Search

International audience; An intact tidal freshwater marsh system (3477 m2) was labelled by adding 15N-ammonium as a tracer to the flood water inundating the ecosystem. The appearance and retention of 15N-label in different marsh components (leaves, roots, sediment, leaf litter and invertebrate fauna) was followed over 15 days. This allowed us to elucidate the direct assimilation and dependence on creek-water nitrogen on a relatively short term and provided an unbiased assessment of the relative importance of the various compartments within the ecosystem. Two separate experiments were conducted, one in spring/early summer (May 2002) when plants were young and building up biomass; the other in late summer (September 2003) when macrophytes were in a flowering or early senescent state. Nitrogen assimilation rate (per hour inundated) was >3 times faster in May compared to September. On both occasions, however, the results clearly revealed that the less conspicuous compartments such as leaf litter and ruderal vegetations are more important in nitrogen uptake and retention than the prominent reed (Phragmites australis) meadows. Moreover, short-term nitrogen retention in these nutrient rich marshes occurs mainly via microbial pathways associated with the litter and sediment. Rather than direct uptake by macrophytes, it is the large reactive surface area provided by the tidal freshwater marsh vegetation that is most crucial for nitrogen transformation, assimilation and short term retention in nutrient rich tidal freshwater marshes. Our results clearly revealed the dominant role of microbes in initial nitrogen retention in marsh ecosystems.

To understand the impact of the northwestern Azores Current Front (NW-AzC/AzF) system on HCO3--and N2-fixation activities and unicellular diazotrophic cyanobacteria (UCYN) distribution, we combined geochemical and biological approaches from the oligotrophic surface to upper mesopelagic waters. N2-fixation was observed to sustain 45–85% of the HCO3--fixation in the picoplanktonic fraction performing 47% of the total C-fixation at the deep chlorophyll maximum north and south of the AzF. N2-fixation rates as high as 10.9 µmol N m-3 d-1 and surface nitrate d15N as low as 2.7‰ were found in the warm (18–24°C), most saline (36.5–37.0) and least productive waters south of the AzF, where UCYN were the least abundant. However, picoplanktonic UCYN abundances up to 55 cells mL-1 were found at 45–200m depths in the coolest nutrient-rich waters north of the AzF. In this area, N2-fixation rates up to 4.5 µmol N m-3 d-1 were detected, associated with depth-integrated H13CO3--fixation rates at least 50% higher than observed south of the AzF. The numerous eddies generated at the NW-AzC/AzF seem to enhance exchanges of plankton between water masses, as well as vertical and horizontal diapycnal diffusion of nutrients, whose increase probably enhances the growth of diazotrophs and the productivity of C-fixers.

The Scheldt estuary is a highly heterotrophic, nutrient-rich, turbid, tidal estuary in a densely populated area (Belgium/The Netherlands). Here we present the results (1) on the long-term changes in nutrient loadings and transformations within the estuary and (2) on nitrogen cycling rate measurements obtained with isotopic tracers. Moreover, we have developed and applied novel techniques that allow direct linking of process rates to the identity and biomass of the organisms involved. Monitoring data and process studies have been used in numerical models to integrate the various biogeochemical cycles and to advance our understanding of the evolving estuarine filter function of the Scheldt estuary.

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.

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‰) but increased significantly with the spring and summer bloom, reaching values as high as +70‰. This enrichment of the ammonium pool in 15N coincided with significant ammonium depletion during summer period, suggesting a close causal relationship. Based on a semi-closed system approach we deduced an apparent fractionation factor associated with NH4+ utilization (i.e. combining effects of uptake and nitrification) of 18.4‰ (SE = 2.0‰), which is similar to values reported in literature. Observed variations of ammonium d15N could account for about 69% of d15N variation in suspended matter.

