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We investigated anaerobic nitrogen (N) cycling in the water column of Lake Kivu, a deep meromictic tropical lake in East Africa. Data were collected at one station in the Northern Basin and one in the Southern Basin, during two sampling campaigns (June 2011—dry season, and February 2012—rainy season). Short‐term incubations of sulfide‐free water with 15N‐labeled substrates revealed high potential denitrification and dissimilatory nitrate reduction to ammonium (DNRA) rates (up to 350 and 36 nmol N produced L−1 h−1, respectively), while anaerobic ammonium oxidation (anammox) was lower (up to 3.3 nmol N produced L−1 h−1). However, anammox rates were 15 nmol N produced L−1 h−1 when 15 NH4+ was added at depths where NH4+ concentrations were very low (< 1 μmol L−1). With the addition of 5 μmol L−1 of 15 NO3- and 10 μmol L−1 of H2S, denitrification and anammox were stimulated in the Northern Basin, while the increase of DNRA rates was less notable. In the Southern Basin, the addition of H2S decreased denitrification rates, probably because of competition with DNRA, which increased, while no effect was observed on anammox. This study puts into evidence the co‐occurrence of denitrification, anammox and DNRA, for the first time in a great tropical lake, and underlines the spatial heterogeneity of these processes. Contrary to numerous reports in literature, we show that anammox can significantly occur in presence of H2S, suggesting that the contribution of anammox in the N cycle may be underestimated.

The Scheldt Estuary (Belgium-The Netherlands), located in one of the most densely populated areas of Europe (over 500 inh./km²), is a highly eutrophic system currently recovering from hyper-eutrophication thanks to improved waste water treatment efforts. Consequently, the carbon cycle in the Scheldt estuary underwent a series of consecutive changes. We analyzed the evolution of organic carbon content and carbon-to-nitrogen ratio in suspended particulate matter, of inorganic carbon speciation (carbon dioxide, bicarbonates and carbonates) and of the 13C/12C isotopic composition of particulate organic and dissolved inorganic carbon, at monthly resolution over the past 20 years. Results show a long term trend superimposed on seasonal patterns. Since the 1998, allochtonous organic carbon inputs decreased progressively in the upstream river basin of the estuary with the reduction of untreated sewage releases, while a simultaneous increase of oxygen and pH, a decrease of DIC (and especially of CO2), and an increase of δ13C-DIC are observed. These all point to reduced respiration and increased primary production in the estuary. Consequences of changes on the global C budget of the estuary are estimated and discussed.

Man-induced changes have a major impact on the biogeochemical cycle of nitrogen. In the framework of the regional OMES programme (Flanders) a long-term study of biological and physico-chemical parameters is conducted on the River Scheldt. The covered section is about 100km between the cities of Ghent and Antwerp with 25 stations sampled on a monthly and even fortnightly (summer period) basis. Knowledge about the natural δ15N and δ18O isotopic composition of nitrate enables to resolve the impact of the different biogeochemical processes acting on the nitrate reservoir, such as uptake, remineralization and denitrification. We measured the nitrate isotopic composition using the bacterial denitrifier method (Sigman et al., 2001) to document the spatial and seasonal evolution of the nitrate isotopic composition along the river for winter, spring, summer and autumn 2014. The major biogeochemical processes involved are discussed.

Benthic nitrogen cycling pathways were studied in a euxinic fjord in western Sweden subject to artificial oxygenation as part of an environmental engineering experiment. Denitrification and dissimilatory nitrate reduction to ammonium (DNRA) were investigated in situ by means of 15N-nitrate additions to benthic chambers. At stations above the oxic/anoxic interface, denitrification dominated nitrate reduction and N2 production both before and after oxygenation, while DNRA accounted for

Nitrogen loads in the Scheldt Estuary decreased since the early 2000s following enhanced waste water treatment. However, nitrate concentrations have increased as a result of increased nitrification and decreased denitrification due to improved oxygen conditions while diffuse nitrate inputs from agriculture did not change. To effectively reduce the nutrient load in the Scheldt Estuary, a more profound knowledge is needed about the internal nitrate sources and sinks. Therefore, we investigated the monthly variation of nitrate oxygen (δ18O) and nitrogen (δ15N) isotopes along the estuarine axis over a period of two years. The results show that nitrification and autotrophic nitrate uptake are the main respective source and sink for nitrate in the estuary, with their relative importance varying with season, distance along the estuarine axis and between years. This implies that a reduction of nitrate concentrations in the Scheldt Estuary can only be achieved through the further reduction of external nutrient andorganic matter inputs. This will lower primary production and ammonium inputs from recycled autochthonous and allochthonous organic matter, and thus ultimately lower nitrate inputs through nitrification.

