JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
Vol. 43, No. 1
AMERICAN WATER RESOURCES ASSOCIATION
February 2007
ESTIMATING RELEVANCE OF ORGANIC CARBON, NITROGEN, AND
PHOSPHORUS LOADS TO A BLACKWATER RIVER ESTUARY1
John Hendrickson, Nadine Trahan, Emily Gordon, and Ying Ouyang2
ABSTRACT: In blackwater river estuaries, a large portion of external carbon, nitrogen, and phosphorus load are
combined in complex organic molecules of varying recalcitrance. Determining their lability is essential to establishing the relationship between anthropogenic loads and eutrophication. A method is proposed in which organic
C, N, and P are partitioned into labile and refractory forms, based upon first-order decay estimated by biochemical oxygen demand relative to total organic carbon, and C:N and C:P ratios as a function of organic carbon lability. The technique was applied in developing total maximum daily loads for the lower St. Johns, a blackwater
Atlantic coastal plain river estuary in Northeast Florida. Point source organic nutrients were determined to be
largely labile. Urban runoff was found to have the highest relative labile organic N and P content, followed by
agricultural runoff. Natural forest and silviculture runoff were high in refractory organic N and P. Upstream
labile C, N, and P loads were controlled by autochthonous production, with 34-50% of summer total labile carbon
imported as algal biomass. Differentiation of labile and refractory organic forms suggests that while anthropogenic nutrient enrichment has tripled the total nitrogen load, it has resulted in a 6.7-fold increase in total labile
nitrogen load.
(KEY TERMS: bioavailability; blackwater rivers; eutrophication; land use; nutrients; nitrogen and phosphorus
loading; organic carbon; organic nutrients; source allocation; watershed development; TMDLs; water quality
modeling.)
Hendrickson, John, Nadine Trahan, Emily Gordon, and Ying Ouyang, 2007. Estimating Relevance of Organic
Carbon, Nitrogen, and Phosphorus Loads to a Blackwater River Estuary. Journal of the American Water
Resources Association (JAWRA) 43(1):264-279. DOI: 10.1111 ⁄ j.1752-1688.2007.00021.x
INTRODUCTION
Accelerated eutrophication arising from nutrient
enrichment of estuaries represents one of the most
significant water quality problems within near coastal
waters world-wide (National Research Council, 2000).
Diagnostic and management approaches often rely on
linking the sources, magnitude and timing of the
external nutrient load to the response of the receiving
water body with dynamic water quality process models. In the evaluations of external nutrient loads to
estuaries in temperate climates, organic nutrients
have typically not been differentiated with regard to
lability, and organic carbon is rarely, if ever included
(Jaworski et al., 1992; Magnien et al., 1992; Valiela
1
Paper No. J05107 of the Journal of the American Water Resources Association (JAWRA). Received July 26, 2005; accepted March 23,
2006. ª 2007 American Water resources Association.
2
Respectively, (Hendrickson) Environmental Scientist V, St. Johns River Water Management District, P. O. Box 1429, Palatka, Florida
32178-1429; (Trahan) GIS Analyst, Jones, Edmunds & Assoc., Gainesville, Florida 32641, USA; and (Gordon, Ouyang) Environmental Scientist I and Environmental Scientist IV, BCI Engineers and Scientists, Lakeland, Florida 33807-5467 (E-Mail ⁄ Hendrickson: jhendrickson@
sjrwmd.com).
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et al., 1992; Boynton et al., 1995; Stepanauskas et al.,
1999; Goolsby et al., 2001). For blackwater rivers of
the southeast U.S. coastal plain, where a large portion
of the nitrogen and phosphorus are contained within
the organic fraction (Graves et al., 2004), and where
natural organic carbon loads can confound the evaluation of oxygen deficits, the failure to account for differences in organic nutrient and carbon lability in the
external load seriously compromises the assessment
of anthropogenic eutrophication effects.
While inorganic nutrients and some low molecular
weight organic compounds are readily assimilated by
aquatic primary producers, organic nutrient forms,
which must first undergo desorption, hydrolysis, bacterial decomposition or photo-decomposition (Bushaw
et al., 1996) for inorganic nutrient regeneration and
utilization, are less readily available. Organic nutrient
re-mineralization is dependent upon the utilization
preference of the parent substrate by general microbial
heterotrophs (DeBusk et al., 2001). A general working
framework has evolved that partitions organic carbon
and nutrients into two pools: a labile pool, that can be
utilized in time frames relevant to water quality
processes of interest in the receiving water, and a
refractory pool, that is decomposed very slowly and
essentially inert for relevant time frames (Wetzel,
1990, p. 737). Carbohydrates, proteins, lipids, nucleic
acids, and pigments, which are in higher proportion in
younger, autochthonous plant material, typically represents the bulk of this labile pool, while humified,
allochthonous OM, largely imported to streams as dissolved and highly colored, leeched, degraded terrestrial plant materials (colored dissolved organic matter,
or CDOM), typically dominates the refractory pool
(Meyer, 1990; Moran and Hodson, 1990; Kaplan and
Newbold, 1995; Moran et al., 1999). In their work on
piedmont and coastal plain blackwater rivers in the
southeast U.S., Sun et al. (1997) demonstrated that
the compositional changes that accompany diagenesis
relate directly to lability, with blackwater stream
CDOM appearing the most refractory per mole carbon,
and this is in agreement with work that has shown
some forms of soil humus in the allochthonous organic
carbon pool to be decades to hundreds of years old
(Raymond and Bauer, 2001). Although natural CDOM
is generally believed to be resistant to microbial
decomposition and largely unavailable for utilization
by phytoplankton in typical estuarine residence times,
these large, heterogeneous organic molecules contain a
substantial amount of nitrogen (N), and to a lesser
degree phosphorus (P), in their structures (DeBusk
et al., 2001), and hence the sheer volume of this material with respect to other organic matter (OM) pools
dictates that its relevance be considered.
No clear definition exists on what constitutes labile
vs. refractory, and whether or not the range between
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the two extremes exists as a continuum or as discrete
states. Labile substrates have been described as those
utilized within timeframes of 1-2 weeks (Sondergaard
and Middelboe, 1995); as utilization through the
exponential growth phase to the stationary phase
(approximately 2 days; Stepanauskas et al., 1999);
approximately 4 days for dissolved organic nitrogen
(DON) of the Delaware River (Seitzinger and Sanders, 1997); or in situ bioreactor residence time
(4-18 h; Volk et al., 1997). Moran and Hodson (1989),
in their investigation of fresh and salt marsh plant
lignocellulose, observed what appeared to be distinct
rates of utilization, suggesting distinct, uniform
chemical classes driving separate utilization rates.
Similarly, Ogura (1975) determined that two distinct
pools of dissolved organic compounds existed in most
aquatic systems.
In order to successfully prescribe pollutant load
limits to reduce accelerated eutrophication and oxygen
deficits in blackwater river estuaries, relevant, labile
organic carbon and nutrients must in some way be distinguished from refractory, unreactive forms. The
separation technique must be robust and generally
applicable to the variety of OM present in the aquatic
environment, and, if possible, reliant upon traditional
chemistry so that it can be applied to existing datasets.
