5 research outputs found
Oxygenation of an anoxic fjord basin strongly stimulates benthic denitrification and DNRA
\ua9 2015 Springer International Publishing Switzerland Hypoxia hampers eutrophication reduction efforts by enabling high nutrient fluxes from sediment to bottom waters. Oxygenation of hypoxic water bodies is often proposed to reduce benthic ammonium and phosphate release. This study investigates the functional response of benthic nitrate-reducing processes to a long-term engineered oxygenation effort in a density-stratified fjord with euxinic bottom waters. Oxygenation was achieved by mixing surface water with deep, euxinic water, which increased oxygen and nitrate concentrations in the deep water column. The presence of nitrate instigated benthic nitrate reduction in the newly oxidized sediments by equally stimulating denitrification and dissimilatory nitrate reduction to ammonium (DNRA). DNRA and total nitrate reduction rates, as well as the contribution of DNRA to total nitrate reduction, decreased with increasing exposure time of the sediments to oxygen. The relative importance of DNRA as a nitrate sink was correlated to nitrate concentrations, with more nitrate being reduced to ammonium at higher bottom water nitrate concentrations. Overall, engineered oxygenation decreased the net efflux of dissolved inorganic nitrogen from the sediments by stimulating net nitrate removal through denitrification
Exceptionally high respiration rates in the reactive surface layer of sediments underlying oxygen-deficient bottom waters
Particle shuttling and oxidation capacity of sedimentary organic carbon on the Baltic Sea system scale
Continental margin sediments receive most of the particulate organic carbon (POC) deposited on the global seafloor, making them crucial locations in the carbon cycle. However, the complex environments in coastal oceans make it challenging to predict the fate of sedimentary organic carbon (OC) in these areas. Here we use data from 21 sites in the Baltic Sea, representing different biological and physiochemical regimes, to explore controls on sedimentary OC cycling. To this end, we combine in situ measured benthic fluxes of dissolved inorganic carbon (DIC; proxy for OC oxidation) with data on sediment properties. In the Gulf of Bothnia, low sedimentary OC oxidation capacities (yearly DIC flux divided by sedimentary POC inventory) were likely caused by a large fraction of terrestrial material in the POC pool, indicated by low sedimentary chlorophyll a content and high (> 10) carbon:nitrogen ratios. The highest OC oxidation capacities were measured at shallow, permanently oxic sites in the Baltic Proper, where bioturbation likely stimulates OC oxidation. The other sites in the Baltic Proper and all stations in the Gulf of Finland displayed increasing OC oxidation capacities with increasing normalised water depth (station depth divided by maximal depth in the basin). This pattern suggests that substantial quantities of POC are shuttled, through repeated cycles of resuspension-redeposition, from shallow erosion-transport (ET) areas to deep accumulation (A) areas. This interpretation was supported by decreasing sediment age and increasing sedimentary inventories of POC and chlorophyll a with normalised water depth. Our calculations indicate that particle shuttling redistributes almost half of the deposited export production from ET areas to A areas in the Baltic Proper, and that substantial amounts of terrestrial organic material are transported through particle shuttling to the deeper parts of the Gulf of Finland and Gulf of Bothnia. Depositional setting and POC origin can thus be central factors in predicting the distribution and fate of OC in coastal and shelf sediments
The fate of fixed nitrogen in marine sediments with low organic loading : an in situ study
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
Investigating hypoxia in aquatic environments: diverse approaches to addressing a complex phenomenon
In this paper we provide an overview of new
knowledge on oxygen depletion (hypoxia) and related phenomena
in aquatic systems resulting from the EU-FP7
project HYPOX (âIn situ monitoring of oxygen depletion in
hypoxic ecosystems of coastal and open seas, and landlocked
water bodiesâ, www.hypox.net). In view of the anticipated
oxygen loss in aquatic systems due to eutrophication and climate
change, HYPOX was set up to improve capacities to
monitor hypoxia as well as to understand its causes and consequences.
Temporal dynamics and spatial patterns of hypoxia were
analyzed in field studies in various aquatic environments, including
the Baltic Sea, the Black Sea, Scottish and Scandinavian
fjords, Ionian Sea lagoons and embayments, and Swiss
lakes. Examples of episodic and rapid (hours) occurrences of
hypoxia, as well as seasonal changes in bottom-water oxygenation
in stratified systems, are discussed. Geologically
driven hypoxia caused by gas seepage is demonstrated. Using
novel technologies, temporal and spatial patterns of watercolumn
oxygenation, from basin-scale seasonal patterns to
meter-scale sub-micromolar oxygen distributions, were resolved.
Existing multidecadal monitoring data were used to
demonstrate the imprint of climate change and eutrophication
on long-term oxygen distributions. Organic and inorganic
proxies were used to extend investigations on past oxygen
conditions to centennial and even longer timescales that
cannot be resolved by monitoring. The effects of hypoxia on
faunal communities and biogeochemical processes were also
addressed in the project. An investigation of benthic fauna is
presented as an example of hypoxia-devastated benthic communities
that slowly recover upon a reduction in eutrophication
in a system where naturally occurring hypoxia overlaps
with anthropogenic hypoxia. Biogeochemical investigations
reveal that oxygen intrusions have a strong effect on the microbially
mediated redox cycling of elements. Observations
and modeling studies of the sediments demonstrate the effect
of seasonally changing oxygen conditions on benthic mineralization
pathways and fluxes. Data quality and access are
crucial in hypoxia research. Technical issues are therefore
also addressed, including the availability of suitable sensor
technology to resolve the gradual changes in bottom-water
oxygen in marine systems that can be expected as a result of
climate change. Using cabled observatories as examples, we
show how the benefit of continuous oxygen monitoring can
be maximized by adopting proper quality control. Finally,
we discuss strategies for state-of-the-art data archiving and
dissemination in compliance with global standards, and how
ocean observations can contribute to global earth observation
attempts