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Temporal variability of denitrification in estuarine sediments
Estuarine, Coastal and Shelf Science, 1991
rates and fluxes of nitrous oxide, nitrate, nitrite and ammonium were determined at two intertidal sites in the Tamar estuary (S.W. England). High sediment nitrate uptake rates were recorded throughout the year, whereas the nitrite and ammonium fluxes were positive (from sediment to water column), with the former resulting from nitrification. Nitrous oxide flux was also positive, being largely attributable to denitrification with some contribution from nitrification or nitrification-denitrification coupling. No relationship was apparent between denitrification rate and nitrate concentration in the overlying water, invalidating the notion that denitrification automatically regulates nitrate during periods of elevated ambient concentration. However, denitrification exhibited a strong covariance with the degree of sediment bioturbation iNereis diversicolor), which was considered to be attributable to increased transport and supply ofnitrate via Nereis burrows. Denitrification accounted for 8.5",, of the annual total nitrate loading to the Tamar estuary, although a maximum of loo",, was observed in summer when the phytoplankton nutrient requirement would also be highest.
Denitrification and oxygen consumption in sediments of two south Texas estuaries
Marine Ecology Progress Series, 1992
Spatial and temporal variations in rates of denitrification and oxygen consumption were measured in sediments of the Nueces and Guadalupe Estuaries in southern Texas, USA. Denitrification rates varied from 4.0 to 71 . l pm01 N2 m'2 h" in the Nueces Estuary and from 4.6 to 34.7 pm01 N, m-' h-' in the Guadalupe Estuary. Denitrification accounted for 29 to 80 'X of total benthic N flux in the study areas. Oxygen consun~ption rates ranged from 176 to 818 pm01 0, m-' h-' in Nueces Estuary and from 208 to 550 pm01 0, m-, h -' in Guadalupe Estuary. In both estuaries denitrification and oxygen consumption rates were generally higher in the upper estuaries where the porosity and organic matter contents of sediments were typ~cally highest. Among 3 measurements in different seasons, the lowest denitrification and oxygen consumption rates were usually observed during winter. Chemical oxygen consumption, as measured using formalin-killed controls, was about 50 % of the total oxygen consumption. A considerable portion (ca 50 %,) of biological oxygen consumption was attributed to nitrification in sediments. We estimated that carbon mineralization by denitrifiers was a s much a s 40 to 179 ' % of that by aerobic heterotrophs. In the Guadalupe Estuary, denitrification removed about 38 % of the measured inputs of organic and inorganic nitrogen. In the Nueces Estuary, the amount of nitrogen removed by denitrification was ca 2 times greater than the measured nitrogen inputs from the Nueces River and precipitation, suggesting that unmeasured anthropogenic inputs along the perimeter of the estuary were very important for maintaining nitrogen balance.
Resource limitation controls the base of food webs in many aquatic ecosystems. In coastal ecosystems, nitrogen (N) has been found to be the predominant limiting factor for primary producers. Due to the important role nitrogen plays in determining ecosystem function, understanding the processes that modulate its availability is critical. Shallow-water estuarine systems are highly heterogeneous. In temperate estuaries, multiple habitat types can exist in close proximity to one another, their distribution controlled primarily by physical energy, tidal elevation and geomorphology. Distinctions between these habitats such as rates of primary productivity and sediment characteristics likely affect material processing. We used membrane inlet mass spectrometry to measure changes in N 2 flux (referred to here as denitrification) in multiple shallow-water estuarine habitats through an annual cycle. We found significantly higher rates of denitrification (DNF) in structured habitats such as sub...
