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Cyanobacterial harmful algal blooms (CyanoHABs) are enhanced by anthropogenic pressures, including excessive nutrient (nitrogen, N, and phosphorus, P) inputs and a warming climate. Severe eutrophication in aquatic systems is often manifested as non-N2-fixing CyanoHABs (e.g., Microcystis spp.), but the biogeochemical relationship between N inputs/dynamics and CyanoHABs needs definition. Community biological ammonium (NH4+) demand (CBAD) relates N dynamics to total microbial productivity and NH4+ deprivation in aquatic systems. A mechanistic conceptual model was constructed by combining nutrient cycling and CBAD observations from a spectrum of lakes to assess N cycling interactions with CyanoHABs. Model predictions were supported with CBAD data from a Microcystis bloom in Maumee Bay, Lake Erie, during summer 2015. Nitrogen compounds are transformed to reduced, more bioavailable forms (e.g., NH4+ and urea) favored by CyanoHABs. During blooms, algal biomass increases faster than internal NH4+ regeneration rates, causing high CBAD values. High turnover rates from cell death and remineralization of labile organic matter consume oxygen and enhance denitrification. These processes drive eutrophic systems to NH4+ limitation or colimitation under warm, shallow conditions and support the need for dual nutrient (N and P) control. [ABSTRACT FROM AUTHOR]

Nitrogen (N) transformations and fluxes at the sediment-water interface (SWI) were measured in Missisquoi Bay, Lake Champlain, to clarify the role of N in cyanobacterial blooms in temperate, eutrophic lakes. N sources (e.g., N fixation and tributary inputs), sinks (e.g., denitrification/anammox), and internal 'links' (e.g., dissimilatory [ABSTRACT FROM AUTHOR]

Mitigating the global expansion of cyanobacterial harmful blooms (CyanoHABs) is a major challenge facing researchers and resource managers. A variety of traditional (e.g., nutrient load reduction) and experimental (e.g., artificial mixing and flushing, omnivorous fish removal) approaches have been used to reduce bloom occurrences. Managers now face the additional effects of climate change on watershed hydrologic and nutrient loading dynamics, lake and estuary temperature, mixing regime, internal nutrient dynamics, and other factors. Those changes favor CyanoHABs over other phytoplankton and could influence the efficacy of control measures. Virtually all mitigation strategies are influenced by climate changes, which may require setting new nutrient input reduction targets and establishing nutrient-bloom thresholds for impacted waters. Physical-forcing mitigation techniques, such as flushing and artificial mixing, will need adjustments to deal with the ramifications of climate change. Here, we examine the suite of current mitigation strategies and the potential options for adapting and optimizing them in a world facing increasing human population pressure and climate change. [ABSTRACT FROM AUTHOR]

Abstract: Nutrient concentrations are often depleted in surface waters during the late summer open-water period in the northern Chukchi Sea. Yet the rate of re-supply of nutrients to the pelagic environment controls phytoplankton community abundance and productivity, which in turn influences the benthic components of this relatively shallow ecosystem. We measured nitrogen cycling rates at four experimental stations on the northeastern Chukchi Sea shelf in the western arctic. At each station, rates for net regeneration, actual uptake, and nitrification were measured using 15N isotope enrichment methods with 24-h bottle incubations under both light and dark conditions. Net regeneration rates throughout the water column ranged between −0.25 and 0.23μmolNL−1 h−1, with highest net positive regeneration occurring under light exposure in surface waters. Actual uptake rates ranged between −0.41 and −0.01μmolNL−1 h−1. Nitrification rates (measured at two stations) were generally highest in the sediment overlying waters, ranging between 0.14 and 0.67μmolNL−1 h−1, and appear to account for most uptake. Calculations of turnover time indicate that N is recycled within one day throughout the water column under natural conditions in the northern Chukchi Sea. Our findings confirm the results of previous studies that suggest a significant portion of surface nutrients on the Chukchi Shelf are products of horizontal advection through the Anadyr Current but also highlight the role of sediment regenerated N in supporting water-column primary production. [Copyright &y& Elsevier]

Total nitrogen (TN) in Lake Okeechobee, a large, shallow, turbid lake in south Florida, has averaged between 90 and 150 μM on an annual basis since 1983. No TN trends are evident, despite major storm events, droughts, and nutrient management changes in the watershed. To understand the relative stability of TN, this study evaluates nitrogen (N) dynamics at three temporal/spatial levels: (1) annual whole lake N budgets, (2) monthly in-lake water quality measurements in offshore and nearshore areas, and (3) isotope addition experiments lasting 3 days and using N-ammonium (NH) and N-nitrate (NO) at two offshore locations. Budgets indicate that the lake is a net sink for N. TN concentrations were less variable than net N loads, suggesting that in-lake processes moderate these net loads. Monthly NO concentrations were higher in the offshore area and higher in winter for both offshore and nearshore areas. Negative relationships between the percentage of samples classified as algal blooms (defined as chlorophyll a > 40 μg l) and inorganic N concentrations suggest N-limitation. Continuous-flow experiments over intact sediment cores measured net fluxes (μmol N m h) between 0 and 25 released from sediments for NH, 0-60 removed by sediments for NO, and 63-68 transformed by denitrification. Uptake rates in the water column (μmol N m h) determined by isotope dilution experiments and normalized for water depth were 1,090-1,970 for NH and 59-119 for NO. These fluxes are similar to previously reported results. Our work suggests that external N inputs are balanced in Lake Okeechobee by denitrification. [ABSTRACT FROM AUTHOR]