In this study, a method is proposed to distinguish
between labile and refractory organic carbon, nitrogen
and phosphorus loads entering a large, blackwater
river estuary, the lower St. Johns River in Northeast
Florida. Poor water quality in the lower St. Johns
River has been identified for over 50 years in reports
published by the U.S. NOAA, Florida State Board of
Health, and later the Florida Department of Environmental Regulation. Various reaches of the LSJR are
listed by the State’s Total Maximum Daily Load
(TMDL) criteria as impaired for one or more of the
following symptoms of eutrophication: chlorophyll a,
turbidity, nitrogen, phosphorus, and dissolved oxygen.
The U.S. Army Corps of Engineers water quality process model CE-QUAL-ICM (Cerco and Cole, 1993) has
been selected to conduct the diagnostic and scenario
modeling investigations necessary to determine these
TMDL reductions. A crucial factor in the selection of
CE-QUAL-ICM was the model’s capability to perform
separate calculations on labile and refractory forms
of organic carbon, nitrogen, and phosphorus. The
methodology developed here is applied to four different
types of loads: (1) small, low-order stream non-point
source loads of the immediate contributing basins; (2)
domestic waste effluent loads; (3) pulp mill effluent
loads; and (4) high-order river loads entering from
upstream. Following this partitioning, a labile nutrient
budget for the estuary is calculated and compared to
the current total nutrient budget, and to the hind-cast,
predevelopment labile nutrient budget.
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MATERIALS AND METHODS
Study Location
The St. Johns River is one of the largest blackwater rivers of the southeast U.S., draining a
24,765 km2 area in northeast Florida. The St. Johns
has a mean discharge of 227 m3 ⁄ s (Nazarian et al.,
2005), and is slow moving and essentially at sea level
for its final 200 km. The lower St. Johns River (hereafter ‘‘LSJR’’) is the estuarine portion of the river,
formed at the confluence of the upper St. Johns and
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Ocklawaha Rivers, encompassing a 7,123 km2 drainage area (Figure 1). Within this reach, the LSJR is
slightly more that 160 km long and has a water surface area, including tributary mouths below head of
tide, of 34,400 hectares. The slow velocity and broad
lacustrine reaches of the LSJR facilitates phytoplankton production, and spring and summer blooms in
this nutrient-rich river often exhibit chlorophyll a
concentrations exceeding 100 lg ⁄ l. The northern
(downstream) portion of the basin is distinguished by
the heavily urbanized cities of Jacksonville, Orange
Park and Middleburg. Roughly three quarters
(64-82%) of the basin’s highly developed land uses
FIGURE 1. The Lower St. Johns River Basin.
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rates appeared consistent with the range in apparent
decay for aquatic organic matter shown in Figure 2,
they were retained in subsequent calculations.
Organic Carbon Partitioning
The rate of organic carbon consumption in oxygenated surface waters is reflected in biochemical oxygen
demand (BOD), with labile substrates consuming
more oxygen per mole of carbon in the test period
(typically 5 days) than refractory substrates. Consumption of organic carbon by bacterial heterotrophs
has generally been found to adhere to first-order
exponential decay (Jewell and McCarty, 1971; Chapra, 1997). Chapra (1997) provides a relationship in
which the concentration of BOD exerted at time t on
the total amount of substrate, or ultimate BOD, is:
Ct ¼ C0 ð1 e
kt
Þ
ð1Þ
where Ct represents the oxygen consumed at time
t, C0 is the maximum oxygen demand exertion
(BODultimate), and k is the substrate-specific decomposition coefficient.
Measurements of ultimate BOD are rarely performed, although total organic carbon should, in theory, be directly related to ultimate BOD, and can
provide a basis for the inter-conversion of organic carbon and oxygen consumption. The molar rate of O2
consumption per organic carbon decomposition of 1:1
(2.67:1 mass ratio) was assumed here, as has been
applied elsewhere in computations of community respiration (Wetzel and Likens, 1990; pp. 210-211; as
per the respiratory quotient (RQ) of Strickland, 1960)
and bacterial carbon utilization (Anderson, 1995;
Foreman et al., 1998).
Figure 2 compares the theoretical rates of change
in substrate organic carbon for BOD exerted over time
for effluents or surface waters with differing predominant organic matter types. First order decomposition
rates range from 0.094 to 0.002 day)1, with domestic
waste and algal organic matter the most labile substrates, and CDOM in runoff of undeveloped-waterwshed blackwater streams largely refractory. This
range is similar to that applied by Cifuentes and Eldridge (1998), and blackwater stream organic carbon
decomposition rate is similar to that of Moran et al.
(1999), who calculated first-order organic matter
decay rates for five rivers of the southeast U.S. ranging between 0.004 and 0.001 day)1. Previous applications of the CE-QUAL-ICM water quality model have
set the labile and refractory organic carbon decomposition rates at 0.075 and 0.001 day)1. Because these
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Percent of ulitmate BOD exerted
or TOC consumed
(medium and high residential, high intensity commercial and industrial) drain to the oligohaline and
mesohaline LSJR. In contrast, 62-98% of the basin’s
agricultural land uses drain to the fresh tidal reach.
AND
100
Algae
k = 0.094 day
80
–1
2ndary STP
60
k = 0.0386 day
–1
Pulp & Paper Eff.
40
–1
k = 0.0096 day
Native DOM
20
k = 0.0022 day
–1
0
0
10
20
30
40
50
60
Time, days
FIGURE 2. Rates of Exertion of BOD for Effluents and Surface
Waters of Predominant Organic Matter Types. Algal rate determined with data from Lake Dora, a hypereutrophic lake in NorthCentral Florida; phytoplankton organic carbon estimated with 50:1
carbon:chlorophyll a. Secondary WWTP effluent values are from a
sampling of 23 point sources of the lower St. Johns River basin.
Pulp and paper determined from a large mill in the lower St. Johns
River basin. Native DOM developed from the mean of BOD and
TOC data for blackwater streams draining undeveloped watersheds
in northeast Florida. Dashed lines represent rates of 0.075 day)1
(upper line) and 0.001 day)1 (lower).