Aquatic Ecology, 2019
Intermittently closed and open lakes/lagoons (ICOLLs) can occur in alternate stable states: clear and turbid, with nitrogen inputs from highintensity agricultural land use often fuelling phytoplankton growth in ICOLLs. Due to their limited water exchange, ICOLLs are particularly susceptible to eutrophication. In these environments, denitrification may remove a substantial proportion of the land-derived nitrogen load, reducing their vulnerability to eutrophication; however, the factors that influence denitrification in ICOLLs are poorly understood. In this study, we addressed the relative importance of physico-chemical and biotic factors related to nitrate-saturated denitrification rates (including temperature, nutrient/ organic matter supply, oxygen conditions, sediment type and benthic macroinvertebrates) in two eutrophic ICOLL ecosystems: one supports some submerged macrophytes, while the other is in a persistent, turbid, phytoplankton-dominated system. Flexible in situ enclosures and denitrification enzyme assay measurements were employed to determine denitrification rates in response to new nitrate pulses, which are commonly observed in these systems. In situ denitrification rates were inhibited in both ICOLLs in winter, whereas in summer they were positively correlated with organic matter availability. Denitrification rates were greater in the shallow, marginal sediments of the ICOLLs. Bioturbating macrofauna significantly enhanced in situ sediment oxygenation and probably transported sediment organic carbon and nitrate simultaneously to sites of denitrification at the sediment oxic-anoxic interface. Our study found that nitrate-saturated sediment denitrification rates were controlled by a hierarchy of Handling Editor: Télesphore Sime-Ngando.
Aquatic Ecology, 1999
In this review of sediment denitrification in estuaries and coastal ecosystems, we examine current denitrification measurement methodologies and the dominant biogeochemical controls on denitrification rates in coastal sediments. Integrated estimates of denitrification in coastal ecosystems are confounded by methodological difficulties, a lack of systematic understanding of the effects of changing environmental conditions, and inadequate attention to spatial and temporal variability to provide both seasonal and annual rates. Recent improvements in measurement techniques involving 15 N techniques and direct N 2 concentration changes appear to provide realistic rates of sediment denitrification. Controlling factors in coastal systems include concentrations of water column NO − 3 , overall rates of sediment carbon metabolism, overlying water oxygen concentrations, the depth of oxygen penetration, and the presence/absence of aquatic vegetation and macrofauna. In systems experiencing environmental change, either degradation or improvement, the importance of denitrification can change. With the eutrophication of the Chesapeake Bay, the overall rates of denitrification relative to N loading terms have decreased, with factors such as loss of benthic habitat via anoxia and loss of submerged aquatic vegetation driving such effects.
Eyre_et_al-2013-Global_Biogeochemical_Cycles.pdf
1] N 2 flux rates (net denitrification) were measured over a diel cycle, seasonally, in 12 benthic habitats across three warm temperate Australian coastal systems. Dark N 2 -N fluxes were strongly controlled by sediment oxygen demand (SOD) across the 3 estuaries, 4 seasons, and 12 benthic habitats (r 2 = 0.743; p < 0.001; n = 142; slope = 0.0170). However, some of the slopes differed significantly between seasons and among estuaries and habitats, and all of the slopes were correlated with the δ 13 C values and C:N ratios of sediment organic matter. Ternary mixing diagrams with the contribution of algal, seagrass, and terrestrial/ mangrove material to sediment organic matter showed that habitats, seasons, and estuaries dominated by a mixture of seagrass and algal material had the lowest slopes, and slopes increase as habitats, seasons, and estuaries have an increasing contribution from terrestrial/ mangrove material. Overall, the slopes of dark N 2 fluxes versus SOD were low compared to previous studies, most likely due to either, or a combination of, the C:N ratio of the organic matter, the mixture of C:N ratios making up the organic matter, the structure of the organic matter, and/or the SOD rates. This study demonstrated that it is not only the quantity but also the type (quality), and maybe the mixture, of organic matter that is an important control on denitrification. As such, rapid global changes to detrital sources to coastal systems due to losses of mangrove, seagrasses, and saltmarshes, and associated increases in algae and macrophytes, are also expected to impact system level losses of nitrogen via denitrification.