International audience; We conducted two (May 2002 and September 2003) pulse additions of 15NH4+ to the flood water inundating a tidal freshwater marsh fringing the nutrient-rich Scheldt River (Belgium) and traced the fate of ammonium in the intact ecosystem. Here we report in detail the 15N uptake into the various marsh components (leaves, roots, sediment, leaf litter and invertebrate fauna), and the 15N retention on a scale of 15 days. We particularly focus on the contributions of the rooted macrophytes and the microbial community in the sediment and on plant litter. Assimilation and short term retention of 15NH4+ was low on both occasions. Only 4?9% of the added 15N trace was assimilated, corresponding to 13?22% and 8?18% of the processed 15N (i.e. not exported as 15NH4+ in May and September, respectively. In May nitrogen assimilation rate (per hour inundated) was >3 times faster than in September. Macrophytes (above- and below ground) were of limited importance for short term 15N retention accounting for 15NH4+ processed by the marsh. The less dominant herbaceous species were more important (on an area basis) than the dominant reed (Phragmites australis). The microbial community colonizing the sediment and litter surfaces were responsible for most nitrogen assimilation and short-term retention in the marsh. The large reactive surface area available for microbial colonization together with direct plant uptake, are the crucial components for nitrogen assimilation, retention and transformation in nutrient-rich tidal freshwater marshes.

A 4-year time series (2001–2004) of the partial pressure of CO2 (pCO2) and air-sea CO2 fluxes is reported in the Scheldt estuarine plume. This system is oversaturated in CO2 with respect to the atmosphere, except during the spring phytoplanktonic bloom, and acts as a net source of CO2 to the atmosphere of 0.7 mol C m-2 yr-1 that represents 7 to 27% of the inner Scheldt estuary CO2 emission. Results also highlight that a high spatial and temporal coverage of the surface pCO2 in coastal ecosystems is crucial for reliable estimations of air-sea CO2 fluxes. The seasonal variations of pCO2 seem to be more dominated by biological activities (photosynthesis/respiration) than by temperature change. A stochiometrically linked C/P mass balance budget failed to provide net ecosystem production estimates consistent with the pCO2 dynamics in this area. It is hypothesized that this discrepancy is related to physiological characteristics of the dominant phytoplanktonic species (Phaeocystis sp.) within the studied area. On the basis of a preliminary dissolved inorganic carbon input/output budget, the annual emission of CO2 toward the atmosphere seems to be largely due to the outgassing of the inputs of CO2 from the inner Scheldt estuary, rather than due to organic carbon degradation.

This paper investigates the estimation behavior of six increasingly complex 15N models, for estimating flux rates between phytoplankton and dissolved N pools in aquatic ecosystems. The development of these models over the last 40 years reflects increasing realism in the pools and fluxes that constitute the N cycle. The purpose of this paper is to assess how the model results are influenced by the underlying assumptions. For example, with respect to uptake of 15N by phytoplankton, is anything gained by assuming that regenerated N become isotopically enriched after the introduction of the 15N label, or is it just as accurate to assume that no source other than the initial 15N label contributes to the enrichment signal in phytoplankton? To conduct an objective assessment of the models, we compared them to (1) a set of reference values generated numerically by a process oriented model, and (2) real experimental data. The results show that for a number of 15N models, properties such as accuracy and precision cannot both be optimized under the same conditions, and a compromise must be struck. Oversimplified models risk bias when their underlying assumptions are violated, but overly complex models can misinterpret part of the random noise as relevant processes. Therefore none of the 15N model solutions can a priori be rejected, but each should carefully be assessed with hypothesis testing. A backward regression strategy based on a statistical interpretation of the cost function (sum of the weighted least squares residuals) was used to select optimal solution subsets corresponding to a given data set.

We quantified the fate and transport of watershed-derived ammonium in a tidal freshwater marsh fringing the nutrient rich Scheldt River in a whole-ecosystem 15N labeling experiment. 15N-NH4+ was added to the floodwater entering a 3,477 m² tidal marsh area, and marsh ammonium processing and retention were traced in six subsequent tide cycles. We present data for the water phase components of the marsh system, in which changes in concentration and isotopic enrichment of NO3-, NO2-, N2O, N2, NH4+, and suspended particulate nitrogen (SPN) were measured in concert with a mass balance study. Simultaneous addition of a conservative tracer (NaBr) confirmed that tracer was evenly distributed, and the Br- budget was almost closed (115% recovery). All analyzed dissolved and suspended N pools were labeled, and 31% of added 15N-NH4+ was retained or transformed. Nitrate was the most important pool for 15N, with nitrification accounting for 30% of 15N -transformation. In situ whole-ecosystem nitrification rates were four to nine times higher than those in the water column alone, implying a crucial role for the large reactive marsh surface area in N-transformation. Under conditions of low oxygen concentrations and high ammonium availability, nitrifiers produced N2O. Our results show that tidal freshwater marshes function not only as nutrient sinks but also as nutrient transformers.