River, tributary and point source effluent water
quality monitoring data collected within the LSJR
basin from 1993 to 2004 comprised the subject dataset for this study. Tributary sampling programs were
designed to characterize water quality for four predominant land uses at a watershed scale: urban, row
crop, dairy and forested (i.e., undeveloped). Urban
runoff samples were collected from streams in the
Jacksonville area, and represented older development, built prior to riparian area protection and
stormwater retention ⁄ detention and treatment that
have been instituted since 1984. Urban land use
included a mix of low to high density residential,
commercial and industrial development. Row crop
was predominantly seepage-irrigated potatoes and
cabbage on ditched and drained coastal plain flatwoods soils. Lands characterized as dairy combined
barn, feedlot, and improved pasture areas. Stations
were selected so that only one development type
(other than the undeveloped condition) occurred
within the watershed, facilitating the bivariate analysis of development density on water quality. Point
source water quality data for 32 domestic waste facilities and three industrial waste dischargers (all pulp
mills) were obtained from monthly operating report
compliance files. Most point source effluent is discharged to the oligo- to polyhaline portions of the
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HENDRICKSON, TRAHAN, GORDON,
river in the vicinity of the city of Jacksonville. During
the time of this sampling, secondary level was the
established treatment level, and effluent limits for
nutrients did not exist. Constituents of interest included biochemical oxygen demand (BOD), total organic
carbon (TOC), particulate organic carbon (POC), total
Kjeldahl-N (TKN), ammonia-N (NH4), nitrate +
nitrite-N (NO2+3), total phosphorus (TP), orthophosphate (PO4), and water color. The following methods
were used in analysis, with numbers in parenthesis
referenced from Greenberg et al., 1992. NO2+3 was
determined by copper-cadmium reduction and diazitozation colorimetry (4500-NO3-E), NH4 by the alkaline
phenol-colorimetric method (4500-NH4-D), TKN by
high temperature sulfuric acid digestion on whole
water (unfiltered) sample followed by NH4 analysis
(4500-Norg-C), PO4 by antimony-phospho-molybdate
complex ascorbic acid-colorimetric method (4500-P-F),
and TP by high temperature sulfuric acid digestion
on whole water sample followed by PO4 analysis.
Nutrient analyses were performed on an O-I Analytical Enviro-Flow FS 3000 auto-analyzer (OI Analytical, CMS Field Products Group, College Station,
Texas, USA). BOD (total BOD; carbonaceous + nitrogenous) was determined by 5-day dark incubation at
20C (5210-B). TOC was determined by high temperature catalytic combustion to CO2 and detection
by infrared detector (5310-B) on a Shimadzu TOC-V
organic carbon analyzer, and POC by combustion and
coulometric detection, and filtered color determined
by the visual comparison method to standard platinum-cobalt solutions (2120-B). Most surface water
chemistry was performed by the St. Johns River
Water Management District (SJRWMD) laboratory,
with some data acquired from the Duval County
(Florida) Environmental Quality Division, and POC
analyzed by the University of Florida’s Department
of Fisheries and Aquatic Sciences. SJRWMD water
quality data below the detection limit are not censored, but are instead reported as determined and
remarked with the STORET ‘‘T’’ code (value reported
is less than the detection limit). This greatly facilitated the ensuing analysis, as many BOD measurements fell below the 2 mg ⁄ l detection limit. In such
cases, the usual 99% confidence range for measurement repeatability is not upheld, though analysis precision at these low ranges is still usually acceptable.
BOD obtained from point source compliance was typically carbonaceous, while surface water analysis
included nitrogenous demand. This was not considered a serious inconsistency, as ammonia concentration was typically low in these samples. Organic
nitrogen was computed as TON = TKN – NH3, and
total non-orthophosphate phosphorus as TNOP = TPPO4. The fraction of TP not in orthophosphate is
referred to as ‘‘total non-orthophosphate-phosphorus,’’
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and abbreviated as TNOP, because unlike nitrogen,
an undetermined portion of the non-inorganic P may
in mineral forms such as calcium or magnesium-complexed polyphosphates. Inorganic nitrogen (DIN) was
calculated as NO2+3 + NH4. The final dataset with
constituent coverage sufficient to proceed with partitioning calculations yielded 789 samples for 31 tributary sampling stations, 22 domestic waste point
sources, and all three pulp mills.
Total organic carbon in surface water samples was
considered to be the sum of carbon within labile
(labile total organic carbon, or LOC) and refractory
substrates (total refractory organic carbon, or ROC),
and that these respective fractions are decomposed
simultaneously, albeit at differing rates. Using the
rates of decomposition of the first-order decay model
of 0.075 day-1 for labile substrates, and 0.001 day-1
for refractory, a pair of simultaneous equations was
set up in the following form:
TOCt¼5 ¼ ROC ð1 eð0:001Þ5 Þ
þ LOC ð1 eð0:075Þ5 Þ
ð2Þ
TOCt¼1 ¼ ROC ð1 eð0:001Þ1 Þ
þ LOC ð1 eð0:075Þ1 Þ
ð3Þ
In equation (2), a TOCt=5 was estimated assuming
that the moles of organic carbon consumed at t = 5
(CBOD5) conformed to moles of oxygen consumed,
consistent with a respiratory quotient = 1, by dividing
BOD5, in mg ⁄ l, by 2.67, to get a TOC5, in mg ⁄ l. When
all TOC is consumed, at t = ¥ (analogous to ultimate
BOD), the exponent term in parenthesis goes to zero,
and TOC¥ = ROC + LOC. The above, paired equations were simplified for computation through the following steps:
ðBOD5 =2:67Þ ¼ ROC ð0:005Þ þ LOC ð0:3127Þ
ð4Þ
TOC ¼ ROC ð1Þ þ LOC ð1Þ
ð5Þ
200 ½ðBOD5 =2:67Þ ¼ ROC ð0:005Þ þ LOC ð0:3127Þ
ð6Þ
TOC ¼ ROC ð1Þ þ LOC ð1Þ
ð7Þ
BOD5 74:906 LOC ð62:54Þ ¼ ROC
ð8Þ
TOC LOC ¼ ROC
ð9Þ
Solving these two equations for LOC produces:
LOC ¼ ðBOD5 74:906 TOCÞ=61:54
ð10Þ
and
ROC ¼ TOC LOC
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The tendency for low lability of AHS and high
lability of young, macronutrient-rich autotroph biomass results in an inverse relationship between relative LOC content and OC:nutrient ratio that is
apparent for surface water and point source effluent
samples. Plots of the natural log of the percent of
LOC vs. the natural log of sample organic C:organic
nutrient ratios exhibited slightly curvilinear relationships (Figures 3a and 3b), and produced R2-values of 0.51 and 0.36 for nitrogen and phosphorus
with second-order polynomial equations. One data
point from stream runoff draining a large dairy and
intensive pasture lands in which the TOC:TNOP
was 4225:1 was omitted from this analysis. From
these equations, organic C:N and C:P for hypothetical, purely labile (%LOC = 100) or refractory
substrates (%LOC = 0) were determined as 4.5 and
37 for nitrogen, and 27 and 617 for phosphorus.
This minimum organic C:N value is similar to that
reported for bacterial cell composition (Fagerbakke
et al., 1996; Kirchman, 2000), while the high C:N
ratio corresponds well with values reported for
refractory organic matter (Thurman, 1985, pp. 292293).
(a) 5
Nitrogen
4
ln (TOC:TON)
In calculations, 2 of the 87 point source samples
and 6 of the 702 tributary samples had BOD5 values
that indicated decay rates less than 0.001 day)1; conversely, three point source samples in the dataset
exhibited CBOD5 values that, when converted to
TOC, exceeded the TOC at the maximum decomposition rate of 0.075 day)1. These values were omitted
from subsequent calculations.