Abstract: Settled particles of fresh, labile organic matter may be a significant source of oxygen demand and nutrient regeneration in seasonally-hypoxic regions caused by nutrient inputs into stratified coastal zones. Studying the dynamics of this material requires sediment sampling methods that include flocculent organic materials and overlying water (OLW) at or above the sediment–water interface (SWI). A new coring device (“HYPOX” corer) was evaluated for examining nitrogen- (N) and oxygen-dynamics at the SWI and OLW in the northern Gulf of Mexico (NGOMEX). The HYPOX corer consists of a “Coring Head” with a check-valve, a weighted “Drive Unit,” and a “Lander,” constructed from inexpensive components. The corer collected undisturbed sediment cores and OLW from sediments at NGOMEX sampling sites with underlying substrates ranging from sand to dense clay. The HYPOX corer could be deployed in weather conditions similar to those needed for a multi-bottle rosette water-sampling system with 20L bottles. As an example of corer applicability to NGOMEX issues, NH4 + cycling rates were examined at hypoxic and control sites by isotope dilution experiments. The objective was to determine if N-dynamics in OLW were different from those in the water column. “Ammonium demand,” as reflected by potential NH4 + uptake rates, was higher in OLW than in waters collected from a meter or more above the bottom at both sites, but the pattern was more pronounced at the hypoxia site. By contrast, NH4 + regeneration rates were low in all samples. These preliminary results suggest that heterotrophic activity and oxygen consumption in OLW in the hypoxic region may be regulated by the availability of NH4 +, or other reduced N compounds, rather than by the lack of sufficient labile organic carbon. [Copyright &y& Elsevier]

Nitrogen (N) dynamics at the sediment–water interface were examined in four regions of Florida Bay to provide mechanistic information on the fate and effects of increased N inputs to shallow, subtropical, coastal environments. Dissimilatory nitrate (NO3 −) reduction to ammonium (DNRA) was hypothesized to be a significant mechanism retaining bioreactive N in this warm, saline coastal ecosystem. Nitrogen dynamics, phosphorus (P) fluxes, and sediment oxygen demand (SOD) were measured in north-central (Rankin Key; eutrophic), north-eastern (Duck Key; high N to P seston ratios), north-western (Murray Key; low N to P ratios), and central (Rabbit Key; typical central site) Florida Bay in August 2004, January 2005, and November 2006. Site water was passed over intact sediment cores, and changes in oxygen (O2), phosphate ( o-PO4 3−), ammonium (NH4 +), NO3 −, nitrite (NO2 −), and N2 concentrations were measured, without and with addition of excess 15NO3 − or 15NH4 + to inflow water. These incubations provided estimates of SOD, nutrient fluxes, N2 production, and potential DNRA rates. Denitrification rates were lowest in summer, when SOD was highest. DNRA rates and NH4 + fluxes were high in summer at the eutrophic Rankin site, when denitrification rates were low and almost no N2 came from added 15NO3 −. Highest 15NH4 + accumulation, resulting from DNRA, occurred at Rabbit Key during a picocyanobacteria bloom in November. 15NH4 + accumulation rates among the stations correlated with SOD in August and January, but not in November during the algal bloom. These mechanistic results help explain why bioreactive N supply rates are sometimes high in Florida Bay and why denitrification efficiency may decrease with increased NO3 − inputs in sub-tropical coastal environments. [ABSTRACT FROM AUTHOR]

We conducted continuous-flow experiments on intact sediment cores from Laguna Madre, Sabine Lake, East Matagorda Bay, and Nueces Estuary to evaluate internal nitrogen (N) sources, sinks, and retention mechanisms in Texas estuaries having different salinities. Mean ammonium (NH4+) flux ranged from slight uptake (negative values) to NH4+ production rates of about 300 µmol m-2 h-1 (units used for all N rates) and increased with salinity (p = 0.10). Net nitrate (NO3-) flux (-20 to 32) and net N2 flux (-70 to 100) did not relate to salinity. Mean net N2 flux was positive but near zero, indicating that N2 sources and sinks are nearly balanced. Total denitrification, N fixation, and potential dissimilatory NO3- reduction to NH4+ (DNRA) rates were estimated after inflow water was enriched with 15NO3- (100 µmol L-1). Total denitrification rates ranged from 0 to 90 versus N fixation rates ranging from 0 to 97. Potential DNRA, measured conservatively as 15NH4+ accumulation, ranged from 0 to 80 and related significantly to salinity (p < 0.01). Increases in total NH4+ release after 15NO3- additions were higher but closely related (r = 0.9998) to 15NH4+ accumulation, implying exchange reactions of DNRA-regenerated 15NH4+ with sediment-bound 14NH4+. The fate of NO3- was related to salinity, perhaps via sulfide effects on DNRA. Potential DNRA was high in southeastern Corpus Christi Bay in August during hypoxia when the sulfide transition zone was near the sediment surface. Nitrogen fixation and DNRA are important mechanisms that add and retain available N in Texas estuaries. [ABSTRACT FROM AUTHOR]

