R. A. Luettich et al
Circulation characteristics of the Neuse River Estuary
Circulation characteristics of the Neuse River Estuary, North Carolina
Richard A. Luettich, Jr.*1, Janelle V. Reynolds-Fleming1,
Jesse E. McNinch2, and Chris P. Buzzelli1
1
Institute of Marine Sciences
University of North Carolina
3431 Arendell Street
Morehead City, NC 28557 USA
2
Waterways Experiment Station
1261 Duck Road
Kitty Hawk, NC 27949 USA
*Corresponding author
tel: (252) 726- 6841 ext. 137
fax: (252) 726-2426
email: [email protected]
Manuscript for Estuaries, 20 April, 2000
R. A. Luettich et al 1
Abstract
An observational program was initiated in 1997 to quantify dominant circulation
characteristics in the Neuse River Estuary (NRE). The study included weekly hydrographic
sampling along the estuary axis, analysis of approximately 14 months of velocity, salinity,
temperature, and water level data from a near surface and a near bottom InterOcean S4 current
meter moored midway along the length of the estuary, and two intensive cross-channel velocity
and hydrographic surveys. Our results indicate that: (1) Astronomical tides constitute less than
2% of the variation in both water level and velocity; (2) Water exchange is dominated by remote
atmospheric forcing for periods of days; (3) Fresh water discharge dominates water exchange for
periods of days to weeks; (4) Variations in velocity with periods of 7 h to 20 h appear as
barotropic standing waves. These motions account for roughly 75% of the total velocity variance
but do not contribute to significant volume exchange due to their short periods; (5) Significant
cross channel circulation occurs and exhibits side to side sloshing; and (6) The estuary has
variable flow types with classical estuarine flow recorded approximately 50% of this study time.
Introduction
The Neuse River Estuary (NRE) is a drowned river valley located at the transition from
the Neuse River to Pamlico Sound in North Carolina (Fig. 1). It is a shallow estuary with a mean
depth of 3.6 m, mean width of 6.5 km, and length of 70 km. Like many estuaries, the NRE has
seen rapid development along its shores and within its watershed during the past 50 years.
Changing and diversifying land use practices that now include forestry, agriculture, industry, and
urban development have placed increasing pressure on estuarine and coastal habitats to
accommodate anthropogenic inputs. It is widely believed that these pressures have resulted in a
R. A. Luettich et al 2
long-term degradation of water quality (Paerl 1983, 1987; Christian et al. 1986; Paerl et al. 1995,
1998). It has frequently been assumed that water quality problems in the NRE are exacerbated by
a relatively long flushing time.
Early studies in Pamlico Sound identified the important role wind has in forcing
circulation in this waterbody and by inference, the lower NRE (Seiwell, 1927; Roelofs and
Bumpus, 1953; Pietrafesa et al., 1986).
The first circulation study in the NRE consisted of two dye releases by Woods (1969). He
found average downstream dye transport rates of 5.4 cm s-1 during a 6 d release and 3.1 cm s-1
during an 18 d release.
Knowles (1975) deployed nine current meters, 7 near bottom and two near surface, in the
NRE for 38 d beginning in August, 1973. He found significant spectral energy near the semidiurnal period and attributed as much as a 20 cm s-1 current to the lunar semi-diurnal tide. This
signal was weakest near the mouth of the NRE and increased with distance upstream. The
strongest evidence for wind forcing was due to the diurnal sea breeze. A mean downstream
current of 1.8 cm s-1 was computed by averaging data from all instruments during the
deployment period. This corresponds to a transit time of 32 d from New Bern (near station 40 on
Fig. 1) to the mouth (just past station 180 on Fig. 1) of the NRE. However, since it is likely that
the majority of his near bottom current meters experienced upstream flow due to estuarine
circulation at least part of the time, this mean current value may not accurately reflect
downstream transport which would tend to be concentrated near the surface.
Kim (1990) examined circulation as part of a study of turbidity maxima in the NRE.
Vertical profiles of velocity and density collected at 20 stations along the estuary axis at monthly
intervals over a 13 mo period frequently indicated vertical density stratification and estuarine
R. A. Luettich et al 3
type circulation (downstream at the surface and upstream near the bottom). Power spectra of
water level time series collected near Minnesott Beach suggested a signal of less than 5 cm in the
approximate semi-diurnal frequency band.
