WATER RESOURCES BULLETIN

VOL. 31, NO.1 ANERICAN WATER RESOURCES ASSOCIATION FEBRUARY 1995

ENVIRONMENTAL FACTORS INFLUENCING SUSPENDED SOLIDS

IN THE LOXAIIATCHEE ESTUARY, FLORIDA'

Jill M. Noel, Robert H. Chamberlain, and Alan D. Stein man2

ABSTRACT: A study was initiated to examine the effects of wind

speed, wind direction, freshwater inflow, and tide height on suspended solid concentration and distribution in the Loxahatchee

estuary, Florida. Recent efforts to increase freshwater flows in this

system raised concerns that suspended solid concentrations would

increase as well, which might result in negative impacts for the

estuary. The data indicated that total suspended solids (TSS) in the

estuary are derived primarily from the inlet and not from freshwater tributaries. In addition, total suspended solids and volatile suspended solids were correlated strongly with salinity, suggesting

that suspended sediments act conservatively throughout this system. No one environmental factor had an overwhelming influence

on suspended solid concentration throughout the estuary; different

regions of the estuary were influenced by different factors. Freshwater inflow was negatively related to TSS in the upper reaches of

the estuary but was positively related to TSS in the central embayment region of the estuary. We attribute this latter finding to the

fact that extremely high inflows both prevented the normal transport upstream of tidal borne suspended sediments and promoted

mixing when the freshwater front moving downstream confronted

the tidal front moving upstream. Wind speed, wind direction, and

tide height had relatively small effects on TSS concentration but

were most influential in reaches upstream of the central embayment, where tidal velocity begins to diminish.

(KEY TERMS: suspended solids; water quality; estuary; Loxahatchee; freshwater inflow; tide; wind.)

INTRODUCTION

The Loxahatchee estuary, located in south Florida,

is valued for its ecological, recreational, and aesthetic

characteristics, as well as for its prime development

potential (McPherson et al., 1982). The Northwest

(NW) Fork of the system is especially valued because

much of the river corridor and surrounding basin is

relatively undeveloped. However, the ecological

integrity of the NW Fork, which is dominated by bald

cypress (Taxodium distichum L.), is threatened by

tide-induced saltwater intrusion. The South Florida

Water Management District (SFWMD) is partly

responsible for the management of the quality and

quantity of freshwater entering into the Loxahatchee

estuary. The SFWMD has increased freshwater flow

into the estuary as a strategy to inhibit the intrusion.

One consequence of this strategy may be elevated concentrations of suspended solids, which can have the

following deleterious effects in estuaries: (1) present

potential hazards to navigation; (2) impact seagrass communities, as well as other benthic communities,

through deposition (McPherson and Sabanskas, 1980;

FDNR, 1984) and depression of dissolved oxygen levels (McPherson and Sabanskas, 1980); and (3)

decrease light penetration through the water column,

thereby limiting primary productivity (Day et at., 1989). However, Sonntag and McPherson (1984a)

reported that suspended solids may not be a serious

problem in the Loxahatchee system because the solids

are derived primarily from the inlet or by hydrological

workings near the inlet, not from freshwater inflow.

Russell and McPherson (1983) estimated that a

total flow of 3.7 m3s4 (130 cfs) from the NW Fork is

necessary to prevent continued salinity encroachment

13 kilometer (km) upriver. Other studies by the U.S.

Geological Survey have addressed the sedimentologic

history (Wanless et at., 1984) and the importance of

freshwater flow, tidal velocity, and tidal volume as

factors controlling suspended sediment concentration

(Sonntag and McPherson, 1984a) in the Loxahatchee

estuary. However, these studies did not address fully

the linkage of freshwater flow and the effect of wind

and tide on the distribution and concentration of suspended solids throughout the estuary. Therefore, suspended solids were included in the water quality

component of an interdisciplinary study that was coniPaper No. 93021 of the Water Resources Bulletin. Discussions are open until October 1, 1995.

2Department of Ecosystem Research and Implementation, South Florida Water Management District, P.O. Box 24680, West Palm Beach,

Florida 33416-4680 (Noel, current address, Department of Botany, Louisiana State University, Baton Rouge, Louisiana 70803).

21 WATER RESOURCES BULLETIN
Noel, Chamberlain, and Steinman

ducted throughout the estuary. The primary objectives of this study were to: (1) define an optimum

freshwater inflow range that will not increase significantly suspended solid concentrations; (2) evaluate

the response of suspended solids to the minimum flow

requirements defined by Russell and McPherson

(1983) to halt saltwater encroachment; (3) determine

the influence of wind, tide, and freshwater inflow on

suspended solids; and (4) establish a baseline for

future studies that evaluate water management practices.

SITE DESCRIPTION

The Loxahatchee estuary is located on the Palm

Beach and Martin County line on the east coast of

Florida (Figure 1). The estuary consists of the North

Fork (N Fork), the Northwest Fork (NW Fork), and

the Southwest Fork (SW Fork). The three forks converge to form the central embayment of the estuary

approximately 3.2 km from Jupiter Inlet. Mean water

depths ranged from 1-3 m throughout the Forks and

the central embayment. The Atlantic Intracoastal

Waterway intersects the estuary between the Jupiter

Inlet and the central embayment.

