J. Great Lakes Res. 24(1):12-31Internat. Assoc. Great Lakes Res., 1998

The Ratio of DDE to PCB Concentrations in Great LakesHerring Gull Eggs and its Use in Interpreting Contaminants Data

Kimberley D. Hughesl , D. Vaughn Weselohl ,*, and Birgit M. Braune2

JCanadian Wildlife Service (Ontario Region), Environment Canada4905 Dufferin St.

Downsview, Ontario M3H 5T4

2Canadian Wildlife Service, Environment CanadaNational Wildlife Research Centre

i 00 Gamelin Blvd.Hull, Quebec KiA OH3

ABSTRACT. The ratio of DDE to PCB (DDE:PCB) concentrations was examined in herring gull(Lams argentatus) eggs collected from thirteen sites on the Great Lakes from 1979 to 1996. This ratio hasbeen shown to have had a number ofbiological interpretations in the past and the data in this study havebeen used to test the validity of these interpretations. The findings suggest that the consistency ofDDE:PCB over many years reflects the relative availability of DDT and PCB in different geographicalareas and provides an indication of a bird's general foraging ecology. The ratio can also be used as areflection of the relative rates of increase or decrease of DDE and PCBs in food over time. A significantincrease was found in the ratio in eggs sampled from sites on the upper Great Lakes and Lake Ontarioduring the study period. This is attributed to PCB levels decreasing faster than DDE levels at these sites.At Lake Erie and its two connecting channels, a significant increase was detected in the ratio but at arate 56% or less than that found at other Great Lakes sites. This lower rate is attributed to DDE and PCBlevels decreasing at high and equal rates. Similarities/differences in the fates of these contaminantsamong the Great Lakes would never have been realized upon an examination of individual contaminantlevels alone. The ratio is valuable as a measure of the relative exposure of the two contaminants in non­migratory birds such as herring gulls. The use of the ratio as an indication of contamination movementthrough the food chain could not be assessed; the ratio cannot be used as a reflection of the interspecificdifferences in the accumulation and metabolism of the two contaminants. Similarities in the patterns ofthese ratios shown by eggs collected from sites in close proximity or within the same lake reinforce thefact that herring gulls are important as monitors of regional contaminant conditions.

INDEX WORKS: DDT, DDE, PCB, herring gull, Great Lakes.

INTRODUCTION

DDT has been recognized for its insecticidalproperties since 1939, and its subsequent popularuse in agriculture led to its widespread contamina­tion of the environment (McEwen and Stephenson1979). PCBs have been in use since 1929, mainly inthe electrical industry, but it was not until the mid­1960s that they were found to be a major environ­mental contaminant (Jensen et al. 1969). Thechemical characteristics of both DDT and PCBs are

"'Corresponding author. E-mail: Chip.Weseloh@ec.gc.ca.

12

such that they persist in nature and bioaccumulatein ecosystems (McEwen and Stephenson 1979, Al­ford-Stevens 1986).

For more than 30 years, the literature has com­monly reported concentrations of DDE, PCBs, andother contaminants found in wildlife tissues. In par­ticular, some of these papers make reference to theratio of DDE and PCB concentrations. DDE:PCBwas referred to as early as 1968 by Risebrough etal. (1968) to express the relative quantities of theinsecticide-origin DDE (breakdown product ofDDT) and the industrial-origin PCB group of com­pounds in biota. This ratio has been credited with awide range of biological interpretations including:

DDE:PCB in Great Lakes Herring Gull Eggs 13

I) A reflection of dietary differences or similari­ties among species in a given locality andtimeframe (Bogan and Bourne 1972, Frank etal. 1975, Ohlendorf et al. 1982, Struger andWeseloh 1985, Weseloh et al. 1989).

2) A reflection of the relative rates of decreaseor increase of DDE and PCBs in food overtime (Gilbertson 1974; Pearce et at. 1979,1989; Weseloh et al. 1989).

3) A reflection of the differences or similaritieswith which DDE and PCBs move through thefood chain (Schreiber and Risebrough 1972,Gilbertson 1974, Bowes and Jonkel 1975,Peakall 1975, Norstrom et at. 1978).

4) A reflection of the interspecies differences orsimilarities in accumulation or metabolism ofDDE and PCBs (Bogan and Bourne 1972,Bourne and Bogan 1972, Holt et at. 1979).

5) Characterization of polluted ecosystems by:i) Reflecting geographic differences or

similarities in the relative availability of DDEand PCBs in the food chain (Risebrough et al.1968; Gilbertson and Reynolds 1972, 1974;Keith and Gruchy 1972; Bowes and Jonkel1975; Vermeer and Peakall 1977; Postupalsky1978; Ohlendorf et at. 1978, 1979, 1982;Lindberg et at. 1985; Struger and Weseloh1985; Ewins et al. 1994; Hebert et al. 1994).

ii) Reflecting relative exposure to contami­nation on breeding grounds, migration routesand wintering grounds (Bourne and Bogan1972, Faber et al. 1972, Gilbertson 1974, Fox1976, Vermeer and Peakall 1977, Ohlendorf etat. 1978, Postupalsky 1978, Findholt andTrost 1985, Weseloh et al. 1989).

Since DDE:PCB is a relative measure, however, itcannot be used independently to show a decrease orincrease in the concentrations of either of the tworesidues.

The Canadian Wildlife Service has been monitor­ing organochlorines, including DDE and PCBs, inwildlife since the early 1970s (Norstrom 1988).Eggs of fish-eating birds have been the main moni­toring tissue (Norstrom 1988, Bishop et at. 1992,Pettit et at. 1994), and in 1974, the herring gull(Larus argentatus) was adopted as the main avianspecies for monitoring organochlorine contaminantsin the Great Lakes (Mineau et al. 1984). The pur­pose of this paper is to describe the temporal andgeographical patterns of DDE:PCB for herring gullson the Great Lakes from 1979 to 1996. These ratioswere drawn mainly from the large organochlorine

data base available for this species (Gilman et at.1977, Weseloh et at. 1979, Mineau et at. 1984,Bishop et al. 1992, Pettit et al. 1994, CWS unpub­lished). The objective of the paper was to evaluatefour out of the five biological interpretations drawnfrom the use of this ratio in the past (see above).With regard to the fifth interpretation, differences inmethodological techniques between laboratorieswould likely confound an attempt to formulate gen­eralizations across trophic levels and, therefore, thisinterpretation (number 3) will not be discussedhere.

METHODS

Thirteen study sites from the five Great Lakes andconnecting channels were selected from the Cana­dian Wildlife Service database (Bishop et al. 1992,Pettit et al. 1994, CWS unpublished) to encompass awide geographical range within the Great Lakes re­gion. These sites are known as Annual MonitorColonies (AMCs) and are: two sites on Lake On­tario (Snake Island and Muggs Island), two sites onLake Erie (Port Colborne Lighthouse and Middle Is­land), three sites on Lake Huron (Channel ShelterIsland, Chantry Island, and Double Island), two siteson Lake Superior (Agawa Rock and Granite Island),and two sites on Lake Michigan (Big Sister Islandand Gull Island). Two colonies are located on con­necting channels and include one unnamed island onthe Niagara River and one site on the Detroit River(Fighting Island) (Fig. 1).

The large decline in DDE and PCBs in herringgull eggs from 1974 to 1978 in the Great Lakes hasbeen reported previously (Weseloh et at. 1979,1990; Hallett et at. 1981; Mineau et at. 1984;Norstrom 1988; Bishop et at. 1992). To examinethe usefulness of DDE:PCB in ecotoxicology, eggswere used that had been collected from 1979 to1996 during which time the decline was less pro­nounced. Generally 9 to 13 eggs were collected ran­domly from each of the sites every year. Oneexception was Channel Shelter Island where sam­pling was not initiated until 1980. Furthermore, in afew cases a site may not have been sampled in agiven year, prior to being included as an AnnualMonitor Colony. Missing years of data include bothNiagara River and Fighting Island in 1980; Big Sis­ter Island in 1979, 1981; Port Colborne Lighthousein 1994; Gull Island in 1979 and 1981; and GraniteIsland in 1983.