Ninety per cent of marine organic matter burial occurs in continental margin sediments, where a substantial fraction of organic carbon escapes oxidation and enters long-term geologic storage within sedimentary rocks. In such environments, microbial metabolism is limited by the diffusive supply of electron acceptors. One strategy to optimize energy yields in a resource-limited habitat is symbiotic metabolite exchange among microbial associations.Thermodynamic and geochemical considerations indicate that microbial co-metabolisms are likely to play a critical part in sedimentary organic carbon cycling. Yet only one association, between methanotrophic archaea and sulphate-reducing bacteria, has been demonstrated in marine sediments in situ, and little is known of the role of microbial symbiotic interactions in other sedimentary biogeochemical cycles. Here we report in situ molecular and incubation-based evidence for a novel symbiotic consortium between two chemolithotrophic bacteria--anaerobic ammonium-oxidizing (anammox) bacteria and the nitrate-sequestering sulphur-oxidizing Thioploca species--in anoxic sediments of the Soledad basin at the Mexican Pacific margin. A mass balance of benthic solute fluxes and the corresponding nitrogen isotope composition of nitrate and ammonium fluxes indicate that anammox bacteria rely on Thioploca species for the supply of metabolic substrates and account for about 57 per cent of the total benthic N2 production. We show thatThioploca-anammox symbiosis intensifies benthic fixed nitrogen losses in anoxic sediments,bypassing diffusion-imposed limitations by efficiently coupling the carbon, nitrogen and sulphur cycles.

Stable sulphur isotopic composition (d34S) of gizzard shad Dorosoma cepedianum was used to investigate the seasonal and ontogenetic variation in the diet of young and adult fish. This study evaluated fish from a hypereutrophic lake that had recently undergone a 40% reduction of large (>300 mm total length, LT) D. cepedianum biomass as part of a biomanipulation experiment, which aimed at reducing internal nutrient loading. Dorosoma cepedianum d34S values showed evidence of ontogenetic changes with young fish (sources than adults (>200 mm LT). The d34S composition of the adult fish suggested an increasing importance of zooplankton in the diet, although benthic food sources remained part of the diet of all D. cepedianum collected in this study. The results indicated that benthic feeding is used by D. cepedianum of all sizes, suggesting that biomanipulation efforts may need to target all sizes of fish to realize benefits.

Lake Hancock is situated in the headwaters of the Peace River, and is identified as having an impact on water quality within the Peace River and Charlotte Harbor. Water quality in Lake Hancock has been verified as impaired for nutrients by the Florida Department of Environmental Protection, due to levels of total nitrogen, total phosphorus and biological oxygen demand that exceed the State of Florida's threshold values for impairment. This study examined the potential benefits of sediment removal on water quality in Lake Hancock. The project included two field experiments: (1) a manipulative experiment to determine if a reduction in nutrient availability could result in decreases in blue-green algal vigor and/or abundance and (2) a manipulative mesocosm experiment designed to evaluate whether partial removal of the lake's sediments would result in improvements in water quality. The findings were consistent with the hypothesis that Lake Hancock is a net sink for phosphorus loads that accumulate in the sediments and the lake is a net source of nitrogen which is exported from the lake. Only by reducing TP loads from both external and internal loading sources could TP levels be reduced to biologically relevant levels.

