2.0 Physical, Chemical, and Biological
Characteristics of the
Lower Cape Fear River Estuary
Matthew R. McIver and Michael A. Mallin
2.1 Introduction
This section of the report includes a discussion of the physical,
chemical, and biological water quality parameters, concentrating on the 1999-2000 Lower
Cape Fear River Program monitoring period. These parameters are interdependent and define
the overall condition of the river. Physical parameters measured during this study
included water temperature, dissolved oxygen, turbidity, salinity, conductivity, and pH.
The chemical makeup of the Cape Fear River was investigated by measuring the magnitude and
composition of nitrogen and phosphorus in the water, as well as concentrations of EPA
priority pollutant metals. Three biological parameters, fecal coliform bacteria,
chlorophyll a and biochemical oxygen demand, were examined to assess the effects of
point and non-point source loading of nutrients, organic materials, and other pollutants
on water quality.
2.2 Materials and Methods
All samples and field parameters collected for the estuarine
stations of the Cape Fear River (i.e. NAV down through M18) were gathered on an outgoing
tide. This was done so that the data better represented the river water flowing downstream
through the system rather than the tidal influx of coastal ocean water. Sample collection
and analyses were conducted according to procedures in the Lower Cape Fear River Program
QA/QC Manual.
Physical Parameters
Water Temperature, pH, Dissolved Oxygen, Turbidity, Salinity, and Conductivity
Field parameters were measured at each site using a YSI 6920 Multiparameter
Water Quality Probe (sonde) and displayed on a YSI 610D datalogger. Individual probes
within the instrument measured water temperature, pH, dissolved oxygen, turbidity,
salinity, and conductivity. At each channel station, physical parameters were measured at
the surface, mid-depth, and bottom, or until the cable connecting the datalogger with the
sonde had been fully extended. Stream stations were sampled at 0.25 m from surface. High
flow occasionally prohibited the probe from reaching the bottom. The instrument was
calibrated prior to and after each sampling trip to ensure accurate measurements.
Chemical Parameters
Nutrients
All nutrient analyses were performed at the Center for Marine Science
Research (CMSR, now Center for Marine Science, CMS) for samples collected prior to January
1996. A local state-certified analytical laboratory was contracted to conduct all
subsequent analyses (February 1996 through May 2000), except for the orthophosphate
procedure which is performed at CMS. The contract laboratory also analyzes for suspended
solids, fecal coliform bacteria, and EPA Priority Pollutant Metals according to
State-accepted procedures. The following methods detail the techniques used by CMS
personnel only.
Water samples were collected ca. 0.2 m below the surface in triplicate
in amber 125 mL Nalgene plastic bottles. After collection, the samples were immediately
placed on ice. In the laboratory, 50 mL of each triplicate was filtered through a separate
1.0 micron pre-combusted glass fiber filter, which were frozen and subsequently used for
chlorophyll a analysis. The triplicate filtrates were pooled in a common glass
flask, mixed thoroughly, and approximately 100 mL was poured into one 125 ml plastic
bottle to be used for orthophosphate analyses. Filtered samples were stored in a freezer
until time of analysis.
Orthophosphate (PO4-3)
All orthophosphate analyses were performed in duplicate using an approved EPA
(1992) method for the Technicon AutoAnalyzer (Method 365.5). In this technique, the
orthophosphate in each sample reacts with ammonium molybdate and antimony potassium
tartrate in an acidic medium (sulfuric acid) to form an antimony-phospho-molybdate
complex. The complex is then reacted with ascorbic acid and forms a deep blue color. The
intensity of the color is measured at a wavelength of 880 nm by a colorimeter and
displayed on a chart recorder. Standards and spiked samples were analyzed for quality
assurance.
Biological Parameters
Chlorophyll a
The analytical method used to measure chlorophyll a is described in
Welschmeyer (1994) and USEPA (1997). Chlorophyll a concentrations were determined
directly from the 1.0 micron glass fiber filters used for filtering samples for
orthophosphate analyses. All filters were wrapped individually in aluminum foil, placed in
an airtight container with drierite granules, and stored in a freezer. During the
analytical process, the glass filters were separately immersed in 10 ml of a 90% acetone
solution for 24 hours. The acetone extracts the chlorophyll a from the glass
filters into solution. At seven stations an additional grinding step was used during
extraction. Each solution was then analyzed for chlorophyll a concentrations using
a Turner AU-10 fluorometer. This method uses an optimal combination of excitation and
emission bandwidths which reduces the errors inherent in the acidification technique.
