MARINE PLASTIC DEBRIS IN INDONESIA: BASELINE ESTIMATES (2010-2019) AND MONITORING STRATEGY (2021-2025)

Indonesia set the mission to reduce marine plastic debris by 70% between 2018-2025 with a global significance to support the United Nations Sustainable Development Goal 14.1. This short communication assesses marine debris baseline estimates in Indonesia before 2020 from available contributions and provides recommendations for monitoring marine debris between 2021-2025. Widely ranging model estimates of plastic debris leakages into seas highlight the importance of data source, the spatial resolution of models and in situ data to provide representative baseline values. Model outputs, recognizing the strengths and uncertainties of available contributions, converge on a baseline value of 0.52 ± 0.36 million tons (Mt) per year prior to 2020 in Indonesia, setting a targeted reduced number of 0.16 Mt of marine debris leakages in 2025. The Indonesian Institute of Sciences showed a preliminary value of plastic debris accumulation in beaches at 113.58 ± 83.88 g/m monthly or equivalent to 0.40 Mt/year by assuming plastic debris is most pervasive within 3 meters from Indonesia’s 99,093 km-long coastlines. While river monitoring data informs land-based plastic debris leakages, stranded beach debris represents a fraction of debris not present in the water column and bottom sediments. Moving forward, monitoring initiatives to mitigate marine debris should leverage on nationwide municipality-level model estimates (e.g., the source to leakage route framework of the National Plastic Action Partnership) as well as in situ river and coastal particularly but not limited to sites co-identified in previous monitoring studies (i.e., Medan, Batam-Bintan, Padang, Jakarta-Seribu Islands, Semarang, Pontianak, Bali, Lombok, Makassar, Manado, Bitung). The latter should be conducted at least seasonally, considering evidence of monsoonal variations of marine debris releases and accumulation in Indonesia. Indonesia's vastness and regional diversity require coordination among stakeholders (government agencies, research institutions, universities, NGOs, citizen scientists) to monitor progress in the environments.


INTRODUCTION
The Indonesian government has set a national action plan to reduce plastic debris by 70% between 2018-2025 with a further ambition to achieve nearzero plastic pollution in Indonesia by 2040, highlighting the importance of measuring outcomes with respect to available baseline data. Without policy interventions and deliverable actions, business as usual scenario has projected increased annual plastic debris leakages into Indonesian seas by 30% from 0.62 to 0.78 million tons (Mt) between -2025(NPAP, 2020. Through the Presidential Decree Number 83/2018, Indonesia aims to set the projection back through community awareness, the intervention of plastic debris at river outlets and marine sources, import taxes, incentives for biodegradable plastics, and the conversion of plastic waste. The initiatives carry a global significance in achieving the UN Sustainable Development Goal 14.1 to reduce land-based plastic debris, considering Indonesia as a major contributor to marine plastic debris (e.g., Jambeck et al., 2015;Lebreton et al., 2017).
We provide an assessment of marine debris baseline estimates in Indonesia prior to 2020 by taking into account available studies and reports with their strengths and uncertainties, and recommendations for monitoring progress towards meeting the 70% reduction target in 2025.

AN ENSEMBLE OF MODEL-BASED ESTIMATES FOR MARINE PLASTIC LEAKAGES IN INDONESIA (2010-2020)
Widely ranging model-based estimates of plastic debris leakages into seas globally (1. 15-12.7 Mt/year) and in Indonesia (0.20-1.29 Mt/year) underline the roles of data source, the spatial resolution of models and in situ data to provide representative baseline values ( Figure 1). Jambeck et al. (2015) laid down a framework for estimating marine plastic debris leakages from mismanaged plastic waste by populations living within 50 km from the coast in 192 countries. While the study adopted the World Bank database reported in Hoornweg and Bhada-Tata (2012), a similar approach by Lebreton and Andradry (2019) combined the Waste Atlas (2006) database with high-resolution population data and yielded lower estimates of marine debris leakages. The earlier study ranked Indonesia as the second-largest contributor to marine plastic debris, while the latter as the ninth worldwide. The National Plastic Action Partnership (NPAP) applied the waste mass flow analysis using secondary data from 514 regencies and cities to capture regional diversity across Indonesia and provided estimates of marine debris leakages are within uncertainty ranges of the previously mentioned studies (NPAP, 2020). An assessment by the World Bank focused on 15 hotspot cities for releasing plastic debris into Indonesian seas in western and central Indonesia by combining modeling and field checks to gain a local understanding of waste management, societal behaviors and plastic debris in the environments (World Bank, 2018). It is worth highlighting the role of physical barriers in waterways in reducing plastic debris releases. By considering artificial barriers such as dams, Lebreton et al. (2017) showed significantly lower global and countryspecific values. The model assumption is in line with Cordova and Nurhati (2019) that showed significantly lower values using in situ river monitoring data in Jakarta Bay that may be attributed to the presence of net booms and river clean-ups. Recognizing their strengths and uncertainties (Table 1), the models show an ensemble mean value of 0.52 ± 0.36 Mt/year as a baseline for marine plastic debris leakages in Indonesia prior to 2020, therefore setting a targeted reduced number of 0.16 Mt of marine debris leakages in 2025. While other studies estimated global marine debris leakages (e.g., Eunomia, 2016;Schmidt et al., 2017;UNEP Ryberg et al., 2018), herein, we included only those with corresponding reported values from Indonesia.  Model using data on waste generated, waste composition, waste management systems, field analysis of waste disposal and capture, and in situ waste sampling in waterways and coastal areas.
LIPI -Cordova et al. (2019) 2018-2019 Beach sites (N=18 spread across Indonesia with Aceh as the westernmost and Biak as the easternmost sites), In situ measurement of stranded beach debris using a line transect alongside low tide line.
Based on field sampling in 18 beaches over the period 2018-2019, the Indonesian Institute of Sciences (LIPI) reported an average monthly plastic debris accumulation of 113.6 ± 84 g/m 2  or 0.40 Mt/year assuming that plastic debris is most pervasive within 3 meters from Indonesia's 99,093 km-long coastlines (BIG and Indonesian Navy, 2018). It is important to note that while river monitoring data informs land-to-sea plastic debris releases, stranded beach debris represents a fraction of debris that is not present in the water column and bottom sediments. The stranded beach debris data also showed that 47.6 ± 11.8% of the collected debris were buoyant plastic materials.

