ALBERT

Description: 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.

Description: 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.

Description: 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.

Description: The use of 15N to measure the flux of nitrogen compounds has become increasingly popular as the techniques and instrumentation for stable isotope analysis have become more widely available. Questions concerning equations for calculating uptake, effect of isotope dilution (in the case of ammonium), duration of incubation, and relationship between disappearance of a nitrogen compound and the 15N uptake measurement have arisen, especially for the research conducted in oligotrophic regions. Fewer problems seem to have occurred in eutrophic areas. However, sufficient literature now exists to allow some generally accepted experimental procedures for 15N studies in eutrophic regions to be laid down. Incubation periods of 2–6 h appear to avoid problems related to isotope dilution and to overcome the bias introduced in some cases by initial high rate or surge uptake. During such incubation periods, assimilation is measured rather than uptake or transport into the cell. Incorporation of 15N into the particulate fraction is usually linear with time over the periods currently used. The 15N method provides a better estimate of incorporation into phytoplankton than 14N disappearance, but a small fraction appears to be lost. Although most workers suggest the loss to be a result of dissolved organic nitrogen production, direct evidence is lacking. If the considerations discussed here are applied with the 15N techniques currently available, reliable estimates of phytoplankton nitrogen flux in eutrophic areas can be obtained.

Description: Most of the marine phytoplankton species for which data are available are rate saturated for photosynthesis and probably for growth with inorganic C at normal seawater concentrations; 2 of the 17 species are not saturated. Photosynthesis in these two species can probably be explained by assuming that CO2 reaches the site of its reaction with RUBISCO (ribulose bisphosphate carboxylase‐oxygenase) by passive diffusion. The kinetics of CO2 fixation by intact cells are explicable by RUBISCO kinetics typical of (eucaryotic) algae, and a CO2‐saturated in vivo RUBISCO activity not more than twice the in vivo light‐ and inorganic‐C‐saturated rate of photosynthesis. For the other species, the high affinity in vivo for inorganic C (and several other attributes) could be explained by postulating active influx of inorganic C yielding a higher concentration of CO2 available to RUBISCO during steady state photosynthesis than in the medium. Although such a higher concentration of internal CO2 in cells with high affinity for inorganic C is found at low (subseawater) levels of external inorganic C, the situation is more equivocal at normal seawater concentrations. In theory, the occurrence of a CO2 concentrating mechanism rather than passive CO2 entry (with consequent glycolate synthesis and metabolism or excretion) could reduce the photon, N, Fe, Mn, and Mo costs of growth, but increase the Zn and Se costs. Thus far, data on costs are available only for photons and N; these data generally agree with the predicted lower costs for cells with high affinity for inorganic C. The ecological significance of these attributes is that most marine phytoplankters are not likely to have photosynthetic or growth rates reduced by the measured decreases in inorganic C in productive seawater, drawdown of inorganic C in productive seawater (or increase as atmospheric CO2 increases) might alter the competitive balance between cells with low and high affinity for inorganic C, and differences in the effectiveness of use of other resources between cells with high and low affinity could cause differences in the rate and extent of resource‐limited growth for communities dominated by high‐affinity or low‐affinity cells.

Description: Nutrient enrichment as a result of anthropogenic activity concentrated along the land‐sea margin is increasing eutrophication of near‐shore waters across the globe. Management of eutrophication in the coastal zone has been hampered by the lack of a direct method to trace nitrogen sources from land into coastal food webs. Stable isotope data from a series of estuaries receiving nitrogen loads from 2 to 467 kg N ha−1 yr−1 from the Waquoit Bay watershed, Cape Cod, Massachusetts, indicate that producer and consumer 15N‐to‐14N ratios record increases in wastewater nitrogen inputs. Nitrate from groundwater‐borne wastewater introduces a 15N‐enriched tracer to estuaries. This study explicitly links anthropogenically derived nitrogen from watersheds to nitrogen in estuarine plants and animals, and suggests that wastewater nitrogen may be detectable in estuarine biota at relatively low loading rates, before eutrophication leads to major changes in species composition and abundance within estuarine food webs.

