Jonathan H. Sharp

Biogeochemical factors that influence the stable nitrogen isotope ratio of dissolved ammonium in the Delaware Estuary

Abstract The isotopic composition (δ15N) of dissolved ammonium (NH4+) in the Delaware Estuary was related to reactions in the nitrogen cycle occurring in different regions of the estuary and at different rates throughout the year. The range of values at any one location (as great as +10 to +40%.) was dependent on either nitrification, algal uptake, and microbial remineralization, or on a combination of these reactions. Specifically, observations of isotopic discrimination during nitrification in the riverine portion of the estuary were similar to those reported in other estuaries. In addition, the first calculation of the isotopic fractionation during algal uptake in the field is reported. Algal assimilation of NH4+ in the estuary had an estimated fractionation factor (ϵ) of −9.1%. This estimated ϵ for the field data and fractionation factors measured in culture ( − 14 to − 20%.) were compared in a numerical simulation of NH4+ transport and uptake in the estuary. Model results for the period of the spring bloom resembled the field data more closely when the isotopic fractionation estimated with the in situ data was used rather than greater isotopic fractionations measured in culture.

Analyses of dissolved organic carbon in seawater: the JGOFS EqPac methods comparison

Abstract Results of a dissolved organic carbon (DOC) methods comparison are presented here in which five high temperature combustion (HTC) instruments and a wet chemical oxidation (WCO) method were used on a series of oceanic samples. The samples were collected during US JGOFS Equatorial Pacific Ocean cruises (EqPac) and most of the authors were involved with DOC analyses for the EqPac Program. Samples were collected with a “clean” protocol and were immediately quick frozen in replicate sample bottles. They were distributed by the first author to the other authors for “blind” analyses later on land on the stored samples. Comparable results (±7.5%) were found by three HTC instruments and the WCO method. There were difficulties with the other two HTC methods for which explanations and improvements are offered. The single most critical element for comparable DOC values appears to be assessment and subtraction of the total instrument blank (or reagent and handling blank for WCO methods). A “zero” carbon (very low C) water sample assisted in having all analysts achieve a uniform assessment of individual instrument or methods blanks. “Conditioning” of the catalyst bed in the combustion tube is critical to achieve consistent low instrument blanks. Failure to thoroughly condition the catalyst bed may be a significant error that can give erroneously high DOC values for oceanic samples. Reference standards available to all analysts also allowed comparison of instrument and methods performance. Contamination problems were demonstrated and it was shown that careful preparation and handling can reduce the potential for errors from contaminated samples. Results indicate that Equatorial Pacific oceanic DOC values in near surface waters are on the order of 60–70 μM C and deep water values on the order of 35–40 μM C. Since the “zero” carbon water contained a small, but measurable, amount of DOC, the sample values reported here may be slightly low. Because the lowest instrument blanks were equivalent to about 10 μM C, it is suggested that even if there were no instrument blank at all and all this “blank” were in the “zero” carbon water, the oceanic sample concentrations could not be underestimated by more than 10 μM C.

Total organic carbon in seawater — comparison of measurements using persulfate oxidation and high temperature combustion

Abstract A method is described for the determination of organic carbon in liquid seawater samples by combustion at a high temperature. Although the method is not easy to perform, it has been successfully employed for a direct comparison to the standard method of persulfate oxidation. With seawater from the central western North Atlantic Ocean, the standard method measured an average of 78% as much carbon as the high temperature method. This indicates organic matter in seawater that is resistant to oxidation by persulfate; some of it appears to be of a colloidal nature. The persulfate resistant organic matter is most noticeable in a region below the surface 100 m and seems to decrease slowly with depth, indicating deep ocean biological utilization of organic matter. Due to a possible procedural error in the standard persulfate oxidation method, the differences indicated here may actually be conservative and the content of total organic carbon in the sea may be as much as twice as high as has been indicated by previous estimates.

Temporal alternation between light- and nutrient-limitation of phytoplankton production in a coastal plain estuary.