The upper and middle reaches of the NW Fork

floodplain support a unique freshwater flora of trees

and plants dominated by Bald Cypress. In 1985, this

section of the NW Fork was designated as a National

Wild and Scenic River. Below river kilometer 13 in the

NW Fork, the cypress community changes to a brackish water environment dominated by Red and Black

Mangroves (Rhizophora mangle L. and Avicennia

germinans (L.) L., respectively). The presence of dead

and stressed Bald Cypress trees indicates that this

area was once a freshwater floodplain (McPherson

WATER RESOURCES BULLETIN 22

Figure 1. Sampling Station Locations in the Loxahatchee River and Estuary, Florida.
Environmental Factors Influencing Suspended Solids in the Loxahatchee Estuar Florida

and Sabanskas, 1980). The lower reaches of the forks

and the central embayment are characterized by the

presence of seagrass beds, primarily Shoal Grass

(Halodule wrightii Aschers.; McPherson et al., 1982).

During periods of low freshwater inflow, brackish

water extends from the Jupiter Inlet 8 km up the SW

Fork to structure 46 (S-46) and approximately 16 km

up the NW Fork (McPherson and Sabanskas, 1980).

The salinity in the N Fork is more uniform than the

other forks because it receives much less average

inflow. Brackish water persists more than 8 km

upstream. The Jupiter Inlet has been kept open since 1947.

Other channel dredge and fill operations in the estuary during the early 1900s have altered the historical

system and marked the beginning of the saltwater

encroachment problem by promoting tidal movement

into the NW Fork (USCOE, 1966; McPherson et al.,

1982).

Canal networks and water regulation structures

are commonly installed in south Florida to channelize

part or all of the marshlands and headwaters of rivers

and estuaries for water supply and flood protection.

In the Loxahatchee estuary, these structures have

resulted in lowered ground water levels and diversion

of historical flows away from the NW Fork, thus promoting additional saltwater intrusion upstream into

the middle reaches of the NW Fork during the dry

season (Vines, 1970). Canal 18 (C-iS) and flood control structure S-46 in the SW Fork were completed in

1958 to convey water for flood protection. In 1974,

Structure G-92 (Figure 1) was installed to divert

water back to the NW Fork from C-18.

Russell and McPherson (1983) examined the total

freshwater input to the estuary from each of the three

forks. After the installation of G-92, they reported

that S-46 operated primarily during the wet season,

contributing 20 to 40 percent of the total inflow to the

estuary. In the wet season, three major tributaries to

the NW Fork (Figure 1) combine to contribute greater

than 60 percent of the freshwater inflows to the system, except during storm events, when S-46 is open. The N Fork contributes less than 3 percent of the

freshwater inflows to the estuary.

METHODS

Eighteen stations were sampled approximately monthly in the estuary (Figure 1) from July 1985

through January 1988 (n=20 to 27 per station) as part

of a water quality survey of the estuary. Salinity, total

suspended solids (TSS), and volatile suspended solids

(VSS) were three of the parameters measured. Stations were spaced approximately 2 km apart

beginning at the inlet and extending into each fork

(Figure 1). Intermediate sampling stations were located at critical sections of the estuary. The last station

in the NW Fork was located just upstream of the normal maximum extent of salt water encroachment during minimum flow (Russell and McPherson, 1983).

Sampling in the SW Fork extended to S-46. The last

sampling station in the N Fork was located as far

upstream as water depths would allow the 6 m boat to

proceed with its 0.5 m draft.

Samples were collected near high tide because

salinity encroachment is at its extreme. Due to the

impossibility of sampling all stations at peak high

tide, samples were collected over a range of tidal elevations surrounding high tide. Typically, sampling

commenced at the inlet and central embayment stations approximately one to three hours before slack

high tide and was finished one hour after slack high

tide at station 32 in the NW Fork (Figure 1). Therefore, the terms high and low tide in the context of this

paper refer to the low and high end of the high tidal

range sampled, not the absolute high and low tide.

Tide height at the time of sampling at a given station

was estimated using the U.S. National Ocean Service

Tide Tables (1985, 1986, 1987, 1988, and 1991).

Five grab samples were collected at each station

using a 1 L bottle lowered to approximately 0.3 m

below the surface. Each grab sample was poured into

the same bucket, and one subsample for analysis was

extracted from the bucket, placed on ice, and returned

to the lab. TSS values were determined gravimetrically (APHA, 1985). VSS are considered a reasonable

estimator of total organic carbon present in a sample

and were determined by ignition of TSS samples at

550o C (APHA, 1985). Salinity also was measured at

the 0.3 m depth at each station using a series 4000

Hydrolab® conductivity meter. Wind speed and direction data (1984-89) were

obtained from the National Oceanic and Atmospheric

Administration (NOAA) recording station at the Palm

Beach International Airport, located approximately

29 km south of the central embayment of the Loxahatchee estuary. Wind speed (measured in m s1) was

averaged on a daily basis, and wind direction was

taken during the fastest observed one-minute wind

speed for the day.

Fetch was estimated based on methods from Smith

(1991). In this estuary, wind-generated waves are limited by the surrounding landforms, and consequently, fetch width is relatively narrow compared to fetch

length. Fetch was calculated at six stations in or near

the central embayment of the estuary (stations 10, 12,

14, 15, 16, and 21) with the assumption that fetch at

the other stations was too limited to be a factor in

wind resuspension of solids.

23 WATER RESOURCES BULLETIN
Noel, Chamberlain, and Steinman

Freshwater inflow data from the SW Fork (S-46)

were provided by the SFWMD. Gate openings, headwater, and tailwater stages were recorded continuously at the structure, and daily mean discharge was

based on a flow rating curve.

