| 1. |
- Bonaglia, Stefano, et al.
(författare)
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The fate of fixed nitrogen in marine sediments with low organic loading : an in situ study
- 2017
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Ingår i: Biogeosciences. - : Copernicus GmbH. - 1726-4170 .- 1726-4189. ; 14:2, s. 285-300
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Tidskriftsartikel (refereegranskat)abstract
- Over the last decades, the impact of human activities on the global nitrogen (N) cycle has drastically increased. Consequently, benthic N cycling has mainly been studied in anthropogenically impacted estuaries and coasts, while in oligotrophic systems its understanding is still scarce. Here we report on benthic solute fluxes and on rates of denitrification, anammox, and dissimilatory nitrate reduction to ammonium (DNRA) studied by in situ incubations with benthic chamber landers during two cruises to the Gulf of Bothnia (GOB), a cold, oligotrophic basin located in the northern part of the Baltic Sea. Rates of N burial were also inferred to investigate the fate of fixed N in these sediments. Most of the total dissolved fixed nitrogen (TDN) diffusing to the water column was composed of organic N. Average rates of dinitrogen (N-2) production by denitrification and anammox (range: 53-360 mu mol Nm(-2) day(-1)) were comparable to those from Arctic and subarctic sediments worldwide (range: 34-344 mu mol Nm(-2) day(-1)). Anammox accounted for 18-26% of the total N2 production. Absence of free hydrogen sulfide and low concentrations of dissolved iron in sediment pore water suggested that denitrification and DNRA were driven by organic matter oxidation rather than chemolithotrophy. DNRA was as important as denitrification at a shallow, coastal station situated in the northern Bothnian Bay. At this pristine and fully oxygenated site, ammonium regeneration through DNRA contributed more than one-third to the TDN efflux and accounted, on average, for 45% of total nitrate reduction. At the offshore stations, the proportion of DNRA in relation to denitrification was lower (0-16% of total nitrate reduction). Median value and range of benthic DNRA rates from the GOB were comparable to those from the southern and central eutrophic Baltic Sea and other temperate estuaries and coasts in Europe. Therefore, our results contrast with the view that DNRA is negligible in cold and well-oxygenated sediments with low organic carbon loading. However, the mechanisms behind the variability in DNRA rates between our sites were not resolved. The GOB sediments were a major source (237 kt yr(-1), which corresponds to 184% of the external N load) of fixed N to the water column through recycling mechanisms. To our knowledge, our study is the first to document the simultaneous contribution of denitrification, DNRA, anammox, and TDN recycling combined with in situ measurements.
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| 2. |
- Niemistö, Juha, et al.
(författare)
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Benthic fluxes of oxygen and inorganic nutrients in the archipelago of Gulf of Finland, Baltic Sea – Effects of sediment resuspension measured in situ
- 2018
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Ingår i: Journal of Sea Research. - : Elsevier BV. - 1385-1101. ; 135, s. 95-106
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Tidskriftsartikel (refereegranskat)abstract
- Benthic fluxes of oxygen and dissolved inorganic nutrients; phosphate (DIP), ammonium (NH4), nitrate+nitrite (NOx), and silicate (DSi); and the effects of resuspension on these were studied in situ with the Göteborg benthic landers in the Gulf of Finland archipelago, Baltic Sea. The benthic fluxes were examined at two shallow stations at depths of 7m and 20m in May and August 2014. Resuspension altered benthic fluxes of oxygen and nutrients in most of the experiments in August, but not in May, which was mainly due to weaker resuspension treatments in spring. Additionally, the benthic nutrient regeneration rates were higher and redox conditions lower in August when the water was warmer. In August, resuspension increased the benthic oxygen uptake by 33–35%, which was, in addition to stronger resuspension treatment, attributed to higher amounts of dissolved reduced substances in the sediment pore water in comparison to conditions in May. Adsorption onto newly formed iron oxyhydroxides could explain the uptake of DIP by the sediment at the 20m station and the lowering of the DSi efflux by 31% at the 7m station during resuspension in August. In addition, resuspension promoted nitrification, as indicated by increased NOx fluxes at both stations (by 30% and 27% at the 7m and 20m station, respectively) and a lowered NH4 flux (by 48%) at the 7m station. Predicted increases in the magnitude and frequency of resuspension will thus markedly affect the transport of phosphorus and silicon and the cycling of nitrogen in the shallow areas of the Gulf of Finland.
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| 3. |
- Nilsson, Madeleine, et al.
(författare)
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Organic carbon recycling in Baltic Sea sediments – An integrated estimate on the system scale based on in situ measurements
- 2019
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Ingår i: Marine Chemistry. - : Elsevier BV. - 0304-4203. ; 209, s. 81-93
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Tidskriftsartikel (refereegranskat)abstract
- In situ measured benthic fluxes of dissolved inorganic carbon (DIC), a proxy for organic carbon (OC) oxidation or recycling rates, are used together with burial rates based on measured sediment accumulation rates (SAR) and vertical distribution of OC in the sediment solid phase to construct a benthic OC budget for the Baltic Sea system. The large variability in recycling rates (4.3 ± 0.87–33 ± 17 mmol C m−2 d−1) and burial rates (1.2 ± 0.8–5.9 ± 1.8 mmol C m−2 d−1) between different sub-basins and between different depositional areas within the basins is accounted for in the budget. Our results indicate that sediments in the Baltic Sea have much higher recycling rates and lower burial rates of OC than previously found. The sediment budget calculations show that 22 ± 7.8 Tg C yr−1 of OC is recycled to the water column due to organic matter oxidation, while long term burial amounts to 1.0 ± 0.3 Tg C yr−1. For the Baltic Sea as a whole, 96% of the particulate OC (POC) deposited on the sea floor (23 ± 7.8 Tg C yr−1; the sum of recycling and burial) is recycled back to the water column. However, the burial efficiency (i.e. the fraction buried of the total deposition) shows large variability between the different basins (2.5–16%). The total benthic POC deposition is approximately 20% higher than the estimated POC source originating from primary production in the water column and riverine input. This difference is likely within the uncertainty range of our budget calculations, however it indicates that the POC sources might be underestimated. The results from this study enhance the understanding of OC delivery, deposition and cycling in the Baltic Sea, and help improving existing Baltic OC budgets.
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