Following collection, the contents of the eggswere stored in hexane-rinsed jars with aluminum

14

Thunder Bay"

~

Milwaukee'"

=

Hughes et at.

Erie

oI

100I

200 kmI

·Oswego

FIG. 1. The location of the herring gull colonies examined in this study. 1) Snake Island; 2) MuggsIsland; 3) Unnamed Island, Niagara River; 4) Port Colborne Lighthouse; 5) Middle Island 6) FightingIsland, Detroit River 7) Channel Shelter Island; 8) Chantry Island; 9) Double Island; 10) Gull Island; 11)Big Sister Island; 12) Agawa Rock; 13) Granite Island.

foil-lined lids and frozen prior to analysis. Eggswere analyzed for a large number of organochlorinecontaminants including DDE and polychlorinatedbiphenyls (PCBs). PCB concentrations were ana­lyzed as Aroclor 1254: 1260 (1: 1) and estimated bya single peak corresponding to PCB 138 (Turle etal. 1991). All analyses for contaminant levels priorto and including 1985 were carried out by the On­tario Research Foundation, Mississauga, Ontariousing standard gas chromatography techniques firstdescribed by Reynolds and Cooper (1975) and latermodified by Norstrom et al. (1980 a, b). After1985 these analyses were carried out by the Cana­dian Wildlife Service at the National Wildlife Re­search Centre using methods described by Peakallet al. (1986). All analytical data were collected on aIlg/g wet weight basis. Detection limits for DDEand PCBs were 0.005 and 0.01 Ilg/g, respectively.

DDE:PCB was determined for each individualegg and an average was calculated using all eggsprior to and including 1985. After 1985 a differentmethod was employed and subsamples of ten eggsfrom a given colony were pooled on an equal wetweight basis and analyzed as a single sample to ob­tain a contaminant value equivalent to an arithmeticmean value. Turle and Collins (1992) have shownthat there are no significant differences betweenmean residue levels determined using individualand pooled samples. In a few cases (23/140) after1985, chemical analyses were performed on bothpooled eggs and individual eggs. The results of in­dividual analyses were used since these data wereavailable. The procedure of pooling eggs for chemi­cal analysis was first initiated in 1982 as part of apilot study at the Niagara River and Port ColborneLighthouse sites (Bishop et al. 1992).

DDE:PCB in Great Lakes Herring Gull Eggs 15

Statistical Analysis

Since, in most cases, there was no variationaround means after and including 1986 (n == 1), lin­ear regression analyses were performed on theDDE:PCB data using a single mean to representeach year. For regression and analysis of covarianceanalyses, one inherent problem is that by examiningthe means only and not the individual data in theanalyses, it was not possible to separate the two dif­ferent sources of variation evident in analyses of in­dividual data: deviations from the regression(among year) and error among the observed ratiosfor a given year. Also, by using means and not indi­vidual data where available, degrees of freedom arelost and thereby increase the likelihood of not re­jecting the null hypothesis and committing a type IIerror. Any bias resulting from the analysis of meanswould be minimal, however, since the among-yearvariation would be expected to contribute far moreto the error term than the within-year variation.Therefore, the analysis of means is an appropriatestatistical method.

Prior to regression analyses, Levene's test for ho­mogeneity of variance was performed comparingyears in which individual eggs were chemically an­alyzed. This test examines among-year variationusing the mean absolute deviations of each individ­ual sample from the mean ratio for each year(Snedecor and Cochran 1980). For those siteswhich were found to have homogeneous variancesamong years the mean ratio values were weightedby the number of eggs included in that mean(Draper and Smith 1981). Pooled samples were alsoweighted by the number of eggs used to producethe pool (Stow et aZ. 1995). Where significant het­erogeneity was detected at a site, means wereweighted by the inverse of the squared standarderror (essentially the variance) for that mean (Neteret aZ. 1990). An estimate of standard error forpooled samples for each site was determined usingdata from the individual samples as follows. Thevariance of all individual samples was weighted bythe degrees of freedom for that sample and usingthis, a "pooled" variance was determined for eachsite. A "pooled" standard error was then estimatedby taking the square root of the pooled variance di­vided by the number of samples in the pool. Allpooled samples for a given site were then weightedby the inverse of the squared pooled standard error.

An important assumption of regression analysis isthat error terms are not autocorrelated over time(Neter and Wasserman 1974). This possibility was

examined in the time series data using the Durbin­Watson test and confirmed that error terms were notcorrelated over time. Hence, regression analysis wasan appropriate method for examining the data.Analysis of covariance was used to compare differ­ences among intercepts of regression lines generatedfor each site (plotted against year) and to test forsignificant site and year effects. Scheffe's multiplecomparison test was used to further examine tempo­ral patterns in the data (Neter and Wasserman 1974).

The patterns of change for DDE and PCB overtime were examined by performing linear regres­sion analyses on the log-normalized (natural log)wet weight concentrations of each contaminant foreach site. The decline in chemical concentrationsfor both of these contaminants appeared to fit anexponential decay model and thus a logarithmictransformation was necessary. For regression analy­ses, natural logarithmic means were weighted bythe number of observations included in that mean.Similarly, as for ratios, pooled samples wereweighted by the number of samples used to producethe pool. Correlation coefficients were examinedfor each of the mean residues with % lipid contentin eggs and it was found that in only seven out oftwenty-six cases (27%) were these two variables (%lipid and contaminant level) significantly corre­lated. Consequently this did not change the patternof representation of arithmetic means and thereforethe data were not lipid standardized. It has beensuggested previously that although organochlorinecontaminants are lipid and not water soluble, it is ofquestionable propriety to lipid normalize contami­nant values when there is no apparent correlationbetween lipid and contaminant concentrations(Hebert and Keenleyside 1995).

All analyses were performed using the SAS sta­tistical package (SAS Institute Inc. 1985) with asignificance level of p < 0.05.

RESULTS

Temporal and Spatial Trends in DDE:PCB

Mean values of DDE:PCB for herring gull eggsdetermined for all thirteen sites in the Great Lakesarea from 1979 to 1996 are shown in Figures 2 and3. A subtle spatial pattern becomes apparent uponexamining the mean value of DDE:PCB for all thir­teen years for all sites (Fig. 4). First, there is a gen­eral increase in the mean ratio from Fighting Islandto Lake Erie sites (Middle Island and Port ColborneLighthouse) to Niagara River to Lake Ontario sites(Muggs Island and Snake Island), proceeding west

16 Hughes et al.

NIAGARA RIVER

to east. Second, the values exhibited by the ratio atthe sites west of Fighting Island (those in the upperGreat Lakes) are greater than those determined forFighting Island. This overall spatial pattern is con­firmed when the ratio for a given year is comparedamong sites (Figs. 2 and 3).

Analysis of covariance (ANCOVA) was used tocompare spatial and temporal trends observedamong sites over the course of this study. Theslopes were not the same when all thirteen siteswere included in the model (Table I; section 1a:significant interaction term). To determine thesource of this heterogeneity, regression analyseswere performed on the data for all sites and an ob­vious geographical trend in the temporal pattern ofthe ratio (Table 2) can be seen. Sites on Lake Erieand those immediately adjacent to Lake Erie (Fight­ing Island and Niagara River) had the smallest re­gression coefficients (slopes) compared with thesites on Lake Ontario and the upper Great Lakes.Furthermore, slopes for Niagara River and PortColborne Lighthouse were not significantly differ­ent from zero. For sites on Lake Ontario and theupper Great Lakes (except Big Sister Island) allslopes were positive and significantly differentfrom zero, indicating that the ratio showed a signifi­cant increase over time at these sites. Therefore,separate analyses were run on each of the threegroups of sites based on the spatial pattern observedfor the regression coefficients. These groups wereas follows: Group I (Lake Ontario)-Muggs andSnake islands, Group 2 (Lake Erie and connectingchannels)-Niagara River, Port Colborne Light­house, Middle and Fighting islands, and Group 3­the remaining sites found on the upper Great Lakes.The slopes were similar within each of these groups(Table 1; section 2a: non-significant interactionterms). Separate ANCOVAs were performed foreach group to determine whether there were signifi­cant differences in the ratio (y intercept) among

FIG. 2. Mean ratios of DDE:PCB (analyzed asAroclor 1254:1260) for herring gulls eggs col­lected from the Detroit River, Lake Erie, NiagaraRiver, and Lake Ontario from 1979-1996. Foryears in which chemical analyses were performedon individual eggs, standard deviations are shown.A reference line has been included for each site toaid the reader in recognizing the DDE:PCB pat­tern over time.

