ALBERT

Beschreibung: Nutrient cycling and export in streams and rivers should vary with flow regime, yet most studies of stream nutrient transformation do not include hydrologic variability. We used a stable isotope tracer of nitrogen (15N) to measure nitrate (NO3−) uptake, storage, and export in a mountain stream, Spring Creek, Idaho, U.S.A. We conducted two tracer tests of 2‐week duration during snowmelt and baseflow. Dissolved and particulate forms of 15N were monitored over three seasons to test the hypothesis that stream N cycling would be dominated by export during floods, and storage during low flow. Floods exported more N than during baseflow conditions; however, snowmelt floods had higher than expected demand for NO3− because of hyporheic exchange. residence times of benthic N during both tracer tests were longer than 100 d for ephemeral pools such as benthic algae and wood biofilms. Residence times were much longer in fine detritus, insects, and the particulate N from the hyporheic zone, showing that assimilation and hydrologic storage can be important mechanisms for retaining particulate N. Of the tracer N stored in the stream, the primary form of export was via seston during periods of high flows, produced by summer rainstorms or spring snowmelt the following year. Spring Creek is not necessarily a conduit for nutrients during high flow; hydrologic exchange between the stream and its valley represents an important storage mechanism.

Beschreibung: We investigated the microbial pathways of nitrogen (N) loss in an April 2005 transect through the Peruvian oxygen minimum zone (OMZ) at 12°S latitude using short anaerobic incubations with 15N‐labeled substrates and molecular‐ecological and lipid‐biomarker studies. In incubations with 15NH4+, immediate production of 14N15N, but not 15N15N, indicated that N2 was produced by the pairing of labeled 15NH4+ with in situ 14NO2− via anaerobic ammonium oxidation (anammox). Supporting this finding, we also found anammox‐related 16S ribosomal ribonucleic acid gene sequences similar to those previously known from other marine water columns in which anammox activity was measured. We identified and enumerated anammox bacteria via fluorescence in situ hybridization and quantitative polymerase chain reaction and found ladderane membrane lipids specific to anammox bacteria wherever anammox activity was measured by our isotope tracer method. However, in incubations with 15NO3− or 15NO2−, in which denitrification would have been expected to produce 15N15N by pairing of oxidized 15N ions, 15N15N production was not detected before 24 h, showing that denitrification of fixed N to N2 was not taking place in our samples. At the time and locality of our study, anammox, rather than denitrification, was responsible for N2 production in the Peruvian OMZ waters.

Beschreibung: Compound‐specific radiocarbon (14C) contents, stable carbon isotopes, and abundances of phytoplankton and vascular plant derived lipid biomarkers (alkenones and fatty acids) were obtained from Santa Barbara Basin and Santa Monica Basin sediments, along with radiocarbon contents of planktic foraminifera and total organic carbon. We investigated core‐top and prebomb sediment intervals at sites from the flanks and depocenters of the basins deposited under contrasting bottom water oxygen concentrations. Bulk organic matter generally has the lowest radiocarbon levels of all sediment constituents measured, whereas planktic foraminifera tend to be the most radiocarbon enriched. Alkenones are systematically depleted in radiocarbon with respect to foraminifera. Short‐chain (C14, C16, C18) fatty acids decrease rapidly in absolute abundance and relative to longer‐chain (〉C24) homologues from core‐top to prebomb samples. The loss of short‐chain fatty acids with depth is associated with 13C depletion of short‐chain fatty acids, indicating preferential preservation of terrestrially derived fatty acids. Short‐chain fatty acids tend to be more 14C‐enriched relative to alkenones in core‐top sediments, whereas longerchain homologues are generally the most radiocarbon depleted of the lipids studied here. Less refractory compounds (e.g., short‐chain fatty acids) are thus enriched in radiocarbon with respect to more recalcitrant biomarkers (alkenones, long‐chain fatty acids). The lower 14C content of more refractory compounds reflects a larger proportion of laterally supplied, preaged material. Greater preservation of labile organic compounds observed at the depocenters than in flank sediments results in the presence of “younger” biomarkers, underlining the important influence of selective degradation of labile compounds on their radiocarbon ages.