Freshwater inflow to the NW Fork was estimated

from continuous stage recorders located in the tributaries. Data were periodically collected from these

stations, and a stage-discharge relationship was calculated for each recorder. The resulting inflow for

these tributaries was summed to estimate total daily

NW Fork flow. When data from a recorder were missing, flow for that tributary was estimated based on

what percent that missing tributary was contributing

just prior to and after the data gap, compared to the

other tributaries. Freshwater inflow was not obtained

for the N Fork (Russell and McPherson, 1983; Sonntag and McPherson, 1984a, b). Tide, wind speed, wind direction, and flow were

separated into classes based on relative magnitude

(Table 1) to explore graphically and statistically the variation and distribution of TSS and VSS in the

estuary. The criteria used to establish these classes

were based on the means and distribution of values

for each factor. Except for wind direction, each factor

was separated into three classes, with the middle

class containing the mean. The range of the classes was further refined so that each class included

approximately an equal number of samples. For wind

direction, the prevailing winds came from the east (56

percent of the time), followed by north (27 percent),

south (6 percent), and west (11 percent). Therefore,

only two classes were formed: (1) wind from the east,

and (2) wind from all the other directions. Flow classes were selected based on flow from S-46. This resulted in an overlap of NIAT Fork flow between the classes,

but whenever S-46 was discharging, the NW Fork had

proportionately increased flows. Flow class 2 includes

the minimum flow requirements identified by Russell

and McPherson (1983) for the NWF.

TABLE 1. Class Criteria for Individual Parameters.

Class

Tide

Height

(m)

Flow

(m3/s)

Wind

Speed

(mis)

Wind

Direction

SWF NWF C)

1 <0.46 0.0 0.0-4.2 3-6 45-125

(east)

2 0.47-0.61 0.0.1.5 2.8-7.1 6-9 126-44

3 > 0.61 1.5-14 4.2-16.4 > 9

WATER RESOURCES BULLETIN 24

The effects of tide, wind speed, wind direction, and

flow on TSS were depicted by creating a graph for

each fork that illustrated mean TSS versus distance

from the inlet. These graphs were based on a select

data set, which included only those dates (n  2) when

Class 1 and 3 of the factor being examined occurred.

By including data from only classes 1 and 3 (i.e.,

under minimum and maximum conditions), we optimized the ability to detect visually effects these factors might have on TSS. In addition, data were

included only from those dates when other factors

were held at low to moderate levels (Table 2), which

reduced the inherent variability of the data and minimized potential interactions.

TABLE 2. Conceptual Design for Graphical Analysis. Rows indicate which parameter classes were held constant

during analysis. NC = not considered for analysis.

Tide Flow

Wind

Speed

Wind

Direction

Variable Examined

Tide

(class 1 and 3) — 2 land2 NC

Flow

(class 1 and 3)

2 — land2 NC

Wind Speed

(class 1 and 3)

NC 2 - NC

Wind Direction

(east and other)

NC 2 1 and 2 — One-way analysis of variance (ANOVA) was used to

examine whether TSS were significantly different

among stations throughout the estuary and to examine if TSS were significantly different in forks located

an equivalent distance from the inlet. In addition, a

series of five one-way ANOVAs was employed to

determine whether tide height, wind speed, wind

direction, flow, or station had a statistically signifi- cant impact on TSS in each fork and the central

embayment. In this analysis, classes for each of these

factors (Table 1) except station were considered as

treatments. Station was included as a factor because

of the inherent variability associated with each location. Significant differences were determined at the

90 percent confidence level in order to be consistent

with the confidence level selected for the multiple

range test. If a factor was associated with a significant change in suspended solids, then Scheffe's multiple range test was applied to determine how classes

differed from one another. A class was determined to
Li

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Environmental Factors Influencing Suspended Solids in the Loxahatchee Estuary, Florida

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25 WATER RESOURCES BULLETIN

be significantly less or greater than another at the 90

percent confidence level as recommended by Day and

Quinn (1989) due to the conservative nature of

Scheffe's test. Multiple stepwise regression also was

employed using the raw, unclassified data as independent variables to determine the amount of TSS variance explained by each of the five factors.

All statistical analyses were performed using Statgraphics® (version 4.2) on an IBM® PC computer

(PS/2, Model 70 486). Normality and homogeneity of

variance were verified using the Kolmogorov-Smirnov

test for goodness of fit and Cochran's test for difference in variance. The data were found to be normally

distributed with homogeneous variance, so transformation was not necessary.

Plant identifications were made according to Wunderlin (1986).

40

30

20

10

14

12

10

__ 8

6

4

RESULTS

Overall Patterns in Estuary

Mean TSS and VSS concentrations in the Loxahatchee Estuary decreased with increasing distance

from the inlet (Figure 2). Mean TSS concentrations

ranged from 33 mg.L-1 at the inlet to 4 mg.L1 in the

NW Fork at river kilometer (RK) 15. Mean VSS values ranged from 12 mg.U1 at the inlet to 2 mg.L1 in

the NW Fork at RK 15. This decreasing gradient was

compared to salinity, a conservative constituent (Figure 3). Simple linear regression of mean TSS and VSS

concentrations against the corresponding salinity

value at the same station resulted in strong positive

correlations (r2 = 0.955 and 0.915, respectively).

40

30

20

10

14

12

10

8

6

2

0

0 4 8 12 16

DISTANCE FROM INLET (km)

Figure 2. Mean TSS (a) and Mean VSS (b) Plotted

Against Distance from the Inlet for Central

Embayment (CB), Northwest Fork (NW Fork),

Southwest Fork (SW Fork), and North Fork (N Fork).

4

2

0

0 10 20 30 40

MEAN SALINITY (g• L)

Figure 3. Regression Line of Best Fit Plotted for Mean

TSS (a) and Mean VSS (b) Versus Salinity. Vertical

bars represent standard deviation;

n ranges from 20 to 28.
Noel, Chamberlain, and Steinman

VSS values throughout the estuary generally represented a constant fraction of the TSS values during

the study. VSS comprised 38 percent of the TSS at the

inlet and a maximum mean of 52 percent at station

32 (RK 15).