!MIDDLE I.

FIGHTING I.

SNAKE I.

\~i \

/, \b.

....."..

UNNAMED ISLAND

LAKE ONTARIO0.30

tIl~ 0.20WClClZ<':li 0.10

0.00 L.....J'-------'---I.-----'----'---'---'----'---L.--'-...J..---'------'-----'-----'---'---'-----'

78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96

YEAR

0.00 '----'---'---'----'-----'----'---'---'----'---L.--'---'---'------'-----'-----'----"'-------'

78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96

YEAR

0.30

0.00 '----'--'--'----'----'-----''------'----'----'----'---'----'----1.---1.---'----'----'------'

78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96YEAR

3LAKE ERIE

0.0

0.00 '---'---'----'-----'----'---'---±""-'----'-----'-..J..-.J....-.l...-,----"'-------'---'------'

78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96

YEAR

DETROIT RIVER

tIl~ 0.20WClClZ<':li 0.10

tIl<.> 0.20a-lii PORT COLBORNECl LIGHTHOUSECl

-0.. \'"

Z< I'w 0.10::!: "'-0""' ..... --<>-- /

0.30

tIl~ 0.20WClClZ<':li 0.10

DDE:PCB in Great Lakes Herring Gull Eggs 17

0.40LAKE SUPERIOR

0.00 ::---==-:!:-:'-:-::':c-::':c-::'---'----'--'-----'--'----'------'---'-----L--'----'-----'78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96

YEAR

SITE

---------

------------------- --

1

I

lI

---

!

cLI,

, -~ , + iff llilr.<?;-e}'" 0~" ~0''v «,-0(1:- ~q}'"

,. , ,.~0' ~~'"~0 ~(;-tJ.J ft0

~ (j-'li~~0~ QO ~,tj ~~ ~'b- ~.:s~ e;. ~'I><$If '/)(:-(;'

0.00

0.05

0.35)

0.30

OJ 0.25

o0­W 0.20ClClZ 0.15

US~ 0.10

LAKE MICHIGAN

0.10

0.30

0.40

OJo0-W 0.20ClClZ

US~

OJ 0.3000-WCl 0.20Clz«w~ 0.10

FIG. 4. Mean ratios of DDE:PCB (± SD) forherring gull eggs collected from each of the thir­teen study sites and pooled together for years1979-1996.

0.00 .~'::--::'::-::'L-J----'----'---'----'----'----'----'----L-----'----L.-::'---L---'------'

78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96

YEAR

LAKE HURON0.40

OJ 0.30o0-WClCl 0.20z«w~

0.10

0.00 .'::-:=-==-=~L_JL_J----'__'__'___'____'_-----'----'----L---L-----'--__L.___'

78 79 80 81 82 83 84 85 86 87 88 89 90 91 9293 94 95 96

YEAR

FIG. 3. Mean ratios of DDE:PCB (analyzed asAroclor 1254:1260) for herring gull eggs collectedfrom Lake Huron, Lake Superior, and LakeMichigan from 1979-1996. For years in whichchemical analyses were performed on individualeggs, standard deviations are shown. A referenceline has been included for each site to aid thereader in recognizing the DDE:PCB pattern over

sites within a group. This test indicated two things.First, significant differences in DDE:PCB were de­tected among sites within Groups 2 and 3 (Table I;section 2b: site term) but not between sites inGroup I (Table I; section 2b: site term). Second,

for all groups, significant changes in DDE:PCBwere detected across time after accounting for dif­ferences among sites using the Type III sums ofsquares (Table I; section 2b: year term).

The results of the ANCOVA, which grouped sitestogether, differed slightly from the results of the re­gression analyses, which examined individual sitepatterns. In Group 2, for instance, although a signif­icant increase was observed for all sites grouped to­gether, Niagara River and Port ColborneLighthouse did not show a significant increase inthe ratio as shown by the regression coefficients(Table 2). Similarly, in Group 3, although overall asignificant increase in the ratio was observed, thiswas not observed at Big Sister Island (Table 2).

Scheffe's multiple comparison tests revealedwhere there were significant differences amongsites within each of these groups (Table 2). Siteswith the same letter within each group were not sig­nificantly different from one another. Notice thatsince ANCOVAs were run separately for eachgroup, between-group comparisons of the ratiowere not possible. For herring gull eggs from allsites on the Great Lakes, values of the ratio overtime were not significantly different between thetwo study sites at each of Lake Ontario, LakeMichigan, and Lake Superior. The two sites onLake Erie differed significantly from each other andboth were significantly different from the ratio ob­served on the Niagara River. At Lake Huron, signif­icant differences were detected in the y-intercept

18 Hughes et al.

TABLE 1. The results of the analysis of covariance(s) examining differences in DDE:PCBmeasured for herring gulls among thirteen study sites on the Great Lakes from 1979-1996.Section 1: Results for all thirteen sites grouped together. The interaction term in (a) whichdenotes the result of a test for homogeneity of slopes reveals that the slopes are not homoge­neous and that a test for differences in the y-intercepts among sites is not valid (i.e, site*yearp = 0.0001). Section 2: Three separate analyses of covariance were run on the sites dividedinto three groups: Group I-Snake and Muggs islands; Group 2-Niagara River, Port Col­borne Lighthouse, Middle and Fighting islands; and Group 3-the remaining sites found onthe upper Great Lakes. A non-significant interaction term indicates that within each of thesegroups (a) the slopes are homogeneous. A test for differences in the y-intercepts within thesegroups (b) reveals where differences are significant. df = degrees of freedom; SS = sum ofsquares; MS =mean square; F =calculated F value; P = probability; NS =not significant.

Section 1.Source df SS MS F P

a) Site 12 0.021 0.002 2.66 0.0025Year 1 0.079 0.079 119.71 0.0001Site*Year 12 0.031 0.003 3.86 0.0001

Section 2.Source df SS MS F P

Group 1: a) Site 1 0.0002 0.0002 0.23 NSYear I 0.024 0.024 31.74 0.0001Site*Year I 0.0003 0.0003 0.34 NS

b) Site I 0.002 0.002 2.97 NSYear I 0.024 0.024 32.39 0.0001

Group 2: a) Site 3 0.002 0.0006 3.09 0.0335Year 1 0.001 0.001 6.74 0.0118Site*Year 3 0.001 0.0004 2.10 NS

b) Site 3 0.052 0.017 84.37 0.0001Year 1 0.001 0.001 6.45 0.0135

Group 3: a) Site 6 0.008 0.001 1.49 NSYear I 0.075 0.075 82.79 0.0001Site*Year 6 0.007 0.001 1.34 NS

b) Site 6 0.218 0.036 39.62 0.0001Year I 0.079 0.079 86.53 0.0001

between two of the three sites (Channel Shelter Is­land and Double Island) while both of these siteswere similar to Chantry Island.

