In cooperation with the U.S. Air Force

Polychlorinated Biphenyls in Aquatic Invertebrates 

and Fish and Observations about Nitrogen and Carbon 

Isotope Composition in Relation to Trophic Structure and 

Bioaccumulation Patterns, Lake Worth and Meandering 

Road Creek, Fort Worth, Texas, 2007–08

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2010–5235

LAKE WORTH

Meandering Road Creek

Woods Inlet

West

Fo rk Trinity River
Front cover: Freshwater food chain showing examples of the types of organisms collected in Lake Worth 

or Meandering Road Creek, Fort Worth, Texas, 2007–08 (drawings by L.S. Coplin, U.S. Geological Survey). 
Polychlorinated Biphenyls in Aquatic 

Invertebrates and Fish and Observations 

about Nitrogen and Carbon Isotope 

Composition in Relation to Trophic 

Structure and Bioaccumulation Patterns, 

Lake Worth and Meandering Road Creek, 

Fort Worth, Texas, 2007–08 

By J. Bruce Moring

In cooperation with the U.S. Air Force

Scientific Investigations Report 2010–5235

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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Suggested citation:

Moring, J.B., 2010, Polychlorinated biphenyls in aquatic invertebrates and fish and observations about nitrogen and 

carbon isotope composition in relation to trophic structure and bioaccumulation patterns, Lake Worth and Meandering 

Road Creek, Fort Worth, Texas, 2007–08: U.S. Geological Survey Scientific Investigations Report 2010–5235, 29 p.
iii

Contents

Abstract ..........................................................................................................................................................1

Introduction ....................................................................................................................................................1

Purpose and Scope .............................................................................................................................4

Description of Study Area ..................................................................................................................4

Previous Studies ..................................................................................................................................4

Methods of Investigation .............................................................................................................................5

Collection and Processing of Samples ............................................................................................6

Collection of Aquatic Invertebrate Samples ..........................................................................6

Collection of Fish Samples ........................................................................................................7

Processing of Aquatic Invertebrate and Fish Samples ........................................................7

Analysis of Polychlorinated Biphenyl Congeners ..........................................................................7

Analysis of Stable Nitrogen and Carbon Isotopes .........................................................................8

Quality Control ......................................................................................................................................8

Polychlorinated Biphenyls in Aquatic Invertebrates and Fish ............................................................10

Observations about Nitrogen and Carbon Isotope Composition in Relation to Trophic 

Structure and Bioaccumulation Patterns .................................................................................12

Trophic Levels and Stable Isotope Composition ...........................................................................12

Horizontal Food Web Position and Stable Isotope Composition ................................................12

Enrichment of Nitrogen-15 and Trophic Structure of Aquatic Invertebrates and Fish ..........13

Nitrogen Isotope Enrichment and Polychlorinated Biphenyl Concentrations in Aquatic 

Invertebrates and Fish .........................................................................................................15

Implications for Ecological Processes and Polychlorinated Biphenyl Concentrations in 

Aquatic Invertebrates and Fish ...................................................................................................18

Summary .......................................................................................................................................................18

References ...................................................................................................................................................20

Appendixes 

1. Relative percent difference for duplicate polychlorinated biphenyl congener and 

isotope data for aquatic invertebrate and fish samples collected in Lake Worth, 

Woods Inlet, and Meandering Road Creek, Fort Worth, Texas, 2007–08 .........................26

2. Laboratory sample blank and spike percent recovery for polychlorinated biphenyl 

congeners and surrogate compounds for two sets analyzed concurrently with 

aquatic invertebrate and fish samples collected in Lake Worth, Woods Inlet, and 

Meandering Road Creek, Fort Worth, Texas, 2007–08 .........................................................27

3. Concentrations of selected polychlorinated biphenyl congeners and number of 

chlorinated substitutes for aquatic invertebrate and fish composite samples 

collected in Lake Worth, Woods Inlet, and Meandering Road Creek, Fort Worth, 

Texas, 2007–08 ............................................................................................................................28

Figures

1. Map showing location of study area and approximate sampling locations for 

aquatic invertebrates and fish in Lake Worth, Woods Inlet, and Meandering Road 

Creek, Fort Worth, Texas, 2007–08 ............................................................................................2
iv

2–6. Graphs showing: 

2. Relation between sum of 15 polychlorinated biphenyl congeners (∑PCBC15) 

in surficial (top 5 centimeters) bed-sediment samples (Braun and others, 

2008) collected in Lake Worth, Woods Inlet, and Meandering Road Creek, 

2006, and ∑PCBC15 in aquatic invertebrate and fish samples collected in 

approximately the same locations, 2007–08, Fort Worth, Texas ...............................11

3. Relation between delta nitrogen-15 (δ15N) and delta carbon-13 (δ13C) in 

aquatic invertebrate and fish composite samples collected in Lake Worth, 

Woods Inlet, and Meandering Road Creek, Fort Worth Texas, 2007–08 .................13

4. Sum of 15 polychlorinated biphenyl congeners (∑PCBC15) compared with 

amount of nitrogen enrichment (represented by an increase in ratio of 

nitrogen-15 to nitrogen-14 [δ15N]) in aquatic invertebrate and fish composite 

samples collected at reference site Lake Worth near State Highway 199 

Bridge (004LW), Fort Worth, Texas, 2007–08 ................................................................15

5. Sum of 15 polychlorinated biphenyl congeners (∑PCBC15) compared with 

amount of nitrogen enrichment (represented by an increase in ratio of 

nitrogen-15 to nitrogen-14 [δ15N]) in aquatic invertebrate and fish composite 

samples collected at Lake Worth in upper and lower Woods Inlet (sites 002WI 

and 003WI), Fort Worth, Texas, 2007–08 .......................................................................16

6. Sum of 15 polychlorinated biphenyl congeners (∑PCBC15) compared with 

amount of nitrogen enrichment (represented by an increase in ratio of 

nitrogen-15 to nitrogen-14 [δ15N]) in aquatic invertebrate and fish composite 

samples collected at Meandering Road Creek (site 001MC), Fort Worth, 

Texas, 2007–08 ...................................................................................................................17

Tables

1. Description of sampling sites and sample size of aquatic invertebrate and fish taxa 

collected in Lake Worth, Woods Inlet, and Meandering Road Creek, Fort Worth, 

Texas, 2007–08 ..............................................................................................................................5

2. Sum of 15 polychlorinated biphenyl congeners in aquatic invertebrate and fish 

composite samples collected in Lake Worth, Woods Inlet, and Meandering Road 

Creek, Fort Worth, Texas, 2007–08 ..........................................................................................10

3. Nitrogen and carbon isotope enrichment in and assigned trophic level for 

aquatic invertebrate and fish composite samples collected in Lake Worth, Woods 

Inlet, and Meandering Road Creek, Fort Worth, Texas, 2007–08 .......................................14
v

Conversion Factors and Explanation of Isotope Units

SI to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.) 

millimeter (mm) 0.03937 inch (in.) 

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square kilometer (km2

) 0.3861 square mile (mi2)

Volume

cubic meter (m3

) 35.31 cubic foot (ft3)

Mass

gram (g) 0.03527 ounce avoirdupois (oz) 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Concentrations of polychlorinated biphenyls are given in micrograms per kilogram (μg/kg). 

Explanation of Isotope Units

Per mil: A unit expressing the ratio of stable-isotope abundances of an element in a sample to 

those of a standard material. Per mil units are equivalent to parts per thousand. Stable-isotope 

ratios, also known as delta values, are computed as follows: 

δXsample = {(Rsample - Rstandard)/Rstandard} x 1,000,

where

δ = delta,

X = heavier stable isotope (for example, carbon-13), and

R = ratio of rare (heavier) isotope to common (lighter) isotope in sample or standard.

The δ values for stable-isotope ratios discussed in this report are referenced to the following 

standard materials:

Element R Standard 

Nitrogen Nitrogen-15/nitrogen-14 Nitrogen in air

Carbon Carbon-13/carbon-12 Vienna Pee Dee Belemnite (Hut, 1987)
Blank Page
Abstract 

During 2007–08 the U.S. Geological Survey, in cooperation with the U.S. Air Force, evaluated the concentration of 

polychlorinated biphenyls (PCBs) in aquatic invertebrates and 

fish from one site in the main body of Lake Worth, two sites 

in a small inlet in Lake Worth (upper and lower Woods Inlet), 

and one site in Meandering Road Creek in Fort Worth, Texas. 

The four sites sampled during 2007–08 were located at or near 

sites where surficial bed-sediment samples had been collected and analyzed for PCBs during previous U.S. Geological 

Survey studies so that PCB concentrations in aquatic invertebrates and fish and PCB concentrations in surficial bed-sediment samples could be compared. Stable nitrogen and carbon 

isotopes were used to help assess differences in the amount 

of these isotopes by species and sampling location. The sum 

of 15 PCB-congener concentrations was highest for aquatic 

invertebrates and fish from the upper Woods Inlet site and lowest for the same aquatic invertebrates and fish from the Lake 

Worth site, where PCBs historically had not been detected 

in lake bed sediment. An increase in the ratio of the heavier 

nitrogen-15 (15N) isotope to the lighter nitrogen-14 (14N) isotope, referred to as enrichment of 15N, was highest in largemouth bass (representing the highest trophic level sampled) at 

all sites and lowest for true midge larvae inhabiting surficial 

bed sediment in the lake (representing the lowest trophic level 

sampled). Enrichment of 15N was less variable in largemouth 

bass and other fish from the highest trophic level compared 

with shorter lived, primary consumer invertebrates from lower 

trophic levels, such as true midge larvae, mayfly nymphs, and 

zooplankton. The delta carbon-13 (δ13C) values measured in 

true midge larvae collected at the Lake Worth and upper and 

lower Woods Inlet sites were more negative compared with the 

δ13C values measured for all other taxa, indicating true midge 

larvae were more depleted of carbon-13 (13C) compared with 

all other aquatic invertebrate and fish. The relative depletion 

of 13C might indicate the carbon sources consumed by true 

midge larvae are different from the carbon sources consumed 

by all other taxon that were sampled. Ratios of stable nitrogen isotopes nitrogen-15 to nitrogen-14 (δ15N) were similar 

between taxa from the Lake Worth site and Woods Inlet sites. 

The sum of 15 PCB-congener concentrations, however, was an 

order of magnitude higher in largemouth bass from the upper 

Woods Inlet site, indicating that PCB-congener concentrations 

in lake bed sediment likely controls biomagnification within 

the lake because of the similarities in trophic structure of the 

resident aquatic community. The biota at the Lake Worth 

reference site, where PCBs were not detected in the surficial 

sediment during previous studies, were less contaminated than 

the biota at sites where PCBs had been detected in the surficial 

sediment. The highest trophic-level consumers (as evidenced 

by the most 15N-enriched δ15N values) showed the maximum 

bioaccumulation.

Introduction

The U.S. Geological Survey (USGS), in cooperation with 

the U.S. Air Force, investigated the concentrations of polychlorinated biphenyls (PCBs) in biota (aquatic invertebrates 

and fish) in selected areas of Lake Worth in Fort Worth, Tex. 

(including Woods Inlet, an arm of Lake Worth that extends 

south from the main body of the lake) and in Meandering 

Road Creek, a tributary to Lake Worth (fig. 1). PCB concentrations in biota samples were compared with PCB concentrations measured in surficial bed-sediment samples collected 

from approximately the same locations in previous USGS 

studies (Harwell and others, 2003; Besse and others, 2005; 

Braun and others, 2008). Aquatic invertebrate and fish samples 

were collected from four sites (site 004LW in the main body 

of Lake Worth, sites 002WI and 003WI in Woods Inlet, and 

site 001MC in Meandering Road Creek) and analyzed for 

selected PCBs. The sampling sites were selected to represent 

the range in PCB concentrations measured in surficial bed 

Polychlorinated Biphenyls in Aquatic Invertebrates 

and Fish and Observations about Nitrogen and Carbon 

Isotope Composition in Relation to Trophic Structure and 

Bioaccumulation Patterns, Lake Worth and Meandering 

Road Creek, Fort Worth, Texas, 2007–08

By J. Bruce Moring
2 Polychlorinated Biphenyls in Aquatic Invertebrates and Fish, Lake Worth and Meandering Road Creek, Fort Worth, TexasFigure 1. Location of study area and approximate sampling locations for aquatic invertebrates and fish in Lake Worth, Woods Inlet, and Meandering Road Creek, Fort Worth, 

Texas, 2007–08. 

EXPLANATION

Site—Site identifier(LW, Lake Worth; WI,Woods Inlet; MC,Meandering Road Creek).See table 1Biota collection sitecollocated with boxcore site. Number in triangle indicatesaverage concentrationof sum of 15 polychlorinated biphenylcongeners, in microgramsper kilogram, in sedimentthat were also analyzed forin biota

10

Fort Worth

Study area

TEXAS

LOCATION MAP

0

0 0.25 0.5 MILE

0.5 KILOMETER

Carswell

Field

Air Force

Plant 4

Dam

Naval Air

Station

Carswell

Field

Air Force

Plant 4

Dam

Naval Air

Station

Interstate Highway 820

State Highway 199

WoodsInlet

MeanderingRoad Creek West Fork

Trinity River 

Lake Worth

WoodsInlet

MeanderingRoad Creek West Fork

Trinity River 

Lake Worth

97°29' 97°28' 97°27' 97°26' 97°25'

32°50'32°49'32°48'32°47'32°46'

004LW

003WI

002WI

001MC

0

10

3318

004LW

003WI

002WI

001MC
Introduction 3

sediment in Lake Worth, Woods Inlet, and Meandering Road 

Creek in previous USGS studies.

PCBs are a class of chlorinated organic compounds, 

and there are 209 distinct PCB compounds called congeners, 

each with from 1 to 10 chlorine atoms attached to two benzene rings. From the 1920s until the late 1970s, about 130 

PCB congeners were widely used in commercial PCB mixtures called Aroclors (Erickson, 1997). In August 2005, the 

Texas Commission on Environmental Quality (2005) issued a 

numeric target of 40 micrograms per kilogram (μg/kg) of total 

PCBs or less in edible fish tissues from Lake Worth. A reduction of PCB concentration in fish tissue to 40 μg/kg or less 

could ultimately allow the Texas Department of State Health 

Services to discontinue the existing fish consumption advisory 

for Lake Worth (Texas Department of State Health Services, 

2000). 