Biochemical Oxygen Demand (BOD)
Five sites were chosen for BOD analysis. One site was located at NC11,
upstream of International Paper, and a second site was at AC, about 3 miles downstream of
International Paper. A third site was located in Livingston Creek (LVC) near its outflow
to the Cape Fear River. Two sites were located in blackwater rivers (NCF117 and B210), and
one site (BBT) was situated in an area influenced by both the mainstem Cape Fear River and
the Black River (Fig. 1.1). The procedure used for BOD analysis was Method 5210 in
Standard Methods (APHA 1995). Samples were analyzed for both 5-day and 20-day BOD. During
the analytical period, samples were kept in airtight bottles and placed in an incubator at
20o C. All experiments were initiated within 5 hours of sample collection.
Samples were analyzed in duplicate. Dissolved oxygen measurements were made using a YSI
Model 57 meter that was air-calibrated. No adjustments were made for pH since all samples
exhibited pH values within or very close to the desired 6.5-7.5 range.
2.3 Results and Discussion
This section includes results from monitoring of
the physical, biological, and chemical parameters at all stations for the time period June
1999 – May 2000. Discussion of this data focuses mainly on the river channel
stations, especially with respect to long term trends. The contributions of the two large
blackwater tributaries, the Northeast Cape Fear River and the Black River, are represented
by conditions at NCF117 and B210, respectively. Conditions at the stream stations are
highly variable and often change rapidly as a result of localized weather phenomena,
therefore, definable patterns and trends are not exhibited at these sites. Chronic water
quality problems at stream stations are discussed and possible causes are suggested.
Hurricanes Dennis and Floyd impacted water quality in the Cape Fear Region on August 29
and September 15, respectively.
Physical Parameters
Water Temperature
Water temperatures at all stations ranged from 5.3 to 31.9 0C and
individual station annual averages ranged from 16.9 to 19.8 0C (Table 2.1). Highest temperatures were found
during August 1999 (all station mean = 28.6) and lowest temperatures were occurred during
February 2000 (all station mean = 6.6). Stream stations were generally cooler than the
river channel stations most likely because of shading and lower nighttime air temperatures
affecting temperatures in the shallower waters.
Salinity
Salinity at the estuarine stations ranged from 0.0 – 35.4 parts per
thousand (ppt) and station annual means ranged from 1.3 – 25.3 ppt (Table 2.2). Salinity was lowest during the late
summer/fall months, a result of rain from Hurricanes Dennis, Floyd and Irene, and highest
during the early/middle summer months. Two stream stations had occasional oligohaline
conditions: The Northeast Cape Fear River at NC 403 and Panther Branch both receive
salt-water effluent from pickle processing facilities.
Conductivity
Conductivity ranged from 0.07 to 39.43 mS/cm at the estuarine stations and
from 0.02 to 7.66 mS/cm at the other stations (Table
2.3). Temporal patterns for conductivity mimicked those of salinity. Dissolved ionic
compounds increase the conductance of surface waters, therefore, conductance decreases
with salinity, often reflecting river flow conditions along with rainfall. Conductivity
can also indicate point source pollution as contaminates often contain ionic compounds. At
several freshwater stations located downstream of point source inputs, conductivities are
often higher than expected in non-saline surface waters. BC117 is below a wastewater
treatment plant and AC is below a pulp mill discharge.
pH
pH values ranged from 3.1 to 8.3 and station annual medians ranged from 3.5
to 7.9 (Table 2.4). Median pH expectedly
increased from the upper river down into the estuary, where low pH swamp water and high pH
seawater mix. No unusually high pH values were recorded at any time. The lowest pH of 3.1
was recorded at the pristine blackwater steam station COL which receives drainage from
extensive riparian swamps. Swampwater can have naturally low pH due to high concentrations
of organic acids.
Dissolved Oxygen
Dissolved oxygen problems are a major water quality concern for the Cape Fear
River (Mallin et al. 1998a; 1998b; 1999a). Dissolved oxygen (DO) concentrations ranged
from 0.2 to 14.2 parts per million (ppm = mg/l) and stations annual means ranged from 4.3
to 9.6 ppm (Table 2.5). Mean annual DO concentrations at the channel stations were
slightly higher than the five-year averages and remained above the North Carolina State
standard of 5 ppm (Figure 2.1). Dissolved oxygen levels at the channel stations are lowest
during summer (Figure 2.2), with the five-year summer average falling below the NC state
standard in the upper estuary and the blackwater tributary rivers (Fig.