Changes in Levels ofDDE and PCB Contaminants

The patterns of change were considered for bothresidues individually from 1979 to 1996 for all siteson the Great Lakes (Table 3). DDE and PCB con­centrations decreased during this period at all sites.The temporal changes of DDE levels (slopes) weresignificantly different from zero in ten out of thir­teen sites (77%). Changes of PCB concentrations

were significantly different from zero at all sites,except Gull Island and Channel Shelter Island. Esti­mates of the change in predicted contaminant levelsusing the regression equations (see % change) dur­ing this period revealed that PCB levels were de­creasing faster than DDE levels, except at PortColborne Lighthouse. This relative pattern accountsfor the general increase in DDE:PCB observed.However, this increase over time is significant onlywhen there is a large difference in the rate of de­crease of DDE and PCB levels at a site. As shownusing regression analyses (Table 2), changes in theratio were significantly different from zero and in-

DDE:PCB in Great Lakes Herring Gull Eggs 19

TABLE 2. Linear regression coefficients (slope) ± SE for DDE:PCB plotted against year and predictedratios for 1979 and 1996 calculated using the derived linear equation for herring gull eggs collected fromthirteen sites on the Great Lakes from 1979-1996. Scheffe's multiple comparison test was used to examinewhether there were significant differences in the y-intercept of the ratio between/among sites. Sites withthe same letter within each group were not significantly different (p < 0.05).

Regression DDE:PCB DDE:PCB Scheffe'sSite Coefficient ±SE 1979+ 1996 Comparison

i) Group 1: Lake OntarioSnake I. 0.0054* 0.0012 0.13 0.22 AMuggs 1. 0.0050* 0.0012 0.1 I 0.20 A

ii) Group 2: Lake Erie and Connecting Channels

Niagara River 0.0013 0.0009 0.10 0.13 APort Col borne Lighthouse -0.0004 0.0008 0.10 0.10 BMiddle I. 0.0013* 0.0003 0.04 0.06 CFighting 1. 0.0015* 0.0005 0.05 0.07 C

iii) Group 3: upper Great Lakes

Channel Shelter I. 0.0037* 0.0007 0.1 I 0.16 AChantry 1. 0.0049* 0.0011 0.1 I 0.20 ABDouble I. 0.0069* 0.0013 0.12 0.24 BCGranite 1. 0.0072* 0.0014 0.14 0.26 CAgawa Rock 0.0047* 0.0016 0.16 0.24 CBig Sister I. 0.0027 0.0017 0.24 0.28 DGull I. 0.0050* 0.0019 0.22 0.30 D

+for some sites (Big Sister, Gull, and Channel Shelter islands) sampling did not occur in 1979; therefore, the year 1980was used in the regression equation to calculate the predicted value of the ratio.

* slope significantly different from zero; p < 0.05.

creasing for sites on Lake Ontario, the upper GreatLakes (except Big Sister Island), Middle Island, andFighting Island. Concurrently, sites that showed asimilar rate of decrease of the two contaminants ex­hibited ratios with slopes that were not significantlydifferent from zero. This pattern occurred at Nia­gara River, Port Colborne Lighthouse, and Big Sis­ter Island. The greatest declines in levels of bothDDE and PCBs were observed at Niagara River,Port Colborne Lighthouse, and Fighting Island rela­tive to all other sites.

DISCUSSION

Spatial and Temporal Trends of the Ratio inHerring Gulls from the Great Lakes

A significant increase was observed in DDE:PCBfrom 1979 to 1996 for herring gulls eggs sampledfrom sites on Lake Ontario and the upper GreatLakes (Groups I and 3, respectively). This increaseresulted from PCB levels declining faster than DDE

at these sites. For sites geographically situated be­tween Groups 1 and 3, that is those on Lake Erieand its connecting channels (Group 2), while anoverall significant change in the ratio was detected,the rate was 56% or less than that found at sites inGroups I and 3 (compare highest regression coeffi­cient at Fighting Island (Group 2) to lowest regres­sion coefficient at Big Sister Island (Group 3)). Fortwo of the four Group 2 sites (Niagara River andPort Colborne Lighthouse), no significant change inthe ratio was detected as both DDE and PCB levelswere declining at similar and very dramatic rates(Tables 2 and 3). These among-group differencescan be explained as follows. A decrease in the ratiofor Group 2 sites relative to other sites could be dueto: factor a) a current local source of or increase inPCB loading or decreased losses of PCBs, factor b)a decrease in DDE loading or increased losses ofDDE, or factor c) both factors a) and b). Each ofthese possibilities will be discussed below.

While the Detroit River has been identified as a

20 Hughes et al.

TABLE 3. Linear regression coefficients (slope) ± SE for log-normalized concentrations of DDE andPCB residue data plotted against year for herring gull eggs collected from thirteen sites on the GreatLakes from 1979-1996. Predicted contaminant levels, expressed in Ilg/g, in eggs for 1979 and 1996 wereestimated using derived linear regression equations. Percent change represents the total change (decline)in predicted contaminant levels from 1979 to 1996.

RegressionPredicted LevelsCoefficient %

Site Residue (lJgIg/year) ±SE 1979+ 1996 Change

( i) Group 1: Lake OntarioSnake I. DDE -0.063* 0.013 8.8 3.0 -66%

PCB -0.096* 0.011 68.9 13.5 -80%Muggs I. DDE -0.067* 0.015 8.0 2.6 -68%

PCB -0.096* 0.012 67.2 13.1 -81%

ii) Group 2: Lake Erie and Connecting ChannelsNiagara River DDE -0.078* 0.011 4.9 1.3 -73%

PCB -0.096* 0.012 48.8 9.5 -81%Port Colborne Lighthouse DDE -0.103* 0.015 5.5 1.0 -82%

PCB -0.098* 0.012 50.3 9.5 -81%Middle I. DDE -0.018* 0.008 2.7 2.0 -26%

PCB -0.043* 0.008 56.9 27.4 -52%Fighting I. DDE -0.082* 0.010 5.2 1.3 -75%

PCB -0.109* 0.011 118.2 18.5 -84%

iii) Group 3: upper Great LakesChannel Shelter I. DDE -0.002 0.018 6.3 6.1 -3%

PCB -0.031 0.015 61.5 37.4 -39%Chantry I. DDE -0.051 * 0.019 3.0 1.3 -57%

PCB -0.085* 0.015 25.7 6.0 -77%Double I. DDE -0.010 0.013 2.7 2.3 -15%

PCB -0.054* 0.012 22.0 8.8 -60%Granite I. DDE -0.032* 0.012 4.9 2.8 -43%

PCB -0.072* 0.013 34.6 10.2 -71%Agawa Rock DDE -0.047* 0.013 4.7 2.1 -55%

PCB -0.073* 0.013 30.1 8.7 -71%Big Sister I. DDE -0.046* 0.019 11.1 5.3 -52%

PCB -0.058* 0.016 50.0 19.0 -62%Gull I. DDE -0.007 0.022 8.4 7.5 -II %

PCB -0.032 0.019 37.2 22.3 -40%

+ see condition in Table 2.* slope significantly different from zero; p < 0.05.

major source of contaminants including PCBs toLake Erie (Oliver and Bourbonniere 1985), a localsource or increase in PCB loading is not responsi­ble for the pattern observed at Group 2 sites. Thehighest rate of decline of PCBs in the Detroit River(Fighting Island) relative to all other sites wasnoted in herring gulls eggs collected from 1979 to1996 (Table 3). Active input of PCBs from currentlocal sources has been severely curtailed since theban on the production of PCBs and legislation

passed on its restricted use in the 1970s (MDEQ1996). Decreased losses of PCBs also were not evi­dent for the Group 2 sites (with the exception ofMiddle Island) as regression coefficients for PCBswere similar if not larger than those for DDE atthese sites (factor a; Table 3).

