Application of the non-invasive Aquatic Eddy Co-variance on complex cold-water benthic habitats

ATLAS work package 2 presentation at ATLAS 3rd General Assembly

There is increasing evidence that cold-water coral (CWC) communities are regions of intensified carbon cycling, but direct measurements of community respiration rates have been limited by the lack of appropriate flux measuring techniques. In fact, in hard substrates habitats traditional approaches, e.g., benthic chambers and microprofiling, are often compromised. The non-invasive Aquatic Eddy Co-variance (AEC) technique can integrate the contribution from complex, mixed communities on both soft and hard substrates. Previous proof-of-concept measurements on CWC habitats have shown that the AEC technique is a valuable tool for investigating local carbon turnover by CWC communities, provided that particular attention is given to the AEC deployment strategy, instrumental setup and data evaluation.
Within ATLAS, we expanded the knowledge base of earlier work to fine-tune our AEC systems for optimal operation in complex cold-water benthic habitats, with specific emphasis on the targeted deep-sea communities at the WP2 study sites: Rockall Bank, Davis Strait, and Condor Seamount (Azores). In particular, we have implemented dissolved oxygen (O2) optodes sensors, which offer improved handling and robustness over traditional Clark-type electrodes. To date, the ATLAS-optimized AEC system has been deployed in a variety of complex coastal and communities such as mussel reefs and sponge beds on submerged rocky outcrops, where benthic habitat complexities comparable to the WP2 sites occurred at shallower, less challenging, depths. This enabled us to fine-tune the deployment setup and AEC data processing to develop a strategy for subsequent deployments at the deeper ATLAS CWC sites.
During the R/V Pelagia 420 cruise AEC activities at the Rockall site focused on the Haas mound (536 m depth) and the smaller Oreo mound (745 m depth). The summit of Haas mound was characterized by dead CWC. Average O2 uptake rates ranged from 12  4 to 25  6 mmol m-2 d-1, reflecting two distinct benthic communities on coral rubble. AEC-based rates were found to be in agreement with both parallel box core incubations and previously reported uptake rates for coral rubble communities, e.g., at the Mingulay Reef Complex. In addition, the bottom roughness associated with the coral-rubble communities, quantified as the bottom roughness length scale (z0) was also consistent with previous assessments. The summit of Oreo mound was characterized by the occurrence of live CWC patches. Uptake rates were expectedly higher, reaching on average 116  38 mmol m-2 d-1. In comparison, the mean global O2 uptake rate for soft sediments at these depths is 2–3 mmol m-2 d-1.
Both technical advances and the added knowledge from the field activities will be instrumental for the upcoming July campaign at Condor Seamount. There we plan to target the main benthic communities at the summit of the seamount, which range from i) well-developed live CWC habitats to ii) transitional habitats on hard substrate, and iii) sandy/unconsolidated sediment. The planned measurements will provide a detailed insight into the carbon turnover rates at the seamount summit and will enable the quantification of its hydrodynamical drivers.