Water Column NO3 Research Articles

Spatial and seasonal pattern of microbial nitrate reduction in coastal sediments in the Vistula River plume area, Gulf of Gdańsk

Estuaries can remove and/or retain land-derived nitrogen (N) and act as filters buffering N loads to the open sea. The N coastal filter can be seasonally variable depending on water temperature and transported loads, two factors acting in synergy and strongly influenced by climate change. The capacity of sediments to mitigate riverine N loads was investigated at four sites in the Vistula River plume area (Gulf of Gdańsk, Southern Baltic Sea). Samplings were carried out in two contrasting seasons: spring and summer, characterized by different water temperatures and nitrate (NO3-) levels. Inorganic N fluxes, and rates of denitrification and dissimilatory nitrate reduction to ammonium (DNRA) were measured in intact sediment cores by means of dark incubations and 15N-nitrate concentration-series experiments. Sampling sites were selected along a gradient of depth (5 to 24 m), that was also a gradient of sediment organic matter content. In both seasons, denitrification rates increased along with depth and from spring (6.5 ± 7.0 µmol m-2 h-1) to summer (20.4 ± 15.4 µmol m-2 h-1), despite lower NO3- concentrations in summer. In spring, at higher NO3- loading, denitrification was likely limited by low water temperature, and elevated sediment oxygen penetration. Coupled denitrification-nitrification prevailed over denitrification of water column NO3- across all sites and seasons, contributing to over 80% of the total denitrification. Notably, no anammox was detected at the sampling sites. DNRA exhibited low to undetectable rates in spring, especially at the shallowest sites. However, during summer, N recycling via DNRA increased and ranged from 0.7 to 14.9 µmol m-2 h-1. The denitrification efficiency (DE), calculated as the ratio between molecular nitrogen (N2) flux and dissolved inorganic N effluxes from sediments, ranged from 0 to 37% in spring, whereas in summer DE did not exceed 16%. Despite the dominance of denitrification over DNRA, the analyzed sediments acted as weak N buffers under in situ dark conditions. However, concentration-series experiments suggested high potential denitrification capacity, exceeding 400 µmol m-2 h-1, in response to short-term, large riverine inputs of NO3-.

Persistence of southern California giant kelp beds and alongshore variation in nutrient exposure driven by seasonal upwelling and internal waves

Kelp beds provide significant ecosystem services and socioeconomic benefits globally, and prominently in coastal zones of the California Current. Their distributions and abundance, however, vary greatly over space and time. Here, we describe long-term patterns of Giant Kelp (Macrocystis pyrifera) sea surface canopy area off the coast of San Diego County from 1983 through 2019 along with recent patterns of water column nitrate (NO3-) exposure inferred from in situ temperature data in 2014 and 2015 at sites spanning 30 km of the coastline near San Diego California, USA. Site-specific patterns of kelp persistence and resilience were associated with ocean and climate dynamics, with total sea surface kelp canopy area varying approximately 33-fold over the almost 4 decades (min 0.34 km2 in 1984; max 11.25 km2 in 2008, median 4.79 km2). Site-normalized canopy areas showed that recent kelp persistence since 2014 was greater at Point Loma and La Jolla, the largest kelp beds off California, than at the much smaller kelp bed off Cardiff. NO3- exposure was estimated from an 11-month time series of in situ water column temperature collected in 2014 and 2015 at 4 kelp beds, using a relationship between temperature and NO3- concentration previously established for the region. The vertical position of the 14.5°C isotherm, an indicator of the main thermocline and nutricline, varied across the entire water column at semidiurnal to seasonal frequencies. We use a novel means of quantifying estimated water column NO3- exposure integrated through time (mol-days m-2) adapted from degree days approaches commonly used to characterize thermal exposures. Water column integrated NO3- exposure binned by quarters of the time series showed strong seasonal differences with highest exposure in Mar - May 2015, lowest exposure in Sep - Dec 2014, with consistently highest exposure off Point Loma. The water column integrated NO3- signal was filtered to provide estimates of the contribution to total nitrate exposure from high frequency variability (ƒ >= 1 cycle 30 hr-1) associated predominantly with internal waves, and low frequency variability driven predominantly by seasonal upwelling. While seasonal upwelling accounted for > 90% of NO3- exposure across the full year, during warm periods when seasonal upwelling was reduced or absent and NO3- exposure was low overall, the proportion due to internal waves increased markedly to 84 to 100% of the site-specific total exposure. The high frequency variability associated with internal waves may supply critical nutrient availability during anomalously warm periods. Overall, these analyses support a hypothesis that differences in NO3- exposure among sites due to seasonal upwelling and higher frequency internal wave forcing contribute to spatial patterns in Giant Kelp persistence in southern California. The study period includes anomalously warm surface conditions and the marine heatwave associated with the “Pacific Warm Blob” superimposed on the seasonal thermal signal and corresponding to the onset of a multi-year decline in kelp canopy area and marked differences in kelp persistence among sites. Our analysis suggests that, particularly during periods of warm surface conditions, variation in NO3- exposure associated with processes occurring at higher frequencies, including internal waves can be a significant source of NO3- exposure to kelp beds in this region. The patterns described here also offer a view of the potential roles of seasonal and higher frequency nutrient dynamics for Giant Kelp persistence in southern California under continuing ocean surface warming and increasing frequency and intensity of marine heatwaves.