Although the tributary organic carbon was considered to be principally allochthonous (of the 265 tributary samples analyzed for chlorophyll a, the
median calculated algal OC ⁄ LOC was 4%, and the
90th percentile relative composition was 30%; two
tributaries with the highest chlorophyll a concentrations had sampling sites in quiescent, open embayments below the head of tide, where autochthonous
production potential was high), a large amount of
the upstream inflowing organic C is autochthonous
in origin. This results from a number of large lakes,
the largest being Lake George, which, at 189 km2,
and just 16 km upstream of the LSJR, is the
predominant lacustrine feature that determines
inflowing organic carbon and nutrient compartmentalization. Within-river TOC was determined
by summing measurements of DOC and POC, as
organic carbon analyzer measurements were found
to underestimate TOC when significant algal biomass was present. Algal biomass (determined from
corrected chlorophyll a or biovolume measurements),
was considered to be labile organic C, and was
separated from non-algal labile organic C assuming
a 50:1 ratio of algal C to corrected chlorophyll
a. Comparison between POC and corrected chlorophyll a demonstrated very good adherence to the
50:1 ratio at when algal biomass dominated the
POC pool.
AND
C:N = 4.5
3
2
C:N = 37
1
y = –0.0478x 2 – 0.2401x + 3.6156
R 2 = 0.51
0
0
1
2
3
4
Organic Nutrient Partitioning
(b) 10
Due to its relatively high content of proteins,
amino acids, lipids, and nucleic acids, labile aquatic
OM typically exhibits low C:N and C:P ratios, and
its decomposition in the aquatic environment tends
to lead to the regeneration of inorganic N and P
(Goldman et al., 1987; Sun et al., 1997). Conversely,
substrates in the aquatic environment with a high
C:N and C:P, such as aquatic humic substances
(Thurman, 1985; pp. 292-294), will tend to immobilize the macro-nutrients during decomposition
(Mann, 1988; Strauss and Lamberti, 2000). Aquatic
humic substances (AHS) exhibit low biological availability, exert low oxygen demand even at high concentrations, and regenerate low levels of mineral
nutrients for subsequent utilization by autotrophs
(Kroer, 1993; Bushaw et al., 1996).
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Phosphorus
ln (TOC:TNOP)
8
C:P = 27
6
4
C:P = 617
2
y = –0.1009x 2 – 0.2109x + 6.4245
R 2 = 0.36
0
0
1
2
3
4
ln (% LOC)
FIGURE 3. Organic Carbon:Nutrient Ratios as a Function of
Calculated Labile Organic Carbon Content for (a) Nitrogen and
(b) Phosphorus. Values in boxes represent regression-calculated
OC:Nutrient ratios at 0 and 100% labile organic carbon content.
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To constrain calculated values of labile organic N
(LON), refractory organic N (RON), labile non-orthophosphate P (LNOP) and refractory non-orthophosphate P (RNOP) such that the sum of labile and
refractory concentrations equaled the laboratory analytical determinations for TKN-NH3 and TP-PO4,
total organic N (TON) and total non-orthophosphate
P (TNOP) were partitioned, rather than multiplied
by, labile and refractory organic C:N and C:P ratios.
This proportional compartmentalization calculation
was:
(
LON ¼
)
LOC
4:5TOC
ROC
LOC
37TOC þ 4:5TOC
TON:
ð12Þ
Following this calculation, RTON could be calculated
by difference with the relationship
RON ¼ TON LON
ð13Þ
or with the complimentary partitioning equation of
the form
(
RON ¼
ROC
37TOC
ROC
LOC
37TOC þ 4:5TOC
)
TON
ð14Þ
Similarly, TNOP was partitioned with the relationships
(
LNOP ¼
LOC
27TOC
ROC
LOC
617TOC þ 27TOC
)
TNOP
ð15Þ
TNOP
ð16Þ
and
(
RNOP ¼
ROC
617TOC
ROC
LOC
617TOC þ 27TOC
)
RESULTS
Organic Carbon and Nutrient Partitioning Patterns
To examine differences in river inflow, tributary
runoff, and effluent N, P, and C forms, summary statistics were prepared and are shown in the box and
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whisker plots of Figure 4. For each of the tributary
sampling stations, seasonal mean flow-weighted concentrations of inorganic N and P forms, and organic
C, N, and P forms were calculated by summing the
product of event concentration and sampling day discharge fraction. Discharge fraction was calculated as
the mean daily discharge for the sampled stream (or
its nearest proxy gauged stream) divided by the sum
of the daily discharges for all sampling dates. Instead
of the four Julian calendar seasons, a three-season
breakdown was used that conforms more closely to
the northeast Florida meteorological conditions: a
cool, moderately wet winter from December through
March, marked by regular frontal systems; a hot, dry
spring-summer from April through July; and a hot,
wet summer-fall from August through November,
marked by convective thunderstorms and tropical
systems. Annual mean concentrations were then calculated for each station from the three seasonal
means. Mean concentrations for domestic waste and
pulp mill effluents were calculated from the individual facility means. Upstream river mean concentrations were determined from biweekly monitoring
from 1995 to 1999.
DIN and PO4 were the highest in domestic waste,
pulp mill effluent, and runoff from row crop and
dairy-dominated watersheds (Figure 4). Concentrations were intermediate in urban runoff, and lowest
in samples from the upstream end of the St. Johns
River. Domestic waste was also high in LOC, but was
the lowest in ROC. Pulp mill effluent exhibited the
highest concentrations of both LOC and ROC. In tributary and river samples, river inflow was found to
have the highest LOC, but moderate levels of ROC.
LOC was also relatively high in urban and dairydominated runoff. LOC levels were moderate in row
crop runoff, and lowest in undeveloped watershed
runoff. Dairy, row crop and undeveloped watershed
runoff exhibited high concentrations of ROC, with the
lowest levels seen in urban runoff.
Labile organic N and P partitioning followed the
general patterns seen in LOC. Pulp mill effluent was
found to contain the greatest amount of LON and
LNOP. Domestic waste was also high in LON and
LNOP, rendering its total composition highly bioavailable, with 98% of TN as TBN (total bioavailable
N; calculated as TIN + LON) and 99% of TP as TBP
(total bioavailable P; PO4 + LNOP). TBN in pulp mill
effluent ranged from 69% to 94% of TN, and TBP
ranged from 88 to 98% of TP. These ranges appear
consistent with NCASI (2004), which found that in
extended decomposition experiments of effluent from
4 mills, between 60% and 84% of TN was TBN, and
between 67% to 91% of TP was TBP. Upstream river
and urban runoff samples were also found to be moderate to high in LON and LNOP, but relatively low
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100
10
DIN
10
LON
1
1
1
0.1
0.1
0.1
LOC
RON
10
10
1
FO
UR
UP
W
W
PM
RC
DA
FO
UR
UP
W
W
PM
FO
UR
UP
W
W
PM
RC
DA
FO
UR
UP
W
W
PM
RC
DA
RC
DA
0.01
0.01
0.01
100
PO4
1
LNOP
1
1
0.1
0.1
0.1
0.01
0.01
0.01
ROC
RNOP
10
1
FO
UR
UP
W
W
PM
RC
DA
FO
UR
UP
W
W
PM
FO
UR
UP
W
W
PM
0.001
RC
DA
FO
UR
UP
W
W
PM
RC
DA
0.001
RC
DA
0.001
FIGURE 4. Patterns in Mean Nitrogen, Phosphorus, and Carbon Forms for Stream Runoff and Point Source Samples. Dark line
signifies the mean concentration, the light center line the median, box ends the 75th percentile, and whisker ends the 95th percentile
values. RC = row crop; DA = dairy; FO = forested or undeveloped; UR = urban; UP = St. Johns River at the basin’s upstream end;
WW = domestic waste final effluent; and PM = pulp mill final effluent. Plot vertical axes in log10 scale is concentration in mg ⁄ l.