Robbins and Bales (1995) reported mean, median and maximum upstream and
downstream velocities from 10 current meters deployed 1.5 m from the bottom along the estuary
for a period of 17 d in October, 1989. Their data contained mean velocities that were much
higher than those recorded by Knowles (1975) and indicated that currents were generally lower
on the north side of the estuary than on the south side and that upstream currents were more
frequent near the northern shore while downstream currents predominated near the southern
shore. Results from a companion two-dimensional, vertically-integrated circulation model
indicated the longitudinal salinity gradient was significant in driving the long term circulation in
the system and that short term circulation responds considerably to wind forcing.
While the previous studies provide a basic impression of the circulation in the NRE,
either the data quality or the analyses leave significant questions unanswered that we have
attempted to address in our present study. Specifically, we seek to understand the following: 1)
Does the semi-diurnal tide provide enough energy to influence circulation and water level? 2)
How do fresh water discharge and wind forcing interact to drive circulation and water level? 3)
What role does wind forcing play in moving the salt wedge along and across the axis of the
estuary versus mixing and destroying vertical stratification? 4) Is there appreciable across
channel circulation in the NRE?
R. A. Luettich et al 4
Methods
Mid-river hydrography study
Physical/chemical properties (temperature, conductivity, dissolved oxygen and pH) of the
water column were sampled weekly at mid-river stations by University of North CarolinaInstitute of Marine Sciences (UNC-IMS), Weyerhaeuser, and North Carolina Division of Water
Quality personnel from June 1997 through December 1998. Observations were made at 0.5 m
intervals from near-surface to near-bottom at mid-river locations (stations 0-180) from Streets
Ferry bridge to the mouth of the NRE (Fig. 1). These efforts expanded a similar bi-weekly
sampling program at a smaller number of stations (0-30, 50-70, and 120; Fig. 1) that had been in
place since 1994. All sampling was conducted using Hydrolab Surveyor 3 data loggers and H20
CTDs with dissolved oxygen and pH sensors. These instruments were calibrated in the lab prior
to each sampling trip.
Current meter mooring
Two InterOcean S4 electromagnetic current meters, positioned near-bottom (75 cm above
bottom) and near-surface (75-125 cm below surface), were moored near Minnesott Beach (Fig.1)
in an average depth of 4 m. The mooring was maintained from August 1997 through September
1998. From August 1997-June 1998, the S4s measured current speed and direction, conductivity,
temperature, and pressure at 2 Hz for 5 min every 30 min. From June 1998 to September 1998,
the S4s collected data at 2 Hz for 3 min every 15 min. Temperature and conductivity values were
verified with an independently calibrated Hydrolab H20 CTD.
R. A. Luettich et al 5
Across-channel ADCP and CTD transects
Current speed and direction throughout the water column were measured using a
shipboard acoustic Doppler current profiler (ADCP). An RD Instruments 1200 kHz broadband
ADCP was boom-mounted and towed along the surface at approximately 4 kts. Ship position and
flow velocities were calculated from Differential Global Positioning System (DGPS) and bottom
tracking, respectively. Depth bins of 25 cm were used and ensemble averaging of pings over a
horizontal distance of 200 m provided a flow speed accuracy of 2 cm s-1. Velocities from the
near-surface and near-bottom depth bins were discarded due to velocity errors associated with
surface disturbances and ship wakes and side lobe reflection at the bottom. Velocity calculations
from the ADCP thus provided an effective range from approximately 75 cm below the surface to
25 cm above bottom.
CTD/DO casts were performed using a Seabird SBE 19 CTD with attached DO sensor
and pumped flow through system. The CTD/DO was lowered continuously at a constant speed of
approximately 1.5 m per min and provided vertical profiles at 2.5 cm resolution. DO and salinity
measurements from the Sea Bird CTD were calibrated and verified in the field with a YSI
dissolved oxygen probe and a Hydrolab H20 CTD with DO sensor that were calibrated
independently in the laboratory.
Currents were measured with the ADCP along transects 100 and 120 two times per day
during June 3-5, 1998 and along transects 100 and 148 every 5-9 h during July 27-30, 1998.
Hydrography was measured by taking CTD casts at 5 stations spaced approximately 800-1200
km apart along each transect within 45 min of each ADCP run.
R. A. Luettich et al 6
Meteorological Data
Meteorological data collected by the Cherry Point Marine Corps Air Station were
obtained from the North Carolina State Climatological Office. These data consist of hourly wind
speed and direction, sky conditions, air temperature, relative humidity and barometric pressure.
The station is located just south of the NRE near the center of the study area as shown in Fig. 1.