The Dual Isotopic Composition of Nitrate in the Gulf of Mexico and Florida Straits

AbstractThe Gulf of Mexico receives considerable riverine nutrient inputs that contribute to eutrophication of northern shelf waters. However, it is unclear whether a significant fraction of these riverine nutrients delivered to the Gulf of Mexico are subsequently exported from the Gulf, potentially influencing the chemistry of the North Atlantic. Here, we report the first dual isotopic composition (“δ15N” and “δ18O”) measurements of nitrate (NO3−) from three profiles collected in different years in the Gulf of Mexico, as well as a new profile from the Florida Straits. We compare these new measurements with prior measurements from the North and South Atlantic, as well as with the magnitude of riverine and other nitrogen fluxes to the Gulf of Mexico. We find that in the upper thermocline, NO3− δ15N measurements from the Gulf of Mexico are intermediate between prior measurements in the Sargasso Sea (lower in NO3− δ15N), and those from the South Atlantic (higher in NO3− δ15N), but are indistinguishable from Florida Straits and Sargasso Sea profiles on deeper isopycnal surfaces. There is little evidence for riverine NO3− influencing the isotopic composition of NO3− exported from the Gulf of Mexico, which, based on its δ15N, would be expected to raise the δ15N of shallow NO3−. Lesser nitrogen fluxes are too small to significantly influence the isotopic composition of water column NO3− in the Gulf of Mexico. These results are consistent with recent modeling work suggesting that the majority of riverine nutrients are retained in the nearshore region.

Quantifying denitrification following floodplain restoration via the two-stage ditch in an agricultural watershed

In agricultural streams, constructed floodplains have been shown to expand bioreactive surface area and enhance nitrate (NO3−-N) removal via microbial denitrification, thereby reducing export to downstream ecosystems. At the Shatto Ditch Watershed (Indiana, USA), 0.6 km of two-stage ditch was constructed at the watershed outlet in 2007, while an additional 3.7 and 2.7 km were constructed in 2017 and 2018, respectively. We quantified denitrification following floodplain construction using experimental incubations of stream sediments and floodplain soils from Spring 2017 to Spring 2019. We measured dissolved dinitrogen gas (N2) concentrations using membrane inlet mass spectrometry (MIMS) and found that denitrification on unconsolidated stream sediments was minimally impacted by stream dredging during two-stage construction in 2017 and 2018. In contrast, even one year after the 2017 construction, denitrification on floodplain soils fell below detection limits; we finally observed measurable rates in Spring 2019. As predicted, stream sediment denitrification was driven by water column NO3−-N concentration, while organic matter limited denitrification on floodplain soils. Floodplains constructed via the two-stage ditch can increase NO3−-N removal by >20% in agricultural watersheds, representing an effective ecological engineering solution to combat downstream eutrophication. However, our results also show that denitrification on floodplain soils may take longer to recover from construction-related activities as organic matter builds up more slowly over time. As such, lags in the recovery or enhancement of ecosystem function are important to consider when documenting restoration success.