in refractory forms. Conversely, undeveloped watershed runoff was low in LON and LNOP, but high in
refractory forms. Row crop and dairy runoff exhibited
levels of RON and RNOP roughly similar to that of
undeveloped watershed runoff, but were elevated
with respect to LON and LNOP.
Mean seasonal and annual flow weighted concentrations of DIN, PO4, LON, LNOP and LOC were
found to increase with increasing watershed development density. Regressions relating concentration to
fraction of watershed developed area (Table 1) exhibited high significance for these nutrient and carbon
forms. Slope values indicated the greatest rate of
increase for watersheds dominated by dairy land use,
while rates of increase for row crop agriculture were
intermediate. Rates of increase in these constituents
were the lowest for urban land covers (combined residential, commercial, transportation and industrial),
but these rates exhibited curvilinear increases with
development density. This may be because of the
compounding effect of impervious surfaces, which
would presumably increase the proportion of urban
runoff contribution with land development intensification. In contrast to inorganic and labile organic
trends, refractory nutrient and carbon forms tended
to remain the same or decrease with increasing development intensity. Slopes for regressions relating
RON, RNOP and ROC concentration to dairy or row
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AMERICAN WATER RESOURCES ASSOCIATION
crop development intensity were either not significant, or exhibited low correlation coefficient values.
For urban land, development intensity was strongly
negatively correlated with RON and ROC concentration.
Seasonal flow-weighted concentrations specific to
each land use category were estimated through a
regression approach conceptually similar to that
applied by McFarland and Hauck (2001). Simple
linear regressions were developed in which the
dependent variable was the sampling station seasonal
flow-weighted mean concentration, and the independent variable the fraction (from 0 to 1) of contributing
watershed land area for each predominant land use
category. Regressions included undeveloped watershed sampling means, resulting in regressions with
y-intercepts that theoretically represented the undeveloped watershed concentration. The specific land
use concentrations, shown in Table 1, were determined by solving the regression equations for the
condition of 100% land use (i.e., fraction = 1). The
most pronounced seasonal patterns were seen in
ROC, RON and RNOP, which tended to be the lowest
in the April-July season, highest in the AugustNovember season. This pattern is consistent with
the annual cycle in rainfall and soil saturation, and
the flooding of near-stream source areas that
facilitates CDOM export. Maximum orthophosphate
271
JAWRA
JAWRA
December-March
April-July
August-November
December-March
April-July
August-November
December-March
April-July
August-November
December-March
April-July
August-November
Season
7
7
7
5
5
5
6
6
6
8
8
6
n
0.034*
0.104
0.044*
9.95 (0.92)
1.90 (0.57)
1.38 (0.91)
4.15 (0.83)
4.82 (0.72)
6.41 (0.88)
0.831 (0.66*)
0.447 (0.64*)
0.458 (0.86*)
TIN
0.092*
0.161
0.083
1.64 (0.91)
0.969 (0.58)
1.34 (0.75)
2.70 (0.98)
2.63 (0.95)
3.82 (0.92)
1.66 (0.78*)
1.35 (0.78*)
1.69 (0.80*)
LON
0.543
0.617
0.719
NS
NS
NS
1.56 (0.59)
NS
2.03 (0.65)
0.145 (0.65)
0.070 (0.78*)
0.059 (0.77)
RON
0.041*
0.043*
0.034*
0.914 (0.91)
0.248 (0.30)
0.891 (0.62)
3.70 (0.77)
2.77 (0.79)
4.65 (0.83)
0.130 (0.58)
0.131 (0.44)
0.186 (0.47)
PO4
0.009*
0.022
0.014*
0.864 (0.89)
0.309 (0.44)
0.420 (0.88)
0.369 (0.90)
0.221 (0.71)
0.618 (0.90)
0.346 (0.78)
0.177 (0.76*)
0.265 (0.70*)
LNOP
0.018
0.024
0.031
0.121
NS
NS
0.085
NS
0.128
NS
0.003
NS
(0.54*)
(0.62)
(0.65)
(0.61)
RNOP
0.33
0.70*
0.39
6.96 (0.95)
1.92 (0.23)
4.92 (0.63)
10.26 (0.91)
13.63 (0.81)
12.94 (0.93)
6.21 (0.86)
5.98 (0.76*)
4.14 (0.90)
LOC
21.56
21.42
28.38
NS
3.18
NS
NS
NS
NS
3.96
2.57
1.49
(0.84)
(0.82*)
(0.86)
(0.75*)
ROC
(Inorganic + labile org.)/total
Labile/total
Urban = Med. - High density residential, commercial and industrial; dairy = intensive pasture, sprayfield and confined animal; row crop = seepage-irrigated row crop agriculture;
undeveloped = native forest, forested wetlands and silviculture.
1
Determined from the extrapolation of regressions relating seasonal flow-weighted mean concentrations and fraction of developed area to the point of 100% watershed development. All concentrations in mg ⁄ l. Correlation coefficients for simple linear regressions between the fraction of watershed categorical land use (independent variable) and
mean seasonal flow-weighted concentration (dependent variable) in parentheses. Asterisks in parentheses (*) represent an exponential regression fit. ‘‘NS’’ signifies a nonsignificant regression slope, in which case the concentration is assumed to be the same as that for the undeveloped case. Undeveloped land use category concentrations
determined from the mean of the 3 intercepts for row crop, dairy and urban regressions. Asterisks in this case represent instances when at least 1 of the 3 regressions produced a negative intercept, in which case the seasonal mean of the 3 least developed watersheds was use to establish this concentration.
Old Urban
Dairy
Row Crop
Undeveloped
Major Land Use
TABLE 1. Flow-weighted Seasonal Nutrient and Organic Carbon Concentration Values for Aggregated Land Use Categories1.
HENDRICKSON, TRAHAN, GORDON,
272
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concentrations are also often observed at this time,
and this may be due to the release of iron-bound
phosphorus as soil redox potential declines. Also noteworthy is the sharp increase in inorganic nitrogen
concentration for the row crop watersheds in the
December-April season, coincident with the cropping
of winter potatoes.