Denitrification and the stoichiometry of nutrient regeneration in Waquoit Bay, Massachusetts
Estuaries, 2002
To determine the removal of regenerated nitrogen by estuarine sediments, we compared sediment N 2 fluxes to the stoichiometry of nutrient and O 2 fluxes in cores collected in the Childs River, Cape Cod, Massachusetts. The difference between the annual PO 4 3Ϫ (0.2 mol P m Ϫ2 yr Ϫ1) and NH 4 ؉ (1.6 mol N m Ϫ2 yr Ϫ1) flux and the Redfield N : P ratio of 16 suggested an annual deficit of 1.5 mol N m Ϫ2 yr Ϫ1. Denitrification predicted from O 2 : NH 4 ؉ flux ratios and measured as N 2 flux suggested a nitrogen sink of roughly the same magnitude (1.4 mol N m Ϫ2 yr Ϫ1). Denitrification accounted for low N : P ratios of benthic flux and removed 32-37% of nitrogen inputs entering the relatively highly nutrient loaded Childs River, despite a relatively brief residence time for freshwater in this system. Uptake of bottom water nitrate could only supply a fraction of the observed N 2 flux. Removal of regenerated nitrogen by denitrification in this system appears to vary seasonally. Denitrification efficiency was inversely correlated with oxygen and ammonium flux and was lowest in summer. We investigated the effect of organic matter on denitrification by simulating phytoplankton deposition to cores incubated in the lab and by deploying chambers on bare and macroaglae covered sediments in the field. Organic matter addition to sediments increased N 2 flux and did not alter denitrification efficiency. Increased N 2 flux co-varied with O 2 and NH 4 ؉ fluxes. N 2 flux (261 ؎ 60 mol m Ϫ2 h Ϫ1) was lower in chambers deployed on macroalgal beds than deployed on bare sediments (458 ؎ 70 mol m Ϫ2 h Ϫ1), and O 2 uptake rate was higher in chambers deployed on macroalgal beds (14.6 ؎ 2.2 mmol m Ϫ2 h Ϫ1) than on bare sediments (9.6 ؎ 1.5 mmol m Ϫ2 h Ϫ1). Macroalgal cover, which can retain nitrogen in the system, is a link between nutrient loading and denitrification. Decreased denitrification due to increasing macroalgal cover could create a positive feedback because decreasing denitrification would increase nitrogen availability and could increase macroalgae cover.
Marine Ecology Progress Series, 2000
Seasonal measurements of organic matter mineralisation by oxygen uptake and denitrification were carried out from July 1996 to March 1998 along a -200 km transect of the River Thames estuary, UK. There was a distinct gradient of decreasing rates of organic matter mineralisation seaward, which was related to the concentration of suspended solids and sedimentary organic carbon ( C ) at each site. There was clear seasonality and highest rates of oxygen uptake (10056 pm01 O2 m-2 h-l) at the muddy sites, but lower rates and non-temperature-dependent oxygen uptake at the sandier sites. Denitnfication, both that driven by nitrate from the overlying water (D,) and that coupled to nitrification in the sediment (D,), followed a sirmlar trend to oxygen uptake, from negliqble rates of approximately 1 1.lmol N m-' h-' for both D, and D, at the furthest offshore site, Site 12, to l 1 407 and 8209 pm01 N m-' h-', respectively, at the inner muddy Site 1. The Thames estuary is heterotrophic and a very efficient organic C filter, trapping and remineralising 77% of its organic C input. Attenuation of fluvial nitrate loads was regulated by freshwater flow. Minimal attenuation (3 %) occurred during peak flows (i.e. during periods of shortest freshwater flushing time) and >loo% attenuation during periods of lowest freshwater flow (longest flushing times). Including the sewage treatment works (STWs) nitrate load in this calculation reduced the degree of attenuation of the nitrate load to, on average, 11 %. Annual rates of D, and D, for an inner area of 125 kmz were 112 and 85 Mm01 N yr-l, respectively, with a total rate of 196 Mm01 N yr-' (2744 t), which was equivalent to 9% of the total dssolved inorganic nitrogen (DIN) load for 1995-96. A mean denitrification rate (D,) of 0.64 m01 N m-' yr-', based on measurements in 4 east coast estuaries, was used to estimate a total rate of denitrification for the entire area of UK east coast estuaries. The total rate of 0.81 Gm01 N yr-' represented 16% attenuation of the total fluvial discharge of nitrate (-6 Gm01 N yr-') to the UK's east coast estuaries (1995-96) and hence a 16% reduction in the UK nitrate load to the North Sea.