A significant statistical difference in TSS existed

among the 18 stations. Scheffe's multiple range test

separated the 18 stations into six overlapping heterogeneous groups with significant decreases in mean TSS occurring as the distance from the inlet

increased. In addition, a second analysis was performed to determine differences among forks. Only

data collected from stations in forks located an equivalent distance from the inlet were used. A one-way

ANOVA revealed a significant difference between the

N Fork, SW Fork, and stations 14-25 in the lower NW

Fork. The multiple range test showed that the mean

TSS in the N Fork was significantly greater than

either the SW Fork or lower NW Fork, and that there

was no significant difference in mean TSS between

the SW Fork and NW Fork (Figure 2).

Influence of Environmental Factors: Overall Patterns

Data for all samples from all stations were pooled

and then partitioned into the environmental classes

(Table 1) to determine the general influence of the

four environmental factors on TSS within the entire

estuary. Mean TSS were significantly greater for flow

Class 1 (lowest flow) than for Classes 2 and 3, and

moderate (Class 2) flows were associated with significantly lower mean TSS concentrations. Mean TSS

values also were significantly different between wind

direction classes, with winds from the east associated

with significantly lower mean TSS values. However,

there was no significant difference between wind

speed classes. TSS associated with tide height were

significantly different, with mean TSS values being

significantly greater during the lowest tides (Class 1).

VSS values for all the stations also were examined

using the same ANOVA approach. The results were

similar to results described above for TSS. VSS percentage of TSS also does not significantly vary

between stations throughout the estuary. Therefore, additional results for VSS will not be reported

because their behavior within the estuary is assumed

to be similar to TSS.

Influence of Environmental Factors: Among

Locations

Tide height had no significant effect on TSS in

four of the five locations examined: inlet/central

WATER RESOURCES BULLETIN 26

embayment, N Fork, lower NW Fork, and upper NW

Fork (Figure 4a; Table 3). In the SW Fork, each consecutive higher tide class had a significantly lower

mean TSS value (Table 3). Similar trends were

observed in the N Fork (Figure 4), but no significant

differences due to tide were detected (Table 3).

Graphically, wind speed exhibited no obvious

effects on TSS in any of the forks (Figure 5). However,

a statistically significant difference in TSS was

detected in the upper NW Fork but in none of the

other locations. High wind speeds in the upper NW

Fork were associated with greater mean TSS values

(Table 4). Wind direction significantly affected TSS in the

lower NW Fork and the SW Fork (Figure 6; Table 5).

In both locations, winds from the east (45-135') were

linked with lower TSS values, although there

appeared to be less relationship between TSS and

wind direction in the SW Fork (Figure 6b) compared

to the lower NW Fork. A similar trend appears in the

N Fork as observed in the SW Fork and lower NW

Fork (Figure 6c), but no statistical differences were

detected.

Graphically, TSS appeared to be influenced by flow

in all locations except the N Fork (Figure 7). The statistical analysis revealed, however, that flow signifi- cantly affected TSS only in the inlet/central

embayment and SW Fork (Table 6). In the SW Fork,

TSS values were less during high flow conditions than

under low flow conditions (Figure 7b). The flow classes in the SW Fork were separated into three groups

with each class of higher flow associated with a lower

mean TSS value (Table 6). In the N Fork, the mean

TSS values did not show a clear difference in response

to flow (Figure 7c). In the inlet and central embayment (stations 1, 6, and 10), TSS values were lowest

at intermediate flows and highest at the greatest flow

class (Table 6). Mean TSS were significantly greater

when flows were higher (Class 3), which is in contrast

to the trend found in the forks.

The effect of station location on TSS values was

significant in the lower NW Fork and the upper NW

Fork (Table 7). In both cases, stations further from

the inlet contained lower levels of TSS (Table 7).

Variance Explained by Environmental Factors

The total amount of variation in the TSS data

explained by all factors ranged from 50 percent in the

lower NW Fork to 8 percent at the inlet (Table 8).

Regression coefficients between flow and TSS were

positive at the central embayment, inlet, and upper NW Fork stations, indicating that high flows are

related to higher TSS at these locations. Conversely,

coefficients between flow and TSS were negative in
-J

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C,)

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C,)

LU

z

LU

C,)

C,,

DISTANCE FROM INLET (km)

12 16 DISTANCE FROM INLET (km)

Figure 4. Mean TSS During High and Low Tide Plotted Against

Distance from Inlet for Central Embayment (CB), Northwest

Fork (a), Southwest Fork (b), and North Fork (c).

Vertical bars represent standard deviation, n = 3 dates.

TABLE 3. Summary of ANOVA Results and Scheffe's Multiple

Range Test Examining Effect of Tide Height on TSS in Each

Location Within the Estuary. Classes used in Scheffe's test

are 1 = < 0.46 m, 2 = 0.47.0.61 m, and 3 =>0.62 m. Values

below class labels are means (mg/L). Classes connected with

a common underline are not significantly different at P = 0.10.

Scheffe's test was conducted only when ANOVA was significant.

Location df MSE F Scheffe's

Inlet and CB 2 57.41 0.63 NS

NP 2 129.64 1.28 NS

SWF 2 1342.16 19.95* 1 2 3

26 22 12

Lower NWF 2 48.39 0.55 NS

Upper NWF 2 11.56 0.96 NS

P <0.001.

Figure 5. Mean TSS During High and Low Wind Speeds Plotted

Against Distance from Inlet for Central Embayment (CB),

Northwest Fork (a), Southwest Fork (b), North Fork (c).

Vertical bars represent standard deviation, n = 2 dates for

high wind speed, and n = 6 dates for low wind speed.