The detrimental effects of aquaculture waste discharges require development of recirculating and/or pretreatment systems. Denitrification, in combination with nitrification, removes fixed nitrogen from culture water in recirculating systems. Three experiments were conducted to measure denitrification rates in biofilter media (“bioballs”) and sediments from a recirculating mariculture system. In this case, “sediments” refer to solids accumulated at the bottom of the biofilter. Bioballs and sediments were collected from the biofilter attached to a 3.7 m diameter shrimp culture tank. Dissolved dinitrogen and oxygen concentrations were calculated from N2/Ar and O2/Ar ratios measured with a membrane inlet mass spectrometer (MIMS) optimized for high precision analysis of dissolved gases in water. Denitrification and oxygen flux were calculated from concentration changes as culture water passed through experimental chambers containing bioballs or sediments. Rates are reported per liter of inflow water flowing through the chambers, rather than per hour, because flow rates and biofilter design vary among aquaculture systems. Mean denitrification rates were 22.5±3.8 and 93.6±2.9 μmol N l−1 (n=6) in bioball and sediment chambers, respectively. Denitrification removed 1.0±0.4% and 3.9±0.3% of the total nitrogen (less N2) passing through bioball and sediment chambers, respectively. Ammonium accumulated in sediment chambers at 27.0±5.4 μmol N l−1, but ammonium flux in bioball chambers was not different from zero. Nitrite accumulation rates in bioball and sediment chambers were 20.8±7.8 and 24.0±2.4 μmol N l−1, respectively. The nitrate removal rate from sediment chambers was 115±35.4 μmol N l−1, but nitrate accumulated in bioball chambers at a rate of 76.8±16.6 μmol N l−1. The data showed that sediments accumulated in biofilters were important sites of nitrogen removal in this system. However, ammonium accumulation and nitrate removal in sediment chambers suggest that dissimilatory nitrate reduction to ammonium (DNRA) may be an important process in sediments accumulated in biofilters. The occurrence of DNRA in biofilters would result in less efficient removal of fixed N. MIMS technology has been used in many analytical chemistry applications but recently was identified as a useful tool for measuring denitrification in natural systems. This work is the first known application of MIMS technology to aquaculture issues and offers advantages over other methods to estimate denitrification in aquaculture systems. [Copyright &y& Elsevier]

Describes a method for estimating denitrification and nitrogen fixation simultaneously in costal sediments. Concept of the method; Measurement procedures; Use of an isotope-pairing technique to dissolved gas measurements with a membrane inlet mass spectrometer.

In this article the authors discuss aspects of the impact of nutrient over-enrichment on the deterioration of water quality in lakes, such as China's Lake Tai, focusing on nitrogen (N2) fixation, harmful algal blooms (HABs), and the non-N2-fixing cyanobacteria Microcystis.

Continental shelves comprise <5% of global ocean area but may account for a disproportionate 30% of primary production, 80% of organic matter burial, and >50% of marine denitrification. The Hanna Shoal region, part of the continental shelf system in the northeast Chukchi Sea, Alaska, is recognized for its high biodiversity and productivity. We investigated the role of sediments in organic matter decomposition and nutrient cycling at five stations on the shallow Hanna Shoal. In particular, we asked (1) how much sediment organic matter is remineralized in the Chukchi Sea, and what factors drive this degradation, (2) do sediments function as a net source for fixed nitrogen (thus fueling primary production in the overlying water), or as a net sink for fixed nitrogen (thereby removing it from the system), and (3) what is the balance between sediment NH 4 + uptake and regeneration, and what factors drive NH 4 + cycling? We conducted dark sediment core incubations to measure sediment O 2 consumption, net N 2 and nutrient (NH 4 + , NO 3 - , NO 2 - , PO 4 3- ) fluxes , and rates of sediment NH 4 + cycling, including uptake and regeneration. Rates of sediment O 2 consumption and NH 4 + and PO 4 3- efflux suggest that high organic matter remineralization rates occurred in these cold (−2 °C) sediments. We estimated that total organic carbon remineralization accounted for 20–57% of summer export production measured on the Chukchi Shelf. Net N 2 release was the dominant nitrogen flux, indicating that sediments acted as a net sink for bioavailable nitrogen via denitrification. Organic carbon remineralization via denitrification accounted for 6–12% of summer export production, which made up ~25% of the total organic carbon oxidized in Hanna Shoal sediments. These shallow, productive Arctic shelves are “hotspots” for organic matter remineralization. [ABSTRACT FROM AUTHOR]

This study evaluated impacts of the BP Deepwater Horizon ( DWH ) oil spill on petroleum hydrocarbons in surface waters of the Louisiana continental shelf in northern Gulf of Mexico. Surface water (~top 5 cm) without visible oil was collected from three cruises in May 2010 during the oil spill, August 2010 after the well was capped, and May 2011 one year after the spill. Concentrations of total dissolved n -alkanes (C 9 –C 35 ) in surface seawater were more than an order of magnitude higher in May 2010 than August 2010 and May 2011, indicating contamination by the DWH oil spill. This conclusion was further supported by more abundant smaller n -alkanes (C 9 –C 13 ), together with pristane and phytane, in May than August 2010 samples. In contrast, even carbon-numbered dissolved n -alkanes (C 14 –C 20 ) dominated the May 2011 samples, and this distribution pattern of dissolved n -alkanes is the first documentation for water samples in the northern Gulf of Mexico. However, this pattern was not observed in May 2011 suspended particles except for Sta. OSS. This decoupling between dissolved and particle compositions suggests that either these even carbon-numbered n -alkanes originated from bacteria rather than algae, or that the alkanes in the shelf were transported from elsewhere. Concentrations of polycyclic aromatic hydrocarbons (PAHs) in suspended particles were 5 times higher on average in May 2010 (83–252 ng L −1 ) than May 2011 (7.2–83 ng L −1 ), also indicating contamination by the DWH oil spill. Application of a biomarker ratio of 17α(H),21β(H)-30-norhopane over 17α(H),21β(H)-hopane confirmed that suspended particles from at least two stations were contaminated by the DWH oil spill in May 2010. Taken together, these results showed that surface waters of the sampling area in May 2010 were contaminated by the oil spill, but also that rapid weathering and/or physical dilution quickly reduced hydrocarbon levels by August 2010. [ABSTRACT FROM AUTHOR]