Freshwater Discharge
Freshwater volume discharge data were collected by the USGS for the Neuse River at
Kinston, NC. This station is approximately 145 km upstream from the mouth of the estuary. The
data were collected at quarter hour intervals. Discharge is largely regulated by the Falls Creek
Dam approximately 129 km upstream of the discharge measuring station.
Velocity Rotations
Water velocity data from the current meter mooring were rotated into along- and acrosschannel components. Component directions were determined by computing power spectra for the
velocity rotated in five degree increments around the compass. The direction producing the
greatest total power (the greatest variance) was chosen as the along-channel direction. The along
channel axis was determined to be NW-SE (300-120 degrees) at the current meter location and is
aligned with the section of the NRE upstream of the 90 degree bend near Minnesott Beach, (Fig.
1).
Wind velocities were also decomposed into along- and across-channel components by
choosing the direction that gave the highest correlation between wind velocity and water level.
This turned out to be NE-SW (50-230 degrees), which is aligned with the section of the NRE
downstream of the 90 degree river bend.
R. A. Luettich et al 7
Results
Mid-river hydrography
The mid-river CTD/DO data provide along-channel snapshots of the hydrography of the
NRE. These results yield a nearly synoptic view of the location of the salt-wedge, the strength of
stratification and dissolved oxygen levels down the length of the estuary.
The data show considerable variability between weekly measurements in both the
location and degree of stratification of the salt-wedge and associated dissolved oxygen levels.
Figure 2 shows an example of the mid-river hydrography. Figures 2a and 2b present sigma T and
dissolved oxygen (DO) data collected July 6-7, 1998. Prior to this sampling period, winds were
toward the southwest for 1-2 days and the mean daily freshwater discharge was weak compared
to the yearly mean. The water column was well mixed and DO levels were near saturation.
Figures 2c and 2d represent data collected August 17, 1998. Prior to this sampling period, winds
were light toward the northeast for 2-3 days and again the mean daily freshwater discharge was
weak compared to the yearly mean. The water column is stratified with the terminus of the salt
wedge extending upriver nearly to New Bern. Bottom waters were nearly anoxic within the salt
wedge.
Mid-river data collected prior to the 1997-98 field program (1994-97) were pooled with
the 1997-98 data set and analyzed for relationships between dissolved oxygen concentration,
stratification and water temperature. Frequency histograms of low oxygen conditions for three
different temperature ranges and six different vertical salinity (density) variations are presented
in Fig. 3. Low dissolved oxygen (DO £ 4 mg l-1) conditions occurred in ~70% of the samples
when water temperatures were above 20°C and the water column was weakly stratified (∆ psu =
R. A. Luettich et al 8
1-2). This increased to ~90% for stronger stratification. Hypoxic to anoxic conditions (DO £ 2
mg l-1) occurred in ~40% of the samples for weak stratification and ~ 85% for strong
stratification during these warm periods. Low oxygen conditions were much less frequent during
cooler temperature, however, they still appeared in 60-70% of the samples when stratification
was strong, even during the winter months. A strong relationship between dissolved oxygen and
stratification was found by Stanley and Nixon (1992) on the Pamlico River Estuary. A
mechanistic/statistical model to predict DO concentration based on temperature and stratification
has been developed by Borsuk et al (in review).
Time Series Data
An example of the time series data (wind velocity, water level, near-surface currents,
near-bottom currents, 30 h low-pass filtered currents, and near-surface and near-bottom salinity)
for the month of August 1997 is presented in Fig. 4. These data indicate that winds blowing
toward the northeast force a set-down of water level and an increase in stratification (Fig.
4a,4b,4f; Julian days 214-217,225-230). These winds enhance the downstream surface current
and may also enhance the upstream bottom estuarine circulation (Fig. 4e). Winds blowing
towards the southwest drive an increase in water level and mix the water column (Fig. 4a,4b,4f;
Julian days 218-223, 230-231,234-241).
Power spectra of wind, water level, water velocity and salinity are presented for two 10
week periods, one during the winter (December 1997-February 1998) and one during the
summer (July-September 1998), in Figs. 5 and 6. These time periods were chosen for three
reasons: 1) complete data records exist for these periods; 2) the winter time period is typically
dominated by winds blowing toward the SW whereas the summer time period is typically
R. A. Luettich et al 9
dominated by winds blowing toward the NE; and 3) the winter time period is characterized by
high freshwater discharge whereas the summer time period is not (Fig. 7).