Total PCB concentrations in surficial bed-sediment 

samples collected from Woods Inlet in Lake Worth and near 

the Lake Worth Dam (fig. 1) have declined exponentially since 

1960, and PCB concentrations in 2000 were about one-third 

of historical peak concentrations (Harwell and others, 2003). 

Despite those declines, mean total PCB concentrations in 

edible fish tissue from Lake Worth have remained about 6 

times (Moring, 2002) to 10 times (Giggleman and Lewis, 

2004) greater than the Texas Commission on Environmental 

Quality acceptable-risk target concentration in edible fish tissue of 40 μg/kg. 

The processes of bioaccumulation and biomagnification 

are the likely reasons PCB concentrations remain elevated 

in edible fish tissue despite declining sharply in the surficial 

bed sediment of Lake Worth. Bioaccumulation involves the 

accumulation of chemicals, such as PCBs, in the tissues of 

organisms as these chemicals are ingested. Biomagnification

involves progressive increases in the concentrations of 

chemicals in the tissues of organisms as these chemicals 

are transferred up through two or more trophic levels in an 

ecosystem (Rand and Petrocelli, 1985). Trophic level refers 

to the position an organism or group of organisms occupy 

in a food web; trophic structure refers to the partitioning of 

biomass between groups of organisms occupying the same 

position in a food web (Preisser, 2008). As a result of biomagnification, lower concentrations of contaminants are 

generally found in organisms from lower trophic levels (in 

the same food web) compared with organisms from higher 

trophic levels. Each trophic level becomes more contaminated than the next lower trophic level, and the increase in 

contaminants up two or more trophic levels is often geometric or exponential in magnitude, particularly for compounds 

with limited water solubility (Dietz and others, 2000). 

Biomagnification also results in higher concentrations in 

organisms at higher trophic levels than would be expected if 

water were the only exposure medium (Gobas and Morrison, 

2000). Fat-soluble (lipophilic) contaminants, such as PCBs, 

are known to biomagnify in aquatic food chains (Burreau and 

others, 1997; Russell and others, 1999; Rognerud and others, 

2002; Borga and others, 2004). PCBs are highly persistent 

contaminants for which food [web] contributions rather 

than waterborne exposures are considered the most relevant 

exposure route for aquatic organisms (Carbonell and others, 

2000). 

The specialization of organisms in biological communities as food producers and consumers allows energy to flow 

and nutrients to cycle through the trophic structure (Ricklefs, 

1979). The trophic structure of an aquatic community is 

defined by organisms representing different ecosystem functions and different trophic levels interacting in a complex food 

web; groups of organisms occupy trophic levels defined by the 

source of energy and nutrients for those organisms (Levine, 

1980). In aquatic ecosystems, phytoplankton (suspended 

algae) are among the organisms at the lowest trophic level, 

converting energy from the sun into useable energy for other 

organisms; for this reason, phytoplankton are referred to as 

primary producers. Other primary producers are macrophytes 

(vascular aquatic plants) and periphytic (benthic) algae (algae 

growing on a submerged surface) (Kohzu and others, 2009). 

Invertebrates and fish are referred to as consumers, but they 

can be separated into additional trophic levels in freshwater systems, including herbivores (plant-eating organisms), 

referred to as primary consumers; invertivores (invertebrateeating organisms), referred to as secondary consumers; and 

piscivores (fish-eating organisms), referred to as tertiary consumers. Among herbivores are primary consumers occupying 

the lowest trophic level of all invertebrates; primary consumers 

serve as primary food items for other invertebrates and fish 

(Broman and others, 1992; Auster and others, 2005). In lakes, 

the range of food for primary consumers includes suspended 

particulate organic matter, benthic particulate organic matter, 

phytoplankton (suspended algae), vascular aquatic plants, and 

benthic algae.

Trophic levels have traditionally been expressed in 

discrete terms representing producers, primary consumers, 

secondary consumers, and tertiary consumers (Ricklefs, 1979), 

symbolized by whole numbers (typically from 1 to 4, with 

1 representing primary producers, 2 representing primary 

consumers, 3 representing secondary consumers, and 4 representing tertiary consumers). In more complex ecosystems, 

additional trophic levels symbolized by whole numbers from 

5 to 8 are possible (Broman and others, 1992; Vander Zanden 

and Fetzer, 2007). Most organisms acquire energy and nutrients from more than one trophic level and thus likely represent 

an intermediate trophic level that can be symbolized by a 

nonwhole number, referred to as a trophic position. 

Metabolic processes of animals enrich the heavy isotopes 

of nitrogen (nitrogen-15 [15N] and carbon-13 [13C]) relative 

to the lighter isotopes (nitrogen-14 [14N] and carbon-12 [12C]) 

(Broman and others, 1992). Because biomagnification of a 

contaminant can be related to trophic level or more precisely, 

trophic position, nitrogen isotope composition provides a 

means to understand the potential for contaminant biomagnification. Patterns of biomagnification of PCBs in aquatic invertebrates and fish can be identified from the relation between 

the PCB concentrations in an organism and the assigned 
4 Polychlorinated Biphenyls in Aquatic Invertebrates and Fish, Lake Worth and Meandering Road Creek, Fort Worth, Texas

trophic levels of one or more groups of organisms. When data 

are sufficient, trophic position calculated from the nitrogen 

isotope composition of an organism (Peterson and Fry, 1987) 

provides a precise measure of the organism’s position in the 

trophic structure. Because carbon isotope composition is 

preserved through multiple trophic levels and positions of 

the food web, it can be used to indicate whether the principal 

source for biomagnification in a freshwater ecosystem is benthic (associated with sediment) or pelagic (associated with the 

overlying water column). 

Purpose and Scope 

This report describes the concentration of the sum of 

15 PCB congeners (∑PCBC15) in aquatic invertebrates and 

fish collected during 2007–08 from Lake Worth near Fort 

Worth (including Woods Inlet) and from Meandering Road 

Creek, a tributary to Lake Worth, and the use of stable isotopes 

of nitrogen and carbon (delta nitrogen-15 [δ15N] and delta 

carbon-13 [δ13C]) in aquatic invertebrates and fish to make 

observations about trophic structure and patterns of bioaccumulation. Aquatic invertebrates and fish were collected at four 

sites where surficial bed-sediment samples had been collected 

and analyzed for PCBs in previous USGS studies (one site in 

the main body of Lake Worth, two in Woods Inlet, and one in 

Meandering Road Creek).

PCB concentrations (∑PCBC15) measured in composite 

samples of aquatic invertebrates and fish were compared with 

PCB concentrations measured in surficial bed-sediment samples collected from approximately the same locations as during previous USGS studies. Trophic levels and δ15N and δ13C 

values were compared with PCB concentrations measured in 

composite samples of aquatic invertebrates and fish to make 

observations on how PCB concentrations varied depending on 

an organism’s position in the food web. The Spearman rank 

correlation was used to evaluate the strength of the relation for 

the following pairs of variables: (1) ∑PCBC15 in surficial bed 

sediment and ∑PCBC15 in aquatic invertebrates and fish taxa 

(taxon [singular] or taxa [plural] refers to a genetically related 

group or groups of one or more organisms with common 

characteristics that differentiate them as a unit) (International 

Commission on Zoological Nomenclature, 2010) from sampling locations in Lake Worth, Woods Inlet, and Meandering 

Road Creek; and (2) δ15N and ∑PCBC15 in composite samples 

of aquatic invertebrate and fish taxa collected from sampling 

locations in Lake Worth, Woods Inlet, and Meandering Road 

Creek. 

Because PCBs are mostly found in sediment, only contributions of PCBs derived from surficial bed-sediment food 

sources were considered; any waterborne contributions to 

PCB concentrations in aquatic invertebrates and fish were not 

evaluated. Effects of differences in age, rates of growth, and 

diet on the PCB concentrations and other similar contaminants 

in aquatic invertebrates and fish were beyond the scope of the 

study. 

Description of Study Area 

Lake Worth is a reservoir on the West Fork Trinity River 

on the western outskirts of Fort Worth, Tex. (fig. 1). In 1914, 

the City of Fort Worth completed the reservoir to serve as a 

municipal water supply; the reservoir also is used for fishing 

and recreation (City of Fort Worth, Texas, 2010). Lake Worth 

has a storage capacity of 47 million cubic meters, a surface 

area of 13.2 square kilometers, and a contributing watershed 

that drains 5,320 square kilometers (Ruddy and Hitt, 1990). 

The area adjacent to the lake to the south and east is mostly 

urban, to the north and northwest mostly residential, and to the 

west mostly rangeland. Lake Worth is in the Cross Timbers 

Level III ecoregion (Omernick, 1987) and is a warm-water 

fishery dominated by fish such as catfish, crappie, and largemouth bass.

Air Force Plant 4 (AFP4) is on the south shore of Lake 

Worth and adjacent to Woods Inlet and Meandering Road 

Creek (fig. 1). Since 1942, aircraft manufacturing operations 

and processes at AFP4 have generated waste materials (fuels, 

oils, solvents, paint residues, and process chemicals) that were 

disposed of on-site (Texas Department of Health, 1998). Two 

PCB leaks or spills, one in 1984 and the other in 1989, have 

been documented on the western side of AFP4 adjacent to 

Meandering Road Creek (Earth Tech Inc., 2005). 

Previous Studies

PCB-contaminated runoff has historically drained from 

AFP4 to Meandering Road Creek, which flows into the Woods 

Inlet of Lake Worth (Harwell and others, 2003; Besse and 

others, 2005; Braun and others, 2008; Braun and Wilson, 

2010). In April 2000, the Texas Department of Health issued a 

fish consumption advisory for Lake Worth because of elevated 

concentrations of PCBs in the edible tissues of selected species of fish (Texas Department of State Health Services, 2000). 

Potential sources of PCBs to Lake Worth (main body and 

Woods Inlet) and Meandering Road Creek include historical 

landfills or historically contaminated soils on AFP4 (Besse 

and others, 2005). Harwell and others (2003) analyzed surficial bed sediment collected from sites throughout much of 

Lake Worth for PCBs. Total PCB concentrations (∑PCBC15) 

in surficial bed sediment ranged from less than the laboratory 

minimum reporting level of 60 μg/kg, the level used for the 

analysis of samples collected in the main body of Lake Worth 

(Harwell and others, 2003, p. 56–66), to 450 μg/kg in Woods 

Inlet and 170 μg/kg in Meandering Road Creek (Braun and 

others, 2008, p. 33–38). The highest PCB concentrations in 

surficial bed sediment were measured at sites in Woods Inlet, 

whereas no detections of PCBs were reported at sites in the 

main body of the lake north of the Interstate Highway 820 

bridge. Besse and others (2005) reported that the highest PCB 

concentrations in surficial bed sediment in Woods Inlet were 

near the mouth of Meandering Road Creek. Braun and others 

(2008) reported detections of PCBs in surficial bed sediment 
Methods of Investigation 5

of Meandering Road Creek and, consistent with Besse and 

others (2005), the highest PCB concentrations were detected 

in Woods Inlet near the mouth of Meandering Road Creek. 

On the basis of PCB concentrations measured in suspended 

sediment at sites on AFP4 near Meandering Road Creek and 

in surficial bed-sediment samples collected in the creek and 

Woods Inlet, Braun and others (2008) concluded that PCBs 

probably entered Woods Inlet primarily from Meandering 

Road Creek, and that runoff from AFP4 is a prominent source 

of PCBs in Meandering Road Creek and Woods Inlet. Braun 

and Wilson (2010) investigated PCBs in suspended-sediment 

samples in storm runoff from outfalls 4 and 5, which drain 

into Meandering Road Creek at AFP4, and found evidence 

that PCB concentrations in storm runoff from AFP4 have 

decreased since engineering controls were implemented during 

2007–08. 

Methods of Investigation

Aquatic invertebrates and fish were collected during 

2007–08 at the following four sites: 004LW Lake Worth near 

State Highway 199 bridge (hereinafter, Lake Worth); 003WI 

Lake Worth in lower Woods Inlet (hereinafter, lower Woods 

Inlet); 002WI Lake Worth in upper Woods Inlet (hereinafter, 

upper Woods Inlet); and 001MC Meandering Road Creek near 

confluence with Lake Worth (hereinafter, Meandering Road 

Creek) (fig. 1; table 1). These sites are at approximately the 

same locations where surficial bed-sediment samples were 

collected and analyzed for PCBs in previous USGS studies 

(Harwell and others, 2003; Besse and others, 2005; Braun 

and others, 2008). PCB concentrations in surficial sediment 

are often positively correlated with the level of exposure of 

many aquatic invertebrates and fish to PCBs (Fowler and 

others, 1978; Larsson, 1984; Helm and others, 2008). The 

four sampling sites for aquatic invertebrates and fish were 

selected to provide a variety of relatively deep water (more 

than 2 meters) and near-shore habitat. A composite sample of 

each taxon from each site was analyzed for 15N, 14N, 13C, 12C, 

and 15 PCB congeners. Composite samples were used instead 

of individual organisms because it was cost prohibitive to collect and analyze individual organisms of multiple taxa across 

the four sites for this study. The upper and lower Woods Inlet 

sampling sites were selected because these sites had some of 

the highest PCB concentrations measured in surficial-sediment 

samples during previous USGS studies. The Lake Worth 

sampling site was selected as a reference site because PCBs 

had not been detected in surficial-sediment samples collected 

at this site during previous USGS studies. The Meandering 

Road Creek sampling site for invertebrates and fish is a reach 

about 100 meters long immediately west of AFP4, about 30 

meters upstream from Woods Inlet, and at or near three previously sampled sediment sites (Braun and others 2008). The 

Meandering Road Creek site for biota sampling was selected 

because PCBs were detected at concentrations exceeding 

the laboratory minimum reporting level in surficial-sediment 

samples collected from the same reach of the creek during 

previous USGS studies, and because aquatic invertebrates and 

fish in the creek had not been sampled historically and would 

likely provide different taxa than the lake sampling sites. Site 

location information and sample size of aquatic invertebrate 

and fish taxa collected at the four sites during 2007–08 are 

listed in table 1. All data from the discrete environmental 

Table 1. Description of sampling sites and sample size of aquatic invertebrate and fish taxa collected in Lake Worth, Woods Inlet, and 

Meandering Road Creek, Fort Worth, Texas, 2007–08. 