Beschreibung: High‐latitude rivers supply the Arctic Ocean with a disproportionately large share of global riverine discharge and terrigenous dissolved organic matter (DOM). We used the abundance of lignin, a macromolecule unique to vascular plants, and stable carbon isotope ratios (δ13C) to trace the high molecular weight fraction of terrigenous DOM in major water masses of the Arctic Ocean. Lignin oxidation products in ultrafiltered DOM (UDOM; 〉1,000 Da) from Arctic rivers were depleted in syringyl relative to vanillyl phenols (S/V = 0.3–0.5) compared to UDOM in temperate and tropical rivers (S/V = 0.5–1.2), indicating that gymnosperm vegetation is a major source of terrigenous UDOM to the Arctic Ocean. High concentrations of lignin oxidation products (83–320 ng L−1) and a depletion of 13C (δ13C = −23.0 to −21.9) in UDOM throughout the surface Arctic Ocean indicate that terrigenous UDOM accounts for a much greater fraction of the UDOM in the surface Arctic (5–33%) than in the Pacific and Atlantic oceans (0.7–2.4%). In contrast, UDOM in deep water from the Arctic Ocean as well as waters from throughout the Greenland Gyre had relatively low concentrations of lignin oxidation products (24–45 ng L−1 ) and was enriched in 13C (δ13C = −21.0 to −20.8). Terrigenous UDOM has a relatively short residence (∼1–6 yr) in surface polar waters prior to export to the north Atlantic Ocean. Assuming that the bulk of Arctic‐derived DOM is compositionally similar to the UDOM fraction, we estimate that 12–41% of terrigenous DOM (2.9–10.3 Tg C yr−1 ) discharged by rivers to the Arctic Ocean is exported to the North Atlantic via surface waters of the East Greenland Current. It appears very little terrigenous DOM from the Arctic is incorporated into North Atlantic Deep Water and distributed globally via deep thermohaline circulation.

Beschreibung: Allochthonous organic carbon can subsidize consumers in aquatic systems, but this subsidy may only be significant in relatively small systems with high organic matter loading. We tested the importance of allochthonous carbon to consumers in a relatively large (258,000 m2) clear‐water lake by adding H13CO3 daily for 56 d. Dissolved inorganic carbon (DIC) was substantially enriched in 13C by the addition, but it was also variable over diel cycles because of exchange with the atmosphere and photosynthesis. By measuring the δ13C value of a physically separated phytoplankton concentrate as well as the δ13C of phospholipid fatty acids, we were able to follow 13C‐labeling dynamics of specific groups of phytoplankton and bacteria. The δ13C values of particulate organic carbon (POC), dissolved organic carbon (DOC), phytoplankton, bacteria, zooplankton, and the invertebrate predator, Chaoborus spp. all increased to a maximum during the addition and declined once the addition ceased. Autochthony (% C derived from internal primary production) of carbon pools (POC, DOC) and consumers was assessed by fitting dynamic models to time series of δ13C. Autochthonous carbon was the dominant source (88‐100%) for POC, gram‐positive bacteria, a copepod, zooplankton biomass, and Chaoborus spp. Autochthonous carbon provided a lower fraction (〈70%) of carbon to DOC, gram‐negative bacteria, and cladoceran zooplankton. In comparison to smaller and more humic lakes, terrestrially derived allochthonous C was less significant to the pelagic food web in this larger, clear‐water lake. Among lakes, the relative importance of autochthonous versus allochthonous carbon to planktonic consumers is positively correlated to the ratio of color (absorbance of light at 440 nm, an indicator of terrestrially derived organic carbon) to chlorophyll.

Beschreibung: We report the results of an integrated modeling and in situ isotope tracer experiment study examining the fate of bacterial carbon in an intertidal sediment. 13C‐glucose was injected into the upper 10 cm of an intertidal sediment, which successfully tagged the bacterial community as evidenced by 13C enrichment of bacterial specific polar‐lipid‐derived fatty acids. Over a period of 4.5 months, 13C enrichment was monitored in sediment organic carbon, bacteria, meiobenthos, macrobenthos, and dissolved inorganic carbon. A mechanistic model accurately simulated label transfer among the biotic and abiotic compartments and was used to derive bacterial production and the loss processes grazing, mortality, respiration, and exchange. Bacterial production averaged 67 mmol C m‐2 d‐1, of which 8% was lost from the sediment by exchange processes, 3% was grazed by meiobenthos and 24% by macrobenthos. The primary fate of bacterial production was mortality (65%) and the released bacterial carbon was recycled back to dissolved organic carbon, resulting in recycling of carbon within the dissolved organic carbon‐bacteria loop. Bacterial respiration was the main loss process from this loop. Although a significant fraction of bacterial carbon production was grazed, our results show that bacterial carbon is primarily a sink of organic carbon in the food web of intertidal sediments.