2.3). Summer
levels during 1999 were normal until the arrival of Hurricane Floyd in September
(Figure 2.2). Working synergistically to lower oxygen levels are two factors: lower oxygen
carrying capacity in warmer water and increased bacterial respiration (or biochemical
oxygen demand, BOD) due to higher temperatures. These hypoxic conditions could have
negative impacts on the biota in the Cape Fear River (see Chapter
4). The dissolved oxygen
sag in the lower river and estuary (Fig.
2.3) begins downstream of the pulp and paper mill
and is exacerbated by inputs from the Black River (Fig.
1.1).
The Northeast Cape Fear and Black Rivers generally have lower dissolved
oxygen levels than the mainstem Cape Fear River (Figure
2.4). These rivers are classified
as blackwater systems because of their tea colored water. As the water passes through
swamps en route to the river channel, tannins from decaying vegetation dissolve in the
water, resulting in the observed color. Decaying vegetation on the swamp floor has an
elevated biochemical oxygen demand and respiration by the bacteria feeding on this organic
matter utilizes oxygen from the water, leading to naturally low dissolved oxygen levels.
Runoff from concentrated animal operations (CAOs) may also contribute to chronic low
dissolved oxygen levels in these blackwater rivers (Mallin et al. 1998b). In the main
river channel, annual mean oxygen levels were highest at the upstream stations NC11 (8.8
ppm) and AC (8.6 ppm) and in the lower estuary at station M23 (7.6 ppm). Lower river and
upper estuary stations IC (7.4 ppm), and NAV (7.0 ppm) had the lowest DO levels. Discharge
of high BOD waste from a paper/pulp mill just above the AC station, as well as
inflow of blackwater from the Northeast Cape Fear and Black Rivers, helps to diminish
oxygen in the upper estuary. As the water reaches the lower estuary higher algal
productivity, mixing and ocean dilution help alleviate oxygen problems.
Hurricanes Dennis and Floyd caused DO levels to plummet to anoxic and
hypoxic levels in October 1999; all channel stations except NC11 had levels below 5.0 ppm
(Table 2.5). The Northeast Cape Fear River
suffered extremely low oxygen levels, becoming nearly anoxic at NCF 117, where minor fish
kills occurred (see Chapter 7). The low dissolved oxygen levels resulting from Hurricanes
Dennis and Floyd were somewhat less severe compared to the levels seen after Hurricane
Fran (1996) and Hurricane Bonnie (1998) hit the Cape Fear Region. Recovery of dissolved
oxygen to normal levels took about one month (Figure
2.2).
Several stations were in violation of North Carolina water quality
standards based on percent of samples in violation during the year June 1999 – May
2000. Definitions used by the North Carolina Division of Water Quality for use-support
consider a water body non-supporting (NS) of its designated use if the water quality
standard for a given parameter is in violation > 25% of the time. It is considered
partially supporting (PS) if the standard is in violation between 11 and 25% of the time.
Stations that can be considered non-supporting for dissolved oxygen (4.0 in blackwater
stations and 5.0 in estuarine stations) were NAV, DP, IC, NCF117, ANC, SAR, GS, N403 and
SR. Stations considered partially supporting for dissolved oxygen include HB, M61, M54,
SPD, BBT, B210, ROC, BCRR, GCO and HAM.
Turbidity
Turbidity levels ranged from 0 to 167 Nephelometric Units (NTU) and
station annual means ranged from 3 to 32 NTU (Table
2.6). Turbidities during this monitoring period were slightly lower than the five-year
mean at most channel stations (Figure
2.5). The NC State standard for freshwater of 50 NTU
was exceeded on three occasions, at the upstream channel stations. The NC State standard
for tidal saltwater of 25 NTU was exceeded on 25 occasions, most often in the upper
estuary (Table 2.6). Although rainfall and river flow are positively correlated with
turbidity in the Cape Fear River (Mallin et al. 1999a; 1999b), turbidity peaks occur most
often in winter and spring (Fig 2.6), the time of lowest rainfall in the Cape Fear Region.
It is likely that decreased vegetation and decreased evapotranspiration during the cooler
months lead to increased runoff and erosion. The contribution of turbidity by the
Northeast and Black Rivers was very low. Typically, blackwater rivers carry a low
turbidity load as a result of the settling action that occurs in the extensive riparian
swamps bordering these rivers. At NCF117 the annual mean was 4 NTU and at B210 the annual
mean was 4 NTU.