Seasonal and spatial controls on N2O concentrations and emissions in low-nitrogen estuaries: Evidence from three tropical systems

Nitrous oxide (N2O) production and emissions are observed in estuary waters, yet little is known about estuary N2O fluxes under low dissolved inorganic nitrogen (DIN) conditions. We present high-resolution spatial surveys of N2O concentrations in three low-DIN (NO3− < 30 μmol L−1) tropical estuaries in Queensland, Australia (Johnstone River, Fitzroy River, Constant Creek), during consecutive wet and dry seasons. Constant Creek had the lowest concentrations of dissolved inorganic nitrogen (DIN; 0.01 to 5.4 μmol L−1 of NO3− and 0.09 to 13.6 μmol L−1 of NH4+) and N2O (93–132% saturation), and associated low mean N2O emissions (0.02–3 μmol m−2 d−1). The Johnstone River Estuary had the highest N2O concentrations (97–245% saturation), with mean emissions of 4 to 6 μmol m−2 d−1, driven by upstream groundwater and riverine sources. In the Fitzroy River Estuary, N2O concentrations (100–204% saturation) and emissions (1–2 μmol m−2 d−1) were associated with wastewater inputs, which had a larger effect during the dry season. Estuary freshwater flushing time was an important factor: when freshwater was relatively stagnant, in-situ N2O emissions were larger than riverine N2O inputs. Conversely, fast freshwater flushing times diminished the role of in-situ emissions, and dissolved N2O largely flushed through the estuary directly to the ocean. Overall N2O emissions from the three tropical estuaries were low compared to previous studies, but were reasonably predicted by DIN concentrations: at low water column NO3− concentrations (i.e. <5 μmol L−1) estuaries can exhibit negative water-air N2O fluxes.

Regulation of Nitrate Uptake by the Seagrass Zostera marina During Upwelling

Prolonged nitrogen (N) fertilization can impact seagrass survival and productivity; however, the effects of N enrichment pulses (e.g., upwelling or sediment resuspension) remain poorly understood. This study examined the effects of short-term (1 h) pulsing of nitrate (NO3−) enrichment, simulating an upwelling event, on dissolved inorganic carbon (DIC) and NO3− uptake capacities, critical in controlling eelgrass productivity. Zostera marina dominates submerged vegetation in coastal lagoons influenced by upwelling in the California Current system. Laboratory incubations were conducted in winter (non-upwelling) and spring (upwelling) with shoots collected from San Quintin Bay meadows, Baja California, Mexico, differentially exposed to upwelled NO3−. Results suggest that NO3− enrichment stimulated DIC and NO3− uptake in winter, reflecting the close relationship between carbon metabolism and NO3− assimilation. Eelgrass shoots showed reduced NO3− incorporation in spring; neither NO3− uptake nor photosynthesis increased when exposed to high NO3−. Saturation of spring shoots at lower ambient NO3− concentrations may be interpreted as a physiological strategy to restrict metabolically costly NO3− incorporation during upwelling; this regulation of NO3− uptake strongly contrasts to the apparently full exploitation of this nutrient by seaweeds also dominant within the bay, as indicated in previous works. Despite their reduced NO3− uptake, eelgrass meadows near the bay mouth acquire NO3− at rates up to 4.2 mmol N m−2 day−1. This represents non-trivial water column NO3− removal compared to the estimated oceanic NO3− supply (~ 7.1 mmol m−2 day−1) during upwelling, highlighting the importance of Z. marina beds in controlling the lagoonal N-budget.

Tracing nitrate sources using the isotopic composition of skeletal-bound organic matter from the calcareous green algae Halimeda

The nitrogen (N) isotopic composition (δ15N) of water column nitrate (NO3−) reveals important information about the source(s) of N, and/or the transformation processes occurring during N transport, to a particular environment. However the physical collection of samples for δ15N–NO3− determination can be logistically challenging and does not address questions about historical δ15N–NO3−. Here we adapt the persulphate/denitrifier protocol, previously developed for scleractinian corals, to analyse the δ15N of organic material in the skeletons of the calcareous macroalgae Halimeda (hereafter referred to as δ15N-SOM). Using the newly established protocol, we show that the Halimeda δ15N-SOM can be used as a proxy for the δ15N of water column NO3−. Based on aquarium and field measurements we show that the maximum offset between the Halimeda δ15N-SOM and the δ15N of water column NO3−, as a result of fractionation during uptake, is 1‰. However this was only tested at low NO3− concentrations (< 10 μmol L−1). Our results demonstrate that in low-nutrient environments, Halimeda skeletal material, including historical material obtained from sediment cores, can provide a reliable proxy record of δ15N–NO3− at the base of the food web. This technique holds much promise for reconstructing spatial and temporal patterns of N supply to coral reef systems worldwide.