Large numbers of young of the year herring (Clupea harengus L.) and sprat (Sprattus sprattus (L.)) typically enter and remain within North Sea estuaries during the winter months. The main purpose of this study was to examine their migration dynamics between the North Sea and the Schelde estuary using C and N stable isotopes. Prior to this, stomach contents were used to verify the isotopic differences between the food sources at the sampling stations. From May 2000 to April 2001 fish were collected monthly in the upper and lower estuary. Muscle tissue and stomach contents were analyzed for d13C and d15N using an EA-IRMS. Based on the stomach contents, it was demonstrated that d15N could not be used as a tracer for fish migration because the longitudinal estuarine d15N gradient had reversed completely during autumn. The d13C gradient, however, was found to be reliable for studying fish movement in the Schelde estuary. Seasonal movements of clupeoids in the Schelde estuary were analyzed by separating the temporal abundance patterns into migration groups based on their muscle isotopic composition. Immigration and emigration seem to occur continuously throughout the year. Most exchange occurred in November. During winter, immigration remained high but gradually decreased. Although the herring and sprat abundance further declined in the estuary during February and March, large seaward emigration was not fully demonstrated. As temporal overlap between immigration and emigration is concluded the results support the hypothesis that migration to estuarine nurseries is individually based.

We monitored the stable nitrogen isotopic composition (d15N) of suspended matter and ammonium in the freshwater stretch of the Scheldt estuary (Belgium) over a full year to investigate for seasonal evolution and possible co-variation between isotopic signatures. The d15N value of ammonium remained rather constant during winter (average = +11.4 pro mille) but increased significantly with the spring and summer bloom, reaching values as high as +70 pro mille. This enrichment of the ammonium pool in 15N coincided with significant ammonium depletion during summer period, suggesting a close causal relationship. Based on a semiclosed system approach we deduced an apparent fractionation factor associated with NH4+ utilization (i.e. combining effects of uptake and nitrification) of 18.4 pro mille (SE = 2.0 pro mille), which is similar to values reported in literature. Observed variations of ammonium d15N could account for about 69% of d15N variation in suspended matter.

