In situ benthic community response to a phytodetritus pulse in the Cabo Verde Abyssal Basin (tropical NE Atlantic)

Ecosystem functioning, i.e. the transfer of material through a system, supports the ecosystem services deep-sea sediments provide, including carbon sequestration, nutrient regeneration, and climate regulation. To date, seven studies globally have researched in situ how various benthic groups contribute to organic matter degradation in abyssal sediments through stable isotope tracer experiments, of which only one in the Atlantic (at the Porcupine Abyssal Plain or PAP). To expand the limited knowledge base on abyssal ecosystem functioning, we performed in situ stable isotope experiments in the Cabo Verde Abyssal Basin (CVAB, tropical North-East Atlantic). The Cabo Verde marine region is an oceanographically interesting region with complex currents, resulting in strong gradients of productivity and unique ecological characteristics. We conducted 2-day in situ incubations with organic substrate (lyophilised diatom culture) labelled with 13C and 15N stable isotopes through five benthic lander deployments to 4,200 m in an area presumed mesotrophic. We assessed sediment community oxygen consumption (SCOC), dissolved inorganic carbon (DI13C) production, nutrient fluxes, and label incorporation into bacteria, large Foraminifera (>300 μm), meiobenthos, and macrofauna. Results were specifically compared across the Atlantic basin to the eutrophic PAP for which all the same system components were reported (Witte et al. 2003). At CVAB, bacteria and meiobenthos dominated phytodetritus processing (91% and 8%, respectively), in contrast to PAP where macrofauna dominated (98%). Phytodetritus remineralisation was two to three times lower at CVAB compared to PAP, most likely due to the low abundance of fast responding macrofauna. However, overall phytodetritus processing efficiency at CVAB was four times greater compared to PAP. Our results support a mesotrophic regime at the CVAB lander site, and provide a unique first insight into ecosystem functioning of tropical (low-latitude) abyssal systems in the Atlantic Ocean. A better understanding of abyssal ecosystem functioning in various ocean regions, to which this study contributes, provides insight into main regulators of abyssal communities and thus may have implications for our understanding of abyssal systems under future climate scenarios.

We would like to express our sincere gratitude to the crew, UTM-CSIC and scientific team aboard the RV Sarmiento de Gamboa for their on-board assistance as well as during the preparation of the iMirabilis2 expedition. The ship time has been provided by the Spanish Ministry of Science and Innovation. The research included in this manuscript received funding from the European Union’s Horizon 2020 iAtlantic project (Grant Agreement No. 818123) awarded to C.O., A.K.S., A.F.B, and J.M.R. A.F.B. was additionally supported by FAPES grant N 85501689. This manuscript reflects the authors’ view alone, and the European Union cannot be held responsible for any use that may be made of the information contained herein. The funding sources had no role in decisions regarding the study design; in the collection, analysis and interpretation of data; in the writing of the report; nor in the decision to submit the article for publication. We would like to thank Tobias Hahn (GEOMAR Helmholtz Centre for Ocean Research Kiel) for advice on the optodes workflow, Dan Harries for advice on meiofauna processing, Dick van Oevelen and Peter van Breugel (Royal Netherlands Institute for Sea Research) for DIC and nutrient analysis, Barry Thornton (James Hutton Instititue) for PLFA and stable isotope analysis, and the Stable Isotope Facility (UC Davis, California, USA) for the stable isotope analysis.

Peer reviewed