Contrasting particulate organic carbon and sediment trophic status in two Antarctic fjords: Hope Bay and Collins Bay

Abstract

Fjords of the Antarctic Peninsula are sensitive to climate change and important carbon sinks, due to their high sedimentation rates. We used several biogeochemical markers to establish sediment trophic status, origin, and sources of suspended and sedimentary organic carbon (OC) in two fjords, Hope Bay (HB) and Collins Bay (CB). Water samples at different depths and bottom sediments were collected in 2019/20 on board the R/V BAP Carrasco. Distinct glacier runoff settings between the two studied fjords were reflected in different physico-chemical water column conditions. Based on its isotopic and biopolymeric composition suspended particulate organic carbon (POC) in both HB and CB, can be considered fresh with high nutritional and energetic quality. Also, POC is derived from recent phytoplankton production, subjected to little bacterial degradation and zooplankton grazing. The predominance of muddy sediments and high total proteins (PRT) concentrations (2.88–3.04 mg g⁻¹) in CB, contrasting with sandy sediments and low PRT concentrations (1.37–1.55 mg g⁻¹) in HB, were consistent with a higher sedimentation rate in CB than in HB. The predominance of PRT-rich and fresh marine sedimentary OC in bottom sediments of the two fjords denoted the occurrence of an important benthic-pelagic coupling. Sedimentary C/N and δ¹³C indicated a main autochthonous marine origin of OC in bottom sediments of the two fjords. Biopolymeric carbon (BPC) concentrations showed that HB sediments are mesotrophic, whereas CB sediments are eutrophic, with a high organic load. Our results show the relevance of getting an integrated vision of both the pelagic and the benthic systems, for a better understanding of OC pathways and fate in Antarctic fjords undergoing rapid environmental changes linked to climate change.