2.0- Physical, Chemical, and Biological Characteristics of the
Center for Marine
Science
University
of
2.1 - Introduction
This section of the report includes a discussion of the physical,
chemical, and biological water quality parameters, concentrating on the
2000-2001 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
2.2 - Materials and
Methods
All samples and field parameters collected for the estuarine stations of
the
Physical Parameters
Water Temperature,
pH, Dissolved Oxygen, Turbidity, Salinity, and Conductivity
Field parameters were measured at each site using a YSI 6920 Multi-parameter 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 station, physical parameters were measured at the surface, mid-depth, and bottom (up to 12 m). High flow occasionally prohibited the probe from reaching the bottom. The instrument was calibrated prior to each sampling trip and calibration points were checked upon return from each trip.
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 June 2001), except for the orthophosphate procedure that was performed at CMS. 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 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 nitrate+nitrite and orthophosphate analyses. All filters were wrapped individually in aluminum foil, placed in an airtight container 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. 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 (Fig. 1.1). 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. 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
2000-June 2001. Discussion of the
data focuses mainly on the river channel stations, but poor water quality
conditions at stream stations will also be discussed.
The contributions of the two large blackwater tributaries, the
Water temperatures at all stations ranged from 1.7 to 34.3 oC and individual station annual averages ranged from 15.7 to 19.1 oC (Table 2.1). Highest temperatures occurred during August (all station mean = 28.2) and lowest temperatures during January (all station mean = 4.9). Stream stations were generally cooler than river stations, most likely because of shading and lower nighttime air temperatures affecting the shallower waters.
Salinity at the estuarine stations ranged from 0 to 35.6 parts per thousand (ppt) and station annual means ranged from 1.5 to 27.4 ppt (Table 2.2). Lowest salinity occurred in April (all stations mean = 2.9) and highest salinity occurred in July (all stations mean = 16.2). Two stream stations, NC403 and SAR, had occasional oligohaline conditions due to discharges from pickle manufacturing facilities. Annual mean salinity for 2000-2001 was higher than the six-year average for 1995-2001 at all stations (Figure 2.1), a result of lower freshwater river flow conditions during this monitoring period. The lower flow conditions during this sampling period influenced a number of parameters, as subsequent sections will show.
Conductivity at all stations ranged from 0.09 to 53.77 mS/cm at the estuarine stations and from 0.05 to 4.71 mS/cm at the freshwater stations (Table 2.3). Temporal conductivity patterns followed those of salinity. Dissolved ionic compounds increase the conductance of water, therefore, conductance increases and decreases with salinity, often reflecting river flow conditions due to rainfall. Conductivity may also reveal point source pollution sources, as is seen at BC117, which is below a municipal wastewater discharge.
pH
pH values ranged from 3.4 to 9.1 and stations annual medians ranged from
3.9 to 8.0 (Table 2.4). pH was
typically lowest upstream due to acidic swampwater inputs and highest downstream
as alkaline seawater mixes with the river water.
The unusually high pH values at LRC are most likely due to industrial
discharges (see also very high dissolved oxygen concentrations).
Low pH values at
Dissolved oxygen (DO) problems are a major water quality concern in the
In the main river channel there is
an oxygen sag that begins at DP below a paper mill discharge and persists into
the mesohaline portion of the estuary. Mean
oxygen levels were highest at the upper river stations NC11 (8.8 mg/L) and AC
(8.3 mg/L) and in the lower estuary at station M23 (7.8 mg/L).
Upper estuary and middle estuary stations NAV (7.0 mg/L) and Marker 61
(6.8 mg/L) had the lowest DO levels. Discharge
of high BOD waste from the paper/pulp mill just above the AC station, as well as
inflow of blackwater from the Northeast Cape Fear and
The Northeast Cape Fear and Black Rivers generally have lower DO levels than the mainstem Cape Fear River (NCF117 mean = 6.3, B210 mean = 7.1). 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 leach into the water, resulting in the observed color. Decaying vegetation on the swamp floor has an elevated biochemical oxygen demand and usurps oxygen from the water, leading to naturally low dissolved oxygen levels. Runoff from concentrated animal feeding operations (CAFOs) may also contribute to chronic low dissolved oxygen levels in these blackwater rivers (Mallin et al. 1998b; 1999a).