We measured fluxes of net dissolved N2 and O2, , and at the sediment–water interface in whole cores in relation to bottom water temperature and salinity in late summer 2010 on the shelf of the eastern Chukchi Sea. Cold (−1.68°C) and high-salinity (33.72) bottom water, characteristic of Pacific winter water, overlaid the sediment over the northeastern region of the shelf while relatively warmer (8.33°C) and less saline (29.88) water with properties of Alaska Coastal Water covered the seabed in the southeastern region. Latitudinal variability of gas fluxes was not linked to differences in temperature or salinity. Relatively higher N2 efflux was measured for sediment located at the water mixing front between cold and warm bottom waters. Benthic O2 consumption in the southeastern region was relatively intense as were and effluxes from the sediments into the water column. Overall, sediments in the Chukchi Sea are sites of intense denitrification but these losses appear compensated by benthic regeneration, as efflux from sediments into the overlying water exceed those of N2. Exposure of sediment cores to incoming light did not affect benthic fluxes significantly compared to dark conditions. [ABSTRACT FROM AUTHOR]

Abstract: An isotope dilution method is described to determine nitrification rates in the northern Gulf of Mexico (NGOMEX) and other eutrophic waters using high performance liquid chromatography (HPLC). A isotope tracer method was not successful for NGOMEX waters due to excessive levels of unlabeled nitrogen (N) captured on sample filters. Nitrification rates were estimated by measuring the dilution of added by (=14NO X ) generated from in situ during sample incubations. NO X in pre- and post-incubation samples was reduced to NH4 + with zinc (Zn) dust following sample acidification. Samples were analyzed by HPLC for concentration and atom% 15N composition. Results were corrected for background natural abundance concentrations measured in sub-samples collected before the reduction step. Nitrification rates were calculated using the difference between initial and final corrected NO X isotopic ratios and concentrations before and after the incubations. Laboratory experiments demonstrated that Zn reduction of NO X to averaged 102% (±0.2–0.8%) within 30min. Different levels of were added to sub-samples of coastal sea water containing a pool of added to simulate a range of nitrification rates. Analysis of the resulting solutions produced a linear relationship (R 2=0.9895; p<0.01) between added concentration and isotope ratio and indicated a detection sensitivity of better than 2 atom%, making the method suitable for eutrophic waters with high nitrification rates. Nitrification rates estimated using both isotope dilution and tracer methods in coastal waters produced comparable results. Nitrification rates of up to 150nmolNL−1 h−1 were measured in the NGOMEX hypoxic zone by isotope dilution. Advantages of the method include: (1) unaltered substrate () concentrations, (2) small sample volume, (3) minimal sample handling, and (4) direct 24h nitrification rate assessment. [Copyright &y& Elsevier]

Abstract: Mechanisms of N removal in permeable sediments were examined using sediment columns packed with southeastern US continental shelf sands. While significant N2 production was observed in oxic SAB sands, the addition of 20μM 15NO3 − tracer in oxic sediment columns for periods of up to 12 days yielded a delayed production of 29N2 and 30N2 which, at the end of the incubation period, was still small relative to previously measured total N2 production rates. Possible explanations for these results include N removal via tightly coupled nitrification–denitrification in microenvironments isolated from the bulk porewater or denitrification mediated by aerobic denitrifiers. The addition of 15NO3 − to porewater at levels above natural concentrations caused N assimilation and increases in sediment respiration rates in oxic columns, indicating N limitation. Delays in achieving steady-state denitrification rates following disturbances may be associated with deep penetration of O2 in advective regimes and amplified by isolation of respiration and denitrification in reactive microenvironments. Isolation of microzone denitrification, delays in achieving steady-state conditions, and the response of sediment community respiration and denitrification rates to increases in low background NO3 − may interfere with the accuracy of denitrification rate measurements in sandy sediments on oligotrophic continental shelves with isotope pairing and traditional core incubation techniques. In columns with anoxic outflow, the immediate appearance of 29N2 and 30N2 following 15NO3 − addition indicates that denitrification in suboxic or anoxic sands relies on external sources of NO3 − rather than only on in situ nitrification, which is the main NO3 − source for denitrification in oxic SAB sands. [Copyright &y& Elsevier]

Abstract: Nitrogen (N) cycling and respiration rates were measured in sediment columns packed with southeastern United States continental shelf sands, with high permeability (4.66×10−11 m2) and low organic carbon (0.05%) and nitrogen (0.008%). To simulate porewater advection, natural shelf seawater was pumped through columns of different lengths to achieve fluid residence times of approximately 3, 6, and 12h. Experiments were conducted seasonally at in situ temperature. Fluid flow was uniform in nearly all columns, with minimal dead zones and channeling. Significant respiration (O2 consumption and ∑CO2 production) occurred in all columns, with highest respiration rates in summer. Most (78–100%) remineralized N was released as N2 in the majority of cases, including columns with oxic porewater throughout, with only a small fraction released as NO3 − from some oxic columns. A rate of 0.84–4.83×1010 molNyr−1, equivalent to 1.06–6.09×10−6 mmolNcm−2 h−1, was calculated for benthic N2 production in the South Atlantic Bight, which can account for a large fraction of new N inputs to this shelf region. Metal and sulfate reduction occurred in long residence time columns with anoxic outflow in summer and fall, when respiration rates were highest. Because permeable sediments dominate continental shelves, N2 production in high permeability coastal sediments may play an important role in the global N cycle. [Copyright &y& Elsevier]

