(a) Sample collection

Sediment cores were collected during the EcoResponse campaign (Assessing the Ecology, Connectivity and Resilience of Polymetallic Nodule Field Systems) aboard the German Research Vessel SONNE from 11 March 2015 to 30 March 2015. Four license areas across 1300 km were targeted (figure 1; BGR, Bundesanstalt für Geowissenschaften und Rohstoffe; IOM.C, InterOcean Metal-Control; GSR, Global Seabed Resources; IFREMER, Institut Français de Recherche pour l'Exploitation de la Mer) and one Area of Particular Environmental Interest (APEI3). The BGR area was sampled at two sub-regions; the ‘Reference area’ and ‘Prospective area’ (BGR.RA: limited mining, BGR.PA: intensive future mining). Samples were collected using the Octopus multicorer (inner diameter: 94 mm) at 23 stations (electronic supplementary material, table S1). Replicates of each area were retrieved from separate multicorer deployments (1 core/deployment, BGR.PA: n = 5, BGR.RA: n = 4, IOM.C: n = 2, GSR: n = 4, IFREMER: n = 5, APEI3: n = 3). The top 5 cm of each core were sliced and frozen in sealed plastic bags at −20°C. Meiofauna was extracted from half of the sediment into sterile Milli-Q water by density-gradient centrifugation (3 × 12 min, 1905 rcf) with the colloidal silica polymer Ludox HS-40 as a flotation medium (specific density 1.18 g cm⁻¹) [25]. Cores for environmental variables were sliced (0–1 cm and 1–5 cm) and stored at −20°C; a 1 ml was subsampled from the 0–1 cm layer and stored at −80°C for pigment analysis (chlorophyll-a, phaeopigments). Extraction was completed using 90% acetone and concentrations were measured fluorometrically using the Trilogy Laboratory Fluorometer at the Max Planck Institute for Marine Microbiology. Grain size analysis was completed using laser diffraction (Malvern Mastersizer Hydro 2000 G). Total organic carbon (TOC) and total nitrogen (TN) was quantified by combustion of freeze-dried samples with the Flash 2000 NC Sediment Analyser.