H51E-01 INVITED 08:00h
Coupled Nitrogen and Oxygen Isotope Measurements of Nitrate Along the Eastern North Pacific Margin
Water column depth profiles along the eastern North Pacific margin from Point Conception to the tip of Baja California indicate elevation of nitrate N-15/N-14 and O-18/O-16 associated with denitrification in the oxygen-deficient waters of the eastern tropical North Pacific. The coupled variations in N-15/N-14 and O-18/O-16 suggest that the O and N isotope effects for denitrification are roughly equivalent at 20-25 permil. This is consistent with our culture study of denitrifiers in seawater medium but stands in contrast to the results of published freshwater studies, which suggest that the isotope effect for N is greater than that for O. The maximum in nitrate O-18/O-16 is somewhat shallower than the maxima in both nitrate N-15/N-14 and the nitrate deficit (as reconstructed from nitrate and phosphate concentrations). This reflects a depth-variant deviation toward higher nitrate O-18/O-16 (or lower N-15/N-14) than can be explained by denitrification if the N and O isotope effects of denitrification are indeed equal. We tentatively interpret the apparent deviation to result from the addition of low-N-15/N-14 nitrate to the shallow thermocline by the remineralization of newly fixed N. A simple model indicates that this remineralization has erased roughly half the nitrate deficit that would otherwise be present at 200 m depth in this region.
H51E-02 08:30h
Nitrogen and oxygen isotope effects associated with nitrate assimilation and denitrification by laboratory cultures of marine plankton.
We report measurements of coupled nitrogen (N) and oxygen (O) isotopic variations in nitrate during a) its assimilation by laboratory cultures of marine phytoplankton, and b) during dissimilatory reduction by cultures of marine and fresh water denitrifiers. Nitrate N ($^{15}$N/$^{14}$N) and O ($^{18}$O/$^{16}$O) isotopic ratios were measured throughout growth using the denitrifier method (Sigman et al. 2001, Casciotti et al. 2002). In the case of denitrifier cultures, accumulated nitrite in the medium was removed to avoid interference with the measurement of N and O isotopic ratios in nitrate, using a new protocol which will be described. We observed large inter- and intra-species variations in the nitrate N and O isotope effects ($^{15}$$\epsilon$ and $^{18}$$\epsilon$, respectively) during its assimilation by marine microalgae. This stands in contrast to the ratio of respective N and O isotope effects (i.e., $^{15}$$\epsilon$:$^{18}$$\epsilon$), which remained remarkably close to unity regardless of the magnitude of $\epsilon$. Unlike nitrate assimilation, nitrate reduction by denitrifiers showed less variability in the magnitude of observed $^{15}$$\epsilon$ and $^{18}$$\epsilon$. Yet $^{15}$$\epsilon$ and $^{18}$$\epsilon$ were also strongly coupled in denitrification, for both marine and fresh water strains. The observed magnitudes of $^{15}$$\epsilon$ and $^{18}$$\epsilon$ and the respective coupling ($^{15}$$\epsilon$:$^{18}$$\epsilon$) will be discussed in the context of algal and bacterial physiology, as well as implications for the interpretation of nitrate N and O isotopes in marine and freshwater systems.
H51E-03 08:45h
Stable Isotopic Detection of Ammonium and Nitrate Assimilation by Estuarine Phytoplankton
We used a stable isotopic approach to determine the relative contribution of nitrate and ammonium to nitrogen requirements of an estuarine phytoplankton community, in Childs River, MA. To obtain a phytoplankton, rather than a mixed seston isotopic signature, we purified chlorophyll a from seston samples and measured its nitrogen isotopic composition. The isotopic signature of phytoplankton increased from winter to summer, but did not vary with salinity. In contrast, the isotopic signatures of nitrate and ammonium decreased linearly with increasing salinity, and only the isotopic signature of ammonium increased in warmer months. We compared the nitrogen isotopic composition of nitrate, ammonium, and phytoplankton, and found that although nitrate dominated the DIN pool, ammonium constituted the majority (60 to 97%) of the DIN taken up by phytoplankton. The fractionation factor for phytoplankton ammonium uptake was 4 \permil. Surprisingly, it appears that phytoplankton took up N mainly as ammonium, and thus acquired their isotopic signature, in the upper reaches of the estuary, and divided and increased abundance during down- estuarine transport based on that initial uptake of nitrogen.
