Jonathan R. Pennock

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.

Chlorophyll distributions in the Delaware estuary: Regulation by light-limitation

Phytoplankton chlorophyll concentrations in the Delaware estuary range over two orders of magnitude and display several maxima over the seasonal cycle. These maxima were found to be regulated both spatially and temporally by light availability. Both the spring chlorophyll maximum, which reaches 50–60 μg chlorophyll l−1 during a Skeletonema costatum dominated bloom, and transient fall blooms (15–20 μg l−1) are focused in mid-estuary. These blooms are regulated spatially by settling out of suspended sediment below the turbidity maximum and both spatially and temporally by physical factors (e.g. river flow) that cause vertical stratification in mid-estuary. In freshwater regions, chlorophyll concentrations display seasonal periodicity correlated with solar irradiance; summer chlorophyll concentrations average 30 μg l−1. These freshwater and mid-estuarine biomass maxima may be correctly predicted using a steady-state light-limitation model. In contrast, summer chlorophyll concentrations in the lower estuary remain below 10 μg l−1 and are not correctly modeled, despite minimum turbidity, and non-nutrient limiting conditions. These chlorophyll concentrations appear to be regulated by a combination of light availability and grazing. Although extremely high anthropogenic nutrient inputs in the freshwater region of the Delaware River provide non-limiting nutrient concentrations throughout the estuary, regulation of phytoplankton growth by light-limitation restricts chlorophyll concentrations below the nuisance levels found in many eutrophic systems.

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.

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.

Temporal and spatial variability in phytoplankton ammonium and nitrate uptake in the Delaware Estuary

Abstract Phytoplankton NH 4 + and NO 3 − uptake was examined along the longitudinal salinity gradient of the Delaware Estuary over several seasonal cycles using 15 N-tracer techniques. Saturated nitrogen uptake rates increased directly with water temperature and reached a maximum of 380 nmol Nl −1 h −1 during summer. This temperature dependence was related primarily to changes in the rate of maximum chlorophyll specific uptake, which varied exponentially between 2 and 70 nmol N [μg Chl h] −1 over a temperature range of 2–28°C. Despite these high uptake rates, balanced growth (C:N⋍7:1) could be maintained over the diel light cycle only by highly efficient nitrogen uptake at low light intensities and dark uptake below the photic zone and at night (dark uptake=25% maximum uptake). Ammonium fulfilled 82% of the annual phytoplankton nitrogen demand in the estuary despite dominance of NO 3 − in the ambient dissolved inorganic nitrogen pool. The predominance of NH 4 + uptake occurred because of the general suppression of NO 3 − assimilation at NH 4 + concentrations in excess of 2 μ m . This suppression, however, was not as universal as has been reported for other systems, and it is suggested that the extremely high NO 3 − concentrations found in the estuary contribute to this pattern. Nitrate was a significant source of nitrogen only during periods of high phytoplankton production in summer, and when NH 4 + concentrations were low towards the end of the spring bloom.

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.

Wind and tidal forcing of a buoyant plume, Mobile Bay, Alabama

AVHRR satellite imagery and in situ observations were combined to study the motion of a buoyant plume at the mouth of Mobile Bay, Alabama. The plume extended up to 30 km from shore, with a thickness of about 1 m. The inner plume, which was 3–8 m thick, moved between the Bay and inner shelf in response to tidal forcing. The tidal prism could be identified through the movement of plume waters between satellite images. The plume responded rapidly to alongshore wind, with sections of the plume moving at speeds of more than 70 cm s−1, about 11% of the wind speed. The plume moved predominantly in the direction of the wind with a weak Ekman drift. The enhanced speed of the plume relative to normal surface drift is probably due to the strong stratification in the plume, which limits the transfer of momentum into the underlying ambient waters.

The effect of salinity on binding of Cd, Cr, Cu and Zn to dissolved organic matter

Salinity can affect binding of metals to humic acid and thus affect bioavailability, however, the effects are poorly understood. The fraction of Cu, Zn, Cd and Cr (at 100 μg/L) complexed with Suwannee River Humic Acid (SRHA) was measured at salinities of 0, 1, 3, 10, and 15 psu using 1000 Dalton ultrafiltration membranes. In distilled water, 10 mg SRHA/L bound at least 40% of each metal. Except for Cu, metal binding decreased to less than 20% at salinities between 1 and 15 psu. Copper binding decreased to 27% at 3 psu, but increased to approximately 60% bound at 15 psu. Copper complexation with natural dissolved organics from a local estuary was also measured and ranged from 25 to 100%.

Microzooplankton grazing and nitrogen excretion across a surface estuarine-coastal interface

The role of the microzooplankton community in regulating phytoplankton biomass was examined across a gradient from a river-dominated estuary to an oceanic-influenced coastal zone. Three stations located along a salinity gradient from the central region of Mobile Bay to 10 km off the coast were sampled from May 1994 to August 1995. Microzooplankton herbivory rates on phytoplankton and microzooplankton excretion of nitrogen derived from phytoplankton were estimated using the dilution technique. Microzooplankton grazing rates (range of station means=0.57–1.10 d−1) and phytoplankton growth rates (0.70–1.62 d−1) both increased across the salinity gradient from the bay station to the offshore station. However, the percent of primary production grazed per day was highest at the bay station (mean=83%) and decreased to a low at the offshore station (mean=64%). Excretion of phytoplankton-derived nitrogen by the microzooplankton was greatest at the bay and bay mouth stations. Excreted nitrogen could potentially supply 39%, 29%, and 20% of phytoplankton nitrogen demand at the bay, bay mouth, and offshore stations, respectively. These results support the idea that herbivorous microzooplankton are important in mediating nitrogen flow to both lower and higher trophic levels. *** DIRECT SUPPORT *** A01BY085 00012