The wind spectra for both time periods are dominated by low frequency energy (periods
longer than approximately 1.5 d and presumably associated with synoptic weather patterns). The
along channel component is closely aligned with the prevailing wind direction and therefore
generally contains more energy than the across channel component. There is no indication of a
diurnal sea breeze signal in the wind spectra from either time period. Water level power spectra
are also dominated by low frequency energy. There is an indication in both time periods of
elevated water level energy around the semi-diurnal frequency. Surface and bottom salinity
during both time periods also show energy primarily at low frequencies. In the winter period,
there is indication of energy at higher frequencies, but does this is not the case in the summer
period. Current velocity energy is much more evenly distributed across the frequency spectrum
than for the wind, water level and salinity. A strong peak exists near 12 h in the along channel
component in the summer and at slightly longer periods (lower frequency) in the winter.
Coherence between the along-channel wind and water level is quite strong in both time
periods at low frequencies (freq 0.01-0.035) with a phase angle of nearly 180 degrees, Figs 8a,c
and 9a,c. This phase angle suggests that a positive along-channel wind (i.e. wind blowing
towards the NE) corresponds to a decrease in water level in the NRE and vice versa. Coherence
between wind and water level is not significant (at the 95% level) during either time period at 24
h (freq ~0.04) suggesting that the water level response at this frequency is not locally wind
driven. Significant coherence between water level and fresh water discharge for frequencies less
than 0.01 (approximately 4 d) in both time periods (Fig. 10) suggests that the underlying
circulation driver for the system is freshwater discharge at these long time scales. At frequencies
R. A. Luettich et al 10
higher than approximately 0.035, water level and along-channel wind as well as fresh water
discharge and along-channel wind show significant coherence at a few isolated frequencies, but
with little consistent pattern.
During both time periods, along-channel near-surface currents are coherent with water
level from roughly 7 h to 20 h (freq 0.05-0.14), Figs. 8b,d and 9b,d. The upstream current leads
the water level by approximately 90 degrees suggesting that these motions represent barotropic
standing waves. Near-surface and near-bottom currents are coherent across roughly this same
frequency range with near-bottom currents leading near-surface currents by 0-40 degrees (Figs.
8f,h and 9f,h). This phase difference suggests the possibility of stratification induced decoupling
between the upper and lower water column with bottom friction important primarily in the lower
water column. In the winter data set, along-channel wind is coherent with near-surface alongchannel currents over roughly the same frequency range that wind and water level are coherent.
The same pattern may also hold in the summer data set although the coherence is not as strong
(Figs. 8e,g and 9e,g). This suggests that low frequency along channel winds drive along channel
surface currents and ultimately water level.
To examine the contribution of the astronomical tides to circulation in the NRE, a leastsquares analysis using seven tidal constituents, O1, K1, N2, M2, S2,M4, and M6, was calculated for
water level for both the winter and the summer periods. The calculated phases and amplitudes
were then used to reconstruct a water level time series due to the tides alone. This reconstructed
tidal signal, Fig. 11, represents less than 1% of the raw water level variance. A similar analysis
of the strength of the astronomical tides in the velocity record yielded a tidal variance of only 12% of the total variance in the raw velocity signal in both the near bottom and near surface
records. Comparatively, 21% (19%) of the water level variance during the winter (summer) and
R. A. Luettich et al 11
86% (83%) of the velocity variance is associated with nontidal energy in the diurnal to sextodiurnal frequency band.
Across-channel flow and hydrography
Both of the shipboard, cross-estuary surveys in the summer of 1998 indicated the
presence of similar across-channel circulation. Therefore, we present only the results of the July
experiment (Figs 12-14).
Sampling was done during a period when the river appeared to be characterized by
estuarine circulation. The along-channel flows at transect 100 indicate this estuarine flow,
although significant variation in flow direction exists across channel as well as vertically within
the water column. A possible pattern is suggested in which upstream flow is focused toward the
southern shore and downstream flow is focused towards the northern shore (Figs. 12a-12d).
The across-channel flow was initially toward the southern shore near the surface and
toward the northern shore near the bottom (Fig. 13a). The corresponding hydrography indicated
upwelling along the northern shore (Fig. 14a). Winds around the time of this transect were light
(5-10 kts) and toward the southwest throughout the morning (Fig. 13e). Within 5 hours, both
wind direction and speed had changed to blowing strongly (15-20 kts) toward the northeast. By
the second transect, the across-channel velocities were reversed from the first transect and were
flowing toward the northern shore near the surface and strongly towards the southern shore near
the bottom (Fig. 13b). Hydrography indicates that the isohalines and isotherms responded
quickly to the across channel flow with upwelling occurring along the southern shore (Fig. 14b).
