of seagrass shoot and enables space for invasive macroalgae to grow. This condition will change the architecture of seagrass vegetation.

Marine microplastics impact to seagrass vegetation

Microplastic is a small plastic particle with < 5 mm in size. There are two types of microplastic based on their sources, primary microplastic (original small plastics produced by the plastic factory) and secondary microplastic (small plastic particles as a result of larger plastic degradation). Due to its tiny size, this plastic particle can be easily distributed in all types of natural environments, including water and sediment. The particle also finds its way to enter the biota body through ingestion and respiration.

In the seagrass habitat, microplastics have also contaminated seagrass sediment. The common types of microplastic shape found in seagrass sediment are fibers, fragments, film, and foam. The particles are also potentially ingested by seagrass-associated fauna. Studies reported that seagrass can be a vector for microplastic transport to herbivore fauna and invertebrate.

Compared to macroplastics, the impact of microplastics on seagrass vegetation is more specific to the seagrass plant and sediment itself, before affecting the entire vegetation. For instance, a laboratory study by Menicagli et al., (2022) showed that microplastics in the sediment of Cymodocea nodosa affect seagrass growth development. Seagrass was observed under stress conditions by showing signs of degeneration of the root system, an increase in leaf loss, and a reduction in photochemical efficiency. Through this experiment, microplastics can reduce the ability of seagrass nutrient uptake from roots and disrupt seagrass metabolism processes. This highlights the harm of microplastic pollution to seagrass plants which will affect the whole vegetation.

           Although marine plastic is not the main disturbances that distructing tropical seagrass ecosystem, its impact is similarly significant to other stressors. Therefore, every individual should really start to take action to reduce the input of plastic waste to our marine environment. The government should take a serious stance on marine plastic waste issue by issuing a special regulation about marine debris management on seagrass vegetation in both coserved and unconserved marine areas. This would eventually improve Indonesia’s biodiversity and vital ecosystem service, especially in the marine environment.