The potential for Lightand nutrient-limitation of phytoplankton production was examined in the Delaware Estuary, USA, by combining a hierarchy of expenmental approaches including smallscale bioassay experiments, ecosystem-level analysis of nutrient concentration and stoichiometric ratios, and light-limitation modeling. Light was found to be the predominate regulator of phytoplankton growth throughout the estuary during the winter period as a result of high turbidity and a wellmixed water column. However, during late spring, phosphorus (P) was found to limit growth. This observation was confirmed at each of the experimental levels, and was related to several factors, including elevated input ratios (230:l) of dissolved inorganic nitrogen (DIN) to PO, in river waters, accumulation of P into phytoplankton, and low rates of P regeneration. During summer, P no longer limited production. At this time DIN:POI ratios and bioassay experiments revealed the potential for nitrogen (N) limitation particularly in the lower estuary while particulate composition ratios and ecosystem nutrient flux estimates gave contradictory evidence. From these data it appears that N was potentially limiting to phytoplankton biomass but that the constant flux of N from upstream and rapid N regeneration maintained non-nutrient-limited steady-state growth. These data document a pattern of recurring system-wide variations in the factors that limit phytoplankton production over several annual sequences. These temporal and spatial variations are related to both light availability as regulated by incldent light, suspended sediment concentration, and depth of the surface mixed-layer and nutrient availability as determined by riverine inputs and in situ biogeochemical processes.

Final dissolved organic carbon broad community intercalibration and preliminary use of DOC reference materials

Abstract A broad community intercalibration exercise for accurate measurement of dissolved organic carbon (DOC) in seawater has been carried out over a period of 5 years. A set of 10 natural samples with DOC content from 40 to 200 μM C were accompanied by two glucose standards and a “zero C” blank; all sealed in glass ampoules. Samples were sent to all interested analysts for “blind” analysis; 62 laboratories in 17 countries participated. A total of 59 separate analyses were determined to be acceptable by screening criteria based on standards and blank; another nine sets of analyses did not pass the screening. The majority of the analyses, both those passing and those that did not, were performed with high temperature combustion (HTC) methods, six sets of analyses were done using wet chemical oxidation methods. From the 53 sets of acceptable HTC analyses, the coefficient of variation (%CV) for analytical comparability of the samples was 10% (“community precision”). It is estimated that the individual replicate injection precision for most instruments was approximately 2% and that no additional variability was caused by differences within the ampoules of individual samples. The additional variability over 2% was likely a result of both random and systematic differences in analytical capabilities from instrument to instrument and from day to day for individual instruments. With an arbitrary selection after the fact, smaller subsets of analysts can show comparability better than 10% and duplicate or triplicate runs on different days of the full sets of samples in several laboratories showed comparability in the 2–6.5% range. Experienced oceanic analysts, with internal or shared reference materials, can now show reproducibility and comparability at a level closer to 2%. Preliminary use of DOC reference materials by 14 participants showed day-to-day reproducibilities for their laboratories in the 2–6% range in most cases; several with poorer reproducibility do not normally perform DOC analyses on samples with concentrations as low as the deep ocean reference used here. Use of these reference materials can also give a demonstration of comparability between laboratories. For credibility of DOC analyses, it is necessary for analysts to use community reference materials and report results of their analytical performance with these references. This paper does not identify individual data nor should it be considered an evaluation of individual laboratories or analysts. The purpose is to show the summary picture of the international community of DOC analysts as it existed in the mid- to late 1990s.

Determination of the isotopic composition of ammonium-nitrogen at the natural abundance level from estuarine waters

Abstract A method was developed to measure the stable nitrogen isotope ratio of dissolved ammonium (NH 4 + ) at the natural abundance level from estuarine waters. This method employed rapid steam distillation with collection of ammonium on zeolite via ion-exchange. The steam distillation step had a recovery of 103±5%; subsequent exchange of the ammonium on zeolite had a yield of 96.4±1.6%. The zeolite with exchanged ammonium was converted to N 2 in quartz tubes at 910°C with CuO and Cu and the isotopic composition of the gas was measured in an isotope ratio mass spectrometer. When analyzing 200 μg of N the accuracy using isotopic standards was within 4% of the true ratio, with an overall precision of ±0.5%. A benefit of this method is that samples can be distilled and preserved onboard ship, thereby minimizing storage artifacts. This method was used in a seasonal study of the isotopic composition of dissolved ammonium from the Delaware Estuary.

The influence of river variability on the circulation, chemistry, and microbiology of the Delaware Estuary

Gravitational circulation of the Delaware Estuary is dominated by a single river, the Delaware River. The seasonal variation in river discharge is large. Consequently, the water column varies between vertically homogenous conditions found during most of the year and strongly stratified conditions found during the high flow of the spring freshet. Both the variation in river discharge and the extent of stratification affect chemical distributions and biological processes in the estuary. With a simple advection-diffusion model, we show that the apparent nonconservative behavior of nitrate in the Delaware Estuary can result from varying endmember concentration and varying river discharge. In addition, we illustrate the relationship between water column stratification, phytoplankton production, and concurrent bacterial activity. Finally, as an indirect chemical response to phytoplankton growth during high river discharge, we show strongly nonconservative patterns for ammonium, phosphate, and silicate in the estuary.