Characterizing denitrification rates in aquatic ecosystems is essential to understanding how systems may respond to increased nutrient loading. Thus, it is important to ensure the precision and accuracy of the methods employed for measuring denitrification rates. The acetylene (C[sub 2]H[sub 2]) inhibition method is a simple technique for estimating denitrification. However, potential problems, such as inhibition of nitrification and incomplete inhibition of nitrous oxide reduction, may influence rate estimates. Recently, membrane inlet mass spectrometry (MIMS) has been used to measure denitrification in aquatic systems. Comparable results were obtained with MIMS and C[sub 2]H[sub 2] inhibition methods when chloramphenicol was added to C[sub 2]H[sub 2] inhibition assay mixtures to inhibit new synthesis of denitrifying enzymes. Dissolved-oxygen profiles indicated that surface layers of sediment cores subjected to the MIMS fiowthrough incubation remained oxic whereas cores incubated using the C[sub 2]H[sub 2] inhibition methods did not. Analysis of the microbial assemblages before and after incubations indicated significant changes in the sediment surface populations during the long flowthrough incubation for MIMS analysis but not during the shorter incubation used for the C[sub 2]H[sub 2] inhibition method. However, bacterial community changes were also small in MIMS cores at the oxygen transition zone where denitrification occurs. The C[sub 2]H[sub 2] inhibition method with chloramphenicol addition, conducted over short incubation intervals, provides a cost-effective method for estimating denitrification, and rate estimates are comparable to those obtained by the MIMS method. [ABSTRACT FROM AUTHOR]

Harmful cyanobacterial blooms (CyanoHABs) are linked to increasing anthropogenic nitrogen (N) and phosphorus (P) inputs. However, CyanoHABs in many large lakes continue despite extensive abatement efforts, mostly focused on external P loading. Internal nutrient cycling can modify nutrient availability and limitation; thus, understanding the relative importance of external vs. internal nutrient loading is essential for developing effective mitigation strategies for CyanoHABs. We estimated long‐term nutrient budgets for Lake Taihu, China, from mass balance models using extensive monitoring of input and output nutrient data from 2005 to 2018 to quantify contributions from internal nutrient loading. The nutrient mass balance showed that 9% and 63% of annual external N and P inputs, respectively, were retained in the lake. Denitrification removed 54% of external N loading and can thus help explain rapid decreases in lake N concentrations and summer N limitation. Water column NH4+ regeneration can help sustain CyanoHABs over the short term and contributed 38–58% of potential NH4+ demand for summer‐fall, Microcystis‐dominated blooms. Internal P release contributed 23–90% of CyanoHABs P demand, although Taihu was a net P sink on an annual scale. Our results show that internal nutrient cycling helps sustain CyanoHABs in Taihu, despite reductions in external nutrient inputs. Furthermore, N is leaving the lake faster than P, thereby creating persistent N limitation. Therefore, parallel reductions in external N loading, along with P, will be most effective in reducing CyanoHABs and accelerate the recovery process in this and other large, shallow lakes. [ABSTRACT FROM AUTHOR]

• Urea is metabolized by the microbial consortium in Lake Taihu. • Urea supports the growth of Microcystis indirectly at a rate comparable to NH 4 +. • Urea fate depends on light conditions (natural light vs. dark). • Heterotrophic bacteria are the dominant organisms affecting urea cycling in Lake Taihu. Lake Taihu, the third largest freshwater lake in China, suffers from harmful cyanobacteria blooms caused by Microcystis spp., which do not fix nitrogen (N). Reduced N (i.e., NH 4 +, urea and other labile organic N compounds) is an important factor affecting the growth of Microcystis. As the world use of urea as fertilizer has escalated during the past decades, an understanding of how urea cycling relates to blooms of Microcystis is critical to predicting, controlling and alleviating the problem. In this study, the cycling rates of urea-N in Lake Taihu ranged from non-detectable to 1.37 μmol N L−1 h−1 for regeneration, and from 0.042 μmol N L−1 h−1 to 2.27 μmol N L−1 h−1 for potential urea-N removal. The fate of urea-N differed between light and dark incubations. Increased 15NH 4 + accumulated and higher quantities of the removed urea-15N remained in the 15NH 4 + form were detected in the dark than in the light. A follow-up incubation experiment with 15N-urea confirmed that Microcystis can grow on urea but its effects on urea dynamics were minor, indicating that Microcystis was not the major factor causing the observed fates of urea under different light conditions in Lake Taihu. Bacterial community composition and predicted functional gene data suggested that heterotrophic bacteria metabolized urea, even though Microcystis spp. was the dominant bloom organism. [ABSTRACT FROM AUTHOR]