(a)
1
(b)
Labile/total
0.8
LNOP
0.6
LON
0.4
0.2
LOC
0
1
Total bioavailable
0.6
0.4
0.2
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TBP
0.8
TBN
Forest
Dairy
Row crop
Urban
0
Fraction of watershed area developed
FIGURE 5. Changes in (a) Percent Labile Organic
C, N, and P and (b) Total Bioavailable N and P
With Increases in Watershed Developed Area.
As a consequence of the increase in inorganic and
labile nutrients and carbon accompanying development, the fractional composition of labile and total
bioavailable nutrients and carbon was found
to increase as development intensity increased
(Figure 5). Despite large absolute differences in concentrations, the relative composition of labile and
total bioavailable forms was found to vary similarly
regardless of development type. Similar to the exponential increase seen for urban runoff LON, LNOP,
and LOC at high development density, the relative
LOC content also increased sharply, and can most
likely be attributed to the concomitant and pronounced decrease in ROC. The relative content of
TBN and TBP increases rapidly with watershed
AMERICAN WATER RESOURCES ASSOCIATION
BLACKWATER ORGANIC CARBON
Partitioning calculations suggest that labile
organic matter represents a much greater proportion
in upstream river inflow than that of within-basin tributaries, with algal biomass comprising a large portion of this labile organic matter. LOC, LON, and
LNOP exhibited annual patterns that are consistent
with the seasonal oscillation in autochthonous or allochthonous source dominance (Figure 6), and which
were more dynamic than the annual temporal patterns seen in tributary runoff. Algal assimilation in
spring and summer resulted in much lower inorganic
N and P concentrations than were observed in tributary runoff. By May of most years, increases in TN,
driven by increased algal LON resulting from cyanobacterial atmospheric nitrogen fixation are evident
(Phlips and Cichra, 2001; Paerl et al., 2003). This
internal N loading sharply increases the relative concentration of TBN imported to the LSJR during the
months of May-August, increasing from roughly 60%
of TN in winter months, to over 80% (Figure 6c).
Autochthonous production also increases the proportion of LOC, up from 17% of TOC in winter, to a peak
of 39% in July. Between July and September, incoming LOC concentration ranged between 5 and 9 mg ⁄ l,
with algal OC representing on average 34-50% of
this. At 2.8 mg ⁄ l, the mean annual incoming LOC
concentration was found to be similar to the elevated
levels for tributary sampling stations of developed
watersheds. However, because of prevailing low inorganic nutrients, TBN and TBP concentrations are
relatively low in comparison to within-LSJR basin
tributary runoff from developed watersheds, at
1.07 mg ⁄ l for TBN and 0.056 mg ⁄ l for TBP.
The relationship between the square root of color
and mean ROC and TOC for streams draining
undeveloped watersheds was found to be linear and
very significant, with an R2-value of 0.979 (Figure 7a).
Mean concentrations within color brackets have been
used to develop this relationship, as color determined
by the visual comparison method produces discrete
values based on platinum-cobalt standard solutions.
The relationship has inherent heteroscedasticity by
virtue of the fact that color standard brackets
increase in width with higher concentrations.
Because undeveloped watershed streams exhibit
low concentrations of LOC (calculations performed in
(a)
2.5
Nitrogen
Concentration, mg/L
River Inflow Nutrient and Carbon Partitioning
NUTRIENT BIOAVAILABILITY
2
TIN
Algal ON
Nonalgal LON
RON
1.5
1
0.5
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
(b)
0.12
Phosphorus
Concentration, mg/L
developed area, particularly for agricultural land
use, reaching between 60% and 80% relative composition at about 30% of watershed developed area
(Figure 5b).
AND
0.09
PO4
Algal OP
Nonalgal LNOP
RNOP
0.06
0.03
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
(c)
% Bioavailable
100
Percent
80
Partitioning Verification: Color Comparison
60
40
In blackwater streams draining undeveloped
watersheds, the organic carbon pool is predominantly
composed of leached, colored humic substances, and a
strong correlation between color and TOC has been
reported in both boreal and blackwater streams
(Rasmussen et al., 1989; Cuthbert and del Giorgio,
1992). In the conceptual dichotomy for organic matter
applied here, ROC is analogous to the aged, refractory portion of the total organic matter pool, or
CDOM, and theoretically, ROC should exhibit a
better correlation to color measurements than TOC.
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Nitrogen
Phosphorus
Organic carbon
20
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Month
FIGURE 6. Mean Monthly Nitrogen and Phosphorus Fractionation
at the Upstream Inlet to the Lower St. Johns River, 1995-99. (a)
Mean monthly nitrogen form concentrations; (b) mean monthly
phosphorus form concentrations; (c) mean monthly percent bioavailable (inorganic + labile organic) N and P. Error bars in (a) and
(b) are 95% confidence bounds on monthly mean total N and P.
273
JAWRA
HENDRICKSON, TRAHAN, GORDON,
(a)
OC Form, mg/l
30
Undeveloped
40
ROC =
1.8697*(Color0.5) – 6.5585
R 2 = 0.9785
(c)
Row crop
40
30
30
20
20
10
10
0
0
0
25
(e) 25
(f) 100
20
Dairy
ROC
10
TOC
Urban
OC Form, mg/l
OUYANG
(b)
40
(d)
AND
Pulp mill
Upstream river
20
20
90
15
15
80
10
10
70
5
5
60
0
0
50
Color class, (Pt–Co units)0.5
FIGURE 7. Comparison of Color and Color-Derived ROC to ROC and TOC. (a) Linear regression model relating the square root
of color to mean TOC and partitioned ROC within color classes for undeveloped LSJR basin watersheds. (b, c, d, e, and f)
Comparison of mean TOC and partitioned ROC within color classes to the color versus ROC model for undeveloped tributaries for
(b) row crop; (c) dairy; (d) urban; (e) upstream river; and (f) pulp mill effluent samples. Solid line in (b) through (f) is the
undeveloped trib color vs. ROC model. Error bars represent 95% confidence intervals for mean concentrations of ROC within classes.
this study determined LOC was on average only 4%
of un-impacted stream TOC), the relationship
between the square root of color and mean ROC and
TOC are nearly identical. However, as LOC increases, the TOC verses color relationship should
degrade, while partitioned ROC should, in theory,
adhere to the color relationship of un-impacted
streams. For row crop and dairy runoff samples, calculated ROC compares well to the un-impacted
stream relationship (Figures 7b and 7c), though
heteroscedasticity is apparent at higher color concentrations. For urban streams (Figure 7d), partitioned
ROC appears higher than the undeveloped stream
relationship for higher color (and ROC) concentrations, but 95% confidence intervals for ROC still
encompass the regression line. For river samples
(Figure 7e), partitioning appears to have produced
slightly higher ROC concentrations for low color classes. These lower color ranges coincide with longer
residence time periods and high phytoplankton biomass. This may indicate a weakness in the partitioning computation when phytoplankton biomass is
high, but may also be due to photodecomposition of
CDOM chromophores, owing to the longer duration
sunlight exposure (Gao and Zepp, 1998; Bertilsson
and Tranvik, 2000; Osburn et al., 2001). Weaker relationships between color and urban stream and river
JAWRA
samples can also be partly attributed to the lower
color range observed for these surface water types.