H51E-04 09:00h
The Nitrogen Isotopes of Nitrate and Total Nitrogen Between Bermuda and Puerto Rico
In samples collected along a meridional transect between Bermuda and Puerto Rico in October 2002, nitrogen isotope (d15N) and concentration measurements were made for nitrate and total nitrogen (TN, which is almost entirely dissolved organic nitrogen, or DON, in the upper 100 m). At each station, nitrate d15N decreased from about 5 per mil at 800 m depth to a minimum of approximately 2 per mil at 200 m depth. The minimum in nitrate d15N at 200 m is presumably due to the input of a low-d15N nitrogen source, either locally or elsewhere in the basin. Between 200 and 100 m, nitrate d15N increased to as much as 6 per mil, most likely because of the assimilation of nitrate by phytoplankton. TN concentrations and d15N are relatively uniform in the upper 100 m along the transect, at approximately 5 uM and 4 per mil (vs. N2 in air), respectively, which are similar to the average annual values for the Bermuda Atlantic Time-Series Study (BATS) station. A slight decrease in TN concentration and increase in TN d15N was observed between the upper 100 m and approximately 300 m, which, in conjunction with mixing, may drive a small but potentially significant flux of N from the surface to subsurface. No meridional gradients in the nitrate or TN data are apparent. This lack of north-south change is in contrast to the presence of apparent meridional gradients in parameters such as winter mixed layer depth, iron deposition and nitrogen fixation rate, which might be expected to affect the concentration and isotopic composition of both TN and nitrate. This disconnect suggests that local nitrogen fixation inputs to the TN and nitrate pool are slow relative to the subtropical circulation. This view is qualitatively consistent with our previous observation of a lack of seasonal variation in TN d15N or concentration at the BATS site.
H51E-05 09:15h
Contribution of Diazotrophs to the Nitrogen Pool Along 10$\deg$N in the Subtropical Atlantic
The Meteor 55 cruise from October 12th to November 17th, 2002 was carried out along 10$\deg$N across the subtropical Atlantic from 56$\deg$W to 19$\deg$W. Here we present a data set from approximately 40 stations that were sampled with a CTD system from surface to 500m depth. Nutrient and particulate organic matter concentrations as well as isotope measurements from POM, NO${3}$, and size fractionated zooplankton were used to estimate the diazotrophic contribution to the particulate nitrogen pools along the transect. An evaluation of the percentage of nitrogen originating from diazotrophs was performed by means of isotope signatures of the three zooplankton size fractions. Significantly higher N-contributions from nitrogen fixation were found at the western end of the transect from 56$\deg$ to 30$\deg$W with 19 to 61% (mean 48%) and only 11-42% (mean 30%) between 25$\deg$ and 19$\deg$W. The diazotrophic contribution to the zooplankton was correlated significantly (n=13, p$<$0.005, r$^{2}$=0.56) with the contribution calculated to the PON, suggesting a direct link between the diazotrophs and the food web. As expected the 25 - 27 potential density surfaces shallowed from west to east with the 26 density surface found below 100m depth in the west and at 50m in the east. Similarly the DIN excess followed this slope and had a significant maximum around these density surfaces implying remineralization of N-rich organic matter produced by diazotrophs. The isolines of the $\delta^{15}$N-NO$_{3}$ signature followed the DIN excess across the Atlantic with a minimum between 100 and 200m. The isotopic data confirm previous findings which find evidence of more intense nitrogen fixation in the western Atlantic compared to the east. However, direct nitrogen fixation rate measurements along the cruise track showed highest rates off Africa. This activity is not mirrored in the natural abundance data. One reason may be the pronounced contribution of upwelled nitrate and the fractionation of the nutrients off Africa, where productivity is high.