[(2, 3, or 4), trophic level; LW, Lake Worth; >, greater than; ---, no data; WI, Woods Inlet; MC, Meandering Road Creek]

Site description

Site 

identifier 

(fig. 1)

Latitude 

(degrees 

minutes 

seconds)

Longitude 

(degrees 

minutes 

seconds)

Number collected per 

invertebrate taxon 

(trophic level), 

August 2007

Number collected per fish taxon 

(trophic level), 

September 2007

True 

midge

larvae

(2)

Zooplankton1

(2 or 3)

Mayfly

nymphs

(2)

Dragonfly

nymphs

(3)

Largemouth 

bass

(4)

Green 

sunfish 

(3)

Longear

sunfish

(3)

Western 

mosquitofish

(3)

Gizzard 

shad 

(3)

Lake Worth near State 

Highway 199 Bridge 

004LW 32°49'17.2" 97°26'43.5" 23 >500 --- --- 11 --- 16 --- 15

Lake Worth in lower 

Woods Inlet

003WI 32°46'59.8" 97°27'17.1" 90 >500 --- --- 10 --- 15 --- ---

Lake Worth in upper 

Woods Inlet

002WI 32°46'41.5" 97°27'15.7" 45 >500 --- --- 10 --- 10 --- 10

Meandering Road Creek 

near confluence with 

Lake Worth

001MC 32°46'36.6" 92°27'13.3" --- --- 10 11 4 23 10 12 ---

1

 Water fleas and copepods. 
6 Polychlorinated Biphenyls in Aquatic Invertebrates and Fish, Lake Worth and Meandering Road Creek, Fort Worth, Texas

samples, including results for quality control, are published in 

the USGS National Water Information System (NWIS) (U.S. 

Geological Survey, 2010). Consistent with all previous USGS 

studies done at Lake Worth and AFP4 (Harwell and others, 

2003; Besse and others, 2005; Braun and others, 2008; Braun 

and Wilson, 2010), estimated PCB congener concentrations 

were used as reported, and values less than the method detection limit were treated as zeros.

All statistical analyses and graphics were done with 

Statistica Version 7.0. The Spearman rank correlation (rho) 

was used to measure the strength of the monotonic association 

between various pairs of variables (Lehmann and D’Abrera, 

1998; StatSoft, 2006). Spearman rank correlation is a nonparametric statistical analysis that uses a rank correlation procedure to evaluate the relation between two variables even if that 

relation is not necessarily linear (Ott, 1993). 

Collection and Processing of Samples

Aquatic invertebrate and fish samples were collected 

in order of lowest to highest expected PCB concentrations, 

based on PCB concentrations measured in surficial sediment 

in previous USGS studies by Harwell and others (2003), Besse 

and others (2005) and Braun and others (2008), to minimize 

cross-site contamination. The first site sampled was Lake 

Worth, the site with the lowest PCB concentrations in surficial 

sediment. The last site sampled was upper Woods Inlet, the 

site with the highest concentrations in surficial sediment. All 

sampling equipment was detergent washed, solvent rinsed 

with methanol, and rinsed with deionized water between sites 

(Moring, 2002). All sampling equipment and sample containers were triple rinsed with ambient water at each site before 

sample collection. All aquatic invertebrates and fish, except 

for zooplankton, were collected in late August and early 

September 2007. Zooplankton samples were collected in April 

2008. Biota samples were composited, freeze-dried, ground, 

and analyzed for the following: 15 PCB congeners, the ratio 

of 15N to 14N (expressed as δ15N), and the ratio of 13C to 12C 

(expressed as δ13C). 

Collection of Aquatic Invertebrate Samples

The different taxa of aquatic invertebrates targeted for 

collection at the Lake Worth, upper and lower Woods Inlet, 

and Meandering Road Creek sites were selected because 

they are primary or secondary consumers that serve as the 

most likely trophic pathway (contribution by food ingestion) 

for the biomagnification of PCBs from surficial sediment 

to secondary and tertiary consumers (Joaquim-Justo and 

others, 1995). Aquatic invertebrates targeted for collection 

at the Lake Worth and upper and lower Woods Inlet sites 

included true midge larvae (table 1) that inhabit surficial 

lake sediment during the larval stage and pelagic zooplankton 

that inhabit the water column over their entire life cycle. The 

true midge larvae (hereinafter midge larvae) collected at the 

Lake Worth and upper and lower Woods Inlet sites were of 

the subfamily Chironominae. Members of this taxon are collectors and filter feeders (Merritt and Cummins, 1996) that 

ingest surficial and suspended sediment. The invertebrates 

collected at the Meandering Road Creek site were mayfly 

nymphs and dragonfly nymphs (table 1). Mayflies collected 

at the Meandering Road Creek site are in the family Caenidae 

and are collector-gatherers or scrapers feeding on periphyton and organic detritus on the surface of rocks and other 

substrates. Dragonfly nymphs of the suborder Anisoptera 

are predators and feed on other invertebrates (Resh and 

Rosenberg, 1984). 

Midge larvae were collected from surficial sediment at 

the Lake Worth and upper and lower Woods Inlet sites with a 

15- by 15- by 23-centimeter stainless-steel box corer by following methods similar to Hogan (2002). The weighted box 

corer was lowered from a boat into the surficial sediment of 

the lake bottom and retrieved. The contents of the box corer 

were placed in a 500-micrometer mesh stainless-steel sieve 

tray. The sediment was rinsed in the sieve tray with ambient 

water; and the midge larvae were collected, placed in a labeled 

polypropylene container, and frozen on-site with dry ice. Box 

coring was repeated within a 300-meter radius of each site 

until a minimum of 3 grams wet weight of midge larvae was 

measured at each of the sites.

Zooplankton were collected at the Lake Worth and upper 

and lower Woods Inlet sites with a 200-micrometer mesh 

plankton net by following methods similar to those described 

in Britton and Greeson (1989). Three horizontal plankton tows 

were taken within a 300-meter radius of each site. Each tow 

involved lowering the plankton net to a depth of 1–3 meters 

and towing the net for approximately 100 meters with a boat. 

At the end of each tow, the contents of the net were placed in a 

labeled polypropylene container. Each composite sample was 

weighed to the nearest gram, entrained organic and inorganic 

debris removed, the sample homogenized by stirring, and a 

5-gram zooplankton subsample retained and frozen on-site 

with dry ice. Frozen zooplankton samples were returned 

to the USGS Texas Water Science Center in Austin, Tex., 

thawed, identified and counted. Zooplankton in each sample 

included members of the orders Cladocera (water fleas) and 

Copepoda (copepods) (table 1). Water fleas range from 0.2 to 

3.0 millimeters in length and are filter feeders that consume 

algae, bacteria, and organic detritus (Pennak, 1989). Copepods 

range from 0.3 to 3.2 millimeters in length; those collected 

in this study included filter feeders (calanoid copepods) that 

feed on other plankton, and predators (cyclopoid copepods) 

that feed by biting and seizing other organisms (Pennak, 

1989). Copepods were the dominant zooplankton taxon in all 

samples. 

All nymphs from the Meandering Road Creek site were 

collected with a 500-micrometer mesh, D-frame dip net 

(Moulton and others, 2002) in coarse bed material that ranged 

from large gravel to small cobble, and the nymphs were 

placed in polypropylene containers. Sampling was continued 

until a minimum of about 2 grams wet weight of mayfly and 
Methods of Investigation 7

dragonfly nymphs were collected, and the samples were frozen on-site with dry ice. 

Collection of Fish Samples

The taxa of fish targeted for collection at the four sites 

were chosen to represent the types of predator and prey trophic 

interactions expected to contribute to the biomagnification 

of PCBs. Fish targeted for collection at the Lake Worth and 

upper and lower Woods Inlet sites included the following: 

Micropterus salmoides (largemouth bass), a tertiary consumer and piscivore; Lepomis megalotis (longear sunfish), 

a secondary consumer, invertivore, and potential prey for 

largemouth bass; and Dorosoma cepedianum (gizzard shad), 

a secondary consumer, planktivore, and potential prey for 

largemouth bass (table 1). No gizzard shad were collected at 

the lower Woods Inlet site. Fish species targeted for collection 

at the Meandering Road Creek site included largemouth bass 

and longear sunfish; Lepomis cyanellus (green sunfish) and 

Gambusia affinis (western mosquitofish) were also collected at 

the Meandering Road Creek site to better represent the different types of fish and thus better represent the trophic structure 

in Meandering Road Creek (table 1). Fish were collected by 

boat electrofishing near shore at the Lake Worth and upper and 

lower Woods Inlet sites and with a backpack electrofishing 

unit at the Meandering Road Creek site. Electrofishing, in this 

study, involved the use of direct current (DC) applied to the 

water with anode and cathode arrays (Moring, 2002). At the 

Lake Worth and upper and lower Woods Inlet sites, fish were 

collected in and around near-shore woody snags, boat docks, 

and concrete riffraff in an area ranging from 50 to 300 meters 

from the sediment collection site associated with each area 

(fig. 1). Fish were collected from multiple habitat types at 

the Meandering Road Creek site, including the main channel 

and channel margins of riffles, runs, and pools over a reach of 

about 100 meters in length, by using methods similar to those 

described in Moring (2006).

Each fish was individually identified by species, measured for its total and standard length (Nielsen and Johnson, 

1983, p. 284–285) to the nearest millimeter, weighed to the 

nearest gram, rinsed with deionized water, double-wrapped 

in heavy-duty aluminum foil, labeled, and frozen on-site with 

dry ice. A fish sample for each species and site was prepared 

by combining the largest fish (whole body samples) that also 

were of a distinct size class into a single composite sample 

of ground tissue. Fish sample sizes ranged from 4 to 23 fish 

per sample; controlling for an equal size class of fish in each 

sample was thought to be more important than an equal number of fish, of varying size and age, in each composite. 

Processing of Aquatic Invertebrate and Fish 

Samples

Upon return to the Texas Water Science Center in Austin, 

study personnel thawed all aquatic invertebrate and fish 

samples, and a minimum of 0.4 gram wet weight was subsampled for the determination and quantitation of the PCB 

congeners. For biological tissue, 1 gram wet weight is normally required for the analysis of PCB congeners, but several 

aquatic invertebrate samples were less than 1 gram; therefore, 

a minimum sample weight of 0.4 gram was determined to be 

acceptable for the analysis of PCB congeners in biological 

samples (Mark Burkhardt, U.S. Geological Survey, written 

commun., 2008). To obtain the minimum of 0.4 gram needed 

for PCB analyses, from 1 to 47 individual organisms per 

sample were included, depending on the taxon. A minimum of 

0.003 gram wet weight was subsampled from each composite 

sample for the analysis of stable isotopes 15N, 14N, 13C, and 12C (Carol Kendall, U.S. Geological Survey, written commun., 

2008). To obtain the minimum of 0.003 gram needed for the 

isotope analyses, from 2 to 45 individual organisms per sample were included, depending on the taxon. The subsamples 

were refrozen pending shipment to the USGS National Water 

Quality Laboratory (NWQL) in Lakewood, Colo., for the 

analysis of PCBs, or the USGS Isotope Research Laboratory 

in Menlo Park, Calif., for the analysis of stable nitrogen and 

carbon isotopes. 

Analysis of Polychlorinated Biphenyl Congeners

The USGS NWQL analyzed the invertebrate and fish 

samples for 15 PCB congeners (appendix 3) using a custom 

analytical method; custom methods have not been completely 

evaluated with USGS guidelines for developing official 

methods. To analyze PCB-congener concentrations, each 

invertebrate and fish sample was solvent-extracted by using 

a pressurized solvent-extraction system, and the 15 PCB 

congeners of interest were extracted from interfering matrix 

components by using a high-pressure water/isopropyl alcohol 

extraction and were isolated by using disposable solid-phase 

extraction (SPE) cartridges. The cartridges were air-dried, and 

sorbed PCB congeners were eluted with methylene chloride 

and diethyl ether through a florisil/alumina/sodium sulfate 

SPE cartridge and reduced in volume to approximately 0.5 

milliliter under a nitrogen stream. The reduced volume extract 

was solvent-exchanged into hexane. The hexane extract was 

reduced to approximately 0.5 milliliter, and the analytes were 

determined and quantitated by using negative chemical gas 

chromatography/mass spectrometry (Mark Burkhardt, U.S. 

Geological Survey, written commun., 2008). For convenience, 

the sum of the concentration of 15 PCB congeners is referred 

to as ∑PCBC15 throughout this report.

PCB-congener concentrations measured in biota samples 

for this study were compared with PCB-congener concentrations measured in surficial-sediment samples collected 

during 2000–2001 from Lake Worth (Harwell and others, 

2003) and with PCB-congener concentrations in surficialsediment samples collected in 2006 from Woods Inlet and 

Meandering Road Creek (Braun and others, 2008). Harwell 

and others (2003) collected 21 surficial-sediment samples 
8 Polychlorinated Biphenyls in Aquatic Invertebrates and Fish, Lake Worth and Meandering Road Creek, Fort Worth, Texas

from 21 sampling locations during 2000–2001 in Lake Worth 

(including the Lake Worth and Woods Inlet sites sampled for 

invertebrates and fish described in this report) with a 14- by 

14- by 20-centimeter Wildco box corer that penetrated about 

20 centimeters into the sediment. Braun and others (2008) 

collected sediment samples in 2004 and 2006−07 during as 

many as three storms at four to five sites (depending on flow 

conditions) that provide inflow to Meandering Road Creek and 

collected surficial-sediment samples for the analysis of PCBs 

from 20 sites distributed between Meandering Road Creek and 

Woods Inlet.

PCBs in the sediment samples collected during 2000–

2001 (Harwell and others, 2003) were determined and quantitated as Aroclor mixtures 1242, 1254, and 1260. Laboratory 

minimum reporting levels ranged from 5 to 20 μg/kg for 

each Aroclor, depending on sample mass, sample matrix, 

and analytical interference (Harwell and others, 2003, p. 33); 

for ∑PCBC15, the laboratory minimum reporting level was 

60 μg/kg. Estimated values are detections equal to or greater 

than the laboratory method detection limit and less than the 

laboratory minimum reporting level. Estimated values differ 

from other quantitative values because the laboratory cannot 

report the estimated value with the same level of confidence. 

Values were reported as less than the laboratory minimum 

reporting level when the constituent was not measured in the 

sample at a concentration equal to or greater than the laboratory method detection limit; the constituent might have been 

present at concentrations less than the laboratory minimum 

reporting level but greater than the laboratory method detection limit, but interferences prevented detection (Childress and 

others, 1999).