While high concentrations of PCBs may havebeen important historically in influencing the pat­tern of DDE:PCB (factor a) from the Detroit Rivereastward, the importance of DDE must also be rec-

DDE:PCB in Great Lakes Herring Gull Eggs 21

ognized. Group 2 sites (not including Middle Is­land) had the highest declines (% change) in DDElevels compared to all other sites (Table 3), which,combined with high PCB declines, contributed tothe low regression coefficients observed for ratiosat these sites (factor b). A rapid loss of contami­nants from this system could be related to the factthat Lake Erie has the highest sedimentation rate ofall of the Great Lakes (Eisenreich et al. 1992). Stra­chan and Eisenreich (1988) found that, using massbalance models for data in the 1980s, 68% and 45%of the loss of total DDT and total PCBs, respec­tively, from the Lake Erie water column was attrib­uted to sedimentation. Furthermore, losses of thesecontaminants by this process were highest in LakeErie relative to all of the other Great Lakes. Lowcontaminant levels detected in the water columnhave been linked to high lake productivity and highsedimentation rates in Lake Erie (Allan et al. 1991).A number of mechanisms must be considered whenattempting to explain the DDE:PCB pattern ob­served in herring gulls eggs. One theory to explainthe significant loss of both DDE and PCBs from theLake Erie ecosystem may be as a result of the asso­ciation between each of these contaminants andparticulate matter (especially those highly organic)which settle out of the water column. During theearly part of this study, this mechanism could ac­count for the high declines of the two contaminantsreported in herring gull eggs collected from Group2 sites. By the late 1980s, however, an increase inwater clarity was evident which was attributed tothe removal of suspended particulate matter fromthe water column as a result of filtration by zebramussels (Dreissena polymorpha) (Leach 1993,Howell et al. 1996). The addition of this species tothe Lake Erie ecosystem has resulted in dramaticchanges in the food web structure including a de­cline in phytoplankton densities (Nicholls and Hop­kins 1993) and a shift in the composition of thebenthic invertebrate community (Howell et al.1996). These changes could affect the ecologicalpartitioning of DDT and PCBs and the subsequentavailability of contaminants to biota (Rowan andRasmussen 1992) and affect further the patternshown by the ratio at Group 2 sites. Recent changesin the stable nitrogen isotopic signature for herringgull eggs collected from the two Lake Erie coloniessuggest changes in prey availability associated withchanges in the forage fish community base (C.E.Hebert, personal communication).

Large scale forces such as weather patterns mayalso affect short-term uptake of contaminants in

gulls (Smith 1995). The short residence time ofLake Erie relative to the other Great Lakes mayalso be important. It is difficult to conclude exactlywhat could be generating the DDE:PCB pattern inthis top predator, especially in light of the tremen­dous changes which have occurred in the Lake Erieecosystem. Factor b appears to best explain the de­crease in the ratio at Group 2 sites compared to theother two sites. Whatever the mechanism(s), thefact that DDE and PCB levels were affected in sim­ilar ways in a consistent manner in Lake Erie is in­teresting, an observation which never would havebeen detected upon an examination of the individ­ual contaminant levels alone.

In contrast to the Detroit River where pointsources provided the greatest input of PCBs to theaquatic environment, the upper Great Lakes receivethe majority of PCB inputs from atmospheric depo­sition (Eisenreich et al. 1981, 1992). While theirlarge surface areas facilitate input of airbourne or­ganic pollutants, significant levels of PCBs in thewater column are also lost through volatilization.Both Lake Michigan and Lake Superior are esti­mated to be a net source of PCBs to the atmosphere(Swackhamer et al. 1988, Achan et al. 1993, Jere­miason et al. 1994). High volatilization rates cou­pled with low input of PCBs from point sourcescould contribute the differences in the DDE:PCBpattern observed at Lake Superior, Lake Michigan,and Lake Huron relative to Lake Erie. Differencesin morphometric parameters among all of the GreatLakes should also be considered. For instance,mean depth and water residence time are two para­meters which affect how a lake will respond tophosphorus inputs and whether the lake is classifiedas eutrophic or oligotrophic (Vollenweider 1976).As mentioned above, lake productivity may affectthe rate of the loss of contaminants from the systemas particles scavenge compounds from the watercolumn, settle, and are buried on the lake bottom.

In terms of the spatial pattern and magnitude ofDDE:PCB at Group 2 sites, historically high inputsof PCBs in the Detroit River may at least partiallyexplain the low ratios observed at Lake Erie and itstwo connecting channels relative to all other sites(Figs. 2, 3, and 4). High PCB levels were especiallyevident in herring gulls eggs collected from Fight­ing Island from 1979 to 1984 (see Appendix). Cont­aminant studies of sediments (Frank et al. 1977,Oliver and Bourbonniere 1985) and herring gulleggs (Mineau et al. 1984, Weseloh et at. 1990) inthe Detroit River and Lake Erie in the 1970s and1980s have shown that PCB levels declined with

22 Hughes et al.

distance eastward from Detroit. A similar spatialpattern has been reported in offshore water samples(J.e. Filkins, personal communication, 1983) and inyoung-of-the-year spottail shiners (Notropis hud­sonicus) (Suns et al. 1981). Areas of high industrialactivity, such as Detroit, are associated with lowDDT:PCB ratios and it has been suggested that anincrease in the ratio should be observed with in­creasing distance from these areas (National Insti­tute of Environmental Health Sciences 1972). Inthis study, a general increase in DDE:PCB was ob­served, moving eastward, from Fighting Island toMiddle Island to Port Colborne Lighthouse to Nia­gara River and to sites on Lake Ontario within anygiven year from 1979 to 1996. An increase inDDE:PCB in surficial sediments, calculated usingmean DOE and PCB levels, was also observed insamples collected from the western to the central tothe eastern basins of Lake Erie (Frank et al. 1977).

A difference in the spatial pattern of DDE:PCBin Lake Ontario compared to Lake Erie was appar­ent since the Niagara River has been identified as amajor source of DOE and PCBs (Whittle and Fitzsi­mons 1983). Sediment analyses revealed that DOEresidues were uniformly distributed throughout thelake, suggesting that there were many possiblesources of contamination around Lake Ontario(Frank et al. 1979). This likely contributes to theobserved high DDE:PCB found in eggs at thesesites.

Spatial and Temporal Trends of the Ratio inOther Birds of the Great Lakes

A similar pattern of ratios increasing temporallyand spatially has been found in other Great Lakesavian species. For instance, this was demonstratedin eggs of common terns (Sterna hirundo) fromLake Huron to Lake Erie to Lake Ontario (Weselohet al. 1989), where the ratios were high in LakeHuron, low in Lake Erie, and high again in LakeOntario. Although a temporal increase was ob­served, in this case, only a few years were exam­ined (1971/72 and 1981) and PCBs had beendetermined using an Aroclor 1260 standard. Postu­palsky (1978) also detected an alternating high,low, high pattern in the ratio in double-crested cor­morants (Phalacrocorax auritus) sampled in LakeHuron, Lake Erie, and Lake Ontario, respectively,in the early 1970s. A temporal increase was alsoobserved at all of these sites, although again, only afew years were examined.

Intraspecific Comparison of DDE:PCB forHerring Gulls across Canada

Comparisons of DDE:PCB of herring gulls eggsacross Canada from 1968 to 1972 suggested that therelative levels of these two contaminants differedacross the continent (Vermeer and Peakall 1977).High ratio values were determined for glaucous­winged gulls (Larus glaucescens, a species closelyrelated to herring gulls) in British Columbia (ap­prox. 0.50) and for herring gulls in the prairies (ap­prox. 0.50) and the Gulf of St. Lawrence (approx.0.38). A relatively low DDE:PCB value of 0.16,however, was determined for herring gulls in theGreat Lakes (Ontario) during this period. Twenty­five years later in 1996, the mean ratio for herringgulls in the Great Lakes was still very low andranged from 0.06 at Middle Island on Lake Erie to0.29 at Granite Island on Lake Superior. In light ofthe high levels of PCBs still detected in the GreatLakes area, the low ratio observed here may be a"signature" for the herring gulls in this region.

DDE:PCB in Birds fromAquatic and Terrestrial Habitats

DDE:PCB has been used to reflect on similaritiesand differences in diets among avian species in agiven locality and time frame (Bogan and Bourne1972, Frank et al. 1975, Ohlendorf et al. 1982,Struger and Weseloh 1985, Weseloh et al. 1989).Frank et al. (1975) found an interesting relationshipwhen examining contaminants in the eggs of terres­trial and aquatic bird species in the Niagara regionof southern Ontario. They found that DDT:PCBwas greater than one for terrestrial feeders, whereasit appeared to be less than one for aquatic-feedingspecies. This was not surprising since in areas oflocal contamination, terrestrial foragers may bemore exposed to DDT which is an agricultural in­secticide while aquatic foragers may be more sus­ceptible to PCBs which have entered the aquaticenvironment primarily as a result of point sourcedischarges or by atmospheric deposition.

