Abyssal seafloor response to fresh phytodetrital input in three Areas of Particular Environmental Interest (APEIs) in the western Clarion-Clipperton Zone (CCZ)

The abyssal seafloor (3500-6000m) remains largely unexplored but with deep-sea mining imminent, anthropogenic impacts may soon reach abyssal communities. Thus, there is a growing need for baseline studies of biodiversity, ecosystem functioning, and connectivity in both potential mining and no-mining areas across the Clarion-Clipperton Zone (CCZ), a key target region for polymetallic nodule mining. In this study, in situ pulse-chase lander experiments were conducted for 1.5 days in three no-mining areas (called Areas of Particular Environmental Interest, or APEIs) in the western CCZ, a region with a seafloor particulate organic carbon (POC) flux gradient. Mean seafloor respiration, macrofaunal abundance, and biomass showed a decreasing trend from the more eutrophic APEI 7 to the more oligotrophic APEI 1, although this trend was not statistically significant (p = 0.18) most likely due to small samples sizes and high variability. Most (96%) of the ¹³C-labelled processed phytodetritus was respired within 1.5 days. Experimental uptake of phytodetritus by macrofauna and bacteria was detected but was lower than in the previously studied and more eutrophic eastern CCZ over similar time scales (1.5 d). Bacteria dominated the short-term (~1.5 d) uptake of organic carbon at the seafloor, yet macrofauna processed more organic carbon per unit biomass than previously found in the eastern CCZ. Our study provides important information on benthic ecosystem functioning in areas set aside from mining in the western CCZ and suggests high variability may occur in the rates of benthic C_org-cycling across the CCZ. We recommend that benthic ecosystem functions be explored across gradients of POC flux which may be a major environmental factor driving ecosystem dynamics in the CCZ.