TABLE 4. Summary of ANOVA Results and Scheffe's Multiple

Range Test Examining Effect of Wind Speed on TSS in Each

Location Within the Estuary. Classes used in Scheffe's test

are 1 = < 6 mIs, 2 = 6-9 mIs, and 3 => 9 mIs. Values below class

labels are means (mg/L). Classes connected with a common

underline are not significantly different at P =0.10. Scheffe's

test was conducted only when ANOVA was significant.

Location df MSE F Scheffe's

Inlet and CB 2 41.12 0.43 NS

NP 2 25.69 0.25 NS

SWF 2 31.67 0.35 NS

Lower NWF 2 30.86 0.34 NS

Upper NWF 2 57.93 544* 1 2 3

1

P <0.01.

27 WATER RESOURCES BULLETIN

Environmental Factors Influencing Suspended Solids in the Loxahatchee Estuary, Florida

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C) E

C,)

Iz

LU

0 4 8
Figure 6. Mean TSS During East Winds and Winds From All

Other Directions Plotted Against Distance from Inlet for

Central Embayment (CB), Northwest Fork (a), Southwest

Fork (b), North Fork (C). Vertical bars represent standard

deviation, n = 3 dates for East winds, and

n = 7 dates for winds from all other directions.

TABLE 5. Summary of ANOVA Results and Scheffe's Multiple

Range Test Examining Effect of Wind Direction on TSS in Each

Location Within the Estuary. Classes used in Scheffe's test are

1 = east, 2 = all others. Values below class labels are means

(mgIL). Classes connected with a common underline

are not significantly different at P = 0.10. Scheffe's test

was conducted only when ANOVA was significant.

Location df MSE F Scheffe's

Inlet and CB 1 57.46 0.60 NS

NF 1 126.86 1.25 NS

SWF 1 432.78 497* 1 2

20 24

Lower NWF 1 863.35 10.60** 1 2 22

Upper NWF 1 9.24 0.77 NS

<0.10; *J <0.012.

WATER RESOURCES BULLETIN 28

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E

U) U, Iz

LU

Figure 7. Mean TSS During High and Low Freshwater Flows

Plotted Against Distance from Inlet for Central Embayment

(CB), Northwest Fork (a), Southwest Fork (b), North Fork (c).

Vertical bars represent standard deviation, n = 2 dates.

TABLE 6. Summary of ANOVA Results and Schoffe's Multiple

Range Test Examining Effect of Flow on TSS in Each Location

Within the Estuary. Classes used in Scheffe's test are 1 = 0 m3s,

2 = 1.4 m3/s, 3 => 1.5 m3/s. Values below class labels

are means (mg/L). Classes connected with a common underline

are not significantly different at P = 0.10. Scheffe's test

was conducted only when ANOVA was significant.

Location df MSE F Scheffe's

Inlet and CB 2 457.23 547* 1

ai

2 3 21fi

NF 2 233.41 2.38 NS

SWF 2 1902.93 32.70** 12

2 3 ZIli

Lower NWF 2 93.99 1.05 NS

Upper NV/F 2 19.01 1.61 NS

<0.01; <0.001.

Noel, Chamberlain, and Steinman

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0) E

U) 1/) Iz

lU

4 8 12

DISTANCE FROM INLET (km)

0 4 8 12

DISTANCE FROM INLET (km)

16
Environmental Factors Influencing Suspended Solids in the Loxahatchee Estuary, Florida

TABLE 7. Summary of ANOVA Results and Scheffe's Multiple Range Test Examining Effect of Stations on TSS in Each

Location Within the Estuary. Numbers used for class labels refer to station numbers within that location. Values below

class labels are means (mg/L). Classes connected with a common underline are not significantly different at

P = 0.10. SchefTe's test was conducted only when ANOVA was significant.

Location df MSE F Scheffe's

Inlet and CB 2 83.68 0.88 NS

NF 2 125.42 1.14 NS

SWF 4 54.03 0.35 NS

LowerNWF 3 810.19 12.16* #14 #21 #23 #25

25 19 16 11

Upper NWF 2 65.17 6.24** #28 #30 #32

1 6 4

<0.001; <0.01.

TABLE 8. Summary of Stepwise Regressions Examining Percent Variance in TSS Data

Explained by Different Variables in Each Location Within the Loxahatchee Estuary.

Central Lower Upper Variable Inlet Embayment NF SWF NWF NWF

S-46 Flow 0.02 0.04 0.34 0.04 0.63 4.85

NWF Flow 0.37 10.47 13.94 36.63 14.86 1.36

Wind Speed 0.63 0.22 0.07 1.46 2.35 7.45

Wind Direction 2.88 3.32 0.26 1.78 2.35 0.33

Tide Height 4.53 1.11 0.19 3.24 3.79 4.60

Stations — 0.58 3.17 1.42 26.57 15.01

the N, SW, and lower NW Forks, indicating that higher inflows are related to lower TSS in these locations.

At the inlet location, tide height explained 4.5 percent of the variation, and wind speed explained an

additional 3 percent. All variables combined accounted for only 8 percent of the variation. The regression

coefficient for the tide height variable was negative,

indicating that higher TSS was correlated with lower

tides. In the central embayment (stations 6 and 10),

flow from the NW Fork and SW Fork accounted for

10.5 percent of the variation in mean TSS. All the

variables combined explained 16 percent of the variation (Table 8).

In the N Fork, flow from the SW Fork and NW

Fork accounted for 14 percent of the variation. All the

variables combined explained 18 percent (Table 8). In

the SW Fork, flow was the single most influential

variable, accounting for 37 percent of the variation in

TSS. All the variables combined accounted for 45 percent (Table 8). In the lower NW Fork (stations 14-25),

flow and the difference between stations accounted for

41 percent of the variation. All variables combined

explained 51 percent (Table 8). In the upper NW Fork,

wind speed and the difference between stations

accounted for 22 percent of the variation in TSS;

adding the other factors to the regression explained

an additional 11 percent of the variance.

