Luis A. Cifuentes

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.

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.

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.

Spatial and temporal variations in terrestrially-derived organic matter from sediments of the Delaware estuary

Terrestrially-derived organic matter in sediments of the Delaware Estuary originates from riverine transport of soils and fresh litter, sewage and industrial wastes, and marsh export of organic matter. The quantity, composition, and spatial distribution of terrigenous organic matter in sediments was determined by elemental (C and N), lignin, and stable carbon isotope analyses. Sediments in the upper Delaware Estuary had low organic carbon content and high lignin content. In contrast, sediments in the lower Delaware Estuary had high organic carbon content and low lignin content. There was a slight decrease in the proportion of syringyl and cinnamyl phenols relative to vanillyl phenols between the upper estuary and lower estuary. Differences in lignin and stable carbon isotope compositions between sediments of the Delaware Estuary and sediments of the Broadkill River estuary (an adjoining salt-marsh estuary) supported previous observations that marshes do not export substantial quantities of organic matter to estuaries. Additional results suggested that lignin-rich sediments were concentrated in the upper estuary, most likely in the zone of high turbidity. Furthermore, algal material diluted lignin-rich sediments, particularly in the lower estuary. The weaker algal signal in bottom sediments compared to that in suspended particulate matter suggested algal material was decomposed either in the water column or at the sediment-water interface. Physical sorting of sediments prior to deposition was also indicated by observations of compositional differences between the upper and lower estuary bottom sediments. Finally, seasonal variations in primary productivity strongly influenced the relative abundance of terrestrial organic matter. In fall, however, the proportion of lignin was greatest because of a combination of greater inputs of terrestrially-derived organic matter, lower river discharge, and a decrease in algal biomass.

Contribution of river phosphate variations to apparent reactivity estimated from phosphate-salinity diagrams

Estuarine mixing of dissolved inorganic phosphate (DIP) was simulated in the low salinity region of the Delaware Estuary. The study was designed to evaluate the extent to which variations in measured river DIP concentration can explain apparent estuarine reactivity in the low salinity region of the estuary inferred from DIP-salinity diagrams. For 12 simulation periods beginning in January 1987 and ending in January 1988, average DIP reactivity inferred from simulated DIP-salinity diagrams was 0·38 μM, with values ranging between − 1·7 (removal) and 0·74 (release) μM. On six of 12 occasions, the inferred reactivity of DIP from the simulated mixing diagram was > 10% of the river DIP concentration. A relative change of 0·6 and 2·0% per day in river DIP was found to produce definite non-linear effects (>I 0%) for periods of increasing and decreasing DIP, respectively. For two dates, the incorporation of simulated mixing into the interpretation of field data actually changed the direction of inferred reactivity. Our study clearly documents, as previously shown by others, the danger of inferring estuarine reactivity of DIP from a DIP-salinity diagram without including knowledge of temporal variations in river DIP concentration.