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Large temporal variations in the nitrogen isotopic composition ([[delta].sup.15]N) of particulate organic matter (POM) and dissolved inorganic nitrogen (DIN) species in Lake Kinneret occurred in response to seasonal phasing of dominant nitrogen cycle processes. The lowest [[delta].sup.15]N POM values (-5.5[per thousand]) were observed in early winter, a consequence of isotopic fractionation by chemoautotrophic microbial [NH.sup.+.sub.4] assimilation, and during the first phase of the nitrification period, by seasonally dominant [NH.sup.+.sub.4] oxidizers. Nitrification itself was strongly fractionating, producing [sup.15]N-depleted [NO.sup.-.sub.3] and strongly enriching water-column [NH.sup.+.sub.4] in [sup.15]N. Toward the end of nitrification and into the phytoplankton bloom period in later winter and early spring, there was a corresponding 15-30[per thousand] jump in POM [[delta].sup.15]N because of assimilation of high-[[delta].sup.15]N [NH.sup.+.sub.4]. Maximal [[delta].sup.15]N POM values were thus measured during the algal blooms of Peridinium gatunense in 2004 and Debarya sp. and Microcvstis sp. in 2005. Toward the end of the blooms in mid- to late spring, POM [[delta].sup.15]N values decreased to values similar to [[delta].sup.15]N for [NO.sup.-.sub.3] (8-12 [per thousand]), indicating a switch to this DIN source. [NO.sup.-.sub.3] assimilation into POM appeared to occur without isotopic fractionation. Late spring was also the denitrification period with [NO.sup.-.sub.3] removal in the newly suboxic hypolimnion. Though [delta][sup.15][NO.sup.-.sub.3] increased in the residual [NO.sup.- .sub.3], complete consumption by denitrification suggests no net effect, as supported by a lack of increase in [[delta].sup.15]N POM at this time. Oligotrophic conditions are found in the epilimnion in late summer and early autumn and low (3.3 [per thousand]) [[delta].sup.15]N POM was found during the bloom of the filamentous [N.sub.2]-fixing cyanobacteria Aphanizomenon ovalisporum and Cylindrospermopsis cuspis. The [[delta].sup.15]N-dissolved organic nitrogen isotopic signature at this time was significantly higher, 8-12 [per thousand], suggesting a phytoplankton source from earlier in the seasonal cycle. Overall, the largest known seasonal variations in N isotope composition have been observed in Lake Kinneret, providing a uniquely useful biogeochemical tool for studying N cycling in lakes as well as detecting long-term changes in N source and cycling in response to watershed land use changes and climate change.

We report a new method for determining the [sup.15]N/[sup.14]N of N[H.sub.4.sup.+] at natural abundance level in both freshwater and seawater. N[H.sub.4.sup.+] is first quantitatively oxidized to N[O.sub.2.sup.-] by hypobromite (Br[O.sup.-]) at pH ~ 12. After the addition of sodium arsenite to consume excess Br[O.sup.-], yield is verified by colorimetric N[O.sub.2.sup.-] determination. N[O.sub.2.sup.-] is further reduced to [N.sub.2]O using a 1:1 sodium azide and acetic acid buffer solution using previously established procedures. The product [N.sub.2]O is then analyzed for isotopic composition using a continuous flow purge and cryogenic trap system coupled to an isotope ratio mass spectrometer. Reliable [[delta].sup.15]N values (standard deviation is 0.3[per thousand] or better) are obtained over an N[H.sub.4.sup.+] concentration range of 0.5-10 [micro]M using 20 mL volumes of either freshwater or seawater samples. Higher concentration samples are readily diluted to lower concentration. Preexisting N[O.sub.2.sup.-] is removed by treatment with sulfanilic acid. There is no interference from any of the nitrogen-containing compounds tested except short-chain aliphatic amino acids (i.e., glycine) which typically are present at very low environmental concentrations. As compared to published methods, our approach is more robust, readily applicable at low concentrations and small sample volumes, and requires less time for preparation and analysis.

We present a novel method for nitrogen and oxygen natural isotopic abundance analysis of nitrate and nitrite of seawater and freshwater at environmental concentrations. The method involves the reduction of nitrate to nitrite using spongy cadmium with further reduction to nitrous oxide using sodium azide in an acetic acid buffer. For separate nitrite analysis, the cadmium reduction step is simply bypassed. Nitrous oxide is purged from the water sample and trapped cryogenically using an automated system with subsequent release into a gas chromatography column. The isolated nitrous oxide is then analyzed on a continuous flow isotope ratio mass spectrometer via an open split. This paper describes the basic protocol and reaction conditions required to obtain reproducible natural abundance level nitrogen and oxygen isotopic ratios from nitrate, nitrite, or both, and the results obtained to support these conclusions. A standard deviation less than 0.2%0 for nitrogen and 0.5%0 for oxygen was found for nitrate samples ranging in concentration from 40 to 0.5 [micro]M (better for nitrite), with a blank of 2 nmol for 50-mL samples. Nitrogen and oxygen isotopic fractionation and oxygen atom exchange were consistent within each batch of analysis. There was no interference from any seawater matrixes. Only one other method published to date can measure the nitrate oxygen isotopic abundance in seawater and none that do so for nitrite alone in the presence of nitrate. This method may prove to be simpler, faster, and obtain isotopic information for lower concentrations of nitrate and nitrite than other methods.