Nitrification and denitrification in estuarine sediments with tube-dwelling benthic animals

Burrowing benthic animals belonging to the same functional group may produce species-specific effects on microbially mediated nitrogen (N) processes depending upon different ecological traits. We investigated the effects of two tube-dwelling organisms, amphipods (Corophium insidiosum) and chironomid larvae (Chironomus plumosus), on benthic N cycling in bioturbated estuarine sediments. Aims of this work were to analyze the interactions among burrowers and N-related microbial processes in two distinct sedimentary environments colonized by benthic animals with different ecological traits. We hypothesized higher rates of nitrification and higher coupled nitrification–denitrification in sediments with C. insidiosum due to continuous ventilation rates. We expected higher denitrification of water column nitrate in sediments with C. plumosus due to lower and intermittent ventilation activity and lower oxygen levels in burrows. To this purpose, we combined process–specific (nitrification and denitrification) with net N flux measurements in intact and reconstructed sediments. Sediments with C. insidiosum had higher rates of oxygen demand and of potential nitrification and higher concentration of pore water NH4+ as compared to sediments with C. plumosus. Sediments with both species displayed comparable net N2 fluxes, mostly sustained by respiration of water column NO3− in sediments with chironomid larvae and by NO3− produced within sediments in sediments with corophiid amphipods. Corophium insidiosum stimulated nitrification nearly 15-fold more as compared to C. plumosus. Overall, our results demonstrate that sediments with burrowing fauna may display similar rates of denitrification, but underlying mechanisms may deeply vary and be species-specific.

Microphytobenthos and chironomid larvae attenuate nutrient recycling in shallow‐water sediments

Abstract In shallow‐water sediments, the combined action of microphytobenthos and bioturbating fauna may differentially affect benthic nutrient fluxes and exert a bottom‐up control of pelagic primary production. In many cases, the effects of microphytobenthos and macrofauna on nutrient cycling were studied separately, ignoring potential synergistic effects. We measured the combined effects of microphytobenthos and chironomid larvae on sediment–water fluxes of gas (O2, TCO2 and N2) and nutrients (NH4+, NO3−, NO2−, and SiO2) in shallow‐water sediments of a hypertrophic freshwater lagoon. Fluxes were measured in the light and in the dark in reconstructed sediments with low (L = 600 ind/m2), high (H = 1,800 ind/m2) and no (C) addition of chironomid larvae, after 3 weeks of pre‐incubation under light/dark regime to allow for microalgal growth. Besides flux measurements, pore water nutrient (NH4+, and SiO2) and dissolved metal concentrations (Fe2+ and Mn2+) were analysed and diffusive fluxes were calculated. Chironomid larvae increased sediment heterotrophy, by augmenting benthic O2 demand and TCO2 and N2 dark production. However, on a daily basis, treatments C and L were net O2 producing and N2 sinks while treatment H was net O2 consuming and N2 producing. All treatments were net C sink regardless of chironomid density. Microphytobenthos always affected benthic nutrient exchange, as significantly higher uptake or lower efflux was measured in the light compared with dark incubations. Theoretical inorganic N, P and Si demand by benthic microalgae largely exceeded both dark effluxes of NH4+, and SiO2 and their net uptake in the light, suggesting the relevance of N‐fixation, water column NO3− and solid‐phase associated P and Si as nutrient sources to benthic algae. Chironomid larvae had a minor effect on inorganic N and P fluxes while they significantly stimulated inorganic Si regeneration. Their bioturbation activity significantly altered pore water chemistry, with a major reduction in nutrient (highest for NH4+ and lowest for SiO2) and metal concentration. Underlying mechanisms are combinations of burrow ventilation and bioirrigation with stimulation of element‐specific processes as coupled nitrification–denitrification, co‐precipitation and inhibition of anaerobic paths such as Fe3+ or Mn4+ reduction or re‐oxidation of their end products. The combined activity of benthic algae and chironomid larvae may significantly attenuate internal nutrient recycling in shallow eutrophic ecosystems, and contribute to the control of pelagic primary production.