A diffusion method for extracting ammonium from marine, estuarine and fresh waters for 15N/14N isotopic ratio determinations at enriched level was developed and validated. The method is based on the conversion of NH4+ to NH3 gas under alkaline conditions, diffusion of NH3 out of the solution to the headspace, NH3 trapping on an acidified GF/D glass fiber filter, and subsequent 15N/14N isotope ratio determination with mass spectrometry. The diffusion period necessary to extract sufficient N in order to accurately measure the atom% 15N was reduced to less than 15h by bubbling the sample with a carrier gas (Air) at room temperature. The technique uses 250mL sample volume and enables accurate atom% 15N measurements in NH4+ pools as small as 1.25μM. A standard operating procedure for ammonium extraction is given involving method performance criteria, such as accuracy, precision, detection limit, quantification limit and robustness. The efficiency of the NH4+ extraction ranged from 40 to 100%. The quantification limit of the method was estimated at around 0.26μmol% N, for an initial 15N abundance of 1%. The within-laboratory reproducibility amounted to 0.03atom% 15N. It was shown that the recovery rate obtained after extraction of the certified reference material (CRM: IAEA-311) solution falls within the 95% confidence interval of the certified values. By applying the developed method to fortified natural water samples of different conductivities, the atom% 15N determinations were precise and accurate for α=1–5%.

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.

Suspended particulate organic matter was sampled monthly between June 1999 and April 2000 in the Scheldt river and estuary to investigate the seasonal and spatial patterns of delta13C and delta15N signatures. delta15N of suspended matter showed large seasonal variation. Minimum values ranged from -0.5 promille in the freshwater zone (spring situation) to +2.3 promille in the mesohaline zone (winter situation). Maximum values (summer situation) ranged from +8.8 promille in the freshwater zone to +12.9 promille in the mesohaline zone. delta13C showed less seasonal variation and ranged overall from -31.1 promille in the freshwater zone to -23.7 promille in the mesohaline zone. During the growth season, decrease of delta13C and increase of delta15N of suspended matter were due to local phytoplanktonic and bacterial biomass. There is strong evidence that the delta15N enrichment of suspended matter during the growth season reflects the delta15N enrichment of the ambient NH4+ pool induced by nitrification and NH4+ uptake. Zooplankton in the mesohaline section of the river was consistently enriched in delta15N 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, delta15N of zooplankton reached a value as high as +25.5 promille, the highest value observed during this study. During spring, present-day delta15N of suspended matter in the oligohaline and mesohaline section increased compared to the 1970s, probably because today nitrification, which enriches the NH4+ pool in delta15N, starts earlier in the season. For summer, the discrepancy between present-day suspended matter delta15N 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 delta15N-depleted sewage will enhance delta15N enrichment of suspended matter during the growth season.

The distribution of nitrification has been measured with the H14CO3- incorporation method in the Seine River and its estuary during summer conditions. The Seine River below Paris receives large amounts of ammonium through wastewater discharge. In the river itself, this ammonium is only slowly nitrified, while in the estuary nitrification is rapid and complete. We show that this contrasting behavior is related to the different hydrosedimentary conditions of the two systems, as nitrifying bacteria are associated with suspended particles. In the river, particles and their attached bacteria either rapidly settle or have a sestonic behavior. Because of the short residence times of the water masses, the dow growing nitrifying population has no time to develop sufficiently to nitrify the available ammonium. The estuary is characterized by strong tidal dynamics. Particles settle and are resuspended continuously with the strong current inversions of ebb and hood. As a result of these dynamics, particles and their attached nitrifying bacteria experience longer residence times in a temporary suspended state than the water masses themselves, providing to slow growing nitrifying bacteria the opportunity to develop a large population capable of nitrifying all the available ammonium.