The above-mentioned relationship betweenDDE:PCB and feeding habits using values gener­ated by Gilbertson and Reynolds (1974) for conta­minants in birds' eggs measured across Canadafrom 1968 to 1972 is illustrated in Table 4. Indeed,many predominantly terrestrially-feeding birds (A),such as owls and hawks, have values greater thanone, while those which feed aquatically (B), includ­ing gulls and petrels, have values less than one.This general pattern has been demonstrated in more

DDE:PCB in Great Lakes Herring Gull Eggs

TABLE 4. DDE:PCB in birds' eggs from terrestrial (A) and pelagic (B) habitatsacross Canadafrom 1968-1972. All values taken from Gilbertson and Reynolds (1974),except (*) values taken from Frank et aI. (1975).

23

(A)

(B)

Species

Goshawk-Accipiter gentilisRed-tailed Hawk-Buteo jamaicensisGolden Eagle-Aquila chrysaetosNorthern Harrier-Circus cyaneusMerlin-Falco columbariusGreat Horned Owl-Bubo virginianusLong-eared Owl-Asio otusShort-eared Owl-Asio flammeusCommon Crow *-Corvus brachyrhynchosAmerican Robin *-Turdus migratoriusEuropean Starling *-Sturnus vulgaris

Fork-tailed Petrel-Oceanodroma furcataLeach's Petrel-Oceanodroma leucorhoaDouble-crested CormorantDouble-crested CormorantEider Duck-Somateria mollissimaHerring GullHerring GullGlaucous-winged GullRing-billed Gull-Larus delawarensisCommon Tern

Location

New BrunswickSaskatchewanAlbertaSaskatchewanQuebecSaskatchewanSaskatchewanSaskatchewanOntarioOntarioOntario

British ColumbiaNew BrunswickOntarioNew BrunswickNew BrunswickOntarioNew BrunswickBritish ColumbiaOntarioOntario

Ratio

76.506.033.519.964.247.749.54

41.701.842.501.54

0.270.610.540.630.140.230.560.480.160.18

recent studies of terrestrial birds (Noble and Elliott1990, Hebert et al. 1994) and aquatic birds (Bourneand Bogan 1972; Ohlendorf et al. 1978, 1982; We­seloh et al. 1979; Heinz et al. 1985; Elliott et al.1989; Pearce et al. 1979, 1989). It should be notedthat this rule is somewhat dependent on local DDTusage and contamination levels: in areas of lowDDT use, the ratio may be equal to or less than one(Hebert et al. 1994). Thus, it appears thatDDE:PCB may be indicative of a bird's foragingecology.

DDE:PCB in Faculative and ObligateAvian Piscivores

The large CWS database on contaminants inGreat Lakes fish-eating birds (Bishop et al. 1992,Pettit et al. 1994) allows us to compare ratios infacultative and obligate avian piscivores. Onewould expect that an almost exclusively picivorousspecies found on the Great Lakes would carry acontaminant load which is predominately composedof PCBs relative to DDTs and/or DDEs (i.e., lowratio); similarly, a species which feeds both aquati­cally and terrestrially, such as the herring gull (Fox

et al. 1990), might show a somewhat higher ratio.This does not appear to be the case. DDE:PCB foreggs of piscivorous birds relative to those of her­ring gulls sampled from all five Great Lakes areshown in Table 5. Piscivorous (or mostly aquatic­feeding) species in this region include: commonterns, double-crested cormorants, Caspian terns(Sterna caspia), and black-crowned night-herons(Nycticorax nycticorax). All of these species gener­ally show higher mean values of DDE:PCB thanherring gulls within the same lake, with the excep­tion of Caspian terns which show similar ratios tothe gulls (Struger and Weseloh 1985). The highestratios are shown by common terns and are at least1.7 times greater than that in herring gulls from thesame location. Previous studies have shown thesame patterns of high ratios for all of these birdsrelative to herring gulls. Double-crested cormorantshave been shown to have higher ratios than herringgulls in the prairies, Great Lakes, and Gulf of St.Lawrence within a given time period (Gilbertsonand Reynolds 1974, Vermeer and Peakall 1977,Heinz et al. 1985). Similarly, this relationship toherring gulls has been shown for black-crowned

24 Hughes et al.

TABLE 5. Comparison ofDDE:PCB for herring gulls and other piscivorous birds from within the samelakelconnecting channel on the Great Lakes. PCB determined as Aroclor 1260 standard for commonterns. For all other species, PCBs determined as Aroclor 1254:1260 (1:1). N represents the number ofeggs sampled.

Location Species Year N Ratio Reference

Lake Ontario Herring Gull 1981 10 0.14 This study(Snake I.)

Lake Ontario Caspian Tern 1981 8 0.13 Struger and Weseloh (1985)(Pigeon I.)

Lake Ontario Double-crested Cormorant 1981 10 0.18 Bishop et at. (1992)(Pigeon I.)

Lake Ontario Black-crowned Night-Heron 1982 12 0.20 Bishop et al. (1992)(Pigeon I.)

Lake Ontario Common Tern 1981 10 0.24 Weseloh et al. (1989)(Toronto Harbour)

Lake Huron Herring Gull 1980 10 0.15 This study(Double I.)

Lake Huron Caspian Tern 1980 10 0.15 Struger and Weseloh (1985)(Cousins I.)

Lake Huron Double-crested Cormorant 1979 9 0.12 Bishop et al. (1992)(Africa Rock)

Lake Huron Black-crowned Night-Heron 1982 0.21 Bishop et al. (1992)(Chantry I.)

Lake Huron Common Tern 1981 10 0.62 Weseloh et al. (1989)(N. Limestone I.)

Lake Erie Herring Gull 1982 10 0.05 This study(Middle I.)

Lake Erie Double-crested Cormorant 1981 10 0.09 Bishop et at. (1992)(Big Chicken I.)

Lake Erie Black-crowned Night-Heron 1982 10 0.06 Bishop et at. (1992)(Middle I.)

Lake Erie Common Tern 1981 10 0.20 Weseloh et at. (1989)(Port Colborne)

Detroit River Herring Gull 1981 10 0.05 This study(Fighting I.)

Detroit River Common Tern 1981 10 0.12 Weseloh et al. (1989)(Fighting I.)

Lake Michigan Herring Gull 1980 10 0.22 This study(Gull I.)

Lake Michigan Caspian Tern 1980 9 0.18 Bishop et at. (1992)(Isle Aux Galets)

Lake Superior Herring Gull 1983 II 0.18 This study(Agawa Rock)

Lake Superior Double-crested Cormorant 1983 0.19 Bishop et at. (1992)(Gravel I.)

DDE:PCB in Great Lakes Herring Gull Eggs 25

night-herons in the lower Great Lakes and NiagaraPeninsula and Green Bay, Lake Michigan (Frank etal. 1975, Gilbertson and Reynolds 1974, Heinz etal. 1985) and finally, for common terns in theprairies, Green Bay, Lake Michigan and NiagaraPeninsula (Frank et al. 1975, Vermeer and Peakall1977, Heinz et al. 1985). Therefore, these piscivo­rous/aquatic feeding birds do not have the predictedlower ratio relative to herring gulls in this region.