Tracing Nitrogen Biogeochemistry During the Beginning of a Spring Phytoplankton Bloom in the Yellow Sea Using Coupled Nitrate Nitrogen and Oxygen Isotope Ratios

AbstractNitrogen biogeochemistry during the beginning of a spring phytoplankton bloom in the Yellow Sea was investigated based on nutrient concentrations, benthic fluxes of nutrients, the nitrogen and oxygen isotope composition of NO3−, and the hydrological conditions. The δ15N and δ18O values for NO3− in the Yellow Sea were more variable in surface waters than in near‐bottom waters and were generally 6.3–8.2‰ for δ15N and 6.2–9.7‰ for δ18O, except in the Yellow Sea Coastal Current Water (YSCCW), where the maximum δ15N and δ18O values found were 13.2‰ and 18.8‰, respectively. Both the δ15N and δ18O values varied among the different water masses with higher values in the YSCCW than in the Yellow Sea Mixed Water and the Yellow Sea Warm Current Water. Higher δ18O relative to δ15N in NO3− in the Yellow Sea reflects contributions from multiple NO3− sources, including nitrification and atmospheric deposition with the later accounting for 71% of total external NO3− input. A simple nitrogen budget indicated the importance of nutrient regeneration from sediments as a source for water column NO3− representing 74% of total NO3− input in the YS.

Eelgrass meadows, Zostera marina (L.), facilitate the ecosystem service of nitrogen removal during simulated nutrient pulses in Shinnecock Bay, New York, USA

Seagrass meadows are important sites of nitrogen (N) transformations in estuaries, however, the role of N loading in driving relative rates of N fixation and denitrification in seagrass habitats is unclear. The current study quantified N fluxes in eelgrass meadows (Zostera marina (L.)) and nearby unvegetated sand in trials representing in situ and N enriched conditions. Net N2 fluxes were low or negative under in situ conditions in both eelgrass and sand. Under N enriched conditions, denitrification was higher than N-fixation, and denitrification in eelgrass was significantly higher than sand. Denitrification of water column NO3− was more significant than coupled nitrification-denitrification in the eelgrass. Denitrification was likely supported by greater organic carbon and N within the eelgrass sediment compared to sand. Eelgrass meadows in Shinnecock Bay may facilitate the ecosystem service of N removal and retention during short-term nutrient pulses that can originate from groundwater discharge and stormwater runoff.

Solute transport into the Jiulong River estuary via pore water exchange and submarine groundwater discharge: New insights from 224Ra/228Th disequilibrium

Pore water exchange (PEX) and submarine groundwater discharge (SGD) represent two mechanisms for solute transport from the seabed into the coastal ocean. However, their relative importance remains to be assessed. In this study, we pursued the recently developed 224Ra/228Th disequilibrium approach to quantify PEX fluxes of 224Ra into the Jiulong River estuary, China. By constructing a full mass balance of water column 224Ra, we were allowed to put various source terms, i.e., SGD, diffusive and advective pore water flow (PEX), and river input in a single context. This led to the first quantitative assessment of the relative importance of PEX vs. SGD in the delivery of solutes into an estuary. We carried out two surveys in the Jiulong River estuary: one in January 2014 (winter survey), the other in August 2014 (summer survey). By virtue of a 1-D mass balance model of 224Ra in the sediment column, we demonstrated that PEX fluxes of 224Ra were highly variable, both temporally and spatially, and can change by 1–2 orders of magnitude in our study area. Moreover, we identified a strong correlation between 224Ra-based irrigation rate and 234Th-based sediment mixing rate. Our results highlighted irrigation as the predominant PEX process for solute transfer across the sediment–water interface.Total PEX flux of 224Ra (in 1010dpmd−1) into the Jiulong River estuary was estimated to be 22.3±3.0 and 33.7±5.5 during the winter and summer surveys, respectively. In comparison, total SGD flux of 224Ra (in 1010dpmd−1) was 11.3±8.6 and 49.5±16.3 in the respective seasons. By multiplying the PEX fluxes of 224Ra by the ratio of the concentration gradients of component/224Ra at the sediment–water interface, we quantified the total PEX fluxes of dissolved inorganic carbon (DIC) and nutrients (NH4+, NO3−, and H4SiO4) into the Jiulong River estuary. In the meantime, net export of DIC and nutrients via SGD were estimated by multiplying the SGD fluxes of 224Ra by the DIC (nutrients)/224Ra ratios in the SGD end-members around this area. Our results revealed that PEX-driven fluxes of solutes rival net SGD input and river input in an estuary. An additional new finding is that water column NO3− in the surface estuary was effectively sequestered due to SGD, probably as a result of intense denitrification occurring in the anoxic subterranean estuary.