Beschreibung: We carried out short‐term (2 d) experiments in nine unproductive lakes in northern Sweden in order to investigate organic carbon sources supporting lake water respiration. Surface water was incubated in gas‐tight bottles in the dark, and the concentration and isotopic composition (δ13C) of dissolved inorganic carbon (DIC) were measured at the start and end of the incubations. Keeling plot analyses revealed that the δ13C of the respired carbon was between ‐28.4% and ‐30.6% in the lakes and that the respired carbon was mainly of allochthonous organic carbon (AlloOC) origin. The respiration of AlloOC corresponded well with metabolic imbalances indicated by negative net ecosystem production (NEP) values in the lake waters. Keeling plot analysis of DIC accumulating in the hypolimnion of two lakes during summer stratification showed δ13C values of around ‐26.6% for excess DIC, implying that the accumulation of DIC was mainly derived from respiration of AlloOC. Our data provide direct evidence that net heterotrophy of these lakes is caused by input and respiration of AlloOC. We conclude that the Keeling plot method is a powerful technique that enables characterization and quantification of the organic carbon sources contributing to respiration in aquatic systems.

Beschreibung: We quantified the fate and transport of watershed‐derived ammonium in a tidal freshwater marsh fringing the nutrientrich Scheldt River in a whole‐ecosystem 15N labeling experiment. 15N‐NH4+ was added to the floodwater entering a 3,477 14 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 Br2 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.

Beschreibung: We report the trophic structure of a myctophid assemblage by measuring the isotopic niches of 14 species living in Kerguelen waters, southern Indian Ocean. Most of the species show distinct isotopic niches that differ by at least one of the two niche axes (δ13C habitat and δ15N trophic position), indicating trophic partitioning within the assemblage. Strong niche segregation occurs within each of the three most common genera of myctophids (Electrona, Gymnoscopelus, and Protomyctophum), illustrating the different mechanisms (habitat and dietary segregation) that allow coexistence of closely related species. Calculated trophic levels (TLs) of myctophids ranged from 3.3 to 4.2, showing that they are secondary and tertiary consumers in the pelagic ecosystem. The positive relationship between TL and standard length of fish points out a structuring effect of size, with larger species (Gymnoscopelus spp.) occupying a higher trophic position than smaller species (Krefftichthys anderssoni and Protomyctophum spp.). Myctophids occupy an intermediate trophic position between macrozooplanktonic crustaceans and seabirds and marine mammals within the pelagic ecosystem. However, the TLs of large myctophids overlap those of crustacean‐eating seabirds [e.g., Eudyptes spp. (crested penguins) and Pachyptila belcheri]. The isotopic niche of myctophids indicates that Aptenodytes patagonicus (king penguin) adults prey upon K. anderssoni when they feed for themselves, thus exemplifying the usefulness of isotopic datasets on potential prey of predators to depict trophic relationships.

Beschreibung: We report the first measurements of coupled nitrogen (N) and oxygen (O) isotope fractionation of nitrate by laboratory cultures of denitrifying bacteria. Two seawater strains (Pseudomonas stutzeri, Ochrobactrum sp.) and three freshwater strains (Paracoccus denitrificans, Pseudomonas chlororaphis, Rhodobacter sphaeroides) were examined. Among four strains of facultative anaerobic denitrifiers, N and O isotope effects were variable, ranging from 5‰ to 25‰, with evidence for a drop in the isotope effects as nitrate concentrations approached the halfsaturation constant for nitrate transport. O isotope effects were similar to their corresponding N isotope effect, such that the progressive increase in nitrate δ18O, when plotted against that in δ15N (where δ18Osample = [(δ18O: δ16Osample/(δ18O:δ16O)reference ‐ 1] × 1000, and δ15Nsample × [(15N:14N)sample/(δ15N:δ14N)reference ‐ 1] × 1000), yielded slopes of 0.86 to 1.02, with a mean value of 0.96. R. sphaeroides, a photo‐heterotroph that possesses only a periplasmic (nonrespiring) dissimilatory nitrate reductase, showed less variability in nitrate N isotope effects, between 13‰ and 20‰, with a modal value of ~15‰. In contrast to the respiratory denitrifiers, R. sphaeroides consistently showed a distinct ratio of δ18O to δ15N change of ~0.62. We hypothesize that heavy N and O isotope discrimination during respiratory denitrification occurs during the intracellular reduction of nitrate by the respiratory nitrate reductase, and the observed magnitude of fractionation is likely regulated by the ratio of cellular nitrate efflux relative to uptake. The data for R. sphaeroides are consistent with isotope discrimination directly reflecting the N and O isotope effects of the periplasmic nitrate reductase NAP, without modification by nitrate uptake and efflux.

