Donald W. Stanley

Phytoplankton, nutrients, and turbidity in the Chesapeake, Delaware, and Hudson estuaries

Abstract Estuaries receive continuous inputs of nutrients from their freshwater sources, but the fate of the inputs is poorly known. In order to document nutrient removal from the water column by phytoplankton, we measured the distributions of turbidity, nutrients, and phytoplankton across the salinity gradients of three estuaries: Chesapeake Bay, Delaware Bay, and the Hudson river estuary. Mixing diagrams were used to distinguish between conservative and non-conservative behavior; i.e. between loss from the water column and export to the estuarine plume on the shelf. In Chesapeake and Delaware Bays, we frequently observed a turbidity maximum in the oligohaline region, a chlorophyll maximum in clearer waters seaward of the turbidity maximum, and a nutrient-depleted zone at the highest salinities. In the Hudson River estuary, mixing diagrams were dominated by lateral waste inputs from New York City, and nutrient removal could not be estimated. In Chesapeake Bay, there was consistent removal of total N, nitrate, phosphate, and silicate from the water column, whereas in Delaware Bay, total N, ammonium, total P, and phosphate were removed. Total N and P removal in the Chesapeake and Delaware are estimated as ca. 50%, except for TP in the Chesapeake, which appeared to be conservative. Phytoplankton accumulation was associated with inorganic nutrient removal, suggesting that phytoplankton uptake was a major process responsible for nutrient removal. In the high salinity zone near and in the shelf plume, an index of nutrient limitation suggested no limitation in the Hudson, slight or no limitation in the Delaware, and widespread limitation in the Chesapeake, especially for P. These observations and information from the literature are summarized as a conceptual model of the chemical and biological structure of estuaries.

Long-term changes in watershed nutrient inputs and riverine exports in the Neuse River, North Carolina.

We compared patterns of historical watershed nutrient inputs with in-river nutrient loads for the Neuse River, NC. Basin-wide sources of both nitrogen and phosphorus have increased substantially during the past century, marked by a sharp increase in the last 10 years resulting from an intensification of animal production. However, this recent increase is not reflected in changes in river loading over the last 20 years. Temporal patterns in river loads more closely parallel short-term changes in point sources and cropland nutrient application despite their overall lower magnitude. Total phosphorus loads have declined at all stations considered, corresponding to a 1988 phosphate detergent ban. Nitrogen load temporal patterns vary by location and the nitrogen fraction considered. The furthest upstream station exhibited nitrogen decreases after the completion of a dam in 1983. At a station just downstream of a rapidly growing urban area, the total nitrogen load has increased since the mid-1980s, primarily as a nitrate concentration increase. This is consistent with concurrent increases in chemical fertilizer use and point source discharges, as well as increased nitrification at treatment plants. This increase in nitrate loading is not reflected at the most downstream station, where no clear nitrogen trends are discernable. The lack of clear downstream nutrient increases suggests that current water quality impairment in the lower river and estuary may result from chronic nutrient overload rather than recent changes in the watershed. If this is true, then the impact of a planned 30% nitrogen loading reduction may not be immediately apparent. We calculate that, given annual variability, detecting a load reduction of this magnitude will take at least four years, and, should nutrients accumulated in the watershed become a significant source, detecting the resulting ecological improvements is likely to take substantially longer.

Stratification and Bottom-Water Hypoxia in the Pamlico River Estuary

Relationships among bottom-water dissolved oxygen (DO), vertical stratification, and the factors responsible for stratification-destratification in this shallow, low tidal-energy estuary were studied using a 15-yr set of biweekly measurements, along with some recent continuous-monitoring data. Hypoxia develops only when there is both vertical water-column stratification and warm water temperature (>15°C). In July, 75% of the DO readings were <5 mg 1−1, and one-third were <1 mg 1−1. Severe hypoxia occurs more frequently in the upper half of the estuary than near the mouth. Both the time series data and correlation analysis results indicate that stratification events and DO levels are tightly coupled with variations in freshwater discharge and wind stress. Stratification can form or disappear in a matter of hours, and episodes lasting from one to several days seem to be common. Estimated summertime respiration rates in the water and sediments are sufficient to produce hypoxia if the water is mixed only every 6–12 d. There has been no trend toward lower bottom water DO in the Pamlico River Estuary over the past 15 yr. *** DIRECT SUPPORT *** A01BY059 00002

Dynamics of NH4+ and NO3− uptake in the water column of the neuse river estuary, North Carolina