Using 224Ra/228Th disequilibrium to quantify benthic fluxes of dissolved inorganic carbon and nutrients into the Pearl River Estuary

The 224Ra/228Th disequilibrium that was recently observed in coastal sediments has been proven to be an excellent proxy for tracing the benthic processes that regulate solute transfer across the sediment–water interface. In order to better utilize this proxy, there is a need to understand the reaction kinetics of 224Ra in sediments. In this study, depth profiles of 224Ra and 228Th in bulk sediments were collected along a transect in the Pearl River Estuary (PRE). Together with bulk sediment measurements, dissolved 224Ra, dissolved inorganic carbon (DIC), and nutrients (NO2−+NO3−, NH4+) in pore water and in the overlying waters were also determined. A marked deficit of 224Ra with respect to 228Th with large spatial variations was observed in the PRE sediments. By use of a diagenetic model for the distributions of dissolved and adsorbed 224Ra in sediments, we infer that adsorption removes 224Ra from aqueous phase at a rate of 0.1±1.1–2000±400d−1. In addition, adsorption of 224Ra exhibits a rate sequence of oxic freshwater>anoxic freshwater>anoxic brackish water, probably reflecting the effect of the redox conditions and ionic strength on the adsorption–desorption kinetics of 224Ra.Benthic fluxes of 224Ra were estimated from the observed deficit of 224Ra in the sediments using a one-dimensional (1D) mass balance exchange model. We demonstrated that irrigation was the predominant process that controls solute transfer across the sediment–water interface, whereas molecular diffusion and sediment mixing together contributed <5% of the total 224Ra fluxes from bottom sediments. We then utilized the 224Ra/228Th disequilibrium approach to quantify the benthic fluxes of DIC and nutrients. We showed that sediment interstitial waters delivered approximately 42±6×109mol of DIC and ∼16±1×109mol of NH4+ into the PRE in the dry season. In contrast, it removed about 13±1×109mol of NO3− from the overlying water column. The benthic flux of DIC is equivalent to ∼18% of the riverine input in this season. In terms of nutrients, our results suggest that bottom sediments are a major sink of water column NO3−, and are a predominant source of NH4+ in the PRE. Overall, this study indicates that irrigation is an important process and must be considered in the mass balance of DIC and nutrients in estuaries.

Nutrient dynamics in the western Canadian Arctic. I. New production in spring inferred from nutrient draw-down in the Cape Bathurst Polynya

Ice-retreat from the Cape Bathurst Polynya occurred in late May and early June 2004, and was quickly followed by a decrease in dissolved nutrient concentration at the sea sur- face. Concentration plots of NO3 � versus PO4 3� , and Si(OH)4 versus NO3 � showed the ratios of nutrient uptake in the surface layer were in proportions expected for diatom growth (N:P 13.1:1; Si:N 1.8:1) and that water-column NO3 � was depleted before PO4 3� and Si(OH)4. The temporal changes in integrated nitrate, phosphate and silicate concentrations were well described by logis- tic models that showed maximum consumption rates of 11.8, 0.82, and 17.8 mmol m �2 d �1 , respec- tively, and a total seasonal draw-down of 210 ± 19 mmol NO3 � m �2 . If we include estimates of NO3 � supply by advection, nitrification and freshwater dilution, then the amount of NO3 � consumed could be 25�33% higher. Uptake of NO3 � above the 1% isolume was balanced by an equivalent (94%) increase in particulate N over a 15 d period beginning at ice break-up. Thus, the amount of particulate spring new production associated with NO3 � disappearance was estimated to be 16.1 ± 1.5 g C m �2 .

