Factors influencing microplastic abundances in the sediments of a seagrass-dominated tropical atoll

Seagrass meadows are one of the world's most diverse ecosystems offering habitats for an extensive array of species, as well as serving as protectors of coral reefs and vital carbon sinks. Furthermore, they modify hydrodynamics by diminishing water flow velocities and enhancing sediment deposition, indicating the potential for microplastic accumulation in their sediments. The build-up of microplastics could potentially have ecological impacts threatening to ecosystems, however little is known about microplastic abundance and controlling factors in seagrass sediments. Here we investigated microplastic characteristics and abundances within sediments underlying four seagrass meadow sites on the Turneffe Atoll, Belize. Sediment cores were collected and sub-sampled to include a range of replicate surface sediments (0–4 cm) and depth cores (sediment depths 0–2, 2–5, 5–10, 10–20 and 20–30 cm). These were analysed using 25 μm resolution μFTIR, with spectral maps processed using siMPle software. Microplastics were prevalent across the sites with an abundance range (limit of detection (LOD) blank-corrected) of < LOD to 17137 microplastics kg−1dw found on the east side of the atoll. However, their abundances varied greatly between the replicate samples. Polyethylene and polypropylene were the most commonly detected polymers overall, although the dominant polymer type varied between sites. There were no differences in the abundance of microplastics between sites, nor could abundance distributions be explained by seagrass cover. However, abundances of microplastics were highest in sediments with lower proportions of fine grained particles (clay, <4 μm) suggesting that hydrodynamics override seagrass effects. Additionally, no patterns were seen between microplastic abundance and depth of sediment. This suggests that microplastic abundance and distribution in seagrass meadows may vary significantly depending on the specific geographical locations within those meadows, and that more complex hydrodynamic factors influence spatial variability at a localised scale.

海草床是世界上最多样化的生态系统之一，为众多物种提供栖息地，同时也是珊瑚礁的保护者和重要的碳汇。此外，它们通过降低水流速度和增加沉积物沉积来改变水动力，这表明其沉积物中可能会积累微塑料。微塑料的积累可能会对生态系统产生生态影响，然而，人们对海草沉积物中微塑料的含量及其控制因素知之甚少。在此，我们研究了伯利兹特内夫环礁四个海草床地点下方沉积物中的微塑料特征和含量。采集了沉积物岩芯并进行了子采样，包括一系列重复的表层沉积物（0 - 4厘米）和深度岩芯（沉积物深度0 - 2、2 - 5、5 - 10、10 - 20和20 - 30厘米）。使用25μm分辨率的μFTIR对这些样本进行分析，并使用siMPle软件处理光谱图。微塑料在各个地点普遍存在，在环礁东侧发现的含量范围（经检测限（LOD）空白校正）从低于检测限到17137个微塑料/千克干重。然而，重复样本之间的含量差异很大。总体而言，聚乙烯和聚丙烯是最常检测到的聚合物，尽管不同地点的主要聚合物类型有所不同。不同地点之间微塑料的含量没有差异，其含量分布也不能用海草覆盖来解释。然而，在细颗粒（黏土，<4μm）比例较低的沉积物中，微塑料的含量最高，这表明水动力作用超过了海草的影响。此外，微塑料含量与沉积物深度之间没有规律。这表明海草床中微塑料的含量和分布可能因床内的具体地理位置而有很大差异，并且更复杂的水动力因素会影响局部范围内的空间变异性。