LARGE CONTRIBUTION OF BENTHIC FORAMINIFERAL AND MACROFAUNAL BIOIRRIGATION TO COUPLING BETWEEN NITROGEN AND MANGANESE CYCLES IN A FJORD ENVIRONMENT

The Gullmar Fjord (Sweden) is known for its periods of seasonal hypoxic conditions in its deep basin, conditions which will likely become worse during climate change. The low oxygen conditions offer an opportunity to study the potential interactions between benthic foraminifera and cycling of different elements. We compared two stations, one oxic and one severely hypoxic, by analyzing the assemblages of living (Cell Tracker Green-labelled) benthic foraminifera and the surrounding geochemical conditions, through the novel techniques 2D-DET gels and Fluorescently Labelled Embedded Cores (FLEC). The benthic foraminifera respond quickly to physico-chemical changes and we demonstrated the impacts of hypoxia on the denitrifying foraminiferal distribution. Moreover, as the sediments were Mn-rich and inhabited by macrofauna, we investigated the potential impact of macrofaunal bioirrigation and if bioturbation could generate deep nitrification microenvironments which could allow the survival of foraminiferal denitrifying species. The 2D-DET gels results in high spatial resolution models with colorimetric measurements illustrating the micro heterogeneity of the sediment. This millimetric resolution enables maps of the nitrate - and manganese concentrations of interstitial waters in a sediment section. The same section is then kept in resin, allowing to observe in 2D: 1) macrofaunal burrows, 2) 2D Mn microdistribution by μXRF. Our study demonstrates that sediments with high nitrate concentrations in the porewater and oxygenated overlying water were dominated by Nonionella sp. T1. Denitrification by this species could account for 50 to 100% of the nitrate loss, estimated from the nitrate gradients. Sediments beneath the hypoxic bottom waters had low inventories of porewater nitrate, denitrifying foraminifera were rare and their contribution negligible. However, in this hypoxic station we observed a deep nitrate-rich microenvironment. This high Mn-rich station might be fuelled by the intense burrow ventilation. We used a numerical approach to evaluate the bioirrigational flux, and it could contribute up to 87 % of the total Mn_d flux. We highlight how fundamental it is to understand the foraminiferal ecology on a microscale to understand biogeochemical cycles and to improve paleoreconstructions.