Nitrogen transformations at the sediment–water interface across redox gradients in the Laurentian Great Lakes

The capacity of a lake to remove reactive nitrogen (N) through denitrification has important implications both for the lake and for downstream ecosystems. In large oligotropic lakes such as Lake Superior, where nitrate (NO3 - ) concentrations have increased steadily over the past century, deep oxygen penetration into sediments may limit the denitrification rates. We tested the hypothesis that the position of the redox gradient in lake sediments affects denitrification by measuring net N-fluxes across the sediment-water interface for intact sediment cores collected across a range of sediment oxycline values from nearshore and offshore sites in Lake Superior, as well as sites in Lake Huron and Lake Erie. Across this redox gradient, as the thickness of the oxygenated sediment layer increased from Lake Erie to Lake Superior, fluxes of NH4 ? and N2 out of the sediment decreased, and sediments shifted from a net sink to a net source of NO3 - . Denitrification of NO3 - from overlying water decreased with thickness of the oxygenated sediment layer. Our results indicate that, unlike sediments from Lake Erie and Lake Huron, Lake Superior sediments do not remove significant amounts of water column NO3 - through denitrification, likely as a result of the

Episodic vertical nutrient fluxes and nearshore phytoplankton blooms in Southern California

Three distinct phytoplankton blooms lasting 4–9 d were observed in approximately 15‐m water depth near Huntington Beach, California, between June and October of 2006. Each bloom was preceded by a vertical NO3 flux event 6–10 d earlier. NO3 concentrations were estimated using a temperature proxy that was verified by comparison with the limited NO3 observations. The lower–water‐column vertical NO3 flux from vertical advection was inferred from observed vertical isotherm displacement. Turbulent vertical eddy diffusivity was parameterized based on the observed background (&lt; 0.3 cycles h−1) stratification and vertical shear in the horizontal currents. The first vertical nitrate flux event in June contained both advective and turbulent fluxes, whereas the later two events were primarily turbulent, driven by shear in the lower part of the water column. The correlation between the NO3 flux and the observed chlorophyll a (Chl a) was maximum (r2 = 0.40) with an 8‐d lag. A simple nitrate–phytoplankton model using a linear uptake function and driven with the NO3 flux captured the timing, magnitude, and duration of the three Chl a blooms (skill = 0.61) using optimal net growth rate parameters that were within the expected range. Vertical and horizontal advection of Chl a past the measurement site were too small to explain the observed Chl a increases during the blooms. The vertical NO3 flux was a primary control on the growth events, and estimation of both the advective (upwelled) and turbulent fluxes is necessary to best predict these episodic blooms.

The Contribution of Benthic Nutrient Regeneration to Primary Production in a Shallow Eutrophic Estuary, Weeks Bay, Alabama

Benthic oxygen, dinitrogen, and nutrient fluxes (NH 4 + , NO 3 − , and PO 4 3− ) were measured monthly during a 1-year period at two locations in Weeks Bay, a shallow (1.4 m) and eutrophic estuary in Alabama. Gross primary productivity (GPP), ecosystem respiration (R), and net ecosystem metabolism were determined from high-frequency dissolved oxygen measurements. Peak water column NO 3 − (55 μM) and chlorophyll a (138 μg/l) concentrations were measured during spring and fall, respectively. Sediments were a net source of NH 4 + (102 μmol m−2 h−1) and PO 4 3− (0.9 μmol m−2 h−1) but a sink for NO 3 − (−30 μmol m−2 h−1). Benthic N2 fluxes indicated net N fixation (12 μmol N m−2 h−1). Sediment oxygen demand (0.55 g O2 m−2 day−1) accounted for <10% of R (7.3 g O2 m−2 day−1). Despite high GPP rates (4.7 g O2 m−2 day−1), the estuary was net heterotrophic. Benthic regeneration supplied, on average, 7.5% and 4% of primary productivity N and P demands, respectively. These results contrast with the conventional view that benthic regeneration accounts for a large fraction of phytoplankton nutrient demand in shallow estuaries.