Because of the relatively small number of pulp mill
effluent samples (only four samples with sufficient
data to perform partitioning), the color versus ROC
relationship is compared only witho the overall average. The close agreement for calculated pulp mill
ROC values with the un-impacted blackwater stream
relationship is particularly surprising, as these effluent concentrations represent a large extrapolation
beyond measured stream values upon which the color
vs. ROC relationship is based.
Whole Basin Nutrient and Organic Carbon Loads
These specific land use concentrations of Table 1
were added to the existing LSJR watershed model
framework (Adamus and Bergman, 1995) to predict
LSJR basin total non-point source load. Basin loads
above background attributable to each major land-use
category were determined by substituting undeveloped land use concentration and runoff parameters in
place of the developed land use parameters within
the watershed model, and subtracting this model
simulation result from the existing condition simulation. Within-basin watershed modeling estimates of
274
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BLACKWATER ORGANIC CARBON
AND
NUTRIENT BIOAVAILABILITY
TABLE 2. Mean Annual Source Loads to the Lower St. Johns River, 1995-99.
Upstream
LSJR Basin
Natural Background
Agriculture
Urban
Point Source
Atmosph. Wet Dep.
Grand Total
Total N
LON
RON
DIN
8,415
4,494
3,369
561
1,324
322
513
1,783
222
1,2578
380
104
383
415
883
86
)53
65
5,775
4,350
61
131
183
1,303
222
2,462
Total P
LNOP
RNOP
PO4
LOC
ROC
370
206
59
104
17,647
95,216
117
97
146
467
3
1,199
22
27
83
98
16
10
10
5
2,183
197
1,693
2,065
45,706
)208
)3092
2,881
435
101
79
60
53
364
3
663
23,786
140,502
All values in metric tons per year.
non-point source loads were combined with point
source and upstream river loads to derive a total
basin mean C, N, and P budget by source, shown in
Table 2. In some cases, most notably for ROC and
RON loads from urban and row crop land uses, loads
are negative, reflecting a decrease in refractory
organic matter export. Overall, it is estimated that
within-LSJR basin ROC load has declined from the
background load by 7%.
Sub-division of nutrient loads into labile and
refractory organic forms greatly effects the interpretation of nitrogen loads to the estuary. The exclusion
of RON to estimate only TBN load reduces the estimate based on TN by 35%. Conversely, if the relevant, potentially available load to the river is
considered to be just the DIN load, the inclusion of
LON to this increases the estimate by 234%. Labile
and refractory partitioning appears to produce a relatively small effect on the interpretation of phosphorus loads, segregating only 6% of the TP load to
RNOP. The greatest TBN load is imported to the
LSJR from upstream sources, while the greatest TBP
load enters in the downstream, meso-polyhaline reach
of the river in the urban Jacksonville area, with most
of this coming from wastewater treatment plant discharge. The greatest source of LON, LNOP and LOC
to the LSJR is upstream inflow, primarily incorporated into algal biomass.
DISCUSSION
To diagnose and manage eutrophication of river
estuaries stemming from nutrient over-enrichment,
the spatially and temporally explicit quantification of
the external load represents a fundamental undertaking. Because estuarine eutrophication is more
succinctly a problem of organic matter overenrichment (Nixon, 1995), with oxygen depression
probably the classic adverse manifestation (Officer
et al., 1984), the requirements of the external load
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quantification apply not only to nitrogen and phosphorus, but also to labile organic carbon that drives
microbial metabolism. In blackwater river estuaries,
eutrophication assessment is complicated by naturally high organic matter loads, and uncertainty
regarding the bioavailability of organic C, N, and P
forms. Previous research has convincingly demonstrated that aquatic organic matter can be rendered
available over a range of time scales, dictating its
relevance to phytoplankton nutrient assimilation and
oxygen deficits within estuaries (Seitzinger and Sanders, 1997; Sun et al., 1997; McKnight et al., 2001)
and that land development can increase the proportion of labile organic matter in the total aquatic
organic matter pool (Seitzinger et al., 2002). Despite
this, general methodologies have not been suggested
to apply these findings, and labile and refractory carbon and nutrient compartmentalization has generally
been ignored in external load budgets. The calculations proposed here, which rely on upon inter-conversion of oxygen demand and organic carbon, and
general trends in C:N and C:P ratios along the labile
carbon gradient, are an attempt to provide such a
methodology.
Because incorporated nitrogen exists in aquatic
environments affiliated with carbon in organic molecules, organic nitrogen partitioning is well correlated
with organic C:N ratio, making this a useful proxy
for establishing biodegradability. Separation of LNOP
is less definitive, and this may be explained by TNOP
incorporated into mineral phases, which is probable
in rivers such as the St. Johns that receive large
amounts of ground waters containing calcium and
bicarbonate (Diaz et al., 1994). This presence of mineral P lowers the apparent OC:TNOP ratio that the
method applied here assumes is characteristic of
the degradability of an organic substrate, and may be
the reason for the poorer correlation between TOC:TNOP and percent LOC. However, given what is
known regarding the relatively high P content of
fresh, labile organic matter, it appears plausible to
accept at least qualitatively that substrates with low
OC:TNOP ratios exert higher oxygen demand and
275
JAWRA
HENDRICKSON, TRAHAN, GORDON,
inferred degradability, that the concentration of
LNOP in environments with low LOC is very low,
and that the proportion of LNOP increases sharply
with land development intensification.
Patterns in LOC determined by the methods used
here produce results similar to those reported by
other investigators in terms of both proportion within
TOC and trends with watershed development and
source. Volk et al. (1997), in a study of mixed
urban ⁄ ag ⁄ forested watersheds in southeast Pennsylvania, found bioavailable DOC to constitute on average 25% of DOC. Sun et al. (1997) reviewed previous
studies of OC bioavailability, and found a range in
LOC from 0% to 86%. For most rivers dominated by
allochthonous OC, the range is closer to between 7%
and 25% (Sondergaard and Middelboe, 1995; Volk
et al., 1997), with blackwater rivers exhibiting the
lowest relative amounts of labile OC (Sun et al.,
1997; Moran et al., 1999), and streams draining
urban areas the highest reported fractions. In this
study, estimated labile organic carbon content ranged
from 3% of TOC in waters draining undeveloped, forested streams, to 52% for highly urbanized streams.
Urbanized streams exhibited high absolute LOC concentrations, and high relative concentrations in part
as a result of simultaneous reductions in ROC. Clinton et al. (2002), in their examination of the changes
in DOC in the shallow groundwater flow path in a
Pacific Northwest floodplain river, concluded that soil
saturation (which controls redox state and subsurface
microbial metabolic activity) and organic matter
source patchiness largely control the export of labile
DOC to the hyporheic zone. Their paradigm may
explain why undeveloped, forested watersheds with
intact riparian areas export relatively low levels of
LDOC.