The availability of iron is known to exert a controlling influence on biological productivity in surface waters over large areas of the ocean and may have been an important factor in the variation of the concentration of atmospheric carbon dioxide over glacial cycles. The effect of iron in the Southern Ocean is particularly important because of its large area and abundant nitrate, yet iron-enhanced growth of phytoplankton may be differentially expressed between waters with high silicic acid in the south and low silicic acid in the north, where diatom growth may be limited by both silicic acid and iron. Two mesoscale experiments, designed to investigate the effects of iron enrichment in regions with high and low concentrations of silicic add, were performed in the Southern Ocean. These experiments demonstrate iron's pivotal role in controlling carbon uptake and regulating atmospheric partial pressure of carbon dioxide., The Southern Ocean exerts a major control on the partial pressure of carbon dioxide (pC[O.sub.2]) in the atmosphere. Because rates of photosynthesis and biological carbon export are low in Antarctic [...]

Author(s): Mark A. Altabet (corresponding author) [1]; Matthew J. Higginson [1]; David W. Murray [2] Most global biogeochemical processes are known to respond to climate change, some of which have [...]

The role of nitrogen stable isotopes in the production of N2 and N2O in Paracoccus denitrificans has been examined. A steady-state reactor, which does not require a dedicated mass spectrometer or an online sample preparation system, has been constructed to measure denitrification effects in P. denitrificans. The reactor is also used in measuring oxygen isotope effects. The data gathered expand the usefulness of stable isotope studies of denitrification under low-oxygen regimes.

Nitrogen-isotope preserved in Southern Ocean sediments, together with geochemical tracers for settling biogenic matter fluxes, reveals patterns of earlier nutrient supply to to phytoplankton in the region. The reduction of the CO2 leak from ocean to atmosphere through higher surface-water stratification at the Polar Front, contributed more to decreased CO2 concentrations than increased export of organic carbon to deep waters from the surface, during the last glacial period.

A study of nitrogen isotopes at three sites in the Arabian Sea indicates changes in the 15N/14N values due to climatic variations, as confirmed by the agreement of the variations with Milankovitch cycle values. This implies that subsurface nitrate level and denitrification vary with climate. These factors may have serious implications on the changes in marine biogeochemical cycles.

Vertical nitrate transport in the North Atlantic, North Pacific and the Indian oceans were observed in Rhizosolenia mats to study the mechanism of nutrient transfer in abundant diatom mats. The study indicated that mats rising to the surface were richer in internal nitrates than those mats which were sinking to the ocean floor. These reuslts indicate that Rhizosolenia mats can provide as mush as 50% of nitrohen requirements of surface water.

During the U.S. JGOFS EqPac program, variations in nitrogen isotopic ratio ([Delta][sup.15]N) and their control by relative [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] drawdown were examined in detail. Near-surface [Delta][sup.15] [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] data clearly conform to Rayleigh isotopic fractionation during phytoplankton utilization of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], with a fractionation factor of about 5[salinity]. This isotopic signal both propagates into particulate nitrogen pools and is a persistent, large-scale characteristic of the system. Decreasing near-surface [[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]] with distance from the equator strongly correlates with increasing [Delta][sup.15]N in euphotic zone particulate organic matter (POM) as well as sinking particles during both Surveys 1 (El Nino) and 2 (non-El Nino) cruises. Despite a doubling of [[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]] accompanying relaxation of El Nino conditions, [Delta][sup.15]N values in near-surface POM and deeply sinking particles as a function of latitude were similar for the two periods. Since [Delta][sup.15]N varies with relative nutrient drawdown and not its concentration, this parameter appears to not have varied significantly. In both cases, about 50% of upwelled [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] was consumed in the immediate vicinity of the equator, as estimated from the [Delta][sup.15]N data. The moored sediment trap time series provided a more highly resolved temporal view of the [sup.15]N dynamics of this system. At the equator and to the south, there is little temporal variation in [Delta][sup.15]N and, hence, in relative [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] utilization over the 1-yr duration of the program. At 2 [degrees] and 5 [degrees] N, there is a large 5[salinity] decrease in [[Delta].sup.15]N during the transition to non-El Nino conditions in response to the northward movement of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]-rich waters, which probably reflects the intensification of the north equatorial current. Overall, % [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] utilization at the equator remained between 40 and 60% over the observation period, despite large hydrographic and dynamical changes. This observation implies a tight control of relative [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] utilization, consistent with iron (Fe) limitation. Since Fe is supplied in this region by upwelling, relative [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] utilization is likely determined by the product of the ratio of Fe to [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] in upwelled waters and the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]:Fe utilization ratio. Given no change in the chemistry of source waters, increases in upwelling, as observed during the transition away from El Nino, will affect surface [[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]] new and export production, but it will not affect relative [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] utilization. In this light, downcore [Delta][sup.15]N records in the equatorial Pacific should reflect past changes in the limitation of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] drawdown by Fe, perhaps through changes in equatorial undercurrent chemistry.

A study was conducted to examine the nitrogen isotope biogeochemistry of sinking particles from the margin of the Eastern North Pacific that includes the waters of Monterey Bay, San Pedro Basin and the Gulf of California. Results indicate that the region's nitrogen isotopic ratio is quite different from previous open-sea observations. It was observed that the eastern North Pacific margin's nitrogen isotopic ratio is loosely linked to the transient indicators of the surface ocean.