The Cape Fear Region has endured the landfall of five hurricanes since
1996, usually accompanied by high rainfall totals. Hurricanes occurred in July 1996,
September 1996, August 1998, August 1999 and September 1999. Only after Hurricane Dennis
were high turbidity levels measured (Figure
2.6). Possibly, flooding allowed floodplain
sediment settlement, and in some cases dilution from rainfall reduced turbidity loads.
The high levels of turbidity measured at NC11 (mean=32 NTU, maximum=167
NTU), indicates that a large portion of suspended material is being transported into the
Lower Cape Fear River system from upstream areas (Mallin et al. 1999b). Rainfall in the
upstream areas of the Cape Fear River has a positive correlation with turbidity in the
lower system (Mallin et al. 1999b). Turbidity levels decline downstream before reaching
the upper estuary at NAV, where increased turbulence may re-suspend turbidity particles.
In the estuary, a turbidity maximum occurs as a result of electrochemical reactions in the
water column due to the influence of charged ionic compounds (i.e. salinity). This leads
to flocculation of particles (the turbidity maximum) and subsequent downstream settling.
In the Lower Cape Fear River system the turbidity maximum occurs somewhere between M54 and
M42 during normal flow conditions. Moving oceanward down the estuary, turbidities decline
to a low at M18, a result of ocean dilution.
Note: The LCFRP uses nephelometers designed for field use, which allows
us to acquire in situ turbidity from a natural situation. North Carolina Division of Water
Quality uses turbidity values from water samples removed from the natural system, put on
ice for a number of hours, and analyzed later using laboratory nephelometers. Standard
Methods notes that transport of samples and temperature change alters true turbidity
readings. Our analysis of samples read using both methods shows that lab turbidity is
nearly always substantially lower than field turbidity, although the same process is used
(nephelometry). Based on field turbidities, one station, M42, could be rated NS for
turbidity, while Stations NAV, HB, BRR, M61, M54, M31, SPD, LRC and BCRR could be rated
PS.
Total Suspended Solids
Total suspended solid (TSS) values ranged from 1 to 191 mg/l, with station
annual means from 2 to 22 mg/l (Table 2.7).
TSS was highest at the uppermost station NC11 and in the upper estuary at NAV. The lower
estuary had generally higher values than the river, most likely a result of increased
biological productivity and increased phytoplankton and zooplankton densities. Lowest
values were seen in the stream stations and in the blackwater rivers. Highest monthly
means for TSS occurred during summer. Both turbidity and TSS peaked after Hurricane Dennis
in September. Although total suspended solids (TSS) and turbidity both quantify suspended
material in the water column, they do not always agree. High TSS does not mean high
turbidity and vice versa. This anomaly may be explained by the fact that fine clay
particles are effective at dispersing light and causing high turbidity readings, while not
resulting in high TSS. On the other hand, large organic particles may be poor at
dispersing light, yet their greater mass results in high TSS.
Light Attenuation
The attenuation of solar irradiance through a water column is measured by a
dimensionless logarithmic function (k) per meter. The higher this light attenuation
coefficient is, the more strongly light is attenuated (through absorbance or reflection)
in the water column. Light attenuation ranged from 0.77 to 8.90 k/m and station
annual means ranged from 1.69 to 4.08 k/m (Table
2.8). High light attenuation did not always coincide with high turbidity. Blackwater
though low in turbidity may increase light attenuation. At NCF6 and BBT, blackwater
stations with moderate turbidity levels, light attenuation was very high (Table 2.8).
Light attenuation coefficients during this monitoring period were approximately equal to
the five-year average at the river and estuarine stations (Figure
2.7). Temporally, light
attenuation was highest after the hurricanes in late summer. This was similar to results
after Hurricanes Fran and Bonnie, when light attenuation was highest system-wide because
of flooding of highly colored swamp water into the main river channels (Mallin et al.
1999c). The high average light attenuation is a major reason why phytoplankton production
in the major rivers and the estuary of the LCFR is generally low. Whether caused by
turbidity or water color this attenuation tends to limit light availability to the
phytoplankton.
Chemical Parameters - Nutrients
Ammonium
Ammonium concentrations ranged from 5 (detection limit) to 1110 m
g/l and station annual means ranged from 38 to 258 m g/l (Table 2.9). Generally, the mean ammonium levels
this monitoring period were higher than the five-year means at the channel stations
(Figure 2.8). Areas with high ammonium levels this monitoring period included AC (mean =
148 m g/l), which is below a pulp mill discharge, LVC (mean = 258 mg/l), which may also be
influenced by the pulp mill or from upstream industrial discharges. The stream sites with
the highest levels of ammonium are mostly influenced by point source discharges. A peak in
the middle estuary may be influenced by the City of Wilmington wastewater discharge and/or
resuspension of sediment-bound ammonium at the turbidity maximum. Ocean dilution accounts
for decreasing levels down into the estuary. The blackwater rivers had substantially
higher ammonium levels during this monitoring period than the five-year means. ANC,
although once a pristine site has apparently been influenced by logging activities.