Description: Organic matter and its carbon and nitrogen isotopic composition were measured in sequential sediment trap and core samples from the Rochester Basin of Lake Ontario to evaluate their usefulness in reconstructing historic changes in lake productivity. The greatest flux of organic matter from the epilimnion occurred during late summer and coincided with whiting events, indicating that calcite precipitation is an effective mechanism for sedimenting organic matter. Carbon isotopes of organic matter were low prior to the onset of stratification, increased to maximum values in late summer, and then decreased following fall overturn. This pattern is controlled mainly by the timing of stratification and primary productivity, which preferentially removes 12CO2 from the epilimnion. The physiological effect of decreased carbon isotopic fractionation with decreasing supplies of [CO2]aq may have also contributed to increased δ13CorgC. Nitrogen isotopes showed a seasonal pattern opposite to that of carbon, whereby δ15N values were low during the summer stratified period and high for the remainder of the year. Seasonal variability in δ15NorgN probably reflects changes in the source of sedimented organic particles, which is dominated by isotopically depleted phytodetritus during the stratified period and isotopically enriched organic matter from heterotrophic or detrital sources during the mixed period. A comparison of organic carbon accumulation rates and δ13CorgC between sediment cores collected in 1987 and 1993–1994 confirms earlier predictions that diagenetic processes reduce the mass accumulation of organic carbon in the zone of oxic pore waters, but will not change the δ13CorgC values. All cores analyzed for δ13CorgC display the reproducible pattern of a progressive increase in the 19OOs, peaking in the early to mid‐1970s, and then decreasing to the present. This pattern matches the historical trends of phosphorus loading to the basin, suggesting that δ13C of organic carbon is a reliable proxy for paleoproductivity and responds to spring phosphorus supplies in the water column. The δ15N of sedimentary organic matter increased linearly from 1840 to 1960 at a rate of 0.3ppt per decade, and remained relatively constant thereafter except for an increase in the upper few centimeters of sediment. The increase in δ15NorgN reflects a combination of factors, including early forest clearance by Europeans, increased sewering by municipalities after 1940, and increased nitrate utilization as productivity increased in the lower Great Lakes. Increased rates of denitrification in the central basin of upstream Lake Erie from the 1930s to the early 1970s may have also contributed to the rise in δ15NorgN values.

Description: Eight stations in the main body of Chesapeake Bay and one on the continental shelf were sampled seven times over a period of 13 months to investigate the nitrogenous nutrition of the phytoplankton. The rates at which the phytoplankton were utilizing NO3−, NO2−, NH4+, and urea N were determined. The data demonstrate that for a large portion of the year there is inadequate N nutrient available to permit a single doubling of the particulate N. Over temperatures from 4°–28°C and salinities from 2–32‰, there was a universally high phytoplankton preference for NH4+ and urea N over NO3− and NO2−. A relative preference index indicated that NH4+ concentrations in excess of 0.5–1.0 µg‐atom N liter−1 almost totally suppressed NO3− utilization. Urea N was used after NH4+ in order of preference, and when the sum of available NH4+ and urea N was insufficient to meet the phytoplankton N nutrient demand, NO3− was used. When the sum of all available N nutrients was less than that required to satiate the phytoplankton demand, NH4+, urea N, NO3−, and NO2− were all utilized at rates proportional to their availability. For the midbay region in October 1973, NO2− was the dominant N nutrient present both in the water and in the diet of the phytoplankton.

Description: The stable isotopes of sulfur, nitrogen, and carbon were used to trace organic matter flow in salt marshes and estuarine waters at Sapelo Island, Georgia. Organic matter inputs from terrestrial sources as detrital input either from forests adjacent to the marshes or from rivers were not detectable by their isotopic signatures in estuarine consumers. The results suggest that there are two major sources of organic matter for the fauna of the marshes and estuarine waters of Sapelo Island: Spartina and algae. The long‐standing debate about the relative importance of Spartina detritus and algae in supporting marsh and estuarine secondary production appears from this analysis to be a draw; both sources are important and their relative importance is determined by feeding mode, size, location, and trophic position of the marsh and estuarine consumers.

Description: The utility of 15N isotope dilution models for the calculation of uptake and remineralization of NH4+ by marine phytoplankton was examined in light of model limitations when applied to field data either when ambient NH4+ levels border on our limit of detection or when there is no statistically significant difference between ambient NH4+ at the beginning and end of an incubation. Through specific examples of field and laboratory data we show that the limitations are a function both of analytical error inherent in the methodology and of changes in rates of uptake and remineralization over the course of a given experiment. We propose modifications to the existing models of NH4+ uptake and remineralization which overcome some of these limitations. The results show that uptake rates have been traditionally underestimated by a factor of ≈2 in routine 15N uptake methodology and that regeneration of NH4+ over relatively brief periods can supply the daily nitrogen requirements of the phytoplankton when there are no losses from the system.