Several stations were in violation of
Turbidity
Turbidity levels ranged from 0 to 108 nephelometric turbidity units (NTU)
and station annual means ranged from 1 to 26 NTU (Table
2.6).
Annual mean turbidity levels for 2000-2001 were lower than the six year
average for 1995-2001 at all river channel stations (Figure
2.4).
Turbidity was high at the upper river stations, reaching a maximum at the
upper estuary, and declining toward the lower estuary.
Turbidity is lowest in the blackwater tributaries (
The high levels of turbidity measured at NC11 (mean=22 NTU, maximum=104 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. 1999). Rainfall in the upper and middle basins of the Cape Fear River has a positive correlation with turbidity in the lower system (Mallin et al. 1998a; 1999a). Turbidity levels decline downstream before reaching the upper estuary at NAV, where increased turbulence may re-suspend turbidity particles. A turbidity maximum typically occurs in the middle estuary. As the freshwater flow meets the tidal saltwater flow, electrochemical reactions in the water column leads to flocculation of particles. In the Lower Cape Fear River system the turbidity maximum occurs somewhere between M54 and M42 during normal flow conditions, yet during the 2000-2001 monitoring period there was no evident turbidity maximum at this location. 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 regulatory agencies are required to use turbidity values from water samples removed from the natural system, put on ice until arrival at a laboratory, and analyzed using laboratory nephelometers. Standard Methods notes that transport of samples and temperature change alters true turbidity readings. Our analysis of samples using both methods shows that lab turbidity is nearly always substantially lower than field turbidity. We therefore recommend that NCDWQ investigate the utilization of field rather than laboratory turbidity in order to obtain data more representative of natural conditions.
Total Suspended Solids
Total suspended solid (TSS) values ranged from 1 to 72 mg/L, with station annual means from 2 to 23 mg/L (Table 2.7). For the river channel stations, TSS was highest in the lower estuary at Marker 18 (mean = 22). The lower estuary had generally higher values than the river; possibly a result of increased biological productivity and increased phytoplankton densities. Estuary dredging is also a source of suspended solids. Low values were seen in the stream stations and in the blackwater rivers. Highest monthly means for TSS occurred in late winter and early spring. Although total suspended solids (TSS) and turbidity both quantify suspended material in the water column, they do not always go hand in hand. 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 or inorganic particles may be less effective at dispersing light, yet their greater mass results in high TSS levels.
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 0.86 to 8.30 k/m and station annual means ranged from 1.57 to 3.53 k/m (Table 2.8). High light attenuation did not always coincide with high turbidity. Blackwater, though low in turbidity, may increase light attenuation through absorption of solar irradiance. At NCF6 and BBT, blackwater stations with moderate turbidity levels, light attenuation was very high. Light attenuation during this monitoring period was lower than the six-year average at all channel stations except NC11 (Figure 2.7), probably a result of lower turbidity through lower river flow. Temporally, light attenuation was highest during late winter and early spring. 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
Total nitrogen (TN) ranged from 220 to 22,900 mg/L and station annual means ranged from 425 to 7,128 mg/L (Table 2.9). Mean total nitrogen was lower this monitoring period than for the five-year mean at all channel stations (Figure 2.6). Previous research (Mallin et al. 1999a) has shown a positive correlation between river flow and TN in the Cape Fear system. 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 subsequent 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. One stream station, BC117, had a very high mean of 5,371 mg/L, presumably from upstream wastewater discharge. Temporal patterns for TN were not evident.
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 21,700 mg/l and station annual means ranged from 12 to 6,299 mg/L (Table 2.10). Station annual means for the 2000-2001 monitoring period were slightly lower than the five-year means, except at NC11, AC, and BBT (Figure 2.7). Highest nitrate levels at NC11 (mean = 705 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 = 209 mg/L) had higher nitrate than the Black River (B210 = 147 mg/L). No clear temporal pattern was observable for nitrate.
