ALBERT

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.

Description: Knowledge of the fractionation of nitrogen isotopes by phytoplankton is a key requirement for the calibration of the new δ15N paleotracer. An essential part of information required in this calibration concerns the magnitude of isotopic fractionation during the incorporation of N substrates by phytoplankton. To this end, the δ15N of batch cultures of Thalassiosira pseudonana grown on nitrate, nitrite, ammonium, and urea was determined. This paper reports the first δ15N study of phytoplankton growth on urea (e.g. organic N substrate). The δ15N of the particulate nitrogen (PN) collected during the logarithmic growth phase, thus for N‐sufficient cells, was lower than the δ15N of the source due to kinetic isotope fractionation. With increasing drawdown of the N substrate, the δ15N of the accumulating PN increased in accordance with the Rayleigh distillation model. Enrichment factors (ε) derived from a least‐squares analysis of the accumulated δ15NPN data were 5.2 ± 0.2‰, 0.9 ± 0.6‰, 20 ± 1‰, and 0.8 ± 0.6‰ for NO3−, NO2−, NH4+, and urea incorporation, respectively. Overall, ε values for nitrate incorporation were consistent with field estimates of ~6‰ and could be used to estimate past relative nitrate utilization as estimated from the 615N of bulk sedimentary organic matter.

Description: Benthic community responses to lake eutrophication are poorly understood relative to pelagic responses. We compared phytoplankton and periphyton productivity along a eutrophication gradient in Greenland, U.S., and Danish lakes. Phytoplankton productivity increased along the phosphorus gradient (total phosphorus [TP] = 2–430 mg m−3), but whole‐lake benthic algal productivity decreased, substantially depressing increases in primary productivity at the whole‐lake scale. In shallow, oligotrophic Greenland lakes, periphyton was responsible for 80–98% of primary production, whereas in Danish lakes with TP 〉 100 mg m−3, phytoplankton were responsible for nearly 100% of primary production. Benthic contributions ranged from 5 to 80% depending on morphometry and littoral habitat composition in lakes with intermediate phosphorus concentrations. Thus, eutrophication was characterized by a switch from benthic to pelagic dominance of primary productivity. Carbon stable isotope analysis showed that the redistribution of primary production entailed a similar shift from periphyton to phytoplankton in the diets of zoobenthos. Benthic and pelagic habitats were energetically linked through food web interactions, but eutrophication eroded the benthic primary production pathway.

Description: Deep‐water nitrate is a major reservoir of oceanic combined nitrogen and has long been considered to be the major source of new nitrogen supporting primary production in the oligotrophic ocean. 15N:14N ratios in plankton provide an integrative record of the nitrogen cycle processes at work in the ocean, and near‐surface organic matter in oligotrophic waters like the Sargasso Sea is characterized by an unusually low 15N content relative to average deep‐water nitrate. Herein we show that the low dΔ15N of suspended particles and zooplankton from the tropical North Atlantic cannot arise through isotopic fractionation associated with nutrient uptake and food web processes but are instead consistent with a significant input of new nitrogen to the upper water column by N2 fixation. These results provide direct, integrative evidence that N2 fixation makes a major contribution to the nitrogen budget of the oligotrophic North Atlantic Ocean.