### 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 windspeed, 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 theestuary 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, suggestingthat suspended sediments act conservatively throughout this system. No one environmental factor had an overwhelming influence on suspended solid concentration throughout the estuary; differentregions of the estuary were influenced by different factors. Freshwater inflow was negatively related to TSS in the upper reaches ofthe estuary but was positively related to TSS in the central embayment region of the estuary. We attribute this latter finding to thefact 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 butwere 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 con

### centrations 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 lev

### els (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 windand tide on the distribution and concentration of sus

### pended solids throughout the estuary. Therefore, suspended solids were included in the water qualitycomponent 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 BULLETINNoel, Chamberlain, and Steinman



### ducted throughout the estuary. The primary objec

### tives of this study were to: (1) define an optimumfreshwater inflow range that will not increase signifi

### cantly 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 forfuture 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), andthe Southwest Fork (SW Fork). The three forks con

### verge 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 brack

### ish 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 diversionof historical flows away from the NW Fork, thus pro

### moting additional saltwater intrusion upstream into 

### the middle reaches of the NW Fork during the dryseason (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 locat

### ed at critical sections of the estuary. The last stationin the NW Fork was located just upstream of the normal maximum extent of salt water encroachment dur

### ing 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 hightide, samples were collected over a range of tidal ele

### vations surrounding high tide. Typically, samplingcommenced at the inlet and central embayment sta

### tions approximately one to three hours before slack high tide and was finished one hour after slack hightide 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 returnedto the lab. TSS values were determined gravimetrical

### ly (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 approximately29 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 lim

### ited 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 BULLETINNoel, 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 continuous

### ly at the structure, and daily mean discharge was based on a flow rating curve. 

### Freshwater inflow to the NW Fork was estimatedfrom continuous stage recorders located in the tribu

### taries. Data were periodically collected from thesestations, and a stage-discharge relationship was cal

### culated for each recorder. The resulting inflow for these tributaries was summed to estimate total dailyNW Fork flow. When data from a recorder were miss

### ing, 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

C) SWF NWF

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 opti### mized the ability to detect visually effects these fac

### tors 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 = notconsidered 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 differentamong 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 becauseof the inherent variability associated with each loca

### tion. Significant differences were determined at the 90 percent confidence level in order to be consistent 

### with the confidence level selected for the multiplerange test. If a factor was associated with a significant change in suspended solids, then Scheffe's multi

### ple range test was applied to determine how classes differed from one another. A class was determined toLi
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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 wasemployed using the raw, unclassified data as indepen### dent variables to determine the amount of TSS variance explained by each of the five factors.

### All statistical analyses were performed using Stat

### graphics® (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 theNW Fork at river kilometer (RK) 15. Mean VSS val

### ues ranged from 12 mg.U1 at the inlet to 2 mg.L1 in the NW Fork at RK 15. This decreasing gradient wascompared 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 =0.955and 0.915, respectively).



### 40

30

20

10

14

12

10

8

6

2



### 0

4 8 

### 0 12 16

DISTANCE FROM INLET (km)

Figure2. 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)

Figure3. 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 testseparated the 18 stations into six overlapping hetero

### geneous groups with significant decreases in mean 

## TSS occurring as the distance from the inletincreased. In addition, a second analysis was performed to determine differences among forks. Onlydata collected from stations in forks located an equiv

### alent 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, andmoderate (Class 2) flows were associated with signifi

### cantly 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 weresimilar to results described above for TSS. VSS per

### centage 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 NWFork (Figure 4a; Table 3). In the SW Fork, each con

### secutive 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 sta

### tistical 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 thanunder low flow conditions (Figure 7b). The flow class

### es 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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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 =3dates.

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 =< 0.46m, 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 =6dates 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 =< 6mIs, 2 =6-9 mIs, and 3 =>9mIs. 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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n = 7dates 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 = allothers. 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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Figure7. 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 = 2dates.

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 = 0m3s, 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** 1 

# 2 2 3 

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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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4 8 12 DISTANCE FROM INLET (km) 

### 0 4 8 12
DISTANCE FROM INLET (km) 16Environmental 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 per

### cent of the variation, and wind speed explained anadditional 3 percent. All variables combined account

### ed 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 thevariables 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 inTSS. 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 inletand 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 overthe narrow range of tidal amplitude and velocity sam

### pled. 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 BULLETINNoel, 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; Biggsand 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 basinsources in the Savannah River estuary. They attribut

### ed 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 watershedsand 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 locat

### ed 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 sus

### pended 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 Susque

### hanna River Estuary, a major source river to the Chesapeake Bay, the normal seasonal TSS averageranges from 10 to 140 mg.L-1, and the average con

### centration 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. Son

### ntag and McPherson (1984a) reported that tropicalstorm Dennis raised the percentage of total TSS contributed to the Loxahatchee estuary by the tributariesto 18 percent. The five-day post storm period account

### ed 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 7aand b show how TSS decreased sharply from the cen### tral embayment to the SW Fork and NW Fork stations. As freshwater flows increase to the NW Forkand SW Fork, the freshwater that is low in TSS pro

### portionately mixes with the tidal water and bothdilute 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 examinedhigher than average tides, which correlate with higher volume. A separate multifactor ANOVA was run to

### examine potential interactions among the environ

### mental 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-inducedresuspension may result in increased TSS as the shallower bottom sediments are more easily disturbed. 

### Our analyses revealed no significant difference inTSS attributed to wind speed and direction in the cen### tral embayment. This is contrary to what was expect

### ed. Sonntag and McPherson (1984a) observedelevated TSS at the inlet due to resuspension of sedi

### ments in the north arm of the Intracoastal Waterway 

### when wind was from the north. They suspected that 

### some portion of this resuspended material wastransported into the central embayment during flood### ing 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 mate

### rial at most stations in the central embayment, lower 

### SW Fork, and lower NW Fork. The amount ofincreased suspended sediment due to wind is evident

### ly overshadowed in the central embayment by the 

### influences of tide velocity and freshwater inflow.Because all the sampling days (except for two) experi### enced wind speeds greater than 5 ms-1, wind proba

### bly resuspends material almost daily. This constant 

### resuspended state at the time of sampling makes itdifficult to distinguish subtle changes in TSS concen

### trations 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 com

### pares 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 sedimentdynamics 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 classwas 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 BULLETINNoel, 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, andtide height normally only slightly affect TSS concen

### trations 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 elevat### ed suspended solids in the upper NW Fork and cen

### tral 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 embaymentand 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.

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