Analysis of Stable Nitrogen and Carbon Isotopes 

Stable isotope ratios are measured as the ratio of the two 

most abundant isotopes of a given element. The most abundant 

isotopes of nitrogen are 15N and 14N, whereas the most abundant isotopes of carbon are 13C and 12C (Clark and Fritz, 1997). 

The USGS Isotope Tracers Project (U.S. Geological Survey, 

2004) provides the following description of stable isotope 

compositions: “The stable isotopic compositions of low-mass 

(light) elements, such as oxygen, hydrogen, carbon, nitrogen, and sulfur, are normally reported as “delta” (δ) values in 

parts per thousand (denoted as ‰) enrichments or depletions 

relative to a standard of known composition. The symbol ‰ 

is spelled out in several different ways: permil, per mil, per 

mill, or per mille. The term “per mill” is the [International 

Standards Organization] term, but is not yet widely used.”

Procedures used for the analysis of stable nitrogen and 

carbon isotopes are described by Kendall and others (2001). 

Isotope composition was reported in “‰” units. Nitrogen 

isotopic compositions are expressed in δ15N relative to 

atmospheric nitrogen (N2) and are reported with a laboratory 

analytical precision of 0.2 ‰. δ15N expresses the ratio of the 

heavier (15N) to the lighter (14N) isotope in a sample to the 

same ratio in a reference standard: 

δ15N = {[(15N/14/N)x/(15N/14/N)Air] – 1}*1,000, (1)

where

x = sample, and

Air = atmospheric nitrogen (N2).

Carbon isotopic compositions are expressed in δ13C ‰ 

relative to the Vienna PeeDee Belemnite International carbon 

standard (Hut, 1987) to express the ratio of the heavier (13C) 

to the lighter (12C) isotope in a sample to the same ratio in a 

reference standard:

δ13C = {[(13C/12/C)x/(13C/12/C)VPDB] – 1}*1,000, (2)

where

x = sample, and

VPDB = Vienna Pee Dee Belemnite International carbon 

standard.

Using nitrogen isotopes as an example, if an organism 

has a higher 15N/14N ratio than the 15N/14N ratio of the reference standard (nitrogen in air), δ15N will be positive; δ15N will 

be negative if the isotope ratio of the sample is less than that 

of the standard. A δ15N value of +30 ‰ means that there are 

30 parts per thousand or 3 percent more 15N in the sample relative to the standard (U.S. Geological Survey, 2004). 

Samples were thawed at room temperature. The organic 

matter was placed in 10-milliliter plastic screw-top vials and 

freeze-dried. The freeze-dried samples were ground using a 

ball mill with a 0.63-centimeter stainless-steel ball. About 

0.5 milligram of freeze-dried sample was weighed into 3- by 

5-millimeter tin capsules. Weights and sample positions were 

recorded on a sample weighing form. The samples were analyzed by combustion in an elemental analyzer at 1,050 degrees 

Celsius (°C). The resulting gases were separated in a gas chromatograph column and the N2

 and carbon dioxide (CO2) were 

compared with reference gases by using a continuous flow 

mass spectrometer. Sample data, including the ratio of stable 

nitrogen isotopes (δ15N) and the ratio of stable carbon isotopes 

(δ13C), were corrected for drift and linearity and entered into a 

database maintained at the USGS Isotope Research Laboratory 

in Menlo Park; observations about sample collection, preparation, and analysis were included in the database.

Quality Control

Three duplicate samples (subsamples) were analyzed: 

one invertebrate sample (a dragonfly nymph sample at the 

Meandering Road Creek site) and two composite fish samples 

(a longear sunfish sample at the Lake Worth site and a green 

sunfish sample at the upper Woods Inlet site). These quality 

control samples were used to evaluate potential bias, variability, or contamination introduced during sample preparation 

and laboratory analysis (appendix 1). Duplicate analyses for 

the dragonfly nymph and longear sunfish samples consisted of 

nitrogen and carbon isotopes. Duplicate analyses for the green 

sunfish sample consisted of 15 PCB congeners. Laboratory 
Methods of Investigation 9

surrogate compound and spike recovery rates and laboratory 

blank samples were also used to evaluate the precision and 

accuracy of laboratory analyses (appendix 2). 

Results obtained for duplicate environmental samples 

were compared with those of the original environmental 

samples by calculating the relative percent difference (RPD) 

for each pair of detected constituents (appendix 1). The RPD 

was calculated as the difference between the environmental 

and duplicate samples divided by the average of the two values 

and is expressed as a percentage: 

RPD = |(C1 – C2)|/((C1 + C2)/2) x 100, (3)

where 

C1 = concentration from environmental sample; and 

C2 = concentration from duplicate sample.

The RPD between the original dragonfly nymph sample 

and its duplicate was 1.17 percent for nitrogen, 3.59 percent 

for carbon, 14.64 percent for δ15N, and -0.11 percent for δ13C 

(appendix 1). The RPD between the original longear sunfish 

and its duplicate was 11.07 percent for nitrogen, 10.57 percent 

for carbon, 6.93 percent for δ15N, and -8.27 percent for δ13C. 

All RPD results were considered acceptable; RPDs less than 

10 percent between an original and duplicate sample for 

stable isotopes are considered excellent results for biological 

tissues media (Steven Silva, U.S. Geological Survey, written 

commun., 2010).

The green sunfish composite sample from the upper 

Woods Inlet site was randomly selected for a comparison of 

the RPD between an original sample and a duplicate sample 

for the 15 PCB congeners analyzed in this study. The RPD 

ranged from 16.05 to 45.51 percent with a median RPD of 

26.45 percent (appendix 1). No adjustments were made to the 

PCB-congener data based on the results for green sunfish. The 

composite sample used for comparison with a duplicate from 

the same samples was randomly selected from all the aquatic 

invertebrate and fish composite samples from the study. The 

RPD for concentration of an organic compound, such as a 

PCB congener, is considered acceptable if it is less than 35 

percent; RPDs between 25 and 35 percent are common for 

PCB congeners in biological tissue samples (Steven Zaugg, 

U.S. Geological Survey, written commun., 2010). All samples 

in this study were composite samples that were homogenized 

prior to analysis; RPDs greater than 10 percent might indicate 

incomplete homogenization of the sample prior to subsample 

collections, but some degree of incomplete homogenization is 

typical for natural, nonreference material samples. No adjustments were made to the invertebrate or fish isotope data based 

on these results (Steven Silva, U.S. Geological Survey, written 

commun., 2010).

The analytical precision of stable isotopes of nitrogen and 

carbon was evaluated for each set of environmental samples 

by analyzing a set of ethylenediaminetetraacetic acid (EDTA) 

standard reference material samples calibrated with respect to 

VPDB for carbon isotopes and to atmospheric N2

 for nitrogen 

isotopes. Paulson and others (2006, p. 27) describes the EDTA 

standard and explains how it is calibrated:

The working standard (EDTA) (41.09 percentage 

carbon by weight, 9.59 percentage N by weight) is 

calibrated with respect to VPDB for carbon isotopes 

and to atmospheric N2 for nitrogen isotopes (δ15N = 

0.7 ‰ and δ13C = -32.2 ‰) [using several] International standards... [International Atomic Energy 

Agency and USGS standards for δ15N; National 

Bureau of Standards and International Atomic 

Energy Agency standards for δ13C]. A size-series of 

EDTA bracketing the nitrogen and carbon content 

of the samples [is] analyzed at an interval of every 

10 samples and used for drift corrections and offsets 

of isotopic ratios and elemental composition. 

Standard deviations of the EDTA sample sets were 

0.19 ‰ for percentage of nitrogen, 0.12 ‰ for δ15N, 0.30 ‰ 

for percentage of carbon, and 0.05 ‰ for δ13C. For the invertebrate and fish samples analyzed for this study, the standard 

deviations of the EDTA sample sets indicated acceptable levels of precision for the isotope constituents (Steven Silva, U.S. 

Geological Survey, written commun., 2010).

To evaluate the accuracy of the extraction process, 

selected PCB congeners and chlorinated pesticides were added 

in the laboratory as surrogates to blank and environmental 

samples. The average surrogate recoveries were 57.4 percent 

for 1,1'-bis(p-chlorophenyl)-2,2,2-trichloroethane (DDT-d8) 

and 73.6 percent for 2,2',3,3',5,5',6,6'-octachlorobiphenyl 

(PCB 202-13C12); all surrogate recoveries are listed in appendix 2. Because a new, custom analytical method was used, 

acceptable ranges for spike and surrogate recoveries had not 

been established, and no past results using the same analytical 

method were available for comparison. The recovery of surrogate compounds is monitored by the laboratory with every 

set of at least 12 environmental samples (Mark Burkhardt and 

Steven Zaugg, U.S. Geological Survey, National Water Quality 

Laboratory, written commun., 2008). 

One laboratory blank sample was analyzed for the 15 

PCB congeners. All PCB-congener concentrations in the blank 

sample were quantified at values less than the applicable PCBcongener method detection limit (shown as a nondetection in 

appendix 2), which ranged from 0.1 to 5.0 μg/kg. 

Laboratory spike samples were analyzed for constituent 

concentrations by gas chromatography and mass spectrometry. 

The percentage of recovery for environmental samples spiked 

with PCB congener 202-13C12 ranged from 68 to 76 percent, which was similar to the range of average percentage 

recovery of 64 to 75 percent for the PCB congeners analyzed 

in the aquatic invertebrate and fish samples (appendix 2). No 

corrections were made to the PCB data on the basis of spiked 

recovery results. 

All the sediment data used in this report were collected 

during previous USGS studies. Quality control for the analysis 

of sediment samples is described in Harwell and others (2003) 

and Braun and others (2008).
10 Polychlorinated Biphenyls in Aquatic Invertebrates and Fish, Lake Worth and Meandering Road Creek, Fort Worth, Texas

Polychlorinated Biphenyls in Aquatic 

Invertebrates and Fish

PCBs are hydrophobic, readily adsorb to fine sediment and other particulates, and presumably would be most 

bioavailable from surficial sediment in Lake Worth (main 

body and Woods Inlet) and in Meandering Road Creek. 

Invertebrates and fish are continually exposed to these sediments directly or indirectly through the aquatic food chain. 

The concentration of ∑PCBC15 ranged from 171.8 μg/kg 

in largemouth bass from the upper Woods Inlet site to a nondetection in midge larvae from the Lake Worth site (table 2). 

The concentration of ∑PCBC15 in midge larvae from the upper 

Woods Inlet site was 55.0 μg/kg, more than 2 times the concentration of 23.6 μg/kg from the lower Woods Inlet site and 

about an order of magnitude higher than ∑PCBC15 in zooplankton from the Lake Worth and upper and lower Woods Inlet 

sites (1.36 to 4.09 μg/kg). The concentration of ∑PCBC15 was 

highest in fish taxa from the upper Woods Inlet sites and lowest for all taxa from the Lake Worth site (table 2). PCBs were 

detected in largemouth bass, longear sunfish, gizzard shad, 

and zooplankton from the Lake Worth site, even though PCBs 

were not detected in the surficial-sediment samples collected 

previously from the site (Harwell and others, 2003) (fig. 2). 

The detection of PCBs in zooplankton and fish from the Lake 

Worth site, despite the fact PCBs were not detected in the 

surficial sediment at this site in previous USGS studies, is 

likely the result of biomagnification of PCBs during predatorprey interactions with progressive accumulation of PCBs from 

lower trophic levels, represented by invertebrates, to higher 

trophic levels, represented by fish. Low PCB concentrations 

(less than the laboratory minimum reporting levels) might be 

present in surficial sediment at Lake Worth, facilitating biomagnification of PCBs near this site. 

The minimum reporting level used by the laboratory for 

the analysis of PCBs in surficial-sediment samples collected 

in Lake Worth during previous USGS studies was higher 

(60 μg/kg) compared with some of the minimum reporting 

levels used by the laboratory at other sites at Lake Worth 

and Meandering Road Creek for surficial bed-sediment 

samples, which ranged from 15 to 75 μg/kg, and much higher 

compared with the minimum reporting levels for PCBs in 

aquatic invertebrates and fish, which ranged from 0.1 to 

10.0 μg/kg. Zooplankton and fish are mobile, and PCBs could 

have accumulated in their bodies while they were in areas 

where PCB concentrations in surficial sediment are higher 

compared with where they were collected, but it is unlikely 

that the biota collected from the Lake Worth site ever inhabited Woods Inlet or Meandering Road Creek. In lakes and 

reservoirs, many zooplankton taxa, including the taxa collected for this study, go through a daily cycle of what has 

been termed “vertical migration,” moving to shallow depths at 

night and to deeper depths during the day to avoid predation 

(Pennak, 1989). In shallow lakes and reservoirs, zooplankton 

go through daily horizontal movements swimming towards 

the open water at dusk and back to the more shallow lake 

margin during the day to avoid predation (Wojtal and others, 

2003). These vertical and horizontal movements should not 

result in large (several kilometers) dispersals of an individual 

zooplankton during its life cycle. Largemouth bass, four other 

centrarchid (sunfish family) species, and forage fish, including 

gizzard shad, were relatively immotile compared to flathead 

catfish, spotted bass, and freshwater drum, based on body burdens of total PCBs in these fishes in relation to distance from a 

highly localized source of PCBs in lake sediment (Bayne and 

others, 1996). Total PCB concentrations in the centrarchids 

and gizzard shad in this study decreased by one-half or more 

compared with concentrations from the source of PCBs in 

sediment similar to the difference in total PCB concentrations 

Table 2. Sum of 15 polychlorinated biphenyl congeners in aquatic invertebrate and fish composite samples collected in Lake Worth, 

Woods Inlet, and Meandering Road Creek, Fort Worth, Texas, 2007–08.

