Abstract
Abstract. Oxygen and nitrate availabilities impact the marine nitrogen cycle at a range of spatial and temporal scales. Here, we demonstrate the impact of denitrifying foraminifera on the nitrogen cycle at two oxygen and nitrate contrasting stations in a fjord environment (Gullmar Fjord, Sweden). Denitrification by benthic foraminifera was determined through the combination of specific density counting per microhabitat and specific nitrate respiration rates obtained through incubation experiments using N2O microsensors. Benthic nitrate removal was calculated from submillimeter chemical gradients extracted from 2D porewater images of the porewater nitrate concentration. These were acquired by combining the DET technique (diffusive equilibrium in thin film) with chemical colorimetry and hyperspectral imagery. Sediments with high nitrate concentrations in the porewater and oxygenated overlying water were dominated by the non-indigenous species (NIS) Nonionella sp. T1. Denitrification by this species could account for 50 %–100 % of the nitrate loss estimated from the nitrate gradients. In contrast sediments below hypoxic bottom waters had low inventories of porewater nitrate, and denitrifying foraminifera were rare. Their contribution to benthic nitrate removal was negligible (< 5 %). Our study showed that benthic foraminifera can be a major contributor to nitrogen mitigation in oxic coastal ecosystems and should be included in ecological and diagenetic models aiming to understand biogeochemical cycles coupled to nitrogen.
Highlights
Hypoxic water (i.e., [O2] < 63 μmol L−1; Diaz et al, 2008; Breitburg et al, 2018) occurs frequently in bottom waters of shallow coastal seas, due to remineralization of organic matter and water stratification
Several monitoring stations are located in the fjord: Släggö (65 m water depth), Björkholmen (70 m water depth) and Alsbäck (117 m water depth); the hydrographic and nutrient data were obtained from the Swedish Meteorological and Hydrological Institute’s (SMHI’s) publicly available database SHARK (SMHI, 2020)
The O2 respiration rate measured from the pool of Nonionella sp
Summary
Hypoxic water (i.e., [O2] < 63 μmol L−1; Diaz et al, 2008; Breitburg et al, 2018) occurs frequently in bottom waters of shallow coastal seas, due to remineralization of organic matter and water stratification. Hypoxia may have large ecological effects (Levin et al, 2009; Rabalais et al, 2010; Zhang et al, 2010), such as an increase in fauna mortality (Stachowitsch et al, 1984; Diaz et al, 2001). Certain microorganisms, e.g., bacteria and foraminifera, can perform denitrification by respiring nitrate (Risgaard-Petersen et al, 2006) and thereby survive in depleted oxygen environments. The effects of decreasing dissolved oxygen availability at spatial and temporal scales will impact biogeochemical cycles such as the nitrogen cycle (Childs et al, 2002; Kemp et al, 2005; Conley et al, 2007; Diaz et al, 2008; Neubacher et al, 2013; Breitburg et al, 2018).
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