Spatiotemporal characterization of sedimentary phytopigments in the southeastern Clarion-Clipperton Zone: Baseline time series and effects of a deep-sea mining test

Sedimentary phytopigments are indicators of organic matter quality and provide critical food for abyssal benthic communities, influencing their abundance, distribution, and diversity. As deep-sea mining emerges as a potential industry, studies addressing baseline conditions and effects of mining activities are crucial for understanding the resulting ecological risks and uncertainties. Here we examined the spatiotemporal variability of sedimentary phytopigments chlorophyll-a and pheophytin over a 3-year time series in the NORI-D contract area in the Clarion-Clipperton Zone of the central Pacific Ocean. Also investigated were the effects of a mining test on these parameters across a spectrum of disturbance zones including direct impact and varying degrees of sedimentation from the mining-generated plume. Baseline samples were collected in October–December 2020 and April–June 2021 from a collector test area (CTA) and preservation reference zone (PRZ), and from CTA in August–September 2022 immediately before a mining test in October 2022. Natural variations in sedimentary phytopigments were observed, with relative stability from 2020 to 2021, followed by a significant decrease in chlorophyll-a in the 2022 baseline samples, reflecting a temporal shift in organic matter quality. Chlorophyll-a concentrations were generally higher in the PRZ compared to the CTA, likely due to environmental heterogeneity. Baseline observations provided a reference for assessing changes in November–December 2022 following the mining test. Chlorophyll-a was diminished in the test-mining track, indicating sediment removal, while concentrations were enriched at a site 200 m away. These findings demonstrate the impact of deep-sea mining on sedimentary organic matter, with removal confined to the mining tracks and organic matter enrichment extending up to 200 m away. Although the abyssal seafloor is typically considered a stable environment, these results reflect fluctuations in organic matter input and quality, underscoring the importance of baseline time-series studies for distinguishing mining-induced changes from background fluctuations.