[PCB, polychlorinated biphenyl; μg/kg, micrograms per kilogram wet weight; LW, Lake Worth; ND, nondetection; ---, no data; WI, Woods Inlet; MC, 

Meandering Road Creek]

Site description

Site 

identifier 

(fig. 1)

Aquatic invertebrates (trophic level) Fish (trophic level)

True 

midge 

larva

(2)

Zooplankton1

(2 or 3)

Mayfly 

nymphs

(2)

Dragonfly

nymphs

(3)

Largemouth

bass

(4)

Green 

sunfish

(3)

Longear

sunfish

(3)

Western 

mosquitofish

(3)

Gizzard 

shad

(3)

Sum of 15 PCB congeners (μg/kg)

Lake Worth near State Highway 199 Bridge 004LW ND 1.36 --- --- 28.8 --- 3.48 --- 6.76

Lake Worth in lower Woods 

Inlet

003WI 23.6 1.47 --- --- 82.7 --- 30.9 --- ---

Lake Worth in upper Woods 

Inlet

002WI 55.0 4.09 --- --- 171.8 --- 115.6 --- 104.7

Meandering Creek near confluence with Lake Worth001MC --- --- 33.0 1.82 81.9 0.45 100.7 85.3 ---

1

 Water fleas and copepods. 
Polychlorinated Biphenyls in Aquatic Invertebrates and Fish 11

(as sum of 15 PCB congeners) in fish between the Lake Worth 

site and the Upper and Lower Woods Inlet sites (fig. 2); a 

distance over water of about 10 kilometers.

PCBs were not detected at concentrations greater than 

or equal to the laboratory minimum reporting level in surficialsediment samples collected in 2000 near the Lake Worth site 

(Harwell and others, 2003). Harwell and others (2003) determined and quantitated PCBs as Aroclor mixtures 1242, 1254, 

and 1260 at a laboratory minimum reporting level of 20 μg/kg 

for each Aroclor; all concentrations were less than 20 μg/kg. 

A lack of detection of PCBs at concentrations equal to or 

greater than the laboratory minimum reporting level in 2000 

near the Lake Worth site might not necessarily indicate a lack 

of occurrence of PCBs in surficial sediment at this site. 

The 15 PCB congeners analyzed for this report are 

predominately found in Aroclor mixture 1260, and these 

congeners have a larger number of substituted chlorine atoms 

compared with congeners generally found in Aroclors 1242 

and 1254 (U.S. Environmental Protection Agency, 2008). 

Congeners with more substituted chlorine atoms have greater 

hydrophobic and bioavailable properties than congeners with 

fewer chlorine atoms (Hill and Napolitano, 1997). In 2006, 

the average ∑PCBC15 concentration in surficial sediment 

was 10 μg/kg in lower Woods Inlet and 33 μg/kg in upper 

Woods Inlet (fig. 1) (Braun and others, 2008). PCB-congener 

concentrations measured in surficial-sediment samples collected in 2003 and 2006 from these two sites in Woods Inlet 

were compared with PCB-congener concentrations measured 

in biota samples collected from these same locations during 

2007–08. The average ∑PCBC15 concentration in surficial 

sediment in Meandering Road Creek in 2006 was 18 μg/kg 

(fig. 1) (Braun and others, 2008). PCB-congener concentrations measured in streambed-sediment samples collected 

in 2006 from 11 sites in Meandering Road Creek (Braun 

and others, 2008) were compared with PCB-congener concentrations measured in biota samples collected from the 

reach of Meandering Road Creek investigated during this 

study. 

The Spearman rank correlation was used to evaluate the 

relation between ∑PCBC15 in surficial sediment collected in 

2006 and ∑PCBC15 in aquatic invertebrates and fish collected 

in 2007–08 from the Lake Worth, upper and lower Woods 

Inlet, and Meandering Road Creek sites. P-values (p) corresponding to each Spearman rank rho value were calculated 

to evaluate the probability of obtaining the computed test 

statistic, or one even less likely, when the null hypothesis of 

no relation between these variables is true (Helsel and Hirsch, 

2002). A p-value less than or equal to .05 indicates a statistically significant correlation, and the strength of the correlation 

increases as the rho value approaches 1. 

Figure 2. Relation between sum of 15 polychlorinated biphenyl congeners (∑PCBC15) in surficial (top 5 centimeters) bed-sediment 

samples (Braun and others, 2008) collected in Lake Worth, Woods Inlet, and Meandering Road Creek, 2006, and ∑PCBC15 in aquatic 

invertebrate and fish samples collected in approximately the same locations, 2007–08, Fort Worth, Texas. 

-5 0 5 10 15 20 25 30 35

SUM OF 15 PCB CONGENERS, IN MICROGRAMS PER KILOGRAM, IN SURFICIAL BED SEDIMENT 

0

20

40

60

80

100

120

140

160

180

SUM OF 15 PCB CONGENERS, IN MICROGRAMS PER KILOGRAM, IN AQUATIC BIOTA

004LW 003WI 001MC 002WI

INVERTEBRATES 

True midge larvae

Zooplankton

Mayfly nymphs

Dragonfly nymphs

FISH

Largemouth bass

Green sunfish

Longear sunfish

Western mosquitofish

Gizzard shad

SAMPLING LOCATION

Meandering Road Creek

Upper Woods Inlet

Lower Woods Inlet

Lake Worth near

SH 199 bridge

001MC

002WI

003WI

004LW
12 Polychlorinated Biphenyls in Aquatic Invertebrates and Fish, Lake Worth and Meandering Road Creek, Fort Worth, Texas

By using the Spearman rank correlation, a statistically significant correlation was found between the ∑PCBC15

concentration in surficial sediment and the ∑PCBC15 concentration in invertebrates and fish from the Lake Worth, upper 

and lower Woods Inlet, and Meandering Road Creek sites 

(rho = .65, p = .002). This correlation indicates bioaccumulation is likely occurring at multiple trophic levels within 

Woods Inlet and Meandering Road Creek and that the fish 

and invertebrates from each site have inhabited the area in 

and around each site with a trophic link to PCBs in the bed 

sediment. The sample-collection sites in Woods Inlet are only 

about 0.5 kilometer apart. Although Meandering Road Creek 

drains into Woods Inlet by a series of culverts, the mayfly 

nymphs (and possibly other invertebrates and fish collected in 

the creek) likely reside in the creek over their life cycle and 

likely do not move into Woods Inlet. The mayfly nymphs of 

the family Caenidae that were collected in Meandering Road 

Creek are shallow lotic (stream-dwelling) organisms (Merritt 

and Cummins, 1996) and generally would not be found in the 

deeper lentic (lake- or reservoir-dwelling) habitat of Woods 

Inlet. 

Of the 15 PCB congeners analyzed in invertebrates and 

fish from the Lake Worth, upper and lower Woods inlet, and 

Meandering Road Creek sites, 3 were penta-, 4 were hexa-, 6 

were hepta-, 1 was octa-, and 1 was a nona-chlorinated PCB 

congener (appendix 3). Penta- and hexa-chlorinated PCB 

congeners often account for more than 50 percent of total 

PCB concentrations in aquatic food webs (Russ and others, 

2002). In zooplankton, fish, and surficial-sediment samples 

from the Lake Worth site and in all invertebrates from the 

upper and lower Woods Inlet and Meandering Road Creek 

sites, the same penta- and hexa-chlorinated PCB congeners 

(congeners 101, 110, 118, 138, 146, 149, and 151) were 

among the most prevalently detected PCB congeners, indicating a close source linkage between these PCB congeners in 

sediment and in invertebrates and fish from multiple trophic 

levels represented at each site.

At the Meandering Road Creek site, the highest PCB 

concentrations were measured for PCB congeners 138, 110, 

and 149 (in that order) in mayfly nymph samples (appendix 3), 

the selected primary consumer for the creek. The highest concentration of PCB congener 110 was measured in largemouth 

bass samples collected at the Meandering Road Creek site. 

The highest concentrations were most frequently measured for 

PCB congeners 138, 110, and 149 (in that order) in multiple 

surficial bed-sediment samples collected from Meandering 

Road Creek (Braun and others, 2008, appendix 2). 

Among the 15 PCB congeners measured in surficial 

bed-sediment samples collected from the upper Woods Inlet 

site, the second and third highest concentrations were measured for congeners 110 and 101, respectively (Braun and 

others, 2008). PCB congeners 110 and 101 were ranked first 

and second in concentration, respectively, after congener 149 

in samples of midge larvae, the primary consumer from the 

upper Woods Inlet site. The PCB congeners 110 and 101 

were ranked second and third in concentration in largemouth 

bass samples collected from the lower Woods Inlet site 

(appendix 3). 

Observations about Nitrogen and 

Carbon Isotope Composition in 

Relation to Trophic Structure and 

Bioaccumulation Patterns

Trophic Levels and Stable Isotope Composition

Attempts to measure trophic positions on the basis of 

stable nitrogen enrichment (Post, 2002) were not successful 

because of difficulties identifying the correct primary consumers. For example, on the basis of δ13C results for the Lake 

Worth and upper and lower Woods Inlet sites (fig. 3), midge 

larvae consume different carbon sources compared with other 

invertebrates and fish and thus do not represent a good primary 

consumer for these sites (fig. 3). Mayfly nymphs were considered the best available taxon to represent the primary consumers at the Meandering Road Creek site; however, on the basis 

of δ13C results, other taxa, such as midge larvae and freshwater 

snails, might have better represented the primary consumers (had they been available to sample), resulting in reliable 

trophic position values. 

Because trophic positions (nonwhole numbers) either 

could not be measured or could not be measured reliably, 

each taxon was assigned a literature-derived trophic level (a 

whole number from 2 through 4) (table 3) to make observations about isotope composition in relation to trophic levels. 

No primary producers (trophic level 1; phytoplankton or 

aquatic plants) were collected. Trophic levels were assigned 

on the basis of food habits for aquatic insect taxa (Merritt 

and Cummins, 1996), zooplankton taxa (Barnes, 1968), and 

freshwater fish species in Texas (Thomas and others, 2007). 

Zooplankton were assigned a trophic level of 3 instead of 2 

(table 3) if predatory copepods were more than 50 percent of 

the zooplankton collected from the site because a predominance of predatory copepods is indicative of tertiary rather 

than secondary consumers. Predatory copepods are intensive, 

selective predators and can regulate the structure of zooplankton prey assemblages (Williamson and Reid, 2001). 

Horizontal Food Web Position and Stable 

Isotope Composition

The horizontal position of biota in a food web is a reflection of their habitat as well as the energy and nutrient sources 

in their habitat. Differences in horizontal food web position 

are often denoted by dietary shifts between prey items occupying similar trophic positions (levels) but living in different 
Observations about Nitrogen and Carbon Isotope Composition in Relation to Trophic Structure and Bioaccumulation Patterns 13

habitats (Lindqvist, 1991). δ13C (unlike δ15N) is conserved 

through multiple trophic levels in an ecosystem and can be 

used to determine the primary source of carbon for the base 

of the food chain, as well as the horizontal food web position 

of biota in an ecosystem (Kidd, 1998). Because there is little 

change in δ13C as carbon is obtained by an organism through 

its diet, δ13C can be used to trace carbon and energy flow from 

primary producers to tertiary consumers (Kidd and others, 

2001). Additional uses of δ13C include differentiating between 

a benthic and a pelagic source of carbon, and defining or 

confirming the principal trophic pathway of energy and bioavailable contaminants (Kidd, 1998) in an ecosystem. For this 

analysis, the primary uses of δ13C were to define the horizontal 

position of selected taxa to help determine if they were good 

candidates as primary consumers.

The δ13C for midge larvae collected at the Lake Worth 

and upper and lower Woods Inlet sites were more negative 

compared with the δ13C of all other taxa, indicating midge 

larvae were more depleted of 13C compared with all other 

sampled aquatic invertebrates and fish. The relative depletion 

of 13C in the midge larvae (fig. 3; table 3) might indicate this 

taxon obtains most of its carbon from dietary sources different 

than those for all other sampled invertebrates and fish (Kidd, 

1998; Bastviken and others, 2003). The δ13C for all other taxa 

were less negative (less depleted) compared with the δ13C for 

midge larvae. The midge larvae were collected in surficial 

sediment from 1 to 5 meters deep, and as detritivores, the 

principal carbon source for this taxon might be phytoplankton 

detritus, benthic algae, or both. The majority of other invertebrate and fish taxa were collected in more shallow zones in the 

lake, inlet, and creek, generally less than 2 meters deep, that 

were most likely dominated by carbon sourced from benthicdwelling periphyton and benthic detritus.

Enrichment of Nitrogen-15 and Trophic Structure 

of Aquatic Invertebrates and Fish

Kidd (1998, p. 357) describes how nitrogen and carbon 

isotopes fractionate in a predictable manner from prey to 

predator and provide a continuous, relative measure of an 

organism’s trophic positioning. As predators feed on other 

organisms, the predator preferentially excretes the lighter 

nitrogen isotope 14N and thereby becomes more enriched in 

the heavier nitrogen isotope 15N than its prey (Steele and 

Daniel, 1978; Vander Zanden and Rasmussen, 2001). An 

increase in the ratio of the heavier isotope (15N) to the lighter 

isotope (14N) in an organism, referred to as enrichment of 

Figure 3. Relation between delta nitrogen-15 (δ15N) and delta carbon-13 (δ13C) in aquatic invertebrate and fish composite samples 

collected in Lake Worth, Woods Inlet, and Meandering Road Creek, Fort Worth Texas, 2007–08. 

-46 -44 -42 -40 -38 -36 -34 -32 -30 -28 -26 -24 -22

DELTA CARBON-13, IN PARTS PER THOUSAND

2

4

6

8

10

12

14

16

18

DELTA NITROGEN-15, IN PARTS PER THOUSAND

2

2

1

1

1 1

1

3

3

4

4

1

4 2

2

2

4

4

3

3

INVERTEBRATES 

True midge larvae

Zooplankton

Mayfly nymphs

Dragonfly nymphs

FISH

Largemouth bass

Green sunfish

Longear sunfish

Western mosquitofish

Gizzard shad

SAMPLING LOCATION

001MC, Meandering Road Creek

002WI, Upper Woods Inlet

003WI, Lower Woods Inlet

004LW, Lake Worth near SH 199 bridge

1

2

3

4
14 Polychlorinated Biphenyls in Aquatic Invertebrates and Fish, Lake Worth and Meandering Road Creek, Fort Worth, Texas

15N (Minagawa and Wada, 1984), was highest in largemouth 

bass (trophic level 4) at all sites and lowest for midge larvae 

(trophic level 2) that inhabit surficial sediment at the Lake 

Worth and upper and lower Woods Inlet sites (table 3). δ15N 

ranged from a maximum of 15.51 ‰ for largemouth bass at 

the Meandering Road Creek site to a minimum of 3.04 ‰ for 

midge larvae at the lower Woods Inlet site. Intermediate δ15N 

values (between the δ15N values measured in midge larvae and 

largemouth bass samples) were measured in samples of green 

sunfish, longear sunfish, western mosquitofish, and gizzard 

shad (all trophic level 3). The highest δ15N value measured 

in zooplankton (trophic level 2 or 3, depending on species 

composition) samples was 14.90 ‰ at the upper Woods Inlet 

site. δ15N values of 14.05 and 13.66 ‰ were measured in 

samples of mayfly nymphs (trophic level 2) and dragonfly 

nymphs (trophic level 3), respectively, at the Meandering Road 

Creek site. The δ15N values measured in samples of mayfly 

nymphs and dragonfly nymphs were similar to the δ15N values 

measured in the majority of samples of fish taxa at all four 

sites, which ranged from 11.47 ‰ for gizzard shad to 15.51 ‰ 

Table 3. Nitrogen and carbon isotope enrichment in and assigned trophic level for aquatic invertebrate and fish composite samples 

collected in Lake Worth, Woods Inlet, and Meandering Road Creek, Fort Worth, Texas, 2007–08.