Several stream stations carried high levels of nitrate on occasion (SAR, NC403, PB, ROC, GCO). NC403 and PB are downstream of industrial wastewater discharges and SAR, ROC and GCO primarily receive non-point agricultural or animal waste drainage. BC117, with high nitrate levels, exceeded the North Carolina State drinking water standard of 10 mg/L on four occasions. The Town of Burgaw wastewater plant, upstream of BC117, has no nitrate discharge limits.
Ammonium concentrations ranged from 5 (detection limit) to 310 mg/L and station annual means ranged from 40 to 141 mg/L (Table 2.11). Generally, the mean ammonium levels this monitoring period were higher than the six-year means at the channel stations (Figure 2.8). Areas with the highest ammonium levels this monitoring period included AC (mean = 134 mg/L), which is below a pulp mill discharge, M54 (mean = 135 mg/L), and M61 (mean = 132 mg/L), in the middle estuary. Ocean dilution accounts for decreasing levels down into the estuary. At the stream stations, areas with high levels of ammonium (>100 mg/L) are mostly influenced by point source discharges. PB also maintains relatively high ammonium levels.
Total Kjeldahl Nitrogen
Total Kjeldahl Nitrogen (TKN) is a measure of the total concentration of organic nitrogen plus ammonium. TKN ranged from 220 to 2,920 mg/L and station annual means ranged from 350 to 1,156 mg/L (Table 2.12). Mean TKN for this monitoring period was lower than the five-year mean at all channel stations (Figure 2.9). 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 drops 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 2.9). The stream stations typically have higher TKN as a result of the influence of swamp water with high organic and ammonium content. There were somewhat higher TKN levels during summer months.
Total Phosphorus
Total phosphorus (TP) concentrations ranged from 5 (detection limit) to 3,420 mg/L and station annual means ranged from 30 to 1,001 mg/L (Table 2.13). TP at the riverine channel stations downstream into the upper estuary at BRR was higher this monitoring period than the six-year mean (Figure 2.10), and TP in the middle and lower estuary was lower. TP was highest coming into the system at NC11 (mean = 208 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. A temporal pattern of higher summer TP is a result of increasing orthophosphate as the spatial pattern of TP is similar to that of orthophosphate.
At the stream stations several areas had high TP including BC117, NC403, PB, and ROC. Most of this high TP is attributable to industrial or wastewater discharges.
Orthophosphate
Orthophosphate ranged from 0 to 3,188 mg/L and station annual means ranged from 12 to 858 mg/L (Table 2.14). The 2000-2001 annual means at the channel stations were higher than the six-year means down into the lower estuary and at the blackwater tributaries (Figure 2.11). In the lower estuary the averages this period were lower than the six-year averages.
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. Orthophosphate levels typically reach maximum concentrations during summertime, 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. 1997; 1999a).
The stream station BC117 had very high orthophosphate levels while SAR, NC403, and ROC had moderately high levels. NC403 and BC117 are strongly influenced by industrial and municipal wastewater discharges, and SAR and ROC by agriculture/animal waste runoff.
Aluminum levels in the Lower Cape Fear system were generally higher in the upper river, with a peak in the upper estuary at NAV, and decreased toward the lower estuary (Table 2.15). Stream stations were generally low except for BC117, which is below the Town of Burgaw wastewater treatment plant outfall.
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 mg/L at some of the estuarine stations in spring and summer (Table 2.19). The freshwater standard of 7 mg/L was never exceeded at the upper river stations. Copper levels at BC117 exceeded the standard in 3 out of twelve months, although the annual mean at this station was below the state standard.
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 mg/L. Iron concentrations generally decreased down-estuary. The stream station LRC had an annual average above 1,000 mg/L.
Water-column concentrations of lead, mercury, and nickel were below the analytical detection limit (Table 2.21, 2.22, 2.23).
Zinc concentrations remained below the state standard at all stations but were elevated at BC117 (Table 2.24).