DISCUSSION

Data from the present study suggest strongly that during most times of the year, TSS in the Loxahatchee estuary are derived primarily from the inlet

and not from freshwater tributaries. This was reflected in both the graphical and statistical analyses of the

data, as well as in the positive correlation of TSS and

VSS with salinity in the Loxahatchee Estuary (Figure 3). Both TSS and VSS acted conservatively over

the narrow range of tidal amplitude and velocity sampled. Therefore, the suspended solids concentrations

measured during this study support the findings by Sonntag and McPherson (1984a) that TSS and VSS

are primarily derived from the inlet or by hydrological

workings near the inlet. This finding is in contrast to

29 WATER RESOURCES BULLETIN
Noel, Chamberlain, and Steinman

estuaries in the northern U.S., where studies have

shown that the majority of sediment originates from

tributaries, not the ocean (Officer et at., 1984; Biggs

and Beasley 1988). However, our results are consistent with those of Olsen et at. (1989), who found that

inputs of contaminants from oceanic sources greatly

exceeded inputs from riverborne or drainage basin

sources in the Savannah River estuary. They attributed this phenomenon to three potential mechanisms:

sediment resuspension in dynamic coastal areas;

sorption onto fine particles; and landward transport

of particulate material along the bottom. The reduced

role of tributaries as sources of TSS in south Florida

can be attributed to the low grade of the watersheds

and associated slow velocities. In addition, the undeveloped status of much of the Loxahatchee basin, and

especially the NW Fork river corridor, helps retain

sediments within the basin (cf. Jordan et at., 1986).

Concentrations of TSS, as well as salinity values,

in the Loxahatchee estuary fall within the normal

range of values for Florida estuaries (Friedman and

Hand, 1989). However, TSS values for the Loxahatchee system were low compared to estuaries located elsewhere. For example, the Severn estuary in

England has TSS as high as 4000 mg.L-1 (Biggs and

Beasley, 1988), the Rio Grande has an average suspended sediment concentration of 42,600 mg.L-1

(Gibbs, 1977), and TSS in the Delaware River Estuary

ranges from 0.5 mg.L' to 230 mg.L-1 along its longitudinal axis (Biggs and Beasley, 1988). In the Susquehanna River Estuary, a major source river to the

Chesapeake Bay, the normal seasonal TSS average

ranges from 10 to 140 mg.L-1, and the average concentration in the Mississippi is 250 mg.L1 (Gibbs

1977).

The greatest concentrations of TSS in the Loxahatchee occurred at the inlet and central embayment

areas. We attribute this primarily to greater tidal flow

compared to the much smaller freshwater discharge

from the tributaries. Sonntag and McPherson (1984a)

reported that tributary inflow normally comprised

only one or two percent of the tidal discharge in the

estuary. Suspended sediments from all tributary

inflow during an average six hour tidal cycle normally

contribute less than one percent of the load that

enters the estuary from the ocean during an incoming

tide. Wanless et at. (1984) attributed this low loading

percentage to the finer size sediments associated with

the Loxahatchee basins.

Sonntag and McPherson (1984a) found that tide

velocity is the dominant factor controlling TSS in the

inlet and central embayment areas. They reported

significantly lower mean TSS values of approximately

21 mg.L-1 at the inlet compared to the 33 mg.L' measured during our study. Throughout the lower reaches

of the forks and central embayment area, Sonntag

WATER RESOURCES BULLETIN 30

and McPherson (1984a) reported mean values of

approximately half what was reported in the present study. The greater values in our study are

attributable to the fact that sampling occurred near

mid-tide, when tide currents and associated TSS were

greatest, as opposed to slack tide when Sonntag and

McPherson's data were collected.

The data collected during our study did not indicate that a permanent turbidity maximum zone exists

within the Loxahatchee Estuary (Figure 3). However,

Wanless et at. (1984) point out that a sandbar that runs longitudinally through most of the central

embayment proves that a deposition area exists in

this area. Figure 2 also indicates that TSS is elevated

in this area. Wanless et at. (1984) reported that the

sediment on the bar is predominantly ocean derived.

Sand begins to fall out of suspension and is deposited

on the bar as currents slow and the water loses its

ability to transport particles. The larger particles are deposited first on the east side of the bar while the

finer size particles are carried farther upstream.

Interestingly, our study did find that the greatest

freshwater inflows resulted in greater concentrations

of suspended solids in the central embayment

(Table 6). Although this finding may seem contradictory compared to the upstream stations, it is based on

the following combination of factors: (1) extremely

high inflows prevent the normal transport upstream

of tidal borne suspended sediments; (2) high inflows

promote mixing in the central embayment, where the

freshwater front moving downstream confronts the

tidal front moving upstream; and (3) appreciably

more sediment is added to the system and reaches the

central embayment during large storm events. Sonntag and McPherson (1984a) reported that tropical

storm Dennis raised the percentage of total TSS contributed to the Loxahatchee estuary by the tributaries

to 18 percent. The five-day post storm period accounted for 24 percent of the total load of suspended solids

contributed to the Loxahatchee from the basin during

the 1981 water year (October 1980-September 1981).

Therefore, high river inflow maximizes suspended

solids in the central embayment and promotes the

deposition of riverine and ocean sediments onto the

sandbar.

