Seafloor Oxygen Consumption of Polymetallic Nodule Ecosystems Measured using Aquatic Eddy Covariance

Polymetallic nodules form complex deep-sea habitats and are increasingly targeted for deep sea mining to source critical metals like nickel, cobalt, and manganese. To guide environmental assessments, the International Seabed Authority requires measurement of the benthic oxygen flux as a sensitive indicator of seafloor respiration and carbon cycling. The Eddy Covariance method potentially offers a powerful way to measure this flux in situ and non-invasively, but the method remains untested in such deep and oligotrophic settings.

The University of Southern Denmark in collaboration with The Metals Company performed the first measurements of benthic oxygen flux using Eddy Covariance within the NORI-D license area in the Clarion-Clipperton Zone at approximately 4250m depth.

The oxygen sensing module on the Eddy Covariance instruments consists of a PyroScience AquapHOx-TX Deep Sea Transmitter and a retractable ultra-high speed fibre oxygen mini-sensor. This setup proved to be robust and produced high quality data.

The extreme water transparency in the Clarion-Clipperton Zone proved challenging for acoustic velocimetry. Evaluation of performance included comparisons to fluxes measured using other more established methods such as benthic chambers and sediment microprofilers. Efforts are underway to make routine measurements of turbulence and benthic oxygen flux using Eddy Covariance in such environments possible.

Karl Attard
University of Southern Denmark

4/9/2025