The across-channel flow weakened considerably during the third transects and slightly reversed
(Fig. 13c), which corresponds to a decrease in wind velocity and a further shift in wind direction.
R. A. Luettich et al 12
Hydrography indicates the cessation of upwelling on the southern shore as evident by relaxation
in the isohalines (Fig. 14c). By the fourth transect, across-channel velocities weakened further,
yet shifted to northward flow at the surface and southward flow at the bottom (Fig. 13d). There is
indication of southern shore upwelling in the hydrographic data (Fig. 14d).
Discussion and Conclusions
This paper reports on results from an observational study of the physical characteristics
of the Neuse River Estuary (NRE), a shallow, lagoonal/riverine estuary in eastern North
Carolina. This study provides a much more thorough description of stratification and dissolved
oxygen, along and across channel circulation and physical forcing mechanisms than has been
previously reported for this system.
Harmonic analysis of approximately a year of water level and current meter data clearly
indicates that semi-diurnal and diurnal astronomical tides are insignificant in the NRE.
Significant low frequency coherence between water level and fresh water discharge in the Neuse
River confirms the coupling between these variables at time scales of days to weeks.
Strong coherence in the synoptic weather band between water level and wind aligned
with the lower NRE (which is also aligned with the long axis of Pamlico Sound) indicates that
these winds are the predominant forcing mechanism for water movement on time scales of days.
Others have shown that water level variations in Pamlico Sound are coherent with synoptic band
winds blowing in the along sound direction (e.g., Pietrafesa et al., 1986). Therefore, we believe
that at these time scales water level (and consequently along estuary circulation) in the NRE is
actually driven by setup of Pamlico Sound rather than direct wind stress upon the NRE. We did
R. A. Luettich et al 13
not find evidence of significant wind energy or estuary response at the diurnal frequency
suggesting that the daily sea breeze was not significant in this area.
Spectra of along-channel water velocity indicate the presence of substantial energy at
diurnal and higher frequencies with elevated levels around 12 h. These velocities are highly
coherent with and approximately 90 degrees out of phase with water level fluctuations
suggesting that at these time scales motion is dominated by barotropic standing waves. While
these relatively short time scale motions do not result in the displacement of large volumes of
water, they comprise an oscillatory motion that can, in areas of significant geometric/bathymetric
irregularities and/or spatially varying low frequency flows, lead to horizontal dispersion of
materials similar to tidal dispersion (Geyer and Signell, 1992).
A possible cause of these motions can be identified by considering the lowest mode
resonant period, T0, of a shallow, barotropic, frictionless waterbody:
T 0 = 2d
gh
where g is the acceleration of gravity, h is the water depth and d is the length of the water body.
Applying this to the long axis of Pamlico Sound plus the lower NRE ( h ≈4.5m , d ≈130km )
gives T o ≈11hrs . Therefore we propose the semi-diurnal signal that was initially identified by
Knowles (1975) and that is pervasive in our data is actually the lowest mode resonance of the
combined NRE/Pamlico Sound system. The higher frequency barotropic standing waves are
probably due to resonances created by shorter length scales in the system. These resonant
motions are commonly called seiches. Preliminary results from a numerical circulation model are
consistent with these expectations and will be reported on fully in the future. Seiche motions are
common phenomena in deeper water bodies. In the Chesapeake Bay, Wang (1979) attributed
R. A. Luettich et al 14
seiches to direct wind stress in the longitudinal (N-S) direction whereas Chuang and Boicourt
(1989) relate similar seiche motion to lateral (E-W) wind.
Despite the shallow water depths and strong barotropic behavior, the bi-weekly to weekly
mid-river data we have collected since 1994 show that the estuary is frequently (salinity)
stratified over the vertical. In the NRE, stratification is due to the existence of a persistent fresh
water discharge (the Neuse River) and the absence of a persistent vertical mixing mechanism
(such as a strong astronomical tide). Wind plays an episodic role in this process as winds
directed down estuary (primarily toward the northeast) enhance downstream fresh water surface
flow and may also enhance upstream near bottom estuarine circulation. Conversely, winds
oriented up estuary (toward the southwest) tend to mix the water column. Thus the NRE can
behave as anything from strongly stratified estuary to a well-mixed estuary depending on which
way the wind is blowing. It has been shown that wind fields acting on the Chesapeake Bay can
both destratify the water column (Goodrich, et al, 1987) and lead to enhanced stratification
(Valle-Levinson, et al, 1998).