During this monitoring period chlorophyll a was generally low to moderate at the river and estuarine stations (Table 2.25). 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. Spatially, highest values are normally found in the mid-to-lower estuary stations because light becomes more available downstream of the estuarine turbidity maximum. During this sampling period highest chlorophyll levels occurred at the upper river station NC11 and the mid-estuary stations Marker 54 and Marker 42. Chlorophyll a production is extremely limited in the large blackwater tributaries. Highest chlorophyll a concentrations were found during spring and summer. There were few differences in mean annual levels at the channel stations from the six-year mean (Figure 2.12).
Substantial phytoplankton blooms
do occur at the stream stations, particularly during summer months (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).
Particularly large stream algal blooms occurred this year at LRC and BCRR
(Table 2.25).
Biochemical Oxygen Demand
Mean annual five-day biochemical oxygen demand (BOD5) concentrations were highest at AC, on average about 30% higher than at NC11 suggesting influence from the pulp/paper mill inputs (Table 2.26). BOD displayed no discernable seasonal trend at any station.
A project aimed at assessing rural stream contributions to BOD began this monitoring period. Results of BOD in several stream stations can be seen in Table 2.26, and more detailed information on this subject is presented in Chapter 3.
Fecal Coliform Bacteria
Fecal coliform (FC) bacterial counts this monitoring period were lower than the five-year average at all stations (Figure 2.13). FC bacteria show a notable spatial trend of highest counts in the upper estuary-lower river area bounded by NAV, HB, BRR, and M61. The state human contact standard (200 CFU) was exceeded nine times (Table 2.27).
All stream stations except LCO surpassed the state standard for human
contact of 200 CFU/100 mL on at least one occasion.
The stream station LRC had a geometric mean count for the year exceeding
this standard. ANC, PB, BCRR, GCO,
and SR all had levels above 1,000 on at least one occasion.
LRC and PB are located below point source discharges, and the other sites
mainly have problems with non-point source pollution. Stations in which fecal
coliform concentrations exceeded 200 CFU/100 mL
>25% of the time (NS) were LRC, ROC, BC117, BCRR and HAM.
Those considered PS for fecal coliforms include NAV, HB, LVC, SAR, NC403,
PB, 6RC, SR, and COL.
The 2000-2001 monitoring period represents the fifth full year of sampling for the Lower Cape Fear River Program (some stations and parameters have been sampled six years, since June 1995). Table 2.28 shows the five/six year mean, minimum, and maximum levels for each parameter, for each station. The figures in this section are intended to show the patterns of select parameters for this five/six year period. Rigorous statistical analysis of these trends is ongoing and will appear in future reports. There are only a few discernable trends and none seem to be very drastic. Three of the five years saw strong influences from hurricanes and therefore the data do not necessarily represent background conditions. The flushing of the Lower Cape Fear River system by these storms certainly affected the ecology of the river acutely, and in some cases for many months and possibly years.
Total nitrogen showed higher and more variable levels during 1996 and 1997, then a leveling off for the period 1998 through 2001 (Figs. 2.14a and b). This variability was due to changes in organic nitrogen (Figs. 2.15a and b). One possible explanation for this change has to do with swamp storage of organic nitrogen. Previous to 1996 there was an extended period of no hurricanes. During this period large amounts of organic nitrogen likely accumulated in riparian swamps of the lower Cape Fear system. Two hurricanes in 1996 and high flows in winter and spring of 1997 may have flushed large amounts of accumulated organic materials from the swamps into the channels; leaving much less organic nitrogen available for flushing during subsequent hurricanes. In contrast, ammonium showed steadily increasing levels during the five-year period at a number of stations (Figs. 2.16a and b). Total phosphorus showed apparent increases at some stations and no apparent trend at others (Figs. 2.17a and b). Again, these data require further statistical analyses before any definite trends can be assigned to them. Figures 2.18a and b offer an opportunity to compare the channel stations in terms of susceptibility to storm-induced low dissolved oxygen. This demonstrates that Stations NCF117, NAV and M54 are particularly hard hit by hypoxia and anoxia during these events. The figures also show that NCF117 on the Northeast Cape Fear River has consistently poorer water quality than B210 on the Black River in terms of dissolved oxygen.
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, G.C. Shank, M.R. McIver, S.H. Ensign and T.D. Alphin. 1999b. 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.
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