[δ15N, delta nitrogen-15; ‰, per mil; TL; assigned trophic level; δ13C, delta carbon-13; LW, Lake Worth; ---, no data; WI, Woods Inlet; MC, Meandering Road 

Creek]

Site description

Site 

identifier 

(fig. 1)

Aquatic invertebrates

True midge larva Zooplankton1 Mayfly nymphs Dragonfly nymphs

δ15N,

‰

[TL]

δ13C, 

‰

δ15N, 

‰ 

(percent 

predatory 

copepods) 

[TL]

δ13C, 

‰

δ15N,

‰

[TL]

δ13C,

‰

δ15N,

‰

[TL]

δ13C, 

‰

Lake Worth near State Highway 199 Bridge 004LW 5.46

[2]

-43.49 9.56

(25)

[2 or 3]

-29.68 --- --- --- ---

Lake Worth in lower Woods Inlet 003WI 3.04

[2]

-41.86 10.73

(43)

[2 or 3]

-29.43 --- --- --- ---

Lake Worth in upper Woods Inlet 002WI 3.55

[2]

-40.35 14.90

(78)

[2 or 3]

-29.17 --- --- --- ---

Meandering Road Creek near confluence 

with Lake Worth

001MC --- --- --- --- 14.05

[2]

-26.36 13.66

[3]

-27.01

Site description

Site 

identifier 

(fig. 1)

Fish

Largemouth

bass

Green 

sunfish

Longear 

sunfish

Western 

mosquitofish

Gizzard 

shad

δ15N,

‰

[TL]

δ13C, 

‰

δ15N,

‰

[TL]

δ13C, 

‰

δ15N,

‰

[TL]

δ13C, 

‰

δ15N,‰

[TL]

δ13C, 

‰

δ15N,

‰

[TL]

δ13C, 

‰

Lake Worth near State Highway 199 

Bridge 

004LW 15.35

[4]

-26.06 --- --- 12.54

[3]

-26.24 --- --- 12.08

[3]

-28.19

Lake Worth in lower Woods Inlet 003WI 15.07

[4]

-26.36 --- --- 11.59

[3]

-23.35 --- --- --- ---

Lake Worth in upper Woods Inlet 002WI 15.48

[4]

-25.06 --- --- 12.34

[3]

-26.12 --- --- 11.47

[3]

-26.43

Meandering Road Creek near confluence 

with Lake Worth

001MC 15.51

[4]

-24.83 14.32

[3]

-24.65 13.93

[3]

-24.65 14.53

[3]

-25.74 --- ---

1

 Water fleas and copepods. 
Observations about Nitrogen and Carbon Isotope Composition in Relation to Trophic Structure and Bioaccumulation Patterns 15

for largemouth bass. δ15N values measured in zooplankton 

samples collected at the Lake Worth and lower Woods Inlet 

sites were 9.56 and 10.73 ‰, respectively, which were lower 

than the δ15N values measured for most of the fish taxa. δ15N 

values measured in midge larvae samples ranged from 3.04 to 

5.46 ‰ for the Lake Worth and upper and lower Woods Inlet 

sites and were lower than the δ15N values measured in all other 

aquatic invertebrates and fish samples. The δ15N values for 

largemouth bass samples from the Lake Worth and upper and 

lower Woods Inlet sites were consistently higher than δ15N values for the aquatic invertebrates. δ15N generally increased as 

trophic level increased. The enrichment of the heavier isotope 

15N from prey to predator (usually 3–5 ‰) provides a continuous variable that can be used to gain insights into the biomagnification of organochlorines, such as PCBs, in an aquatic food 

web (Kidd and others, 2001).

The range in δ15N values was smaller for higher trophic 

level organisms such as largemouth bass and the other fish 

taxa compared with the range in δ15N values for the aquatic 

invertebrate taxa (table 3). For example, the range in δ15N 

values measured in largemouth bass was 0.44 ‰ compared 

with 5.34 ‰ for zooplankton. Greater variation in δ15N is 

expected in primary consumers, such as midge larvae and 

zooplankton, compared with tertiary consumers, such as 

largemouth bass and other fish, because the invertebrates 

are shorter lived, and δ15N in the invertebrates more closely 

reflects the temporal and spatial variability in the concentration of stable nitrogen isotopes in short-lived primary 

producers (Vander Zanden and Rasmussen, 1999; Post, 

2002). Zooplankton were collected about 6 months later than 

the other taxa collected from the Lake Worth and Woods 

Inlet sites (April 2008 compared with September 2007), 

and seasonal and intergenerational differences in concentrations of stable isotopes were not accounted for in this study. 

Dragonfly nymphs from the Meandering Road Creek site are 

predators and secondary consumers (trophic level 3); δ15N for 

this taxon (13.66 ‰) was expected to be higher than δ15N for 

mayfly nymphs (14.05 ‰), a primary consumer (trophic level 

2) at this site, and lower than the δ15N for tertiary consumers (11.47 to 15.51 ‰, trophic level 3 or 4, depending on fish 

species). 

Nitrogen Isotope Enrichment and 

Polychlorinated Biphenyl Concentrations 

in Aquatic Invertebrates and Fish

Because trophic levels are an indication of expected 

biomagnification in a food web, they can be used to make 

observations regarding patterns of bioaccumulation (Broman 

and others, 1992; Rolff and others, 1993). At the Lake Worth 

and upper and lower Woods Inlet sites, the ∑PCBC15 concentration increases with nitrogen enrichment (as indicated by 

δ15N) in accordance with trophic level, from aquatic invertebrates to fish, with the highest ∑PCBC15 concentrations and 

δ15N values measured in largemouth bass (figs. 4 and 5). 

4 6 8 10 12 14 16

DELTA NITROGEN-15, IN PARTS PER THOUSAND

0

5

10

15

20

25

30

SUM OF 15 PCB CONGENERS, IN MICROGRAMS PER KILOGRAM,

MEASURED IN COMPOSITE FISH AND INVERTEBRATE SAMPLES 

True midge larvae

Gizzard shad

Zooplankton Longear sunfish

Largemouth bass

Figure 4. Sum of 15 polychlorinated biphenyl congeners (∑PCBC15) compared with amount of nitrogen enrichment (represented by an 

increase in ratio of nitrogen-15 to nitrogen-14 [δ15N]) in aquatic invertebrate and fish composite samples collected at reference site Lake 

Worth near State Highway 199 Bridge (004LW), Fort Worth, Texas, 2007–08.
16 Polychlorinated Biphenyls in Aquatic Invertebrates and Fish, Lake Worth and Meandering Road Creek, Fort Worth, Texas

Zooplankton (trophic level 2 or 3, depending on taxa 

composition) had relatively high δ15N values but low ∑PCBC15 

concentrations compared with other invertebrate and vertebrate taxa. In zooplankton, the ∑PCBC15 concentration was 

highest (4.09 μg/kg) at the upper Woods Inlet site (table 2) 

where previous USGS studies indicated PCB concentrations 

in surficial sediment were highest. The number of zooplankton and associated sediment samples (n = 3 for each) were 

insufficient to statistically evaluate the relation between the 

concentration of ∑PCBC15 in zooplankton from the Lake Worth 

and Woods Inlet sites and the concentration of ∑PCBC15 in 

surficial sediment measured at these sites in previous studies 

(fig. 2). Zooplankton collected in this study feed on pelagicdwelling phytoplankton or other zooplankton (Kling and 

others, 1992) and generally are not in physical contact with 

surficial sediment. The relatively high δ15N (table 3) but low 

∑PCBC15 concentration (table 2) in the zooplankton samples 

from the Lake Worth and Woods Inlet sites might indicate 

that zooplankton occupy multiple trophic levels (Sprules and 

Bowerman, 1988). It also might indicate less bioavailability 

(degree to which a chemical can be taken up by biota) of 

PCBs from the water column where zooplankton feed compared with the bioavailability of PCBs from sediment where 

benthic organisms feed, or that PCB concentrations in pelagic 

food sources are lower compared with those in benthic food 

sources. Predatory copepods accounted for 25 to 78 percent 

of the relative abundance of all zooplankton taxa at the Lake 

Worth and Woods Inlet sites (table 3). A decrease in δ15N in 

zooplankton samples collected from the upper Woods Inlet, 

lower Woods Inlet, and Lake Worth sites (in that order) corresponds to a decrease in the percentage of predatory copepods present in each zooplankton sample from these sites. The 

large relative abundance of predatory copepods might explain 

the relatively high δ15N for zooplankton compared with other 

taxa, whereas the lack of any trophic connection to surficial 

sediment would explain the relatively low concentration of the 

∑PCBC15 for this taxon.

Whereas nitrogen enrichment as δ15N in longear sunfish (trophic level 3) and largemouth bass (tropic level 4) is 

similar in magnitude, the ∑PCBC15 concentration is higher in 

largemouth bass and longear sunfish from the upper and lower 

Woods Inlet sites (30.9 to 171.8 μg/kg) compared with the 

∑PCBC15 concentration in these fish from the Lake Worth site 

(3.48 to 28.8 μg/kg). Because the invertebrate and fish communities at the Woods Inlet and Lake Worth sites are trophically similar, the PCB concentrations measured in longear 

sunfish and largemouth bass and the biomagnification of PCBs 

at the higher trophic levels in the Woods Inlet and Lake Worth 

sites are likely controlled by differences in the concentration 

of PCBs in surficial sediment. Total PCB concentrations in 

largemouth bass and other centrarchids (family of sunfishes) 

was significantly negatively correlated to the distance from a 

Figure 5. Sum of 15 polychlorinated biphenyl congeners (∑PCBC15) compared with amount of nitrogen enrichment (represented by 

an increase in ratio of nitrogen-15 to nitrogen-14 [δ15N]) in aquatic invertebrate and fish composite samples collected at Lake Worth in 

upper and lower Woods Inlet (sites 002WI and 003WI), Fort Worth, Texas, 2007–08.

2 4 6 8 10 12 14 16

DELTA NITROGEN-15, IN PARTS PER THOUSAND

20

40

60

80

100

120

140

160

180

SUM OF 15 PCB CONGENERS, IN MICROGRAMS PER KILOGRAM

0

True midge larvae (003WI)

True midge larvae (002WI)

Largemouth bass (003WI)

Zooplankton (002WI) Zooplankton (003WI)

Longear sunfish (002WI)

Longear sunfish (003WI)

Gizzard shad (002WI)

Largemouth bass (002WI)
Observations about Nitrogen and Carbon Isotope Composition in Relation to Trophic Structure and Bioaccumulation Patterns 17

source of PCBs in sediment in a lake in Alabama (Bayne and 

others, 2002), but this was not the case for flathead catfish 

(Pylodictis olivaris) collected from the same sites. These 

results provide indirect evidence that PCB residues in fish can 

be used as an indicator of fish motility where there is a large 

gradient in PCBs in lake bed sediment. 

Nitrogen enrichment as δ15N was more variable in aquatic 

invertebrates and fish from the Meandering Road Creek site 

compared with δ15N values measured in these biota samples 

at the other sites (fig. 6). The δ15N values measured in aquatic 

invertebrates and fish from the Meandering Road Creek site 

did not follow expected δ15N levels for each taxon based on 

what is known about the diet and trophic level of the taxa. 

For example, δ15N values measured in mayfly nymphs were 

similar to those measured in longear sunfish and green sunfish, 

and the ∑PCBC15 measured in the mayfly nymphs was about 

one-half the ∑PCBC15 measured in some fish (longear sunfish, 

western mosquitofish, and largemouth bass) from this site. 

This similarity in nitrogen enrichment among different trophic 

levels might be an indication of the open nature of stream 

ecosystems, compared with lakes, that contributes to larger 

variability in short- and long-term nitrogen and carbon sources 

(Vander Zanden and Fetzer, 2007). 

The Spearman rank correlation was used to evaluate the 

relation between δ15N and ∑PCBC15 in composite samples 

of different species of invertebrates and fish collected from 

the various sampling sites in Lake Worth, Woods Inlet, and 

Meandering Road Creek. At the Lake Worth reference site, 

where PCB contamination had not been detected previously, there was a statistically significant correlation between 

nitrogen enrichment (represented by an increase in δ15N) and 

∑PCBC15 in aquatic invertebrates and fish collected from the 

site (rho = .90, p = .05) (fig. 4). There was not a statistically 

significant correlation between δ15N and ∑PCBC15 in aquatic 

invertebrates and fish from the Woods Inlet sites (rho = .50, 

p = .17) (fig. 5). Removing zooplankton from the analysis of 

the relation between δ15N and ∑PCBC15 for the Woods Inlet 

sites did not improve the correlation results (as pelagic feeders with little contact with surficial sediment, this taxon was 

not thought to represent an important trophic pathway for the 

biomagnification of PCBs in the fish of Lake Worth). Nitrogen 

enrichment, represented by an increase in δ15N in aquatic 

invertebrates and fish from the Meandering Road Creek site, 

was not significantly positively correlated (rho = .14, p =.78) 

to the concentration of ∑PCBC15 in aquatic invertebrates and 

fish from the site (fig. 6).

Despite visual evidence of a monotonic association 

(positive correlation) between the ∑PCBC15 in biota and the 

∑PCBC15 in surficial bed sediment (fig. 2), the small sample 

sizes (n = 5 for the Meandering Road Creek site, n = 7 for the 

Woods Inlet sites, and n = 5 for the Lake Worth site) likely 

contributed to the lack of statistically significant correlations 

Figure 6. Sum of 15 polychlorinated biphenyl congeners (∑PCBC15) compared with amount of nitrogen enrichment (represented by an 

increase in ratio of nitrogen-15 to nitrogen-14 [δ15N]) in aquatic invertebrate and fish composite samples collected at Meandering Road 

Creek (site 001MC), Fort Worth, Texas, 2007–08.