Ecosystem function measurements can enhance our understanding of nitrogen (N) delivery in coastal catchments across river and estuary ecosystems. Here, we contrast patterns of N cycling and export in two rivers, one heavily influenced by wastewater treatment plants (WWTP), in a coastal catchment of south Texas. We measured N export from both rivers to the estuary over 2 yr that encompass a severe drought, along with detailed mechanisms of N cycling in river, tidal river, and two estuary sites during prolonged drought. WWTP nutrient inputs stimulated uptake of N, but denitrification resulting in permanent N removal accounted for only a small proportion of total uptake. During drought periods, WWTP N was the primary source of exported N to the estuary, minimizing the influence of episodic storm-derived nutrients from the WWTP-influenced river to the estuary. In the site without WWTP influence, the river exported very little N during drought, so storm-derived nutrient pulses were important for delivering N loads to the estuary. Overall, N is processed from river to estuary, but sustained WWTP-N loads and periodic floods alter the timing of N delivery and N processing. Research that incorporates empirical measurements of N fluxes from river to estuary can inform management needs in the face of multiple anthropogenic stressors such as demand for freshwater and eutrophication. [ABSTRACT FROM AUTHOR]

Preventing harmful algal blooms (HABs) is needed to protect lakes and downstream ecosystems. Traditionally, reducing phosphorus (P) inputs was the prescribed solution for lakes, based on the assumption that P universally limits HAB formation. Reduction of P inputs has decreased HABs in many lakes, but was not successful in others. Thus, the "P-only" paradigm is overgeneralized. Whole-lake experiments indicate that HABs are often stimulated more by combined P and nitrogen (N) enrichment rather than N or P alone, indicating that the dynamics of both nutrients are important for HAB control. The changing paradigm from P-only to consideration of dual nutrient control is supported by studies indicating that (1) biological N fixation cannot always meet lake ecosystem N needs, and (2) that anthropogenic N and P loading has increased dramatically in recent decades. Sediment P accumulation supports long-term internal loading, while N may escape via denitrification, leading to perpetual N deficits. Hence, controlling both N and P inputs will help control HABs in some lakes and also reduce N export to downstream N-sensitive ecosystems. Managers should consider whether balanced control of N and P will most effectively reduce HABs along the freshwater-marine continuum. [ABSTRACT FROM AUTHOR]

Phytoplankton (eutrophication, biogeochemical) models are important tools for ecosystem research and management, but they generally have not been updated to include modern biology. Here, we present a dynamic, mechanistic, molecular-level (i.e. gene, transcript, protein, metabolite) model of Anabaena - nitrogen interaction. The model was developed using the pattern-oriented approach to model definition and parameterization of complex agent-based models. It simulates individual filaments, each with individual cells, each with genes that are expressed to yield transcripts and proteins. Cells metabolize various forms of N, grow and divide, and differentiate heterocysts when fixed N is depleted. The model is informed by observations from 269 laboratory experiments from 55 papers published from 1942 to 2014. Within this database, we identified 331 emerging patterns, and, excluding inconsistencies in observations, the model reproduces 94% of them. To explore a practical application, we used the model to simulate nutrient reduction scenarios for a hypothetical lake. For a 50% N only loading reduction, the model predicts that N fixation increases, but this fixed N does not compensate for the loading reduction, and the chlorophyll a concentration decreases substantially (by 33%). When N is reduced along with P, the model predicts an additional 8% reduction (compared to P only). [ABSTRACT FROM AUTHOR]

We evaluated O2 metabolism and mixing of river and seawater in the northern Gulf of Mexico (NGOMEX) in May and August of 2010. A strong correlation between the δ²H and δ18O of water and salinity indicated simple mixing of the two water sources. The isotope values for the freshwater source suggested that Mississippi River water predominated over the Atchafalaya River water in at our study sites in both May and August. Knowledge of the δ18O of water is required for determination of the ratio of primary production to respiration (P:R) from the δ18O and concentration of O2. We observed a strong correlation between δ18O of water and salinity, which indicates that subsequent studies of P:R may simply be able to predict the isotopic composition of water from salinity in this region. P:R values were determined from the concentration and isotopic composition of O2 and varied from 0.88 to 1.10. The range in P:R and progression from high values in May to net heterotrophy in August is consistent with enhanced primary production in spring followed by respiration in excess of production in summer. Averages of -5.8‰ and -3.4‰ were observed for the isotopic enrichment factor, εobs, for sub-pycnocline respiration in May and August, respectively. In contrast to previous studies we found that εobs is not an effective indicator of the proportion of water column to sediment respiration, but rather reflects the total rate of sub-pycnocline respiration (water column plus sediment respiration). Nonetheless, as εobs is an indication of total sub-pycnocline respiration insight into oxygen-removal can be obtained without the need for incubation. Even though our research cruises occurred approximately three weeks after the initiation and one month after cessation of the Deepwater Horizon oil spill, P:R and εobs values were each within ranges reported previously in NGOMEX. Thus, a direct link between the oil spill and O2 metabolism or hypoxia development was not apparent. [ABSTRACT FROM AUTHOR]

Nitrogen (N) pollution in aquatic ecosystems has attracted much attention over the past decades, but the dynamics of this bioreactive element are difficult to measure in aquatic oxygentransition environments. Nitrogen-transformation experiments often require measurement of 15N-ammonium (15NH4+) ratios in small-volume 15N-enriched samples. Published methods to determine N isotope ratios of dissolved ammonium require large samples and/or costly equipment and effort. We present a novel ("OX/MIMS") method to determine N isotope ratios for 15NH4+ in experimental waters previously enriched with 15N compounds. Dissolved reduced 15N (dominated by 15NH4+) is oxidized with hypobromite iodine to nitrogen gas (29N2 and/or 30N2) and analyzed by membrane inlet mass spectrometry (MIMS) to quantify 15NH4+ concentrations. The N isotope ratios, obtained by comparing the 15NH4+ to total ammonium (via autoanalyzer) concentrations, are compared to the ratios of prepared standards. The OX/MIMS method requires only small sample volumes of water (ca. 12 mL) or sediment slurries and is rapid, convenient, accurate, and precise (R2 = 0.9994, p < 0.0001) over a range of salinities and 15N 14N ratios. It can provide data needed to quantify rates of ammonium regeneration, potential ammonium uptake, and dissimilatory nitrate reduction to ammonium (DNRA). Isotope ratio results agreed closely (R = 0.998, P = 0.001) with those determined independently by isotope ratio mass spectrometry for DNRA measurements or by ammonium isotope retention time shift liquid chromatography for water-column N-cycling experiments. Application of OX/MIMS should simplify experimental approaches and improve understanding of N-cycling rates and fate in a variety of freshwater and marine environments. [ABSTRACT FROM AUTHOR]