Another factor that may contribute toward increasing ammonium levels system-wide is
airborne ammonium from the large number of concentrated animal operations (CAOs) in the
basin.
Temporally, lower ammonium usually occurs during colder months
(Figure 2.9) , yet a winter peak in 1996 shows this pattern does not always hold up. Higher levels
of oxygen in the water column will facilitate the conversion of ammonium to nitrate, and
therefore during hot months with low oxygen, ammonium levels should be higher. The
unpredictability of ammonium levels on a seasonal basis is likely due to the multitude of
variables that may affect ammonium levels including input/sources, oxygen conditions,
bacterial activity, and algal productivity.
Nitrate+Nitrite
Nitrate+Nitrite (henceforth referred to as nitrate) is the main species of
inorganic nitrogen in the Lower Cape Fear River. Concentrations ranged from 5 (detection
limit) to 12,100 m g/l and station annual means ranged from 6 to 2,909 mg/l (Table 2.10). Station annual means for the
1998-99 monitoring period were similar to the five-year means (Figure
2.10). Highest
nitrate levels at NC11 (mean = 617 mg/l) indicate that much of this nutrient is imported
from upstream. Moving downstream from NC11, nitrate levels decrease, most likely as a
result of uptake by primary producers and tidal dilution. The blackwater rivers carried
low loads of nitrate compared to the mainstem Cape Fear stations, though the Northeast
Cape Fear River (NCF117 mean = 308 mg/l) had higher nitrate than the Black River (B210 =
245 mg/l). No clear temporal pattern is observable for nitrate (Figure
2.11), although we
can see the effects of Hurricanes Fran, Bonnie, and Floyd, as nitrate levels drop to very
low levels. These drops may be a result of dilution and/or lack of oxygen, which would
favor the ammonium nitrogen species over nitrate. A February 2000 peak (Fig. 2.11) was
largely the result of unusually high nitrate concentrations at a number of locations
(Table 2.10), particularly those draining numerous concentrated animal operations (CAOs).
This may have resulted from mobilization of nitrogen deposited on floodplains during
Hurricane Floyd, and carried into the streams during high January and February rains.
Two stream stations, PB (mean = 1,198 mg/l) and BC117 (mean = 2,909
mg/l), carried extremely high levels of nitrate on several occasions, exceeding the North
Carolina state drinking water standard of 10 mg/l twice. Both sites are strongly
influenced by wastewater discharges upstream. Nitrate at several other tributary stations
was noticeably elevated. Most of these stations are in the Northeast Cape Fear River
watershed and may be influenced by agriculture and/or factory-style livestock operations,
which can contribute high nutrient levels to local watersheds. High nitrate concentrations
are becoming more common at SAR, an upper Northeast Cape Fear River station draining
numerous hog farms (Table 2.10).
Total Kjeldahl Nitrogen
Total Kjeldahl Nitrogen (TKN) is a measure of the total
concentration of organic nitrogen plus ammonium. TKN ranged from 71 to 2,060 mg/l and
station annual means ranged from 377 to 1,201 mg/l (Table 2.11). Mean TKN for this monitoring
period was lower than the five-year mean at all channel stations (Figure 2.12). Ammonium was generally higher for
the channel stations this monitoring period and therefore organic nitrogen species must
have been somewhat lower to account for low TKN. TKN concentration decreases down through
the estuary, likely due to ocean dilution and food chain uptake of nitrogen. Measured TKN
levels in the blackwater rivers are higher than in the mainstem Cape Fear River as a
result of the high concentration of organic materials dissolved in the water (Figure
2.12). The stream stations typically have higher TKN as a result of the influence of swamp
water with high organic and ammonium content. There is somewhat higher TKN during summer
months (Figure 2.13). TKN has steadily declined at the Lower Cape Fear River channel
stations over the five monitoring periods, 1995-2000.
Total Nitrogen
Total nitrogen (TN) ranged from 160 to 13,100 mg/l and station
annual means ranged from 590 to 5,371 mg/l (Table 2.12). Mean total nitrogen was lower
this monitoring period than for the five-year mean at all channel stations (Figure
2.14).