13.0 13.5 14.0 14.5 15.0 15.5 16.0

DELTA NITROGEN-15, IN PARTS PER THOUSAND

0

20

40

60

80

100

120

SUM OF 15 PCB CONGENERS, IN MICROGRAMS PER KILOGRAM

Dragonfly nymphs 

Mayfly nymphs

Green sunfish 

Longear sunfish

Western mosquitofish Largemouth bass
18 Polychlorinated Biphenyls in Aquatic Invertebrates and Fish, Lake Worth and Meandering Road Creek, Fort Worth, Texas

between the δ15N in biota and the ∑PCBC15 in surficial bed 

sediment (as evidenced by low rho values and high p-values). 

Stronger statistical correlations between δ15N and the ∑PCBC15

concentration in aquatic invertebrates and fish might be 

obtained by collecting and analyzing individual invertebrates 

and fish rather than the composite sample data that were 

obtained in this study, which greatly reduced the sample sizes 

available for statistical analyses. 

Implications for Ecological 

Processes and Polychlorinated 

Biphenyl Concentrations in Aquatic 

Invertebrates and Fish

The results of this study provide insights on the importance of understanding the intermediate ecological processes 

between a bioavailable source of PCBs in surficial sediment 

and the transfer of these contaminants through the aquatic 

food chain, ultimately to a top-tier tertiary consumer, such as 

largemouth bass. The results of this study provide evidence 

for the variability in nitrogen and carbon enrichment and 

corresponding PCB concentrations measured in whole tissue 

samples of taxa, such as largemouth bass, from different sites 

and among different taxa from the same site, more so for the 

invertebrates and fish from the Meandering Road Creek site 

compared with those from the Lake Worth and upper and 

lower Woods Inlet sites. Variability in PCBs concentration in 

surficial sediment at the Lake Worth, upper and lower Woods 

Inlet, and Meandering Road Creek sites affects the bioavailability and ultimately the PCB concentrations in the aquatic 

invertebrates and fish that reside at each of these sites. In addition to PCB concentrations in sediment, variability in nitrogen 

enrichment at the various trophic levels and differences among 

the types of aquatic invertebrate and fish at each site affected 

the bioavailability and biomagnification of PCBs. 

Much of the emphasis, to date, in PCB-related aquatic 

studies in Texas reservoirs has been concerned with the occurrence and concentration of these contaminants in sediment 

(Harwell and others, 2003; Braun and others, 2008) or in 

fish commonly caught and consumed by the public (Texas 

Department of State Health Services, 2007). The results of this 

study provide supporting information that can assist regulators in the risk assessment process by highlighting the variable 

nature of PCB accumulation in aquatic invertebrates and fish, 

even in a relatively small aquatic ecosystem, such as the study 

area investigated for this report. 

Summary

During 2007–08 the U.S. Geological Survey (USGS), 

in cooperation with the U.S. Air Force, evaluated the concentration of polychlorinated biphenyls (PCBs) in aquatic 

invertebrates and fish (biota) collected from one site in the 

main body of Lake Worth (004LW), two sites in a small inlet 

in Lake Worth (lower Woods Inlet [002WI] and upper Woods 

Inlet [003WI]), and one site in Meandering Road Creek 

(004LW) in Fort Worth, Texas. Aquatic invertebrates and fish 

(biota samples) collected at each site included taxa representing primary, secondary, and tertiary consumers. Biota samples 

were composited, freeze-dried, ground, and analyzed for 15 

PCB congeners and stable isotopes of nitrogen (nitrogen-15 

[

15N] and nitrogen-14 [14N]) and carbon (carbon-13 [13C] and 

carbon-12 [12C]). 

PCB concentrations measured in the biota samples during 

this study were compared with PCB concentrations measured 

in sediment samples obtained from approximately the same 

sampling locations during previous USGS studies of Lake 

Worth and Meandering Road Creek. Additionally, differences 

in the amounts of 15N compared to 14N (expressed as δ15N) and 13C to 12C (expressed as δ13C) in biota samples were assessed 

by species and sampling location and compared to PCB 

concentrations in biota samples to make observations about 

trophic structure and patterns of bioaccumulation. 

The Lake Worth (reference) site is near the State 

Highway 199 bridge, and two sites are in Woods Inlet (upper 

and lower), which receives runoff from Meandering Road 

Creek. The fourth site is in Meandering Road Creek near the 

confluence with Lake Worth and upstream of Woods Inlet. 

PCB-contaminated runoff has historically drained from Air 

Force Plant 4 to Meandering Road Creek, which flows into 

Woods Inlet of Lake Worth. The four sites were selected 

because they represent the range in PCB-congener concentrations measured in surficial sediment in Lake Worth, Woods 

Inlet, and Meandering Road Creek during previous USGS 

studies and, thus, the likely range in the bioavailability and 

biomagnification of PCBs in aquatic invertebrates and fish. 

Aquatic invertebrate and fish taxa collected at the Lake 

Worth, upper and lower Woods Inlet, and Meandering Road 

Creek sites were chosen to represent the types of predator 

and prey trophic interactions expected to contribute to the 

biomagnification of PCBs. Aquatic invertebrates targeted for 

collection at the Lake Worth and upper and lower Woods Inlet 

sites included true midge larvae, zooplankton, mayfly nymphs, 

and dragonfly nymphs and the fish taxa targeted for collection 

at these sites included largemouth bass, longear sunfish, and 

gizzard shad. Fish targeted for collection at the Meandering 

Road Creek site included longear sunfish and largemouth bass; 

western mosquitofish and green sunfish were also collected 

to better represent the different types of fish and thus better 

represent the trophic structure at the site. Primary consumers, 

such as true midge larvae, zooplankton, and mayfly nymphs, 

are the essential trophic pathway for the biomagnification 

of PCBs from surficial sediment and the water column to 

secondary consumers, such as dragonfly nymphs, and tertiary 

consumers, such as largemouth bass and other fish taxa. 

The sum of the concentration of 15 PCB congeners 

(∑PCBC15) was highest in largemouth bass from the upper 

Woods Inlet site (171.8 μg/kg); PCBs were not detected in 
Summary 19

true midge larvae at the Lake Worth site. ∑PCBC15 in true 

midge larvae at the upper Woods Inlet site (55.0 μg/kg) was 

more than 2 times as high as that at the lower Woods Inlet 

site (23.6 μg/kg), and about as much as an order of magnitude 

higher compared with ∑PCBC15 in zooplankton at the Lake 

Worth and upper and lower Woods Inlet sites, which ranged 

from 1.36 to 4.09 μg/kg. ∑PCBC15 was highest in fish taxa at 

the upper Woods Inlet site, and lowest in all taxa at the Lake 

Worth site. PCBs were detected in largemouth bass, longear 

sunfish, gizzard shad, and zooplankton at the Lake Worth 

site, even though PCBs were not detected in the surficialsediment samples collected previously from the site. Using the 

Spearman rank correlation, a statistically significant correlation was found between ∑PCBC15 concentrations in surficial 

sediment and in invertebrates and fish from the Lake Worth, 

upper and lower Woods Inlet, and Meandering Road Creek 

sites (rho = .65, p = .002). This correlation indicates bioaccumulation is likely occurring at multiple trophic levels within 

Woods Inlet and Meandering Road Creek. The dominant 

PCB congeners in fish, invertebrate, and surficial-sediment 

samples from the Lake Worth, upper and lower Woods Inlet, 

and Meandering Road Creek sites were the same penta- and 

hexa-chlorinated PCBs, indicating a close source linkage 

between sediment and the multiple trophic levels at each 

site. The detection of PCBs in zooplankton and fish from the 

Lake Worth site despite the fact PCBs were not detected in 

the surficial sediment at this site in previous USGS studies is 

likely the result of biomagnification of PCBs during predatorprey interactions with progressive accumulation of PCBs from 

lower trophic levels, represented by invertebrates, to higher 

trophic levels, represented by fish. Low PCB concentrations 

(less than the laboratory minimum reporting levels) might be 

present in surficial sediment at the Lake Worth site, facilitating 

biomagnification of PCBs near this site. 

δ15N generally increased as trophic level increased. An 

increase in the ratio of the heavier 15N isotope to the lighter 14N isotope in an organism, referred to as enrichment of 15N 

(denoted as δ15N), was highest in a biota sample from the highest trophic level sampled (largemouth bass, trophic level 4) 

and lowest in biota from the lowest trophic level sampled (true 

midge larvae, trophic level 2). δ15N ranged from a maximum 

of 15.51 ‰ for largemouth bass from the Meandering Road 

Creek site to a minimum of 3.04 ‰ for true midge larvae from 

the lower Woods Inlet site. Intermediate δ15N values (between 

the δ15N values measured in true midge larvae and largemouth 

bass samples) were measured in samples of longear sunfish, 

green sunfish, western mosquitofish, and gizzard shad (all 

trophic level 3). The highest δ15N value measured in zooplankton (trophic level 2 or 3, depending on species composition) 

samples was 14.9 ‰, which was measured in a zooplankton 

sample from the upper Woods Inlet site. 

Among biota samples, the range in δ15N values was 

smaller for largemouth bass and the other fish taxa compared 

with the range in δ15N values for the aquatic invertebrate taxa. 

For example, the range in δ15N values measured in largemouth 

bass was 0.44 ‰ compared with 5.34 ‰ for zooplankton. 

Greater variation in δ15N is expected in primary consumers, 

such as true midge larvae and zooplankton, compared with 

tertiary consumers, such as largemouth bass and other fish, 

because the invertebrates are shorter lived, and δ15N in the 

invertebrates more closely reflects the temporal and spatial 

variability in the concentration of stable nitrogen isotopes in 

short-lived primary producers that they consume. Unlike 

stable 15N, stable isotope 13C is conserved through multiple 

trophic levels in an ecosystem. Increases in the ratio of the 

heavier 13C isotope to the lighter 12C isotope (referred to as 

δ13C) were used to define the horizontal position of selected 

taxa to help determine if they were good candidates to use as 

primary consumers. The δ13C for true midge larvae collected 

in the Lake Worth and upper and lower Woods Inlet sites 

were more negative compared with the δ13C of other taxa, 

indicating true midge larvae were more depleted of 13C compared with all other sampled aquatic invertebrate and fish. 

The relative depletion of 13C in the true midge larvae might 

indicate this taxon obtains most of its carbon from dietary 

sources different from those of all other sampled invertebrates and fish. The δ13C for all other taxa were more positive 

compared with the δ13C for true midge larvae. The true midge 

larvae were collected in surficial sediment from 1 to 5 meters 

in depth, and as detritivores, the principal carbon source for 

this taxon might be phytoplankton detritus, benthic algae, or 

both. The majority of other invertebrate and fish taxa were 

collected in more shallow zones in the lake, inlet, and creek, 

generally less than 2 meters in depth, that were most likely 

dominated by carbon sourced from benthic-dwelling periphyton and benthic detritus. 

Because the concentrations of bioaccumulating contaminants such as PCBs vary by trophic level, patterns of PCB 

concentrations in organisms representing different trophic 

levels with different amounts of nitrogen and carbon isotope 

enrichment can be used to make observations regarding patterns of bioaccumulation. At the Lake Worth and upper and 

lower Woods Inlet sites, the ∑PCBC15 concentration increases 

with nitrogen enrichment (as indicated by higher values δ15N) 

in accordance with trophic level, from aquatic invertebrates to 

fish, with the highest ∑PCBC15 concentrations and δ15N values 

measured in largemouth bass. 

Zooplankton (trophic level 2 or 3, depending on species 

composition) had relatively high δ15N but low ∑PCBC15 concentrations compared with other invertebrate and vertebrate 

taxa. In zooplankton, the concentration of ∑PCBC15 was highest (4.09 μg/kg) at the upper Woods Inlet site where previous 

USGS studies indicated PCB concentrations in surficial sediment were highest. The number of zooplankton and associated 

sediment samples (n = 3 for each) were insufficient to statistically evaluate the relation between the ∑PCBC15 concentration 

in zooplankton from the Lake Worth and Woods Inlet sites and 

the ∑PCBC15 concentration in surficial sediment measured at 

these sites in previous studies. Zooplankton feed on pelagicdwelling phytoplankton or other zooplankton and generally are 

not in physical contact with surficial sediment. The relatively 

high δ15N but low ∑PCBC15 concentration in the zooplankton 
20 Polychlorinated Biphenyls in Aquatic Invertebrates and Fish, Lake Worth and Meandering Road Creek, Fort Worth, Texas

samples collected from the Lake Worth and Woods Inlet sites 

might indicate that zooplankton occupy multiple trophic levels. It also might indicate less bioavailability (degree to which 

a chemical can be taken up by biota) of PCBs from the water 

column where zooplankton feed compared with the bioavailability of PCBs from sediment where benthic organisms feed, 

or that PCB concentrations in pelagic food sources are lower 

compared with those in benthic food sources. A decrease in 

δ15N in zooplankton samples collected from the upper Woods 

Inlet, lower Woods Inlet, and Lake Worth site (in that order) 

corresponds to a decrease in the percentage of predatory copepods present in each zooplankton sample from these sites. The 

large relative abundance of predatory copepods might explain 

the relatively high δ15N for zooplankton compared with other 

taxa, whereas the lack of any trophic connection to surficial 

sediment would explain the relatively low ∑PCBC15 concentration for this taxon.

Whereas nitrogen enrichment as δ15N in longear sunfish 

(trophic level 3) and largemouth bass (tropic level 4) is similar 

in magnitude, the ∑PCBC15 concentration is higher in largemouth bass and longear sunfish from the Woods Inlet sites 

(30.9 to 171.8 μg/kg) compared with the ∑PCBC15 concentration in these fish collected from the Lake Worth site (3.48 to 

28.8 μg/kg). Because the invertebrate and fish communities 

at the Lake Worth and Woods Inlet sites are similar, the PCB 

concentrations measured in longear sunfish and largemouth 

bass and the biomagnification of PCBs at the higher trophic 

levels in the Lake Worth and Woods Inlet sites are likely controlled by differences in the PCB concentrations in surficial 

sediment.

