Short‐term response of an abyssal macrofaunal community to a simulated phytodetrital pulse in the Cabo Verde Abyssal Basin, Northeast Tropical Atlantic Ocean

Abyssal ecosystems comprise more than 50% of the Earth's area and constitute an important reservoir in the global carbon cycle. With ocean productivity expected to decrease due to global warming, these ecosystems could face significant impacts in the coming decades. Benthic macrofauna are a key component of the seafloor carbon and nitrogen cycles, but limited in situ measurements result in high global uncertainty on the rates of metazoan C assimilation at abyssal depths. We sampled the macrofaunal community at the Cabo Verde Abyssal Basin (CVAB), finding a higher abundance of macrofaunal organisms (polychaetes and crustaceans) compared to other abyssal basins of the Atlantic Ocean. We assessed their short-term response to a simulated phytodetrital pulse during a two-day tracer in situ experiment at 4200 m depth. 13C and 15N-labeled diatoms were used as a food source, and the uptake of these elements by the macrofaunal community was quantified. Results showed that surface deposit feeding polychaetes contributed the most to the biomass (75%) and C and N uptake (70% and 83%), revealing their importance to organic matter cycling in the abyss. Enrichment was modest in most macrofauna; however, the uptake of labeled diatoms by some organisms was detected after 48 h. Our findings suggest that CVAB might receive more pelagic input than other abyssal basins underlying oligotrophic regimes. This study establishes a baseline for both macrofauna characterization and benthic ecosystem functioning in abyssal sediments around Cabo Verde and underscores the crucial role of macrofaunal-sized benthic organisms in C uptake and assimilation within the tropical abyssal Atlantic.