The Gulf of Mexico is affected by hurricanes and suffers seasonal hypoxia. The Deepwater Horizon oil spill impacted every trophic level in the coastal region. Despite their importance in bioremediation and biogeochemical cycles, it is difficult to predict the responses of microbial communities to physical and anthropogenic disturbances. Here, we quantify sediment ammonia-oxidizing archaeal ( AOA) community diversity, resistance and resilience, and important geochemical factors after major hurricanes and the oil spill. Dominant AOA archetypes correlated with different geochemical factors, suggesting that different AOA are constrained by distinct parameters. Diversity was lowest after the hurricanes, showing weak resistance to physical disturbances. However, diversity was highest during the oil spill and coincided with a community shift, suggesting a new alternative stable state sustained for at least 1 year. The new AOA community was not significantly different from that at the spill site 1 year after the spill. This sustained shift in nitrifier community structure may be a result of oil exposure. [ABSTRACT FROM AUTHOR]

Abstract: Understanding the mechanisms of organic matter decomposition in hypoxic waters becomes more important globally with increasing hypoxia in coastal regions. Peptide decomposition rates were measured in low-oxygen bottom water and compared to those in oxygenated surface water of a stratified 18-m water column at a hypoxic northern Gulf of Mexico station. Water from each depth was spiked with small peptides alanine–valine–phenylalanine–alanine (AVFA) and VFA and incubated onboard for ca. 80h to determine peptide decomposition rates, decomposition products, and bacterial community structure shifts. Decomposition rates of amended peptides were twice as high in the bottom water as in the surface water, and the pathways of peptide decomposition differed. In surface waters, the small peptides were hydrolyzed into individual amino acids extracellularly by aminopeptidase, based on the analysis of hydrolyzed fragments; hydrolysis rates remained constant throughout the incubation period. In contrast, in bottom waters the amended peptides were metabolized rapidly into ammonium by bacteria through either direct uptake or tight coupling of extracellular hydrolysis and subsequent uptake after an initial adaptation period (~22h). Dissolved oxygen level did not affect peptide decomposition patterns significantly in surface or bottom water, when it was manipulated by sparging with air or nitrogen gas. The high efficiency of peptide decomposition in bottom waters corresponded to the rapid growth of several genera of Alpha- and Gammaproteobacteria. These bacterial species may have caused rapid peptide decomposition, but the exact mechanisms remain unclear. The high efficiency of peptide decomposition in hypoxic regions illustrates biogeochemical feedback to hypoxia formation in stratified coastal regions. [Copyright &y& Elsevier]

Hypoxia is a global problem resulting from excessive nutrient inputs to coastal regions, but the biogeochemical mechanisms of hypoxia development are not well understood. The primary location of oxygen consumption (i.e., sediments versus water column) is still debated and may depend on the analytical approach used. In this study, oxygen respiration was measured using incubations combined with membrane inlet mass spectrometry in sediments, water overlying sediments, and the water column in the Gulf of Mexico hypoxic zone. Water column respiration ranged from 0.09 to 4.42 μmol O2 l−1 h−1 (mean = 0.77 ± 0.07 (standard error)) and was significantly higher shortly after two hurricanes. Overlying water respiration ranged from 0.31 to 2.46 μmol O2 l−1 h−1 (mean = 0.70 ± 0.09) and accounted for 3.7 ± 0.8% of total below-pycnocline respiration. Sediment oxygen consumption, measured using a continuous-flow incubation technique, was lowest after the two hurricanes and ranged from 408 to 1800 μmol O2 m−2 h−1 (mean = 834 ± 83.8 μmol O2 m−2 h−1). Sediments accounted for 25 ± 5.3% of total below-pycnocline respiration, and sediment oxygen consumption was related negatively to ambient bottom-water oxygen concentration. This negative relationship contradicts previous literature and suggests that high sediment oxygen consumption is driven by abundant, fresh organic material and regulates bottom-water oxygen concentration, rather than the common assumption that bottom-water oxygen concentration determines sediment oxygen consumption. The results from this study suggest that storms and mixing events may lead to conditions suitable for hypoxia redevelopment in as little as two days after disturbances, with the water column playing a critical role in system hypoxia development and maintenance. [ABSTRACT FROM AUTHOR]