Total nitrogen concentrations remained fairly constant down the river and declined into
the lower estuary, most likely reflecting uptake of nitrogen into the food chain through
algal productivity and grazing by planktivores as well as through dilution. The pulp mill
above AC is a source of TN, increasing levels at this station over levels at NC11. The
blackwater rivers carried total nitrogen loads somewhat lower than those found in the
mainstem Cape Fear River (NCF117 mean = 1,037 mg/l, B210 mean = 847 mg/l). One stream
station, BC117, had a very high mean of 5,371 mg/l, presumably from upstream wastewater
discharge. Temporal patterns for TN are not evident (Figure
2.15), yet there is a
conspicuous decline in TN in November 1997 to April 1998 minimum, followed by a gradual
increase. A similar decline in TKN (Figure 2.13) and the lack of a decline in nitrate or
ammonium suggests that the declining constituent of TN is organic nitrogen. Reasons for
this trend are unknown.
Orthophosphate
Orthophosphate ranged from 1 to 2,940 mg/l and station annual means
ranged from 9 to 721 mg/l (Table 2.13).
The annual means at the channel stations this monitoring period were similar to the
five-year means with some stations higher and some stations lower (Figure
2.16).
Orthophosphate levels typically reach maximum concentrations during summertime (Figure
2.17), when anoxic sediment releases bound phosphorus. Also, in the Cape Fear River,
summer algal productivity is limited by nitrogen, thereby allowing the accumulation of
orthophosphate (Mallin et al. 1999a).
A regression of summer orthophosphate for the previous monitoring
period indicated that levels of this nutrient had slightly increased (Mallin net al.
1999b). Low orthophosphate concentrations after Hurricane Floyd helped to decrease the
annual mean for the current monitoring period and the regression is not as
strong (Figure 2.18). Much of
the orthophosphate load is imported into the Lower Cape Fear system from upstream areas as
NC11 typically has the highest levels. Yet during this monitoring period, the higher
levels at AC suggests a source between the two stations, possibly the pulp mill upstream
of AC. The Northeast Cape Fear River had higher orthophosphate levels than the Black
River. Orthophosphate can bind to suspended materials and is transported downstream via
turbidity, thus high levels of turbidity at the uppermost river stations may be an
important factor in the high orthophosphate levels. Turbidity declines toward the estuary
because of settling, and orthophosphate concentration also declines. In the estuary,
primary productivity helps reduce orthophosphate concentrations by assimilation into
biomass.
BC117 (mean = 721 mg/l) had very high concentrations of orthophosphate
and SAR (mean 158 mg/l), on the Northeast Cape Fear River, had moderately high
orthophosphate levels. Although there is a textile discharge upstream, it is likely that
the high number of factory-style livestock farms in the area contribute as a source of
this nutrient.
Total Phosphorus
Total phosphorus (TP) concentrations ranged from 5 (detection
limit) to 3,480 mg/l and station annual means ranged from 36 to 996 mg/l (Table 2.14). TP at the riverine channel stations
down to NAV was lower this monitoring period than the five-year mean (Figure
2.19), while
in the middle estuary TP was higher, and in the lower estuary it was lower. TP was higher
than average in the Northeast Cape Fear River and lower than average in the Black River.
TP was highest coming into the system at NC11 (mean = 172 mg/l) and declined into the
estuary toward the ocean. Some of this decline is attributable to the settling of
phosphorus-bearing turbidity, yet incorporation of phosphorus into the food chain is also
responsible. TP can bind with turbidity and be transported downstream; these two
parameters are positively correlated in the estuary (Mallin et al. 1999a). A temporal
pattern of higher summertime TP (Figure
2.20) is a result of increasing orthophosphate as
the spatial pattern of TP is similar to that of orthophosphate. At the stream stations
BC117 (mean = 996 mg/l) exhibited high TP and SAR (mean = 293 mg/l) had moderate levels.
Pickle plant discharge is likely the source of periodic high TP levels at NC403 (mean =
324 mg/l).
Organic versus Inorganic Nutrient Composition
Organic nitrogen composition by station ranged from 46% to 75% of total
N (Figure 2.21). The lowest levels were at NC11 (46%), increasing into the estuary at M54
(59%), and highest at the river mouth (75%). Organic nitrogen in blackwater rivers is
naturally high (NCF117 = 72% and B210 = 73%) because of the complex dissolved compounds
that drain from riparian swamps.