The role of freshwater inflow on TSS is reversed in

the systems upstream of the central embayment. TSS

decreases as flow increases in the forks. Figures 7a

and b show how TSS decreased sharply from the central embayment to the SW Fork and NW Fork stations. As freshwater flows increase to the NW Fork

and SW Fork, the freshwater that is low in TSS proportionately mixes with the tidal water and both

dilute the TSS concentration and inhibit the movement upstream of tidal-borne sediments.
Environmental Factors Influencing Suspended Solids in the Loxahatchee Estuary, Florida

Low to moderately high flows did not significantly

decrease the already low concentrations of TSS in the

NW Fork (Table 6), although mean TSS values were

consistently lower in the high flow class compared to

the low flow class. The freshwater inflow volume

examined during this study seldom approached the

volume measured by Sonntag and McPherson (1984a)

required to dramatically increase sediment loading.

However, stepwise regression analysis of the data

indicated that the highest flows sampled during this

study were related to elevated TSS concentrations.

In the SW Fork, low tides resulted in higher mean

TSS values, which was contrary to expectation. We

hypothesized that because suspended sediments were

ocean-derived, TSS concentration would increase as

tide height increased. McLusky (1989) reported that

during higher tides the volume of water in an estuary

increases, thus reducing the concentration of suspended sediments. During this study, we examined

higher than average tides, which correlate with higher volume. A separate multifactor ANOVA was run to

examine potential interactions among the environmental factors measured. This analysis revealed a

significant interaction between tide height and wind

speed, suggesting that TSS became elevated during

moderate wind speed and lower, non-slack tide.

Therefore, as water levels decrease, wind-induced

resuspension may result in increased TSS as the shallower bottom sediments are more easily disturbed.

Our analyses revealed no significant difference in

TSS attributed to wind speed and direction in the central embayment. This is contrary to what was expected. Sonntag and McPherson (1984a) observed

elevated TSS at the inlet due to resuspension of sediments in the north arm of the Intracoastal Waterway

when wind was from the north. They suspected that

some portion of this resuspended material was

transported into the central embayment during flooding tide. Our fetch calculations indicated that sufficient energy can be generated by wind speeds greater

than 6 m•s1 from the east to resuspend bottom material at most stations in the central embayment, lower

SW Fork, and lower NW Fork. The amount of

increased suspended sediment due to wind is evidently overshadowed in the central embayment by the

influences of tide velocity and freshwater inflow.

Because all the sampling days (except for two) experienced wind speeds greater than 5 ms-1, wind probably resuspends material almost daily. This constant

resuspended state at the time of sampling makes it

difficult to distinguish subtle changes in TSS concentrations due to wind using the methods employed in

this study. The significant increase in mean TSS that occurred

as wind speed increases in the upper NW Fork

was not expected in this narrow, meandering, and

sheltered portion of the estuary. Examination of the

interaction of wind speed and tributary flow indicated

that most of the highest flows occurred when wind

speed also was high, which may partly account for

this result.

CONCLUSIONS

The following conclusions are based on the results

of this study and the reports by the USGS (Russell

and McPherson, 1983; Sonntag and McPherson,

1984a,b; Wanless et al., 1984) . Further evaluation is

needed to determine effects that these conclusions

may have on other water quality parameters and

biota before recommendations are developed and

implemented.

(1) The concentration of suspended sediment compares well with other Florida estuaries but is less

than temperate systems probably because of the

smaller allochthonous contribution of sediments from

upland sources. This difference may be due to the

smaller drainage basin of the Loxahatchee, the

basin's low topographic relief, differences in soil type,

and the small urban density in the Loxahatchee

Slough (watershed) and the NW Fork river corridor.

(2) Suspended sediments in the estuary are primarily derived from the ocean or hydrologic workings

near the inlet.

(3) TSS and VSS are highly correlated with salinity. Therefore, suspended sediments act conservatively

throughout the system. However, sandbars within the

Loxahatchee estuary indicate that sediment sinks do

exist.

(4) Tide velocity dominates suspended sediment

dynamics in the inlet and central embayment. Freshwater flow is secondary in importance throughout the

system and dominates in the mid-estuarine areas

upstream of the central embayment.

(5) Freshwater deliveries to the NW Fork that are

maintained in the Class 2 range with a constant year

round flow of 3.4 m3s1 (120 cfs) to prevent further

saltwater encroachment will not significantly increase

suspended sediments in the estuary. This flow class

was actually associated with significantly lower suspended sediment concentrations in the mid-estuarine

areas of the SW Fork and lower NW Fork. Moderate

flows do not significantly change concentrations in

the remaining areas of the upper NW Fork, N Fork,

central embayment, or inlet.

(6) Class 2 inflows through S-46 (less than 1.5

m3•s-1 (55 cfs)) were associated with significantly

lower suspended sediment concentrations in the SW

Fork. However, both base flow and storm water flow

31 WATER RESOURCES BULLETIN
Noel, Chamberlain, and Steinman

from the Loxahatchee Slough historically has been,

and should continue to be, directed first to the NW

Fork and secondarily through S-46, until future

reports recommend alternatives beneficial to both

forks.

(7) Suspended sediments increase upstream of the

central embayment when flow volumes decrease to

Class 1 because incoming tides are unobstructed in

their movement of suspended sediment upstream.

(8) The factors of wind speed, wind direction, and

tide height normally only slightly affect TSS concentrations and are also most influential upstream of the

central embayment area where tidal velocity begins to

diminish.

(9) High flows (Class 3) are correlated with elevated suspended solids in the upper NW Fork and central embayment. Sonntag and McPherson (1984a)

reported that flows that approached 22 m3-s1 (775

cfs) from the NW Fork were responsible for a 16-fold

increase in sediment loading from the tributaries.

Our study found Class 3 flows from the NW Fork that

approach 16 m3-s-1 (565 cfs) have the potential to

increase suspended sediment concentration in the

upper NW Fork. Combined Class 3 flows from both

the SW Fork and NW Fork, which totaled more than

approximately 20 m3-s-1 (700 cfs), elevated suspended

sediment concentrations in the central embayment

and may influence deposition of sediment in the central embayment and elsewhere in the system.