THE ESTUARINE INTERACTION OF NUTRIENTS, ORGANICS, AND METALS: A CASE STUDY IN THE DELAWARE ESTUARY

Abstract: In the estuarine environment, biogeochemical processes alter concentrations of soluble nutrients, organic matter, and trace metals. Some constituents show geochemical reactivity and are filtered out by “flocculation” type reactions; these may be considered as a geochemical “filter”. Other constituents show biochemical reactivity and are filtered out by organismic processes; these may be considered as a biochemical “filter”. Through use of data from the Delaware Estuary, the geochemical filter is illustrated as it affects humic acids, phosphate, and iron; the biochemical filter as it affects ammonium, phosphate, silicate, and urea. Contrasting examples are presented for the transition elements copper and nickel which show little filtration, despite the potential for bioreactivity. Cadmium and phosphate are used to illustrate a combined biogeochemical filter.

A preliminary methods comparison for measurement of dissolved organic nitrogen in seawater

Abstract Routine determination of dissolved organic nitrogen (DON) is performed in numerous laboratories around the world using one of three families of methods: UV oxidation (UV), persulfate oxidation (PO), or high temperature combustion (HTC). Essentially all routine methods measure total dissolved nitrogen (TDN) and calculate DON by subtracting the dissolved inorganic nitrogen (DIN). While there is currently no strong suggestion that any of these methods is inadequate, there are continuing suspicions of slight inaccuracy by UV methods. This is a report of a broad community methods comparison where 29 sets (7 UV, 13 PO, and 9 HTC) of TDN analyses were performed on five samples with varying TDN and DIN concentrations. Analyses were done in a “blind” procedure with results sent to the first author. With editing out one set of extreme outliers (representing 5 out of 145 ampoules analyzed), the community comparability for analyzing the TDN samples was in the 8–28% range (coefficient of variation representing one standard deviation for the five individual samples by 28 analyses). When DIN concentrations were subtracted uniformly (single DIN value for each sample), the comparability was obviously worse (19–46% cv). This comparison represents a larger and more diverse set of analyses, but the overall comparability is only marginally better than that of the Seattle workshop of a decade ago. Grouping methods, little difference was seen other than inconclusive evidence that the UV methods gave TDN values for several of the samples higher than HTC methods. Since there was much scatter for each of the groups of methods and for all analyses when grouped, it is thought that more uniformity in procedures is probably needed. An important unplanned observation is that variability in DIN analyses (used in determining the final analyte in most UV and PO methods) is essentially as large as the variability in the TDN analyses. This exercise should not be viewed as a qualification exercise for the analysts, but should instead be considered a broad preliminary test of the comparison of the families of methods being used in various laboratories around the world. Based on many independent analyses here, none of the routinely used methods appears to be grossly inaccurate, thus, most routine TDN analyses being reported in the literature are apparently accurate. However, it is not reassuring that the ability of the international community to determine DON in deep oceanic waters continues to be poor. It is suggested that as an outgrowth of this paper, analysts using UV and PO methods experiment and look more carefully at the completeness of DIN conversion to the final analyte and also at the accuracy of their analysis of the final analyte. HTC methods appear to be relatively easy and convenient and have potential for routine adoption. Several of the authors of this paper are currently working together on an interlaboratory comparison on HTC methodology.

Qualitative and numerical analyses of the effects of river inflow variations on mixing diagrams in estuaries

Abstract The effects of river inflow variations on alkalinity/salinity distributions in San Francisco Bay and nitrate/salinity distributions in Delaware Bay are described. One-dimensional, advective-dispersion equations for salinity and the dissolved constituents are solved numerically and are used to simulate mixing in the estuaries. These simulations account for time-varying river inflow, variations in estuarine cross-sectional area, and longitudinally varying dispersion coefficients. The model simulates field observations better than models that use constant hydrodynamic coefficients and uniform estuarine geometry. Furthermore, field observations and model simulations are consistent with theoretical ‘predictions’ that the curvature of propery-salinity distributions depends on the relation between the estuarine residence time and the period of river concentration variation.