A rough idea of the "estuarine" nature of the NRE can be obtained from the year-long
data set of near surface and near bottom current velocities we have obtained. After low pass
filtering (30 h) these data to remove seiches and other high frequency components, the direction
of the near surface current was compared with the direction of the near bottom current.
Estuarine circulation (defined as downstream near the surface and upstream near the bottom)
occurred 52 percent of the time; riverine circulation (defined as downstream near the surface and
near the bottom) occurred 36 percent of the time; anti-riverine circulation (defined as upstream
near the surface and near the bottom) occurred 7 percent of the time; reverse estuary circulation
R. A. Luettich et al 15
(defined as upstream near the surface and downstream near the bottom) occurred 5 percent of the
time.
The presence of stratification has significant ecological implications, many of which are
due to the relationship between stratification and the dissolved oxygen concentration below the
pycnocline. At water temperatures above 20 0C, benthic and lower water column oxygen
consumption is sufficiently strong to cause significant oxygen reduction when vertical
stratification is as little as 1-2 psu. Even at cold water temperatures (e.g., below 10 0C) when
microbial oxygen consumption is strongly curtailed, significant instances of low dissolved
oxygen to hypoxic bottom water occur during periods of stronger stratification.
Finally, this study has documented the existence of rapid (less than 6 hr) cross-river
sloshing and accompanying near shore, bottom water upwelling in the portion of the NRE
between Cherry Point and New Bern. Due to the 90 degree bend in the NRE near Cherry Point,
the prevailing northeast-southwest winds are oriented nearly across channel in this section of the
estuary. Thus the same winds that ultimately drive the majority of the longitudinal circulation
are oriented to drive cross-channel flow upstream of Cherry Point. In this case winds blowing
toward the southwest are expected to cause upwelling on the northern shore and winds blowing
toward the northeast should cause upwelling on the southern shore. These upwelling events will
intermittently fill the near shore water column with bottom water. If the dissolved oxygen
concentration in this bottom water is sufficiently low, the upwelling may be lethal for fish that
are engulfed by these waters. Alternatively, repeated exposure to upwelled waters with sublethal
dissolved oxygen concentrations may have chronic effects on fish and/or their habitat.
R. A. Luettich et al 16
Acknowledgements
We thank Crystal Fulcher for assistance with the hydrographic field work, data analysis, and data
management, Jennifer McNinch and Joe Purifoy for assistance with the circulation and
hydrographic field work and data analysis, and Jim Hench for his insightful comments regarding
this manuscript. We thank the North Carolina State Climate Office for meteorological data from
Cherry Point and Alex Cardinell of the US Geographical Survey in Raleigh for the Kinston
discharge data. Funding was provided by the CISNet program (Environmental Protection
Agency grant R-826938-01-0) and by the Neuse River MODMON program (UNC Water
Resources Research Institute grant UNC-CH-040-197-1). A presentation of data obtained during
the MODMON program can be found in Luettich et al. (2000) and on the web at
www.marine.unc.edu/neuse/modmon/modmon.html.
Literature Cited
Borsuk, M.E., C. A. Stow, R. A. Luettich, H. W. Paerl, and J. L. Pinckney. 2000. Modeling
oxygen dynamics in an intermittently stratified estuary: Estimation of process rates using
field data. (submitted to Estuarine, Coastal and Shelf Science).
Christian, R. R., W. Bryant, and D.W. Stanley. 1986. The relationship between river flow and
Microcystis aeruginosa blooms in the Neuse River, NC. Water Resources Research
Institute, Report 223, Raleigh, North Carolina.
R. A. Luettich et al 17
Chuang, W. S. and W. C. Boicourt. 1989. Resonant Seiche Motion in the Chesapeake Bay.
Journal of Geophysical Research , 94(C2): 2105-2110.
Geyer, W. R. and R. P. Signell. 1992. A Reassessment of the Role of Tidal Dispersion in
Estuaries and Bays. Estuaries, 15(2), 97-108.
Goodrich, D. M., W. C. Boicourt, D. Hamilton and D. W. Pritchard. 1987. Wind-Induced
Destratification in Chesapeake Bay. Journal of Physical Oceanography, 17, 2232-2240.
Jarrett, J. T. 1966. A study of the hydrology and hydraulics of Pamlico Sound and their relation
to the concentration of substances in the sound: North Carolina State University, Dept. of
Civil Eng., M.S. thesis, Raleigh, North Carolina, 156 pages.