The composition of phosphorus compounds (inorganic versus organic) was
fairly constant at all stations, with organic phosphorus composition ranging from 56% to
67% of total P (Figure 2.22). In the Cape Fear River the lowest organic phosphorus
composition was at NC11 (56%) with a gradual increase into the estuary at M54 (64%), and
maximum percentages near the ocean at M18 (67%). Organic phosphorus composition was higher
in the Black River (B210= 65%) than in the Northeast Cape Fear River (NCF117= 54%).
Organic nitrogen percentages in the blackwater rivers were much higher than organic
phosphorus (NCF117 - 72% vs. 54%, B210 - 73% vs. 65%). Higher ratios of organic versus
inorganic compounds for both nitrogen and phosphorus in the estuary, along with higher
algal biomass, reflect assimilation of nitrogen and phosphorus into the food chain via
primary productivity.
A clear temporal pattern is discernable for inorganic nitrogen to
inorganic phosphorus ratios (Figure
2.23). The ratio is highest in late winter/spring, and
during late summer dips below the Redfield ratio of 16:1 N:P. These ratios would
theoretically lead to phosphorus limitation of primary productivity during the
winter-spring period and nitrogen limitation in summer. Our experimental data demonstrate
that summer nitrogen limitation does occur, but in winter primary production is
light-limited (Mallin et al. 1999a). However, phosphorus limitation has been demonstrated
experimentally in springtime (Mallin et al. 1999a).
Chemical Parameters - EPA Priority Pollutant Metals
Aluminum levels in the Cape Fear system were
generally high at NC11 and decreased downstream to NAV, where levels were again elevated (Table 2.15). Stream stations were generally low
except for BC117, which is below the Burgaw Wastewater Treatment plant outfall (Table
2.15).
Arsenic, cadmium, and chromium all maintained concentrations below
detection limits at all stations throughout the year (Tables 2.16, 2.17, and 2.18).
Copper concentrations periodically exceeded the state tidal saltwater
standard of 3 m g/L at some of the estuarine stations on occasion (Table 2.19). The freshwater standard of 7 m g/L
was exceeded three times at BC117 (rating this station partially supporting) and twice at
ROC, likewise rating it PS. Estuarine stations rated PS included NAV, HB, M54, M42 and
M31.
The LCFRP is an iron-rich system (Table 2.20). All of the freshwater stations
except for NCF117 maintained average iron concentrations above the state standard of 1000
ug/L. Iron concentrations generally decreased down-estuary (Table 2.20).
Water-column concentrations of lead and mercury were mostly below the
analytical detection limit (Table 2.21, 2.22).
Nickel concentrations exceeded the state standard twice at freshwater
stations and exceeded the salt water standard once in the estuary (Table 2.23).
Zinc concentrations remained below the state standard at all of the
estuarine stations and most of the freshwater stations (Table 2.24). However, Station LRC exceeded the
state standard once (Table 2.24).
Biological Parameters
Chlorophyll a
During this monitoring period chlorophyll a was generally low at the
river and estuarine stations of the LCFRP (Table
2.25; Figure 2.24). Production of chlorophyll a biomass is low to moderate in
this system primarily because of light limitation by turbidity in the mainstem and high
organic color and low inorganic nutrients in the blackwater rivers. Highest chlorophyll a
concentrations are typically found during the period June through September (Fig.
2.25).
During the past year chlorophyll a was lower than the long-term average, possibly a
result of Hurricanes Dennis and Floyd causing extremely low levels September-November 1999
(Table 2.25; Figure 2.25). Spatially, highest values are normally found in the
mid-to-lower estuary stations (Figure 2.24), because light becomes more available
downstream of the estuarine turbidity maximum. Chlorophyll a production is
extremely limited in the large blackwater rivers (Table 2.25; Figure 2.24).
Substantial phytoplankton blooms do occur at the stream stations,
particularly during summer months (see SR, PB, and GS - Table
2.25). These streams are
generally shallow, so mixing does not carry phytoplankton cells down below the critical
depth where respiration exceeds photosynthesis. Thus, when flow conditions permit,
elevated nutrient conditions (such as are periodically found in these stream stations) can
lead to algal blooms. In areas where the forest canopy opens up large blooms can readily
occur. When blooms occur in blackwater stream stations, they can become sources of BOD
upon death and decay, reducing further the low summer dissolved oxygen conditions common
to these waters (Mallin et al. 1998b).
Biochemical Oxygen Demand
Average five-day biochemical oxygen demand (BOD5) concentrations were
approximately 1.0 mg/L at stations NC11, B210 and NCF117 (Table 2.26). Unexpectedly, average BOD5 and
BOD20 were much less elevated at AC compared with NC11 compared with previous years
(Mallin et al. 1999b), although several BOD20 were extremely higher at AC than at NC11.
