Microplastics as a sedimentary component in reef systems: A case study from the Java Sea

Abstract

Microplastic pollution has been reported from coral reef systems all over the tropics. Exposure to microplastics has several negative impacts on coral health, such as bleaching, tissue necrosis, or an impairment of the coral's immune system. Despite this potential risk for reef systems, the controlling processes for microplastics dispersion and accumulation in reef sediments are still largely under-studied. Presented here is a study of microplastics (125 µm to 5 mm) distribution in two tropic atoll reef platforms in Kepulauan Seribu, Indonesia. Sediment samples were collected in different facies zones within the reef platform. Microplastics were concentrated using density floatation and characterized by light and scanning electron microscopy. Some particles were identified as polypropylene using micro-Fourier transform infrared spectroscopy. All recovered microplastics were classified as secondary microplastics, likely derived from marine and local sources, with fibres as the most abundant type. Microplastics are showing similar transport and accumulation behaviour as fine siliciclastic grains. The abundance of microplastic is controlled by the proximity to the source area of larger plastic debris and hydrodynamic processes. Microplastics are not only present in low energy environments but also high energy settings such as the reef crest. Processes that contribute to accumulation in reef sediments are biofouling, interlocking and the creation of compound grains. Microplastics are present in sediment close to the seafloor (0 to 3.5 cm) but also at depths between 3.5 cm and 7.0 cm. Microplastic particles from below 3.5 cm are unlikely to be remobilized under modal weather conditions in the studied equatorial reefs. Subtidal reef sediment therefore can be regarded as a permanent sink for microplastics. The study shows that microplastics in coral reef environments deserve careful consideration since microplastics pose an additional threat to corals and their ability as framework builders in reef systems.

Microplastics: impacts on corals and other reef organisms

“…Plastic particles that enter the oceans can become deposited in sediments, either directly due to their density or as a result of biofouling, and ingestion/egestion by biota (Figure 2). A wide range of concentrations have been reported for coral sediments [47,49,51,[55][56][57][58][59] (Table 1); however, due to the lack of standardised methods and units for reporting it is difficult to make direct comparisons. The predominant morphotype and polymer type vary between studies.…”

“…Sediments >3.5 cm depth act as permanent sinks as they are unlikely to be resuspended under modal sea conditions. Under extreme conditions, the sediments act as a source of MPs back to the local area [ 59 ]. The distribution of MPs within the coral reef system is influenced by the presence of habitat-forming species.…”

Microplastics in mangroves and coral reef ecosystems: a review

“…Fourier Transform Infrared (FTIR) spectroscopy came in, to provide a more detailed chemical characterization of microplastics in environmental samples, and were very successful for polar microplastics. Attenuated Total Reflection (ATR) FTIR spectroscopy, an advanced spectroscopic technique was found to be an effective detection technique for polymeric detection in microplastics, but their higher cost of detection hindered their application (Vidyasakar et al 2018;Utami et al 2021). In Micro-FTIR techniques, an optical microscope was combined with an FTIR spectroscopy, providing a good detection of even the smallest sizes of microplastics present in the samples (Daniel et al 2021).…”

“…Being non-selective feeders, corals consume smaller size organisms, such as zooplanktons, and gets fed by microplastic particles present in water due to their size similarities. These microplastics when ingested and accumulated within corals, cause severe tissue damage, affect their immune system, reduce their food intake, affect growth, cause necrosis and bleaching (Utami et al 2021). The ingestion and retention time of microplastics within marine organisms, greatly depends on the size of organisms, microplastic size and concentration of microplastics in the aqueous environment.…”

“…As reef dwellers within this environment have varied feeding mechanisms, the ingestion of microplastics varied based on their trophic levels, and experienced an induction in apoptosis. An analysis by Utami et al (2021) on detection of microplastics in reefs of Java sea, Indonesia, stated that secondary microplastics were the major sources for contamination in this region, during which biofouling of the microplastics increased the particle density to reach the sea floor and entangle within the carbonate sediments (Utami et al 2021).…”