Kim, S. Y. 1990. Physical Processes and Fine-Grained Sediment Dynamics in the Neuse River
Estuary, NC: University of North Carolina, Curriculum of Marine Sciences, PhD. thesis,
Chapel Hill, North Carolina, 128 pages.
Knowles, C. E 1975. Flow dynamics of the Neuse River, North Carolina, Sea Grant Publication
UNC-SG-75-16, Sea Grant Program, Raleigh, North Carolina.
Luettich, R. A., J. E. McNinch, J. L. Pinckney, M. J. Alperin, C. S. Martens, H. W. Paerl, C. H.
Peterson, and J. T. Wells. 2000. Neuse River Estuary Modeling and Monitoring Project,
Final Report: Monitoring Phase, Water Resources Research Institute, Raleigh, North
Carolina, 190 pages.
R. A. Luettich et al 18
Paerl, H. 1983. Factors regulating nuisance blue-green algal bloom potential in the lower Neuse
River. Report No. 177, Water Resources Research Institute, Raleigh, North Carolina, 139
pages.
Paerl, H. 1987. Dynamics of blue-green algal (Microcystis aeruginosa) blooms in the lower
Neuse River, NC: causative factors and potential controls. Report No. 299, Water
Resources Research Institute, Raleigh, North Carolina, 164 pages.
Paerl, H. W., M. A. Mallin, C. A. Donahue, M. Go, and B. L. Peierls. 1995. Nitrogen loading
sources and eutrophication of the Neuse River Estuary, NC: Direct and indirect roles of
atmospheric deposition, Report 291, Water Resources Research Institute, Raleigh, North
Carolina, 116 pages.
Paerl, H.W., J. Pinckney, J. Fear, and B. Peierls. 1998. Ecosystem response to internal watershed
organic matter loading: Consequences for hypoxia in the eutrophying Neuse River
Estuary, North Carolina, Marine Ecological Progress Series 166:17-25.
Pietrafesa, L. J., G. S. Janowitz, T. Y. Chao, R. H. Weisberg, F. Askari, and E. Noble 1986. The
physical oceanography of Pamlico Sounds, UNC Sea Grant Report, UNC-SG-WP-86-5,
Sea Grant Program, Raleigh, North Carolina, 126 pages.
Robbins, J. C. and J. D. Bales. 1995. Simulation of hydrodynamics and solute transport in the
R. A. Luettich et al 19
Neuse River Estuary, North Carolina, US Geological Survey, Open File Report 94-511,
Raleigh, North Carolina, 85 pages.
Roelofs, E. W. and D. F. Bumpus. 1953. The hydrography of Pamlico Sound. Bulletin of Marine
Sciences of the Gulf and Caribbean, 3(3), 181-205.
Seiwell, H.R. 1927. A brief report on the physical hydrography of Pamlico Sound and its
tributaries. Typed report to the Dept. Geol., Univ. N.C. and U.S. Bureau of Fisheries.
Unpublished.
Stanley D. W. and S. W. Nixon. 1990. Stratification and Bottom water hypoxia in the Pamlico
River Estuary. Estuaries, 15(3), 270-281.
Valle-Levinson, A., J. L. Miller and G. H. Wheless. 1998. Enhanced Stratification in the lower
Chesapeake Bay following northeasterly winds. Continental Shelf Research, 18, 16311647.
Wang, D. D. 1979. Wind-driven circulation in the Chesapeake Bay, winter 1975. Journal of
Physical Oceanography, 9, 564-572.
Woods, W. J. 1969. Current Study in the Neuse River and Estuary of North Carolina. Report no.
13. Water Resources Research Institute, Raleigh, North Carolina, 34 pages.
R. A. Luettich et al 20
Figure Legends
Fig. 1. The Neuse River Estuary site location. The map shows locations of the S4 current meter
mooring, across-channel hydrographic transects, meteorologic station and mid-river sample
stations in which CTD/DO casts were conducted weekly from June 1997-December 1998.
Fig. 2. Mid-river hydrographic plot from July 6-7, 1998 and August 17, 1998. New Bern is
approximately 11 km and Cherry Point is approximately 40 km downstream of Streets Ferry
Bridge (Station SFB). a) sigma T and b) dissolved oxygen show unstratified conditions and near
saturated levels of dissolved oxygen existed down the length of the NRE following a period of
winds blowing toward the southwest. c) and d) show stratified and hypoxic conditions existed
below the salt-wedge following a period of light winds blowing toward the northeast.