Beschreibung: We used variation in algal d13C between river habitats to study the spatial scale of energy flow through river food webs. We found a strong negative relationship between herbivore d13C (which reflects algal d13C) and water velocity in three productive Northern California rivers but not in unproductive streams. The contrast among habitats suggests that water velocity affects algal d13C most strongly when CO2 availability is low relative to photosynthetic rates. Our results help explain the wide variation in published river biota d13C and show that past studies using carbon isotope analyses may have significantly underestimated the importance of algal‐derived carbon to river food webs. While flow‐related variation in d13C complicates this common application of carbon isotope analysis, we show that it provides a natural tracer of the flux of algal production derived from different habitats within rivers to higher trophic levels. Measurements of consumer d13C showed that most invertebrate and vertebrate consumers relied on local production, except for filter‐feeding insects and steelhead trout, which relied on production derived from multiple sources. Stable carbon isotopes may thus be used to spatially delineate the habitats that support river food webs, providing previously unavailable information for understanding and managing river ecosystems.

Beschreibung: Marine diatoms generally form large blooms during periods of cool temperature (〈20°C), high NO3− fluxes (〉25 μM‐N), and turbulent mixing, but the adaptations that allow diatoms to bloom under these conditions are not well understood. We have conducted both NO3− uptake kinetics and direct short‐term temperature manipulation studies on field diatom‐dominated populations from Chesapeake and Delaware Bays during both spring and fall blooms. Absolute rates of NO3− uptake by a Rhizoseleni‐dominated population did not appear to saturate even at concentrations as high as 180 µM‐N. We observed contrasting patterns of NO2−, NH+, and urea utilization as a function of experimental temperature (ambient ± 9°C). Over the temperature range of 7–25°C, absolute uptake rates of NO3− (ρNO3−) decreased an average of 46% with increasing temperature from 7 to 25°C (nine individual experiments), while ρNH4+ and ρUREA increased with increasing temperature by an average of 179 and 86% (eight individual experiments), respectively. Based on these observations and the nature of the physical environment, we hypothesize that these diatom‐dominated populations were taking up NO3− in excess of nutritional requirements, the reduction of which may serve as a sink for electrons during transient periods of imbalance between light energy harvesting and utilization. We suggest that the increase in non‐nutritional NO3− uptake increases proportionately with the magnitude of the imbalance between light energy harvesting and imbalance. This hypothesis reconciles previous observations of low C:N uptake ratios, high release rates of dissolved organic nitrogen or NO2− by diatom‐dominated assemblages, other observations of nonsaturating NO3− kinetics in field populations, and the apparent "preference" for NO3− by the netplankton size fraction. The two phenomena described here, nonsaturable kinetics and a negative relationship between NO3− uptake and short‐term temperature shifts, have important ecological implications. The hypothesized ability of these diatom‐dominated populations to better modulate the flow of photosynthetic electron energy, via NO3− reduction, in variable environments may provide a competitive advantage to diatoms and could potentially explain why diatoms frequently dominate in regions of cool temperature, high NO3− flux, and turbulent mixing. Also, models of new production may need to incorporate terms for temperature dependence of NO3− uptake. Finally, if a significant fraction of NO3− uptake is regulated by non‐nutritional mechanisms in the cell, and if some fraction of nitrogen reduced by this mechanism is subsequently released in the form of NO2−, NH+, or dissolved organic nitrogen (DON), then estimates of new production based solely on NO3− uptake could be seriously biased.