Denitrification in Alluvial Wetlands in an Urban Landscape

Riparian wetlands have been shown to be effective "sinks" for nitrate N (NO3-), minimizing the downstream export of N to streams and coastal water bodies. However, the vast majority of riparian denitrification research has been in agricultural and forested watersheds, with relatively little work on riparian wetland function in urban watersheds. We investigated the variation and magnitude of denitrification in three constructed and two relict oxbow urban wetlands, and in two forested reference wetlands in the Baltimore metropolitan area. Denitrification rates in wetland sediments were measured with a 15N-enriched NO3- "push-pull" groundwater tracer method during the summer and winter of 2008. Mean denitrification rates did not differ among the wetland types and ranged from 147 +/- 29 microg N kg soil(-1) d(-1) in constructed stormwater wetlands to 100 +/- 11 microg N kg soil(-1) d(-1) in relict oxbows to 106 +/- 32 microg N kg soil(-1) d(-1) in forested reference wetlands. High denitrification rates were observed in both summer and winter, suggesting that these wetlands are sinks for NO3- year round. Comparison of denitrification rates with NO3- standing stocks in the wetland water column and stream NO3- loads indicated that mass removal of NO3- in urban wetland sediments by denitrification could be substantial. Our results suggest that urban wetlands have the potential to reduce NO3- in urban landscapes and should be considered as a means to manage N in urban watersheds.

Anaerobic Ammonium Oxidation (Anammox) in Chesapeake Bay Sediments

Anaerobic ammonium oxidation (anammox) has recently been recognized as a pathway for the removal of fixed N from aquatic ecosystems. However, the quantitative significance of anammox in estuarine sediments is variable, and measurements have been limited to a few estuaries. We measured anammox and conventional denitrification activities in sediments along salinity gradients in the Chesapeake Bay and two of its sub-estuaries, the Choptank River and Patuxent River. Homogenized sediments were incubated with (14/15)N amendments of NH4+, NO3-, and NO2- to determine relative activities of anammox and denitrification. The percent of N2 production due to anammox (ra%) ranged from 0 to 22% in the Chesapeake system, with the highest ra% in the freshwater portion of the main stem of upper Chesapeake Bay, where water column NO3- concentrations are consistently high. Intermediate levels of relative anammox (10%) were detected at locations corresponding to tidal freshwater and mesohaline locations in the Choptank River, whereas anammox was not detected in the tidal freshwater location in the Patuxent River. Anammox activity was also not detected in the seaward end of Chesapeake Bay, where water column No3- concentrations are consistently low. The ra% did not correlate with NH4+ accumulation rate in anoxic sediment incubations, but ra% was related to water column NO3- concentrations and salinity. Anammox bacterial communities were also examined by amplifying DNA extracted from the upper Chesapeake Bay sediment with polymerase chain reaction (PCR) primers that are specific for 16S rRNA genes of anammox organisms. A total of 35 anammox-like sequences were detected, and phylogenetic analysis grouped the sequences in two distinct clusters belonging to the Candidatus "Scalindua" genus.

Quantification of denitrification in permeable sediments: Insights from a two‐dimensional simulation analysis and experimental data

Using a two‐dimensional simulation analysis, we investigated the effects of sediment flushing on denitrification and the implications for two methods commonly used to measure denitrification in intact sediment cores: the N2:Ar‐ratio method and isotope pairing technique (IPT). Our simulations of experimental chamber incubations showed that advective flushing of the sediment can significantly increase sediment denitrification driven by NO3− from the water column (up to a factor of 5), but that nitrification and coupled nitrification‐denitrification is reduced under conditions of sediment flushing (up to a factor of 6). N2 fluxes across SWI may differ significantly from actual rates of denitrification for periods lasting from 1 up to more than 5 d after changes in parameters such as sediment flushing rate and water column NO3− concentrations. Simulations of the isotope pairing technique, showed that the rate of labeled N2 production, after the addition of 15NO3− may take up to ~24 h to reach steady state, depending on NO3− concentrations in the water column and sediment flushing rate. Measurements of denitrification in sand using IPT confirmed that short term incubations (11 h) underestimated the actual denitrification. Furthermore, model simulations were able to give a good estimate of measured N2 fluxes across SWI at different flushing rates under non‐steady state conditions, confirming the ability of the model to realistically simulate experimental situations.