Fig. 3. Frequency diagrams from the mid-river sampling program. Data represent the frequency
of low dissolved oxygen (a) and hypoxic (b) conditions under varying temperature ranges and
varying stratification (represented by ∆psu=bottom salinity – surface salinity). Sample size, n, is
shown for each temperature range and ∆psu combination.
Fig. 4. Time series from the S4 mooring near Minnesott Beach during August 1997 showing a)
wind velocity (direction wind is blowing towards), b) water level, c) near-surface currents, d)
near-bottom currents, e) near-bottom and near-surface currents filtered with a 30 h low-pass
filter, and f) near-surface and near-bottom salinity.
R. A. Luettich et al 21
Fig. 5. Power spectra and 95% confidence intervals for wind, water level, salinity and water
velocity from a 10-week winter period (December 1997-February 1998). Wind data are from
Cherry Point MCAS and all other variables are from the S4 mooring near Minnesott Beach.
Fig. 6. Power spectra and 95% confidence intervals for wind, water level, salinity and water
velocity from a 10-week summer period (July-September 1998). Wind data are from Cherry
Point MCAS and all other variables are from the S4 mooring near Minnesott Beach.
Fig. 7. Fresh water discharge from the NRE at Kinston, NC for 10-week winter (December 1997February 1998) and summer (July-September 1998) periods. The station is located 145 km
upstream from the mouth of the NRE.
Fig. 8. Coherence squared and phase differences (degrees) for a 10-wk winter period (December
1997-February 1998). Coherence squared values above 0.4 indicate data are coherent at the 95%
significance level. Phase differences are not reliable for coherence below the 90-95%
significance level.
Fig. 9. Coherence squared and phase differences (degrees) for a 10-wk summer period (JulySeptember 1998). Coherence squared values above 0.4 indicate data are coherent at the 95%
significance level. Phase differences are not reliable for coherence below the 90-95%
significance level.
R. A. Luettich et al 22
Fig. 10. Coherence squared and phase differences (degrees) from a 10-wk winter period
(December 1997 – February 1998) and summer period (July – September 1998). Coherence
squared values above 0.4 indicate data are coherent to the 95% significance level. Phase
differences are not reliable for coherence below the 90-95% significance level.
Fig. 11. Comparison of de-meaned raw water level signal from 10-wk winter and summer
periods with reconstructed tidal signal as computed from least squares analysis of seven major
tidal constituents (O1, K1, N2, M2, S2, M4, M6). A 1 m mean has been added to the reconstructed
tidal signal for plotting convenience.
Fig. 12. Along-channel velocity component at Transect 100 in July 1998. Survey times are EDT.
The white dashed line represents the zero-contour.
Fig. 13. Across-channel velocity component at Transect 100 in July 1998. Survey times are EDT.
The white dashed line represents the zero-contour.
Fig. 14. Water column hydrography at Transect 100 in July 1998.
R. A. Luettich et al 23
Figure 1.
R. A. Luettich et al 24
Figure 2.
n=47
n=59
n=145
n=38
n=28
4 mg/L or less
90
80
70
60
a.
50
n=24
n=3
n=5
n=7
n=25
10
n=8
20
n=22
n=117
30
n=29
40
n=17
n=103
frequency (% of samples)
n=53
100
n=169
R. A. Luettich et al 25
0
0 to 1
1 to 2
2 to 3
3 to 4
4 to 5
over5
0-10 deg C
10-20 deg C
20-30 deg C
70
n=59
n=145
n=28
80
n=38
2 mg/L or less
90
n=47
50
n=53
60
b.
n=5
n=22
n=8
10
n=7
n=25
20
n=117
30
n=3
n=24
n=29
40
n=17
n=103
frequency (% of samples)
n=169
100
0
0 to 1
1 to 2
2 to 3
3 to 4
delta salinity (psu)
Figure 3.
4 to 5
over5
R. A. Luettich et al 26
Figure 4.
R. A. Luettich et al 27
Figure 5.
R. A. Luettich et al 28
Figure 6.
R. A. Luettich et al 29
Figure 7.
R. A. Luettich et al 30
Figure 8.
R. A. Luettich et al 31
Figure 9.
R. A. Luettich et al 32
Figure 10.
R. A. Luettich et al 33
Figure 11.
R. A. Luettich et al 34
Figure 12.
R. A. Luettich et al 35
Figure 13.
R. A. Luettich et al 36
Figure 14.