δ15N was more variable in aquatic invertebrates and fish 

collected from the Meandering Road Creek site compared 

with δ15N values at the other sites. The δ15N values measured 

in aquatic invertebrates and fish from the Meandering Road 

Creek site did not follow expected δ15N levels for each taxon 

based on what is known about the diet and trophic level of the 

taxa. For example, δ15N values measured in mayfly nymph 

samples were similar to those measured in longear sunfish and 

green sunfish, and the ∑PCBC15 measured in the mayfly nymph 

samples was about one-half the ∑PCBC15 measured in some 

fish (longear sunfish, western mosquitofish, and largemouth 

bass) from this site. This similarity in nitrogen enrichment 

among different trophic levels might be an indication of the 

open nature of stream ecosystems, compared with lakes, that 

contributes to larger variability in short- and long-term carbon 

and nitrogen sources. 

Despite visual evidence of a monotonic association 

(positive correlation) between the ∑PCBC15 in biota and the 

∑PCBC15 in surficial bed sediment, the small sample sizes 

(n = 5 for the Meandering Road Creek site, n = 7 for the 

Woods Inlet sites, and n = 5 for the Lake Worth site) likely 

contributed to the lack of statistically significant correlations 

between the δ15N in biota and the ∑PCBC15 in surficial bed 

sediment (as evidenced by low rho values and high p-values). 

Stronger statistical correlations between δ15N and the concentration of ∑PCBC15 in aquatic invertebrates and fish might be 

obtained by collecting and analyzing individual invertebrates 

and fish rather than the composite sample data that were 

obtained in this study, which greatly reduced the sample sizes 

available for statistical analyses. 

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Appendixes 1–3
26 Polychlorinated Biphenyls in Aquatic Invertebrates and Fish, Lake Worth and Meandering Road Creek, Fort Worth, Texas

Appendix 1. Relative percent difference for duplicate polychlorinated biphenyl congener and isotope data for aquatic invertebrate and 

fish samples collected in Lake Worth, Woods Inlet, and Meandering Road Creek, Fort Worth, Texas, 2007–08.

[LW, Lake Worth; RPD, relative percent difference; WI, Woods Inlet; MC, Meandering Road Creek; PCB, polychlorinated biphenyl; μg/kg, micrograms per 

kilogram; --, not analyzed for; ‰, per mil; δ15N, delta nitrogen-15; δ13C, delta carbon-13]

Lake Worth near State 

Highway 199 Bridge 

(004LW)

Lake Worth in upper Woods Inlet 

(002WI)

Meandering Road Creek near 

confluence with Lake Worth 

(001MC)

Longear sunfish Green sunfish Dragonfly nypmhs

Original 

environmental 

sample

Duplicate 

sample RPD

Original 

environmental 

sample

Duplicate 

sample RPD

Original 

environmental 

sample

Duplicate 

sample RPD

PCB congeners (μg/kg in sample)

PCB 101 -- -- -- 26.30 16.55 45.51 -- -- --

PCB 110 -- -- -- 23.20 16.04 36.52 -- -- --

PCB 118 -- -- -- 6.94 5.91 16.05 -- -- --

PCB 138 -- -- -- 15.80 13.21 17.89 -- -- --

PCB 146 -- -- -- 3.57 2.66 29.21 -- -- --

PCB 149 -- -- -- 25.20 18.30 31.74 -- -- --

PCB 151 -- -- -- 3.79 2.77 30.96 -- -- --

PCB 170 -- -- -- 6.59 5.30 21.68 -- -- --

PCB 174 -- -- -- 3.91 3.21 19.63 -- -- --

PCB 177 -- -- -- 2.04 1.73 16.50 -- -- --

PCB 180 -- -- -- 11.90 9.12 26.45 -- -- --

PCB 183 -- -- -- 2.68 2.11 23.85 -- -- --

PCB 187 -- -- -- 6.21 4.96 22.40 -- -- --

PCB 194 -- -- -- 2.79 1.96 35.10 -- -- --

PCB 206 -- -- -- 1.37 .86 45.25 -- -- --

Constituents (percent in sample)

Nitrogen 12.11 10.84 11.07 -- -- -- 9.43 9.32 1.17

Carbon 41.56 46.20 10.57 -- -- -- 48.64 48.92 3.59

Isotopes (‰ in sample)

δ15N 12.11 12.98 6.93 -- -- -- 12.66 14.66 14.64

δ13C -25.15 -27.32 -8.27 -- -- -- -26.16 -26.19 -.11

Appendix 1
Appendix 2 27

Appendix 2

Appendix 2. Laboratory sample blank and spike percent recovery for polychlorinated biphenyl congeners and surrogate compounds 

for two sets analyzed concurrently with aquatic invertebrate and fish samples collected in Lake Worth, Woods Inlet, and Meandering 

Road Creek, Fort Worth, Texas, 2007–08.

[PCB, polychlorinated biphenyl; ND, nondetection less than method detection limit; DDT-d8, 1,1'-bis(p-chlorophenyl)-2,2,2-trichloroethane; PCB 202-13C12, 

2,2',3,3',5,5',6,6'-octachlorobiphenyl]

Sample blank percent recovery Environmental sample spike percent recovery

Set 1 Set 2 Set 1 Set 2

PCB congener

PCB101 ND ND 75.60 138.00

PCB110 ND ND 92.40 150.00

PCB151 ND ND 66.70 58.10

PCB149 ND ND 70.30 64.40

PCB118 ND ND 61.20 67.80

PCB146 ND ND 61.70 70.80

PCB138 ND ND 71.80 71.20

PCB187 ND ND 62.40 64.00

PCB183 ND ND 59.50 61.00

PCB174 ND ND 64.30 64.40

PCB177 ND ND 59.00 62.70

PCB180 ND ND 58.20 63.40

PCB170 ND ND 47.00 63.50

PCB194 ND ND 58.00 63.30

PCB206 ND ND 59.10 62.50

Surrogate compound

DDT-d8 56.10 59.60 55.20 58.80

PCB 202-13C12 68.20 76.20 73.80 76.20
28 Polychlorinated Biphenyls in Aquatic Invertebrates and Fish, Lake Worth and Meandering Road Creek, Fort Worth, Texas

Appendix 3

Appendix 3. Concentrations of selected polychlorinated biphenyl congeners and number of chlorinated substitutes for aquatic 

invertebrate and fish composite samples collected in Lake Worth, Woods Inlet, and Meandering Road Creek, Fort Worth, Texas, 2007–08.

[Concentrations in micrograms per kilogram (μg/kg) wet weight; PCB, polychlorinated biphenyl; LW, Lake Worth; <, less than laboratory method detection 

limit; ---, no data; E, estimated value less than laboratory minimum reporting level; WI, Woods Inlet; MC, Meandering Creek] 

Site 

description

Site 

identifier 

(fig. 1)

Method 

detection 

limit

(μg/kg)

PCB 

congener

Chlorinated 

substitute

Aquatic invertebrates Fish

Midge

larvae

Zooplankton

Mayfly

nymphs

Dragonfly

nymphs

Largemouth

bass

Green

sunfish

Longear

sunfish

Western 

mosquitofish

Gizzard 

shad

Lake 

Worth 

near 

State 

Highway 199 

Bridge 

004LW 4.0 101 Penta- <4 <4 --- --- 4.44 --- <4 --- <4

10.0 110 Penta- <10 <10 --- --- E7.9 --- <10 --- <10

.4 118 Penta- <.4 E.14 --- --- 1.69 --- E.27 --- E.64

.2 138 Hexa- <.2 .84 --- --- 1.25 --- .59 --- E1.17

.2 146 Hexa- <.2 E.12 --- --- .72 --- .58 --- E.3

2.0 149 Hexa- <2 E.03 --- --- E1.25 --- <2 --- E1.17

.2 151 Hexa- <.2 E.1 --- --- E.86 --- E.03 --- E1.25

.2 170 Hepta- <.2 <.2 --- --- .81 --- <.2 --- E.09

.2 174 Hepta- <.2 <.2 --- --- E.45 --- E.02 --- E.54

.2 177 Hepta- <.2 <.2 --- --- .28 --- .93 --- E.2

.2 180 Hepta- <.2 E.02 --- --- 1.74 --- .48 --- E.17

.4 183 Hepta- <.4 E.04 --- --- E.27 --- E.29 --- E.29

.2 187 Hepta- <.2 E.01 --- --- 1.93 --- .28 --- .54

.4 194 Octa- <.4 E.06 --- --- E.14 --- <.4 --- E.23

.2 206 Nona- <.2 <.2 --- --- <.2 --- <.2 --- .33

Site 

description

Site 

identifier 

(fig. 1)

Method 

detection 

limit

(μg/kg)

PCB 

congener

Chlorinated 

substitute

Aquatic invertebrates Fish

Midge

larvae

Zooplankton

Mayfly

nymphs

Dragonfly

nymphs

Largemouth

bass

Green

sunfish

Longear

sunfish

Western 

mosquitofish

Gizzard 

shad

Lake Worth 

in lower 

Woods 

Inlet 

003WI 4.0 101 Penta- E3.2 <4 --- --- 7.4 --- 4.66 --- ---

10.0 110 Penta- E2.5 E.14 --- --- E7.78 --- E1.27 --- ---

.4 118 Penta- .93 E.21 --- --- 5.05 --- 2.67 --- ---

.2 138 Hexa- 2.12 .37 --- --- 16.5 --- 4.42 --- ---

.2 146 Hexa- .27 .20 --- --- 3.02 --- 1.4 --- ---

2.0 149 Hexa- 2.71 E.05 --- --- 11.2 --- 3.82 --- ---

.2 151 Hexa- .38 .20 --- --- 2.1 --- .76 --- ---

.2 170 Hepta- .49 E.04 --- --- 5.24 --- 2.14 --- ---

.2 174 Hepta- .46 E.05 --- --- 2.21 --- .64 --- ---

.2 177 Hepta- E.18 <.2 --- --- 1.37 --- .58 --- ---

.2 180 Hepta- .62 .02 --- --- 8.92 --- 4.14 --- ---

.4 183 Hepta- E.20 E.09 --- --- 2.36 --- .89 --- ---

.2 187 Hepta- .65 E.02 --- --- 5.95 --- 2.49 --- ---

.4 194 Octa- E.16 E.08 --- --- 2.52 --- .69 --- ---

.2 206 Nona- E.01 <.2 --- --- 1.04 --- .33 --- ---
Appendix 3 29

Appendix 3. Concentrations of selected polychlorinated biphenyl congeners and number of chlorinated substitutes for aquatic 

invertebrate and fish composite samples collected in Lake Worth, Woods Inlet, and Meandering Road Creek, Fort Worth, Texas, 

2007–08—Continued.

Site description

Site 

identifier 

(fig. 1)

Method 

detection 

limit

(μg/kg)

PCB 

congener

Chlorinated 

substitute

Aquatic invertebrates Fish

Midge

larvae

Zooplankton

Mayfly

nymphs

Dragonfly

nymphs

Largemouth

bass

Green

sunfish

Longear

sunfish

Western 

mosquitofish

Gizzard 

shad

Lake Worth 

in upper 

Woods 

Inlet 

002WI 4.0 101 Penta- 10.1 E0.50 --- --- 35.7 --- 22.3 --- 20.07

10.0 110 Penta- 10.4 E.77 --- --- 23.5 --- E7.89 --- 19.62

.4 118 Penta- 3.8 E.37 --- --- 8.74 --- 4.53 --- 6.42

.2 138 Hexa- 7.14 .70 --- --- 15.8 --- 22.0 --- 14.5

.2 146 Hexa- 1.24 E.12 --- --- 6.01 --- 3.38 --- 3.12

2.0 149 Hexa- 9.42 .59 --- --- 27.9 --- 17.0 --- 21.75

.2 151 Hexa- 1.16 E.11 --- --- 4.43 --- 3.95 --- 3.28

.2 170 Hepta- 2.08 E.12 --- --- 8.36 --- 4.56 --- 5.95

.2 174 Hepta- 1.41 E.07 --- --- 2.54 --- 2.14 --- 3.56

.2 177 Hepta- .78 E.05 --- --- 2.08 --- 2.22 --- 1.88

.2 180 Hepta- 3.83 .24 --- --- 16.2 --- 8.82 --- 10.51

.4 183 Hepta- .77 E.07 --- --- 4.5 --- 2.98 --- 2.39

.2 187 Hepta- 1.9 E.14 --- --- 11.2 --- 8.14 --- 5.58

.4 194 Octa- .70 E.04 --- --- 3.58 --- 2.5 --- 2.37

.2 206 Nona- .25 .20 --- --- 1.22 --- 1.12 --- 1.12

Site 

description

Site 

identifier 

(fig. 1)

Method 

detection 

limit

(μg/kg)

PCB 

congener

Chlorinated 

substitute

Aquatic invertebrates Fish

Midge

larvae

Zooplankton

Mayfly

nymphs

Dragonfly

nymphs

Largemouth

bass

Green

sunfish

Longear

sunfish

Western 

mosquitofish

Gizzard 

shad

Meandering Road 

Creek 

near 

confluence 

with 

Lake 

Worth

001MC 4.0 101 Penta- --- --- 5.04 <4 14.4 <4 16.0 17.9 ---

10.0 110 Penta- --- --- E6.38 <10 15.7 <10 17.1 20.6 ---

.4 118 Penta- --- --- 2.98 E.06 6.06 E.02 6.7 6.29 ---

.2 138 Hexa- --- --- 6.83 <.2 11.5 .03 11.8 8.55 ---

.2 146 Hexa- --- --- .70 E.14 2.4 E.05 2.95 2.36 ---

2.0 149 Hexa- --- --- 5.34 <2 13.2 <2 18.6 14.2 ---

.2 151 Hexa- --- --- .54 .30 1.02 <.2 2.85 1.49 ---

.2 170 Hepta- --- --- 1.04 <.2 3.23 <.2 3.95 1.89 ---

.2 174 Hepta- --- --- .24 <.2 1.17 <.2 2.24 1.34 ---

.2 177 Hepta- --- --- .25 <.2 .57 .23 1.43 .96 ---

.2 180 Hepta- --- --- 2.24 .35 6.88 E.09 7.35 5.48 ---

.4 183 Hepta- --- --- .36 <.2 1.16 E.02 1.36 .95 ---

.2 187 Hepta- --- --- .88 .39 3.33 E.01 6.77 3.27 ---

.4 194 Octa- --- --- E.17 E.28 .89 <.4 1.06 E.02 ---

.2 206 Nona- --- --- <.2 E.18 .36 <.2 .51 <.2 ---
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