H51E-06 09:30h
Denitrification in the Sediments of a Nitrate-rich River Draining an Agricultural Watershed
The interface between surface water and ground water in stream systems has been shown to exert strong control over the quality and quantity of waters at the surface and recharging to aquifers. We are investigating the removal of nitrate in a river draining the Pajaro Valley, a coastal, agriculturally-rich watershed that features elevated nitrate levels and groundwater pumping in excess of recharge (overdrafting). During summer base flow (discharge $\sim$0.3 m$^{3}$/sec), when no significant precipitation has fallen in the basin for 2 to 3 months, the Pajaro River consistently loses 0.1 to 0.2 m$^{3}$/sec of its discharge to the underlying alluvial aquifer between Chittenden Road and Murphy's Crossing Rd, a 10 km stretch east of Watsonville, CA. Nitrate concentrations during base flow, which are typically in excess of the EPA limit for drinking water (10 mg NO$_{3}$-N/L), decrease by $\sim$30% along this stretch. Molar DIN:TDP ratios during summer baseflow are typically 300-400, suggesting strong phosphorus limitation (relative to nitrogen) to assimilative uptake in this system. We observed zones of nitrate depletion (i.e. nitrate concentrations lower than, but other solutes equal to, stream concentrations) at various points in the subsurface of the stream during low-flow conditions. Nitrate in these zones was isotopically heavier ($\epsilon$=-21$\permil$ and -10$\permil$ for $^{15}$N and $^{18}$O, respectively), suggesting that denitrification (and/or DNRA) is the mechanism of nitrate removal. As nitrate concentrations were reduced downstream, residual nitrate was not fractionated as strongly as in the subsurface ($\epsilon$=-9$\permil$), which could be caused by the incomplete return of denitrified (and thus heavier) nitrate to the main channel after seeping into the streambed. $\delta$$^{18}$O values in surface water increased a greater amount over the stretch later in the water year ($\epsilon$=-16$\permil$ in October, compared to -4$\permil$ in July), suggesting nitrification might act as a nitrate source to this reach at this time. Thus, true denitrification rates may be greater than estimates based on observed nitrate removal in this reach.
H51E-07 09:45h
Isotopic Tracing of Sources of Riverine Nitrate: The Example of the Oldman River Basin, Alberta, Canada
The isotopic composition of dissolved nitrate in aquatic ecosystems can be a valuable tracer for its sources and its biogeochemical history. Nitrate derived from atmospheric deposition, synthetic fertilizers, soil nitrification, and from sewage or manure can often be distinguished if both nitrogen and oxygen isotope ratios are determined. Additionally, denitrification result in characteristic patterns of progressively increasing nitrogen and oxygen isotope ratios as nitrate concentrations decrease. The objective of this study was to determine the sources of riverine nitrate and the processes it may have undergone in the Oldman River Basin in southern Alberta, Canada, using a combination of hydrological, chemical, and isotopic techniques. In the mainly forested upstream portion of the Oldman River Basin, riverine nitrate was characterized by low concentrations (0.2 mg/L) and low nitrogen isotope ratios (+3 per mil). In the agriculturally used lower part of the Oldman River Basin, concentrations and nitrogen isotope ratios of riverine nitrate increased significantly, particularly in the winter months. Sampling of tributaries revealed that canals and drains in the irrigation districts were often characterized by nitrate concentrations exceeding 35 mg/L and nitrogen isotope ratios near +15 per mil. Many of the fields in the irrigation districts in the lower part of the Oldman River Basin are regularly manured at rates exceeding 15 Mg/ha/year. Topsoils had total N concentrations of 4 mg/g and N isotope ratios approaching +15 per mil as a consequence of repeated manure applications. Concentrations of dissolved nitrate in an aquifer underneath an agricultural field approached 50 mg/L and nitrogen isotope ratios ranged between +11 and +14 per mil, whereas oxygen isotope ratios were negative. This suggests that manure applied to agricultural fields in the lower part of the Oldman River basin is the dominant source of groundwater nitrate in the underlying aquifers. This nitrate infiltrates into the irrigation and drainage canals particularly in the non-irrigation season, when water levels are low. Agricultural return-flow provides a feasible explanation for elevated concentrations and nitrogen isotope ratios of dissolved nitrate in the drainage canals, which constitute tributaries to the Oldman River. The increased nitrate load in the tributaries is ultimately responsible for increased concentrations and elevated nitrogen isotope ratios of riverine nitrate in the Oldman River during the winter months.