In an attempt to more fully understand the dissolved inorganic nitrogen dynamics of the Neuse River estuary, 15NH4+ and 15NO3− uptake rates were measured and daily depth-integrated rates calculated for seven stations distributed along the salinity gradient. Measurements were made at 2–3-wk intervals from March 1985 to February 1989. Significant dark NH4+ uptake occurred and varied both spatially and seasonally, accounting for as much as 95% of light uptake with the median being 33%. Apparent NH4+ uptake ranged from 0.001 μmol N 1−1 h−1 to 4.2 μmol N 1−1 h−1, with highest rates occurring during late summer-fall in the oligohaline estuary. Apparent NH4+ uptake was significantly related to NH4+ concentration (p<0.01); however, the regression explained <3% of the variation. Daily-integrated NH4+ uptake ranged from 0.1 mmol N m−2 d−1 to 133 mmol N m−2 d−1 and followed the trend of apparent uptake. Annual NH4+ uptake of the estuary was significantly lower in 1988 than for any other year. Dark uptake of NO3− was only 14% of maximum light uptake. Apparent NO3− uptake rates ranged from 0.001 μmol N 1−1 h−1 to 1.84 μmol N 1−1 h−1 with highest rates occurring in the oligohaline estuary. Apparent NO3− uptake was significantly related to NO3− concentration (p<0.01); however, the regression explained <5% of the variation. In general, NO3− uptake was only 20% of total dissolved inorganic nitrogen (DIN) uptake. Daily-integrated NO3− uptake ranged from 0.1 mmol N m−2 d−1 to 53 mmol N m−2 d−1 and followed similar patterns of apparent uptake. Annual NH4+ uptake was 11.39 mol N m−2 yr−1, 10.28 mol N m−2 Yr−1, 10.93 mol N m−2 yr−1, and 7.38 mol N m−2 yr−1, and 1.84 mol N m−2 yr−1, with the 4-yr mean being 10.0. Annual NO3− uptake was 3.12 mol N m−2 yr−1, 3.40 mol N m−2 yr−1, 1.96 mol N m−2 yr−1, and 1.84 mol N m−2 yr−1, with the 4-yr mean being 2.6. The total annual DIN uptake was more than twice published estimates of phytoplankton DIN demand, indicating that there is an important heterotrophic component of DIN uptake occurring in the water column. The extrapolation of nitrogen demand from primary productivity results in serious underestimates of estuarine nitrogen demand for the Neuse River estuary and may be true for other estuaries as well.

Long‐term trends in Pamlico River Estuary nutrients, chlorophyll, dissolved oxygen, and watershed nutrient production

Trends in Pamlico River estuary ammonia nitrogen (NH4), nitrate nitrogen (NO3), phosphate phosphorus (PO4), chlorophyll a (chl a) and dissolved oxygen (DO) during the past 20–24 years were analyzed, and estimates of annual N and P production in the watershed over the past century were computed. The goal of the study was to determine whether or not the estuary is becoming more eutrophic. NO3 has decreased in the upper and middle regions of the estuary by 3–6% yr−1 since 1970, and NH4 has decreased throughout the estuary at an annual rate of 5.5–7.7% yr−1. Since 1967 PO4 has increased by 2% yr−1 in the lower two thirds of the estuary due to discharges from a phosphate mining facility. Thus the inorganic N:P ratio has decreased, which suggests that N is now potentially more limiting than in the past. In the upper estuary, chl a increased at a rate of 6.6% yr−1 since 1970, and bottom water DO decreased very slightly; neither showed trends in the middle and lower estuary regions. The weight of the evidence is that the Pamlico has not become more eutrophic during the past two decades. This finding is corroborated by the lack of a trend since 1970 in calculated N and P production from point and nonpoint sources in the watershed. Watershed nutrient production is estimated to have increased severalfold between 1880 and 1970, but appears to have stabilized after 1970, due primarily to decreased application of fertilizer on croplands.

MICROBIOLOGICAL CHANGES OCCURRING AT THE FRESHWATER-SEAWATER INTERFACE OF THE NEUSE RIVER ESTUARY, NORTH CAROLINA

: Previously, other researchers have observed rapid biogeochemical changes at the freshwater-seawater interface (FSI) of the Tamar Estuary in England. The postulated cause of these changes is a sequence of processes triggered by mass mortality of freshwater phytoplankton. Such mortality can be viewed as an example of selective “filtration” operating at the FSI. We examined a number of physical, chemical, and microbiological variables in the Neuse River Estuary, North Carolina, within the context of this hypothesis. Nitrate nitrogen, orthophosphate, and number of phytoplankton taxa all decreased significantly (p<0.05) going downriver across the FSI, while bacterial density and productivity each rose sharply (p<0.05). Other parameters that were monitored (phytoplankton density, chlorophyll a, ammonia nitrogen, and light absorption coefficient), changed in the vicinity of the FSI, but none of the changes were statistically significant at the 0.05 level. In the laboratory, we tested the effects of low salinities on the riverine microbial community. Of seven variables, only bacterial density and bacterial productivity showed any significant treatment effects. Overall, we could not confirm the hypothesis that exposure to salt in the FSI affects the microbial community. Alternate hypotheses involving nutrient availability, increased residence time of water, light availability, and population interactions are discussed.