Because of the tendency for labile organic matter to
possess higher amounts of N and P relative to C, labile
nutrient fractions in runoff from developed watersheds
are higher than labile carbon fractions, on average
50% for LON and 75% for LNOP. LON ranged from
20% to 36% of TON for mostly undeveloped watersheds, from 36% to 66% for dairy and row crop-dominated watersheds, and from 38% to 92% for urbanized
watersheds. Stepanauskas et al. (2000), in their study
of Scandinavian rivers, estimated the percent of labile
dissolved organic nitrogen (DON) as between 19% and
55%. Because of the very low levels of TNOP observed
in undeveloped watershed runoff, it cannot be said
with certainty what percent is composed of LNOP,
although it appears that typical concentrations are
probably less than 10 lg ⁄ l. For urbanized streams,
TNOP was found to range from 66% to 97% of LNOP.
Trends identified here for the high relative amounts
of LON in urban area runoff are similar to that
observed by Seitzinger et al. (2002). Particularly signiJAWRA
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OUYANG
ficant was the similar conclusion regarding the relatively high levels of LON in urban ⁄ suburban runoff,
compared to agricultural development. However, our
analysis suggested that per area developed, urban
land use produced lower absolute concentrations of
both LON and LNOP than agriculture, and that the
high relative concentrations were because of the
decline in refractory forms. Tufford et al. (2003) identified a decline in dissolved organic N in urbanized relative to undeveloped streams of coastal plain South
Carolina, suggesting a similar lowering of RON, and
Graves et al. (2004) observed lower levels of color
(assumed a proxy for ROC) in runoff from urban areas
relative to pasture and wetland areas in south Florida.
Three factors are hypothesized to be largely responsible for this: (1) reduced terrestrial plant organic
matter inputs; (2) the expansion of impervious surfaces, which reduces the mingling of shallow ground
water with organic matter-rich soil layers; and (3) the
disruption of stream-riparian area processes due to
wetland destruction and channelization. Owing to
these factors, hydrophyllic organic acids that dominate
the refractory organic matter pool in the undeveloped
watershed setting (Aitkenhead-Peterson et al., 2003)
are replaced by labile organic matter favored in
surface flow import. Possible sources that might be
favored in surface water flow paths include effluent
from clogged drainfields and leaking septic tanks,
sanitary sewer line leaks, pump station overflows, pet
feces, grass clippings, or industrial site waste.
At some point in the continuum between roughly
third-order stream (typical size of contributing
streams within the LSJR basin) and sixth-order river
(size of the St. Johns at the upstream end of the
LSJR), secondary production dependence appears to
shift from allochthonous to autochthonous organic
matter sources, adhering to the tenets of the river
continuum concept (Vannote et al., 1980). One may
presume that the point of this transformation will
depend upon stream flow, season, lacustrine characteristics and levels of nutrient enrichment. It is a factor that must be kept in mind in watershed modeling
to derive nutrient load budgets, as it determines the
extent to which terrestrial characteristics alone can
predict stream chemical profile. Beyond this point, instream nutrient and organic matter processing must
be linked to watershed modeling, or direct monitoring
must be used to characterize incoming load.
If the labile and refractory partitioning proposed
here can be believed, then these results lead to a
dramatically different perspective of the sources and
degree of nutrient enrichment in the LSJR basin, and
perhaps in other blackwater river systems. Watershed modeling performed on the pre-development
scenario within the basin produces a mean annual
estimated (1995-99 hydrologic conditions) TN load of
276
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BLACKWATER ORGANIC CARBON
1,324 MT, one-third of the present day (1995-99 mean
annual) TN load from point and nonpoint sources of
3,940 MT. Arguably more relevant to estuarine eutrophication, however, is the increase TBN load, which
has risen from the predevelopment estimated mean
annual total of 441 MT ⁄ yr, to the present day level of
2,960 MT ⁄ yr, or a 6.7-fold increase. A bioavailable-P
only comparison suggests a smaller increase in
relative anthropogenic loading effect, with present
day TP load constituting a 7.1-fold increase over the
predevelopment scenario (up from 117 MT ⁄ yr to
827 MT ⁄ yr), compared with the TBP increase of 7.8fold (up from 100 MT ⁄ yr to 785 MT ⁄ yr). Thus, it is
clear from the compartmentalization performed here
that relative increases in potentially bioavailable
nutrient load, particularly for nitrogen, are greatly
underestimated in blackwater river systems when
budgets are based on total nutrient concentrations
alone. This finding has important implications for
eutrophication modeling, particularly if undeveloped
stream conditions are used to hind-cast natural background receiving water productivity levels. There are
also important implications in the TMDL process for
ascribing assimilative capacity, and, perhaps more
importantly, allocation for N and P ( and perhaps
LOC, perhaps as BOD), which should be based upon
the reactivity of nutrient loads.
Historical accounts of ‘‘jubilee,’’ the mass herding of
estuarine organisms fleeing hypoxic waters (May,
1973), and the copious supply of natural organic matter characteristic to coastal plain watersheds, has led
to the perception that low oxygen episodes are a natural phenomena in southeast U.S. estuaries. While it
is plausible that blackwater river estuaries should
exhibit net heterotrophy, Sucsy and Hendrickson
(2004) concluded through water quality modeling that
the refractory organic matter-dominated composition
that would have prevailed in the pre-European settlement LSJR would not have been sufficient to sustain
the in situ decomposition necessary to drive low dissolved oxygen levels that are now observed. The high
relative amounts of LOC calculated at the upstream
end of the LSJR arising from algal production, and the
large increase in the within-basin load of TBN and
TBP leading to phytoplankton production within the
LSJR, appears to be a much more likely source of labile
organic matter that drives low oxygen episodes. Loworder reaches of blackwater river systems may exhibit
naturally low dissolved oxygen as a result of advective
volume displacement with waters of low oxygen
content from adjacent, net-heterotrophic, stream-side
source areas. However, in higher order freshwater reaches at or near the head of tide, within-river volume is
typically sufficient to buffer such low oxygen concentration water displacement, and the oxygen regime is
dictated more by autochthonous production and respirJOURNAL
OF THE
AMERICAN WATER RESOURCES ASSOCIATION
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NUTRIENT BIOAVAILABILITY
ation cycles (Meyer and Edwards, 1990). In the LSJR,
nutrient enrichment leading to accelerated algal
organic matter production stands as a more plausible
source of oxygen deficits, and as such should be viewed
as a degraded condition, and subject to remediation
through nutrient pollution control.
ACKNOWLEDGMENTS
We wish to thank Dr. Wayne Magley of the Florida Department
of Environmental Protection for assistance in acquiring point
source effluent data, and Mr. Dana Morton and Ms. Betsy Deuerling of the City of Jacksonville for collecting and supplying essential surface water quality data. We also wish to thank Dr. Peter
Sucsy, Mr. Dean Campbell, and the anonymous reviewers for the
JAWRA for very helpful suggestions in analysis and review.
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