How can this apparent contradiction be ex­plained? First, it is very difficult to compare ratiosfor piscivorous species with those of herring gullsbecause, not only do their diets differ, but, perhapsmore importantly, because all of these species, ex­cept herring gulls, are migratory. Therefore,organochlorine residue levels measured in thesebirds may reflect a combination of contaminantsfound not only on their breeding grounds but alsoon migratory routes and wintering grounds (Rise­brough et al. 1968, Ewins et al. 1994). These birdsmay be subject to varying proportions of DDT/DDEand PCBs in the environment. For example,Caspian and common terns migrate for at least 6months of the year to central and northern SouthAmerica (Ludwig 1965, Haymes and Blokpoel1978), where DDT has been used extensively (Bur­ton and Philogene 1986). Double-crested cor­morants winter in Florida and along the Gulf Coast(Dolbeer 1991) where both DDE and PCBs havebeen identified in over-wintering cormorant tissues(King et al. 1987). Findholt and Trost (1985) havesuggested that black-crowned night-herons are ex­posed to DDT on wintering grounds in southwest­ern U.S. and Mexico. High levels of DDT at thesenon-breeding grounds might contribute to thehigher DDE:PCB that common terns, black­crowned night-herons and double-crested cor­morants show relative to herring gulls. Eggs areneeded from strictly piscivorous non-migratoryspecies in the Great Lakes in order to compare toherring gulls to discern whether the ratio would belower for these birds than for gulls, as expected. Inany case, these findings stress the importance ofusing a non-migratory species, such asthe herring gull, as an indicator of environmentalcontamination.

EVALUATION OF PREVIOUS USES

At the beginning of this paper five interpretationswere described of how DDE:PCB has been pre­sented in the past. The validity of these interpreta­tions will be examined, excluding number three,

based on these findings along with those found inthe literature. First with regard to use of the ratio asa reflection of dietary differences among species, itis likely that in areas of moderately to high levels ofDDT/DDE and PCB contamination, DDE:PCB maybe indicative of a bird's foraging ecology (i.e., aseither an aquatic or strict terrestrial feeder). How­ever, it was not possible to use the ratio to distin­guish foraging habits between faculative andobligate avian piscivores. The inability to detectdifferences of this type may have been becausefood preference of aquatic feeders was confoundedby migratory habits.

Second, the ratio has been used as a reflection ofthe relative rates of decrease or increase of DDEand PCBs in food over time. While this is a validuse of the ratio, an examination of temporalchanges in individual contaminant levels is neces­sary in order to provide some insight on the patternshown by the ratio. The observed increase in theratio at sites on Lake Ontario and the upper GreatLakes is a result of PCB levels declining faster thanDDE levels from 1979 to 1996. While a significantincrease was observed at sites on Lake Erie and itsconnecting channels, this increase was greatly re­duced (relative to other sites) largely as a result ofthe two contaminants declining at equal rates dur­ing this period. Exactly what accounts for these ge­ographic differences in trends of DDE:PCB is notclear. Contaminant, phosphorus, and food web dy­namics as well as lake morphometry, changes inherring gull diet, and large-scale forces such asweather events are but a few mechanisms whichmust be considered in discerning trend differencesamong the lakes. Nevertheless, contaminant ratiosand multi-chemical comparisons, in general, arevaluable since they may provide insight into simi­larities/differences in the apparent fates of thesechemicals in different ecosystems.

Third, the usefulness of the ratio in interspecificcomparisons of birds is unclear, since the compar­isons involved birds that differ in migratory pat­terns. However, the literature suggests that based onthe patterns of accumulation of PCBs shown by avariety of birds including herring gulls, great blueherons (Ardea herodias), and peregrine falcons(Falco peregrinus), there appears to be little differ­ence in PCB accumulation or metabolism in thesebirds (Norstrom 1988). Furthermore, the who1e­body half lives of DDE and PCBs in herring gulls(Norstrom et al. 1986, Clark et al. 1987, Brauneand Norstrom 1989) are quite similar (200 to 300days). Therefore, it appears that this ratio cannot be

26 Hughes et al.

used to assess differences in accumulation and me­tabolism among avian species.

Finally, the ratio does appear to be useful to com­pare the relative availability of DDE and PCBs indifferent geographical areas. Due to the high avail­ability of PCBs, the lower Great Lakes, especiallyLake Erie, show a low mean value of DDE:PCB inherring gull eggs, as well as in other avian species'eggs, relative to the other Great Lakes. Similarly ona larger scale as a result of high PCB levels, herringgulls from the Great Lakes region show a lowerratio (signature) than herring gulls found elsewherein Canada. It is these differences in levels ofDDT/DDE and PCBs, and consequently their toxiceffects, which place birds from different geographi­cal areas at different levels of risk. Ultimately, how­ever, as in the case of temporal studies of the ratios,it is necessary to examine individual contaminantlevels before drawing any definite conclusions re­garding their ratios.

Differences in DDE:PCB between migratory andnon-migratory birds, i.e., the failure of the ratios toalign as predicted for facultative and oligate pisici­vores, emphasize the importance of using non-mi­gratory species as indicators of regionalenvironmental contamination. Contaminant levelsmeasured in eggs of migratory species found in theGreat Lakes during the breeding season may be par­tially influenced by exposure to contaminants onmigration routes and wintering grounds, thus dis­torting the relative positions of the ratios. Since thesame individual gulls do not generally feed at oppo­site ends of the lakes (D.V.W. personal observation)and since similarities were found in the temporalpatterns shown by the values of the ratios at threeof the five Great Lakes, the relative levels of thetwo contaminants in the diet of herring gulls maybe similar within each of these lakes. For Lake Erie,the pattern of exposure of DDE and PCB over timein herring gulls differed from one end of the lake tothe other; these differences were not as dramatic atLake Huron, although among-site differences inDDE:PCB were detected. For all of the GreatLakes, general similarities in the patterns of theratio shown by eggs collected from the same lake orat sites in close proximity to one another providefurther evidence that herring gulls are important asmonitors of regional contaminant conditions.

ACKNOWLEDGMENTS

We would like to thank Craig Hebert, PeterEwins, Ross Norstrom, Helen Rodd, and Donna

Stewart for reviewing an early draft of this paper.Brian Collins provided statistical advice. We wouldalso like to thank L. Jackson and D. Smith for theircomments on this manuscript.

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Submitted: 10 January 1996Accepted: 7 January 1998Editorial handling: Thomas J. Murphy

APPENDIX. Concentrations of DDE and PCB, expressed in pg/g, for herring gull eggs collected from thirteen sites on the Great ~0

Lakes from 1979-1996. Eggs were either analyzed individually or as a pooled sample. In cases where N > 1, N represents the numberof eggs analyzed individually and mean contaminant levels ± SD are shown. In cases where N = 1, a pooled sample of ten eggs wasanalyzed and no estimate of variance is given. All data are from Bishop et al. (1992), Pettit et al. (1994) and CWS unpublished.

1979 1980 1981 1982 1983 1984 1985 1986

SNAKE I. DDE 8.83 ± 2.96 7.12±4.07 11.75 ± 4.54 8.75±2.19 5.07 ± 1.77 7.21 ± 2.02 7.15±3.00 4.71 ± 2.16PCB 62.90 ± 17.78 53.49 ± 23.62 85.62 ± 40.65 61.48 ± 12.10 46.18 ± 18.11 53.34 ± 15.67 34.59 ± 14.90 29.45 ± 9.25N 10 10 10 10 11 10 10 10

MUGGST. DDE 9.05 ± 3.47 8.17 ± 5.82 10.26 ± 5.16 11.47±4.40 4.49 ± 1.61 5.32 ± 2.28 4.90 ± 1.33 4.00 ± 1.04PCB 76.29 ± 29.74 59.69 ± 28.83 72.17 ± 29.98 64.47 ± 20.47 39.01 ± 15.27 48.87 ± 20.13 36.57 ± 8.62 25.04 ± 5.46N 10 9 10 9 II 10 10 10

NIAGARA RIVER DDE 4.01 ± 1.33 5.68 ± 2.64 3.65 3.87 ± 1.14 3.35 ± 1.08 4.12 ± 1.09 2.73 ± 1.02PCB 50.47 ± 22.51 50.35 ± 25.10 45.50 34.06 ± 20.37 37.27 ± 14.52 29.13±11.02 22.55 ± 10.62N 10 0 10 1 II 10 10 10

PORT COLBORNE L. DDE 3.44 ± 1.00 3.40 ± 1.51 4.74 ± 1.72 7.46 3.05 ± 1.31 4.43 ± 1.37 3.62 ± 1.13 3.21PCB 37.59 ± 6.22 37.59 ± 15.75 44.36 ± 12.88 59.70 36.76 ± 21.37 45.13 ± 14.66 29.66 ± 12.81 23.70N 10 9 10 I 10 10 10 I