Reversing the effects of eutrophication can be challenging and requires the reduction of external nutrient sources and internal nutrient loading. Omnivorous gizzard shad Dorosoma cepedianum can facilitate nutrient loading from the sediments as a consequence of their foraging activity at the sediment-water interface and subsequent excretion of nutrients in the water column. This feeding activity may contribute considerably to the release of nutrients from the sediments in eutrophic Florida lakes. Biomanipulation via removal of gizzard shad has been proposed as a management strategy for improving water clarity by reducing internal nutrient loading from the sediments. Preliminary studies at Lake Denham, Florida, suggested that strong biomass reductions of gizzard shad using haul seines may reduce phytoplankton biomass. Recently, biomanipulations have been attempted on several lakes of the Harris Chain of Lakes, Florida using gill nets, but the results of these efforts have yet to be experimentally evaluated. Understanding how fish life history metrics respond to density reductions is critical to understanding the potential impact of biomanipulation on lake food webs. We used a whole-lake gizzard shad reduction experiment,hereafter referred to as a biomanipulation, to 1) assess impacts of a commercial gizzard shad removal on their population dynamics (i.e., recruitment, growth, mortality), 2) measure diet contents of gizzard shad to indicate mode of feeding, 3) explore the potential for gizzard shad removal to influence lake water quality, and 4) evaluate the potential for bycatch impacts on black crappie Pomoxis nigromaculatus fisheries. We tested the hypothesis that gizzard shad removal at Lake Dora would result in compensatory changes in reproductive rates of the gizzard shad population. We sought to understand the mechanisms for compensatory responses by evaluating changes in growth, reproductive investment, maturation schedules, larval fish densities, juvenile survival, and recruitment of gizzard shad.Lakes Eustis and Harris were used as reference sites with no commercial fishing. Commercial fishing with gill nets (minimum of 4 inch mesh) occurred in March and April of 2005 and January - March 2006. We collected data on gizzard shad population dynamics at all three lakes from November 2004 to May 2007. The total harvest of gizzard shad from Lake Dora was estimated at 124,989 kg (54 kg/ha) in 2005 and 135,095 kg (58 kg/ha) in 2006. Leslie depletion analysis estimated an exploitation rate on vulnerable-sized fish of 0.61 (95% confidence interval = 0.42 to 0.73) in 2005 and 0.46 (95% confidence interval = 0.30 to 0.63) in 2006. Total biomass reduction for the gizzard shad population was about 40% from both years of harvest combined. Compensatory responses of individual vital rates were weak following biomanipulation with the exception of length-at-maturity. Gizzard shad at Lake Dora matured at a size 40 mm smaller in 2007 than in 2005, and we observed no changes in size at maturity for Lakes Eustis or Harris. We detected no change in growth, the gonadosomatic index (an index of fecundity), and juvenile survival, but we found a small decrease in average larval fish density after fishing at Lake Dora. Despite small changes in vital rates, we found increased gizzard shad recruitment to age-1 from 2005 to 2007 at Lake Dora, indicating no reduction in gizzard shad recruitment despite substantial decrease in population egg production. Changes in individual vital rates that led to increased recruitment may have been very small or were obscured by sampling variation. Age-1 recruitment estimates were uncertain due to low vulnerability of these small fish to the experimental gill nets. Further sampling in 2008 and 2009 will track these cohorts as they become more vulnerable to the gill nets at age 2. If future samples confirm preliminary conclusions from age-1 recruitment estimates, we would conclude that the population compensated through increased reproduction and maintained constant or possibly increased recruitment despite a 40% total biomass reduction. This finding would have important implications for biomanipulation efforts because compensatory reproduction may dampen biomass reductions by maintaining or increasing the numbers of age-0 gizzard shad, even if the mechanisms for compensation are difficult to detect in field data. Future sampling will refine our conclusions regarding recruitment compensation of the gizzard shad population at Lake Dora. We used a simulation model to evaluate the relative performance of alternative gizzard shad removal strategies. The 4-inch mesh nets used in the commercial fishery showed dome-shaped vulnerability schedules for gizzard shad, with fish not fully vulnerable to the gear until age 3.5 and vulnerability declining after age 4. Our results show that gill net fisheries for gizzard shad are unlikely to cause large total biomass reductions for gizzard shad (i.e., ? 75% declines) under current gear and fishery configurations. Achieving a 75% reduction in total shad biomass, which is often the target in biomanipulation efforts, could only be achieved by 1) use of smaller mesh sizes, especially 3-inch mesh, 2) very high fishing mortality rates, and 3) fishing every year. We chose a 75% biomass reduction target based on literature reviews of many previous biomanipulation studies, but the degree of reduction required to reduce phytoplankton biomass in Florida lakes is unkown. Our results suggest that long-term total gizzard shad biomass reductions are unlikely to exceed 40-50% at Lake Dora or similar lakes without substantial increases in the fishing mortality (i.e., fishing effort) and decreases in gill net mesh size. Gizzard shad diets were evaluated using stable isotopes of sulfur and gut content analysis. Gizzard shad ?34S values confirmed the ontogenetic changes in the diet composition reported in literature. During the summer of 2006, gizzard shad ?34S signatures showed clear evidence of an ontogenetic shift from water column to benthic food items. The ?34S values of young gizzard shad were initially high (9-10o/oo) and associated with pelagic modes of feeding, but declined rapidly to values between 0.1o/oo and 2.4o/oo once a TL size of 100-200 mm was reached,suggesting increased importance of benthic feeding. Gut content analysis showed that nearly all gizzard shad stomachs contained evidence of both pelagic and benthic feeding. Gizzard shad in the 100 - 200 mm length class probably derive most of their food from the microflora associated with sediment detritus, whereas larger fish likely spend more time in the water column foraging on zooplankton (copepods and cladocerans), although their foreguts still contained plant and mud detritus. The size relationship with ?34S suggested some size-dependent diet shifts to zooplankton in gizzard shad populations. Our analysis of zooplankton and water quality data from 2003 to 2007 showed no changes in water quality and macrozooplankton biomass following gizzard shad removal at Lake Dora. The removal resulted in no change in chlorophyll a concentration, Secchi depth, or total phosphorus concentration. There were also clearly no changes in copepod or cladoceran biomass. Macrozooplankton communities may be controlled by a number of other factors including juvenile gizzard shad, threadfin shad D. penetense, invertebrate predators, or density of inedible filamentous algae. We expected gizzard shad removal to reduce water column phosphorus and chlorophyll a based on previous studies evaluating omnivore removals. Our results suggest that either 1) these effects are not likely via gizzard shad removal in Florida lakes, or 2) the biomass reduction was not strong enough to elicit a response in the phytoplankton community or total phosphorus concentrations. Although gizzard shad clearly contribute to internal phosphorus loads in eutrophic lakes, the magnitude of this loading relative to external inputs, sediment fluxes, and wind resuspension is unknown. Our results suggested that these other phosphorus loads substantially exceeded those attributable to the two-year gizzard shad removal at Lake Dora. Black crappie is the primary sport fish targeted by recreational anglers at Lake Dora, and our results show that the population could be negatively impacted by increases in exploitation resulting from either the recreational fishery or bycatch from the commercial gill net fishery for gizzard shad. The estimated recreational exploitation rate in 2006 was approximately the total sustainable exploitation rate, and increases due to recreational fishing and/or commercial bycatch greatly increase the probability of recruitment overfishing. Resource managers must evaluate policy trade-offs to consider the benefit of the gizzard shad removal and the negative impacts of bycatch mortality on recreational fisheries. Total bycatch estimates in 2006 (January - March) were nearly twice as high as total bycatch estimates in 2005 (March - April). These results suggest that bycatch could be reduced by timing the commercial fishing season to prevent fishing during winter and early spring when black crappie are more abundant in open-water areas where gill netting occurs. Bycatch impacts on black crappie fisheries may be acceptable if the gizzard shad reduction is successful in improving water clarity and increasing aquatic macrophyte abundance. Possible management alternatives are to 1) discontinue the gill net fishery to eradicate bycatch and prevent any harm to the black crappie recreational fishery, or 2) increase commercial effort and gizzard shad exploitation to optimize the success of the biomanipulation. The results of this study showed that continuing the program at the current level of commercial effort did not optimize either management objective at Lake Dora. Results of this study show that current commercial fishing gear configurations for gizzard shad reductions are unlikely to achieve large (> 75%) reductions in total gizzard shad biomass. We cannot conclude that biomanipulation is not a viable management tool for restoration of Florida lakes, but our results clearly show that 40% biomass reduction over two years did not significantly influence lake nutrients and zooplankton abundance at Lake Dora. Future biomanipulations targeting water quality improvements should seek to maximize biomass reductions for gizzard shad and should be conducted using control lakes to verify any shifts that occur. Lower mesh sizes and higher commercial fishing effort are recommended, but resource managers should recognize that substantial impacts to black crappie fisheries could occur.