Abstract: This study investigated potential transformation processes and fates of nitrate at the sediment–water interface of Copano Bay during a period of drought by conducting continuous-flow and slurry experiments combined with a 15NO3 − addition technique. Rates of 15NO3 −-based denitrification, anaerobic ammonium oxidation (ANAMMOX) and potential dissimilatory nitrate reduction to ammonium (DNRA) were in the range of 27.7–40.1, 0.26–1.6 and 1.4–3.8μmol 15Nm–2 h–1, respectively. Compared with the total 15NO3 −fluxes into sediments, dissimilatory processes contributed 29–49% to loss of the spiked 15NO3 −. Based on the mass balance of 15NO3 −, microbial assimilation was estimated to consume about 50–70% of the added 15NO3 −, indicating that most of nitrate was incorporated by microorganisms in this N-limiting system. In addition, significant correlations of nitrate transformation rates with sediment characteristics reflect that the depth related behaviors of nitrate transformations in core sediments were coupled strongly to organic matter, iron (Fe) and sulfur (S) cycles. [Copyright &y& Elsevier]

Abstract: A new approach, combining 15NH4 + isotope dilution and continuous-flow techniques, provided estimates of “actual” and “net” NH4 + flux and sediment NH4 + demand (SAD) at the sediment–water interface (SWI) of sites in the northern Gulf of Mexico (NGOMEX). The sites included a hypoxic site (C6), two sites with intermediate oxygen levels (B7 and F5), and a normoxic site (CT). Control cores without isotope addition and other cores from the same site treated with 15NH4 + labeled overflowing water differentiated between net and actual regeneration flux and actual vs. potential uptake flux of NH4 +. Experiments were conducted in 2008 before (July) and after (September) two successive hurricanes (Gustav and Ike) and in January and August, 2009. Actual regeneration was significantly higher than net flux at most sites. Net flux did not differ significantly in most sites/dates, but the actual regeneration, and the actual and potential uptake, showed temporal and spatial variation; the flux at the hypoxic site was more active than non-hypoxic sites. SAD, the difference between potential and actual NH4 + uptake flux, was higher at the hypoxic site than at non-hypoxic sites before and after the hurricanes in 2008 and during the hypoxia season in 2009. SAD related negatively to bottom water DO values. Conclusions: (1) net flux often underestimated actual regeneration, (2) hurricane activity decreased N dynamics, and (3) microbial N limitation status at the hypoxic site related to NH4 + removal processes that were independent of oxygen (e.g., anaerobic heterotrophic uptake or anammox). These results indicate a rather consistent NH4 + demand at the SWI during the hypoxic season and suggest that reduced nitrogen may limit microbial dynamics in the region. [Copyright &y& Elsevier]

Abstract: Harmful cyanobacterial blooms, reflecting advanced eutrophication, are spreading globally and threaten the sustainability of freshwater ecosystems. Increasingly, non-nitrogen (N2)-fixing cyanobacteria (e.g., Microcystis) dominate such blooms, indicating that both excessive nitrogen (N) and phosphorus (P) loads may be responsible for their proliferation. Traditionally, watershed nutrient management efforts to control these blooms have focused on reducing P inputs. However, N loading has increased dramatically in many watersheds, promoting blooms of non-N2 fixers, and altering lake nutrient budgets and cycling characteristics. We examined this proliferating water quality problem in Lake Taihu, China’s 3rd largest freshwater lake. This shallow, hyper-eutrophic lake has changed from bloom-free to bloom-plagued conditions over the past 3 decades. Toxic Microcystis spp. blooms threaten the use of the lake for drinking water, fisheries and recreational purposes. Nutrient addition bioassays indicated that the lake shifts from P limitation in winter–spring to N limitation in cyanobacteria-dominated summer and fall months. Combined N and P additions led to maximum stimulation of growth. Despite summer N limitation and P availability, non-N2 fixing blooms prevailed. Nitrogen cycling studies, combined with N input estimates, indicate that Microcystis thrives on both newly supplied and previously-loaded N sources to maintain its dominance. Denitrification did not relieve the lake of excessive N inputs. Results point to the need to reduce both N and P inputs for long-term eutrophication and cyanobacterial bloom control in this hyper-eutrophic system. [Copyright &y& Elsevier]

[Display omitted] • A new method to determining 15NO 3 − for isotope-enrichment experiments is presented. • Immediate advantages of this method are rapid, accurate, and cost-effective. • It was applied successfully to measuring nitrification and NO 3 − immobilization. Using 15N stable isotope as a tracer to quantify N transformation rates in isotope-enrichment experiments improves understanding of the N cycle in various ecosystems. However, measuring 15N-nitrate (15NO 3 −) in small volumes of water for these experiments is a major challenge due to the inconvenience of preparing samples by traditional techniques. We developed a "REOX/MIMS" method by applying membrane inlet mass spectrometry (MIMS) to determining 15NO 3 − concentrations in a small volumes of water from isotope-enrichment experiments after converting the dissolved inorganic N to N 2. The nitrates (NO 3 − + NO 2 −) were reduced to NH 4 + with zinc powder, and the ammonium (NH 4 +) was then oxidized to N 2 by hypobromite iodine solution. The resulting 29N 2 and 30N 2 were measured via MIMS. This optimized protocol provides a sensitive (~0.1 μM) and precise (relative standard deviation = 0.1–4.37%) approach to quantify 15NO 3 − concentrations (0.1–500 µM) in water samples over a wide range of salinities (0–35‰) and in 2 M KCl solution with excellent calibration curves (R2 ≥ 0.9996, p < 0.0001). The method was combined with 15NO 3 − isotope-enrichment incubation experiments to measure gross nitrification and gross NO 3 − immobilization rates in various ecosystems. It was rapid, accurate, and cost-effective. Future applications of this efficient approach will inform scientists, modelers and decision makers about mechanisms, sources, fates, and effects of NO 3 − delivered to or produced in numerous aquatic and terrestrial ecosystems. [ABSTRACT FROM AUTHOR]