ACKNOWLEDGMENTS

This manuscript is dedicated to the memory of Walt Dineen. lie

recognized the need for environmental research by SFWMD in

south Florida estuaries and its importance as the foundation for

improving estuarine management. Data collection would not have

been possible without Dan Crean's effort in the field. We are also

very appreciative to the mechanics, especially Jeff Stokes, who kept the boats and trucks operational. Dan Crean and Kathy Haunert

also compiled the data and assisted in the analysis. This report also

benefitted from the input of Tony Federico, Dan Haunert, Frank

Lund, Ben McPherson, Ron Mierau, Garth Redfield, Joel Van

Arman, and the comments of three anonymous reviewers. A special

thanks is owed to Nick Aumen for providing guidance in writing

this report and reviewing the many drafts.

LITERATT.JRE CITED

American Public Health Association (APHA), American Water

Works Association, and Water Pollution Control Federation

(APHA, AWWA, WCPA), 1985. Standard Methods for the Examination of Waste Water (16th Edition). American Public Health

Association, Washington, D.C.

Biggs, R. B. and E. L. Beasley, 1988. Bottom and Suspended Sediments in the Delaware River and Estuary. In: Ecology and

Restoration of the Delaware River Basin, S. K. Majumdar, E. W.

Miller, and L. E. Sage (Editors). Pennsylvania Academy of Science, pp. 116-131.

WATER RESOURCES BULLETIN 32

Day, J. W., Jr, C. A. S. Hall, W. M. Kemp, and A. Yanez-Arancibia,

1989. Estuarine Ecology. John Wiley and Sons, 558 pp.

Day, R. W. and G. P. Quinn, 1989. Comparisons of Treatments After

an Analysis of Variance in Ecology. Ecological Monographs

59(4):433463.

Florida Department of Natural Resources (FDNR), 1984. Loxahatchee River-Lake Worth. Creek Aquatic Preserve Management Plan, 117 pp.

Friedman, M. and J. Hand, 1989. Typical Water Quality Values for

Florida's Streams, Lakes and Estuaries. Florida Department of

Environmental Regulation, Standards and Monitoring Section,

Bureau of Surface Water Management, 23 pp.

Gibbs, R. J., 1977. Suspended Sediment Transport and the Turbidity Maximum. In: Estuaries, Geophysics, and the Environment.

National Academy of Sciences, Washington, D.C., pp. 104-109.

Jordan, T. E., J. W. Pierce, and C. L. Correll, 1986. Flux of Particulate Matter in the Tidal Marshes and Subtidal Shallows of the

Rhode River Estuary. Estuaries 9:310-319.

McLusky, D. 5., 1989. The Estuarine Ecosystem. Chapman and

Hall, New York, New York, 215 pp. McPherson, B. F., and M. Sabanskas, 1980. Hydrologic and Landcover Features of the Loxahatchee River Basin, Florida. U.S.

Geological Survey Water-Resources Investigations Open File

Report 80-1109.

McPherson, B. F., M. Sabanskas, and W. A. Long, 1982. Physical,

Hydrological, and Biological Characteristics of the Loxahatchee

River Estuary, Florida. U. S. Geological Survey Water-Resources

Investigations Open-File Report 82-350. Officer, C. B., D. R. Lynch, G. H. Setlock, and G. R. Helz, 1984.

Recent Sedimentation Rates in the Chesapeake Bay. In: The

Estuary As a Filter, V. S. Kennedy (Editor). Academic Press Inc.,

Orlando, Florida, pp. 131-157.

Olsen, C. R., M. Them, I. L. Larsen, P. D. Lowry, P. J. Mulholland,

N. H. Cutshall, J. T.Byrd, and H. L. Windom, 1989. Plutonium,

Load-210, and Carbon Isotopes in the Savannah Estuary: Riverborne Versus Marine Sources. Environmental Science and Technology 23: 1475-1481.

Russell, G. M. and B. F. McPherson, 1983. Freshwater Runoff and

Salinity Distribution in the Loxahatchee River Estuary, Southeastern Florida, 1980-82. U. S. Geological Survey WaterResources Investigations Open-File Report 83-4244. Smith, J., 1991. Wind-Wave Generation On Restricted Fetches.

U.S. Corps of Engineers, Misc. Paper CERC-91-2.

Sonntag, W. H., and B. F. McPherson, 1984a. Sediment Concentrations and Loads in the Loxahatchee River Estuary, Florida,

1980-82. U. S. Geological Survey Water-Resources Investigations Report 84-4167.

Sonntag, W. H., and B. F. McPherson, 19Mb. Nutrient Input from

the Loxahatchee River Environmental Control Sewage-Treatment Plant to the Loxahatchee River Estuary, Southeastern

Florida. U.S. Geological Survey Water-Resources Investigations

Open-File Report 84-4020.

U.S. Army Corps of Engineers (USCOE), 1966. Survey Report on

Jupiter Inlet, Florida. Memorandum Report.

U. S. National Ocean Service, 1985, 1986, 1987, 1988, 1991. High

and Low Water Prediction, East Coast of North and South

America.

Vines, W. R., 1970. Surface Water, Submerged Lands, and Waterfront Lands: Area Planning Board, Palm Beach County, 182 pp.

Wanless, H., V. Rossinsky, Jr., and B. F. McPherson, 1984. Sedimentologic History of the Loxahatchee River Estuary, Florida. U. S.

Geological Survey Water-Resources Investigations Report 84-

4 120.

Wunderlin, R. P. 1986. Guide to the Vascular Plants of South Florida. University of South Florida, University Presses of Florida.