The fate and transport of watershed-derived ammonium in a tidal freshwater marsh fringing the nutrient rich Scheldt River, Belgium, was quantified in a whole ecosystem N-15 labeling experiment. In late summer (September) we added N-15-NH4+ to the flood water entering a 3477 m(2) tidal freshwater marsh area, and traced the ammonium processing and retention in four subsequent tide cycles. In this paper we present the results for the water-phase components of the marsh system and compare them to a similar experiment conducted in spring/early summer (May). Changes in concentration and isotopic enrichment of NO3- + NO2-, N2O, N-2, NH4+ and suspended particulate nitrogen (SPN) were measured in concert with a mass balance study. All analyzed N-pools were labeled, and 49% of the added 15NH(4)(+) was retained or transformed. The most important pool for N-15 was nitrate, accounting for 17% of N-15-transformation. N-2, N2O and SPN accounted for 2.4, 0.02 and 1.4%, respectively. The temporal and spatial patterns of N-15 transformation in the water phase component of the system were remarkably similar to those observed in May, indicating good reproducibility of the whole ecosystem labeling approach, but the absolute ammonium transformation rate was 3 times higher in May. While the marsh surface area was crucial for nitrification in May this was less pronounced in September. Denitrification, on the other hand, appeared more important in September compared to May.

The objective of this study is to determine the extent and the major pathways of inorganic nutrients (with a focus on nitrogen) retention in a small eutrophic lowland river dominated by the growth of macrophytes. For this, we calculated for different seasons, daily nutrient fluxes (mainly N and P) through a 1.5 km section of a eutrophic macrophyte-dominated lowland river. Additionally, for each season, we measured the specific contribution of ammonium and nitrate uptake of macrophytes and phytoplankton using 15N tracers and we determined the net exchange fluxes between the water and the sediments using sediment core incubations. Finally, a mass balance integrating the incoming nutrient sources (upstream river, tributaries and groundwater inflow), the sediments, phytoplankton and macrophytes compartment was constructed.