MARINE PLASTIC DEBRIS MONITORING STRATEGY (2021-2025)
Monitoring plastic debris in marine environments in the coming years shall meet the criteria of comparability with baseline estimate, representative spatial coverage, and ease of methods of existing frameworks. Monitoring efforts should employ nationwide municipalitylevel model estimates to quantify sources and transport, and in situ monitoring in the environments (e.g., rivers, coastal areas). The nationwide system model developed by NPAP enables to monitor plastic debris by municipalities and sectors following the system change scenario using local government data. The World Bank report referred river waterways as 'non-tidal zones' and coastal areas as 'tidal zones' underlines that in situ monitoring data could serve as ground truth for land-derived plastic waste releases and accumulation in the environments. In other words, in situ river and coastal monitoring data are not complimentary, but both are necessary considering the information that they provide related to model outputs. The ease of visual counting of macroplastic debris in the field makes it possible to scale up monitoring data through citizen science programs.

WHERE AND WHEN TO MONITOR
Future in situ river and coastal monitoring would benefit from comparison with previous monitoring efforts; however, this does not limit new monitoring sites to provide more representative spatial coverage (Figure 2). There were western and central sites surveyed by both the World Bank and LIPI report: Medan, Batam-Bintan, Padang, Jakarta-Seribu Islands, Semarang, Pontianak, Bali, Lombok, Makassar, Manado and Bitung. We highlight the need for including sites based on population density and small islands concentrated in eastern Indonesia.
In situ monitoring also would benefit from having a time continuity considering monsoonal variations of marine debris release and accumulation in Indonesia (e.g., Cordova and Nurhati, 2019;Kurniawan and Imron, 2019).

MONITORING METHODS
Currently, several protocols for in situ monitoring involve characterization of the collected debris (plastic, metal, glass, timber, etc.), plastic categories (plastic bottles, styrofoam, etc.), and estimation of plastics by abundance and weight such as the UNEP (Cheshire et al., 2009), OSPAR protocol (2010, NOAA (Lippiatt et al., 2013), and GESAMP (2019) protocols. These approaches may differ slightly in debris categories. An introduction of a new category that represents personal protective equipment may be useful with regard to the prolonged COVID-19 pandemic.
Besides manual collection techniques, the use of an unmanned aerial system (UAS) offers the collection of high-resolution, time-effective, costefficient monitoring of over larger sections of natural and built environments compared to manual surveys (Manfreda et al., 2018;Tmušić et al., 2020). Recent works showed the application of UAS to map debris of less than 2.5 cm using aerial images maps produced by drones (Bao et al., 2018;GESAMP, 2019;Andriolo et al., 2020). Furthermore, the UAS could identify hotspots and pathways of marine debris Gonçalves et al., 2020;Tmušić et al., 2020).

River Debris in situ Monitoring
Plastic debris monitoring has used networks of stationary stations such as bridges (e.g., Cordova and Nurhati, 2019;van Emmerik et al., 2019), floating booms (Gasperi et al., 2014) and possibly vessels, as well as active tracking methods (e.g., Ivar do Sul et al., 2014). For instance, Gasperi et al. (2014) used a network of 26 existing floating booms to collect river debris. The tracking methods may actively monitor debris moving through the river system using the global positioning system (GPS) and the release of plastic marked objects that can be recovered and identified. For example, Ivar do Sul et al. (2014) used painted plastic materials to study preferential flow routes, accumulation and removals of plastic debris in mangrove forests with changes in hydrometeorological conditions.

Monitoring Stranded Beach Debris
For stranded beach debris monitoring, the surveyed beaches should be consistent in length (i.e., no less than 100 m), setting (i.e., sandy or pebble for Indonesia), and not regularly cleaned. If the beach has been cleaned, it is advised to survey after two months or depending on the sampling frequency to allow debris accumulation. The pervasiveness and composition of plastic debris can be determined using at least triplicates of large quadrats of a certain size. A team of trained observers could classify, count and weight debris the collected debris from the quadrants.
Besides the typical debris classification by types, studies have also classified debris by countries of origin and pre-disposal categories for stranded beach debris. Lee et al. (2012) classified debris by the country of origin from information on the manufacturing country and printed language to identify foreign debris. Debris can also be classified by pre-disposal categories, including end-user products, packaging material, fishery products and industrial materials other than fisheries. End-user products are consumer goods thrown away after use. Packaging materials are products for containment, safety, handling and distribution of goods from manufacturers to customers (e.g., food wrappers, PET bottles). Because beaches accumulate large amounts of fishing-related debris, the categories can be isolated from other industrial items to identify attributing industries (Jang et al., 2014).