BOD displayed no discernable seasonal trend at any of the six stations (Table 2.26). There
was not a major increase in BOD following Hurricane Floyd, contrary to what we have seen
after other hurricanes (Mallin et al. 199b; 1999c). Sources for this BOD increase were
swine waste inputs, malfunctioning human sewage systems, and increased non-point source
contributions from swamp watersheds and agricultural areas. It is likely that dilution
from the enormous amount of rainfall and wetland cleansing of the floodwaters may have
helped reduce BOD increases.
Fecal Coliform Bacteria
Fecal coliform (FC) bacterial counts in the LCFR system this monitoring
period were lower than the five-year average at all stations. FC bacteria show a notable
spatial trend of highest counts in the upper estuary-lower river area encompassed by NAV,
HB, BRR, and M61 (Figure 2.26). The state human contact standard was exceeded twice, at AC
and DP. The mainstem river stations otherwise display relatively low counts (Table 2.27). FC levels following Hurricane Floyd
were unexpectedly low, consistent with other water quality parameters (Chapter
7).
The stream stations that yield chronically elevated fecal coliform
counts are mainly those downstream of point sources. LRC (below the site of the former
Stevecoknit), BC 117 (below the Burgaw wastewater treatment plant), and BCRR all had
geometric mean counts for the year exceeding the state standard for human contact of 200
CFU/100 mL (Table 2.27). BCRR had lower counts than BC117, but was also in violation of
the health standard. As there are no major point sources listed upstream of BCRR,
non-point sources may be responsible for these high counts. Based on the human contact
standard of 200 CFU/100 ml, Stations N403, PB, LRC, BC117, BCRR, and HAM were
non-supporting (NS). Stations LVC, GS, ROC, GCO, SR and BRN were partially supporting
(Table 2.27).
2.4 - References Cited
APHA. 1995. Standard Methods for the Examination of Water and
Wastewater, 19th ed. American Public Health Association, Washington, D.C.
Mallin, M.A., L.B. Cahoon, M.R. McIver, D.C. Parsons and G.C. Shank.
1997. Nutrient limitation and eutrophication potential in the Cape Fear and New River
Estuaries. Report No. 313. Water Resources Research Institute of the University of North
Carolina, Raleigh, N.C.
Mallin, M.A., M.H. Posey, M.L. Moser, G.C. Shank, M.R. McIver, T.D.
Alphin, S.H. Ensign and J.R. Merritt. 1998a. Environmental Assessment of the Lower Cape
Fear River System, 1997-1998. CMSR Report No. 98-02. Center for Marine Science Research,
University of North Carolina at Wilmington, Wilmington, NC.
Mallin, M.A., L.B. Cahoon, D.C. Parsons and S.H. Ensign. 1998b.
Effect of organic and inorganic nutrient loading on photosynthetic and heterotrophic
plankton communities in blackwater rivers. Report No. 315. Water Resources Research
Institute of the University of North Carolina, Raleigh, N.C.
Mallin, M.A., L.B. Cahoon, M.R. McIver, D.C. Parsons and G.C. Shank.
1999a. Alternation of factors limiting phytoplankton production in the Cape Fear Estuary. Estuaries
22:985-996.
Mallin, M.A., M.H. Posey, M.L. Moser, L.A. Leonard, T.D. Alphin,
S.H. Ensign, M.R. McIver, G.C. Shank and J.F. Merritt. 1999b. Environmental Assessment of
the Lower Cape Fear River System, 1998-1999. CMSR Report No. 99-01, Center for marine
Science Research, University of North Carolina at Wilmington, Wilmington, N.C.
Mallin, M.A., M.H. Posey, G.C. Shank, M.R. McIver, S.H. Ensign and
T.D. Alphin. 1999c. Hurricane effects on water quality and benthos in the Cape Fear
Watershed: Natural and anthropogenic impacts. Ecological Applications 9:350-362.
U.S. EPA 1997. Methods for the Determination of Chemical Substances
in Marine and Estuarine Environmental Matrices, 2nd Ed. EPA/600/R-97/072. National
Exposure Research Laboratory, Office of Research and Development, U.S. Environmental
Protection Agency, Cincinnati, Ohio.
Welschmeyer, N.A. 1994. Fluorometric analysis of chlorophyll a
in the presence of chlorophyll b and phaeopigments. Limnology and Oceanography
39:1985-1993.