Stable N isotopes are frequently used to assess seasonal variation in the trophic levels of aquatic consumers. This requires a thorough investigation of the variability of the d15N signature of the organisms at the base of the food web. However, since suspended matter is usually a mixture of terrestrial detritus, sewage, phytoplankton and microzooplankton, d15N signatures of the primary producers are difficult to assess. Since for estuaries under anthropogenic stress NH4 will often be the main nutrient assimilated by the primary producer community, the NH4 d15N can be used as an efficient proxy of the d15N of primary producers.We measured the seasonal evolution of d15N signatures of NH4 and calculated the in situ isotopic fractionation for NH4 taking into account assimilation and nitrification. Then, we compared the calculated seasonal pattern of d15N of phytoplankton with the d15N pattern of copepods to assess the relative importance of phytoplankton in the diet of copepods. In addition, we show that the increase in the d15N signature of SPOM in eutrophic estuaries relative to pristine systems is mediated through the phytoplankton community and related to the intensity of NH4 consumption processes enriching the d15N of NH4 .

Suspended particulate organic matter was sampled monthly between June 1999 and April 2000 in the Scheldt river andestuary to investigate the seasonal and spatial patterns of d13C and d15N signatures. d15N of suspended matter showed large seasonal variation. Minimum values ranged from -0.5 pro mille in the freshwater zone (spring situation) to + 2.3 pro mille in the mesohaline zone (winter situation). Maximum values (summer situation) ranged from + 8.8 pro mille in the freshwater zone to + 12.9 pro mille in the mesohaline zone. d13C showed less seasonal variation and ranged overall from -31.1 pro mille in the freshwater zone to -23.7 pro mille in the mesohaline zone. During the growth season, decrease of d13C and increase of d15N of suspended matter were due to local phytoplanktonic and bacterial biomass. There is strong evidence that the 15N enrichment of suspended matter during the growth season reflects the 15N enrichment of the ambient NH4 + pool induced by nitrification and NH4+ uptake. Zooplankton in the mesohaline section of the river was consistently enriched in 15N relative to suspended matter but followed its seasonal trend. During summer and autumn the isotopic offset between zooplankton and the suspended particulate organic matter was consistent with a pattern of selective feeding on phytoplankton. During summer, d15N of zooplankton reached a value as high as + 25.5 pro mille, the highest value observed during this study. During spring, present-day d15N of suspended matter in the oligohaline and mesohaline section increased compared to the 1970s, probably because today nitrification, which enriches the NH4+ pool in 15N, starts earlier in the season. For summer, the discrepancy between present-day suspended matter d15N values and those observed in the 1970s was even larger, especially in the oligohaline and freshwater reaches, probably as a result of improved O2 conditions now favouring nitrification. Likewise, the present decreased input of 15N-depleted sewage will enhance 15N enrichment of suspended matter during the growth season.

The Scheldt estuary is a highly polluted macrotidal estuary draining one of the most densily populated areas in the world (425 inhab/km²). The present nitrogen load to the estuary is approximately of 50 kT N / year from which about 20 % is under the form of ammonium. This ammonium is almost completely nitrified in the estuary, even in winter conditions, when low temperatures are known to limit the activity of nitrifying bacteria. This is probably linked to the fact that water residence times are very long (75 days on the average) sothat even with reduced nitrification rates, depletion of ammonium is still possible. The oxidation of ammonium to nitrate by nitrifying bacteria results in an enrichment of N-NH4 in its heavy isotope 15N. Indeed, the heavy isotope 15N is discriminated against the light 14N isotope during the oxidation process. Measurements of the delta 15N in particulate organic matter and copepods shows that the heavy isotopic 15N signature of ammonium is most probably incorporated in the entire microbial food web. This indicates that, at least for some periods of the year, the microbial food web of the estuary is based on microorganisms (phyto- and bacterio-plankton) finding their N requirements by assimilating NH4+ rather than any other inorganic or organic N source.