MIDDLE I. DDE 2.76 ± 0.59 2.60 ± 0.65 3.06 ± 0.93 2.63 ± 1.03 1.66 ± 0.85 2.03 ± 1.03 2.04 ± 0.57 2.32PCB 59.28 ± 12.77 54.29 ± 11.56 68.61 ± 15.74 58.81 ± 23.88 37.92 ± 15.41 47.26 ± 15.85 47.15±1O.46 43.00N 10 10 10 10 10 10 10 I

FIGHTING I. DDE 5.93 ± 1.50 5.42 ± 1.57 3.99 ± 1.33 3.16 ± 0.89 3.45 ± 1.28 3.47 ± 2.23 2.37 g:PCB 138.72 ± 44.95 111.02 ± 33.36 75.86 ± 25.76 65.36 ± 18.00 84.79 ± 21.19 48.17 ± 17.06 41.30N 10 0 10 10 II 10 10 I ~:=-

CHANNEL SHELTER T. DDE 8.90 ± 3.77 7.30 ± 2.05 8.10 ± 2.91 6.06 ± 1.94 5.41 ± 2.06 4.81 ± 1.37 5.95 ~

'"PCB 69.55 ± 23.07 65.28 ± 19.06 71.57 ± 31.44 48.58 ± 13.54 54.84 ± 27.03 48.40 ± 15.30 46.00 ~....N 10 10 10 II 10 10 I

~CHANTRY I. DDE 2.54 ± 1.69 2.83 ± 1.44 4.08 ± 1.86 5.03 ± 3.23 2.22 ± 1.01 2.55 ± 1.68 2.47 ± 1.29 2.00PCB 31.08 ± 22.94 23.41 ± 14.54 27.70 ± 9.85 38.06 ± 33.71 16.86 ± 3.45 21.54 ± 9.94 14.25 ± 9.55 12.30N 10 10 10 10 11 10 10 I

DOUBLE I. DDE 2.06 ± 0.76 2.60 ± 1.28 3.56 ± 1.40 3.84 ± 2.06 3.25 ± 2.02 2.58 ± 0.72 3.08 ± 1.37 2.10PCB 26.24 ± 7.36 17.41 ± 7.75 23.08 ± 7.63 30.51 ± 15.22 19.69 ± 6.56 18.36 ± 6.37 19.56 ± 5.77 11.70N 10 10 10 10 II 10 10 I

GRANITE I. DDE 6.39 ± 3.66 3.65 ± 0.99 5.23 ± 0.93 6.75 ± 4.75 3.40 ± 0.91 3.30 ± 1.84 3.30PCB 59.05 ± 16.92 27.48 ± 9.30 31.33 ± 5.61 39.70 ± 24.50 17.81 ± 5.04 19.51 ± 8.71 14.30N 10 10 10 10 0 10 10 I

AGAWAROCK DDE 7.26±5.41 3.70 ± 3.54 6.26 ± 3.09 5.83 ± 2.19 3.17±1.10 2.49 ± 0.85 2.97 ± 0.71 3.13PCB 58.43 ± 33.07 23.68 ± 12.17 36.35 ± 17.76 29.78 ± 8.37 21.42 ± 19.58 16.01 ± 5.41 12.26 ± 2.57 13.90N 10 10 10 10 11 10 I

BIG SISTER I. DDE 11.38 ± 2.61 17.89 ± 5.81 6.76 ± 1.77 9.34 ± 4.05 7.93 ± 7.44 7.05PCB 56.95 ± 12.06 66.89 ± 23.38 27.66 ± 9.48 34.00 ± 11.15 36.82 ± 37.21 27.00N 0 10 0 10 10 10 10 I

GULLI. DDE 12.95 ± 6.48 13.84 ± 3.83 6.19±1.37 6.37 ± 2.33 5.92 ± 1.59 7.91PCB 58.70 ± 23.07 63.93 ± 29.78 32.65 ± 19.20 28.93 ± 7.44 26.51 ± 7.36 27.50N 10 10 II 10 9 I

Continued

Appendix (continued).

1987 1988 1989 1990 1991 1992 1993 1994 1995 1996

SNAKE I. DOE 2.85 5.15 5.20 3.37 3.47 ± 2.31 5.02 6.62 4.01 2.59 2.62PCB 17.34 26.67 31.24 18.15 \6.22 ± 7.00 20.58 23.34 19.95 14.76 13.99N 1 1 1 1 13 1 1 1 I 1

MUGGS I DOE 2.35 3.34 5.35 3.35 3.58 ± 1.54 4.99 3.92 3.64 1.86 3.43PCB 15.62 20.39 33.65 18.73 17.98 ± 5.72 21.77 18.78 19.52 12.37 18.26N 1 1 1 1 13 1 1 1 1 1

NIAGARA RIVER DOE 1.51 1.69 2.08 2.02 1.73 1.78 1.67 1.81 1.16 1.51PCB 12.83 12.45 19.15 15.72 13.49 15.20 14.65 14.56 11.04 11.28N 1 1 1 1 1 1 1 1 1 1 gPORT COLBORNE L. DOE 1.74 ± 0.95 1.93 3.15 1.56 1.55 1.21 ± 0.57 0.98 0.88 1.15PCB 15.63 ± 5.62 17.59 33.18 17.31 14.99 13.80 ± 4.59 11.12 11.03 10.02 ~N 10 1 I 1 1 13 I 1 I '.

"'tlMIDDLE I. DOE 1.70 ± 0.47 2.21 2.24 2.47 2.70 2.22 ± 0.81 2.00 2.18 1.97 1.35 ~

PCB 27.88 ± 6.28 37.40 45.24 42.87 38.11 35.50 ± 12.50 32.26 34.71 36.12 21.00 ~

N 10 1 1 1 1 13 1 1 1 1....:=

FIGHTING I. DOE 2.24 ± 0.84 3.18 2.24 1.99 1.14 2.28 ± 0.92 2.14 1.68 1.53 1.16 ~PCB 33.77 ± 9.01 60.56 56.63 39.48 22.17 30.65 ± 6.59 26.80 25.38 20.73 17.36

.,~

N 10 1 1 1 1 13 1 1 1 I l::l-CHANNEL SHELTER 1. DOE 4.00 4.55 ± 1.73 7.02 5.89 8.04 7.42 14.83 ± 11.13 7.78 4.23 4.35 t""PCB 31.26 37.87 ± 13.25 51.94 47.10 48.07 45.18 86.05 ± 51.63 47.42 24.63 28.20 l::l

;z,;o.N 1 16 1 1 I 1 13 1 I 1 ~

'"CHANTRY I. DOE 1.04 1.12 ± 0.69 0.77 1.61 1.87 2.06 2.67 ± 2.69 1.27 1.57 1.35 :J::PCB 7.71 8.01 ± 10.81 6.25 I 1.21 10.15 10.19 9.72 ± 5.88 8.26 9.50 7.30 ~

N 1 13 1 1 I 1 10 I I 1 ~....DOUBLE I. DOE 1.60 1.69 ± 0.76 2.37 2.1 I 2.07 2.66 3.67 ± 2.51 3.10 1.63 2.66 :=

~PCB 8.95 9.27 ± 2.33 14.13 11.47 9.87 10.21 12.18 ± 6.69 14.15 8.41 12.80

~N I 13 1 1 1 I 10 1 1 1 ;::

GRANITE 1. DOE 2.90 2.75 ± 1.28 -3.17 2.40 4.25 3.76 5.20 3.01 2.68 ± 1.25 2.69 -PCB 13.41 15.54 15.15 11.69 ± 4.44 15.50 14.72 20.39 13.32 12.19±3.88 9.41 ~N I I 1 13 1 I I 1 13 1 ~

'"AGAWAROCK DOE 2.15 2.71 2.59 2.51 ± 1.46 2.94 3.61 2.97 1.77 2.29 ± 1.48 2.88PCB 11.29 11.32 15.02 11.39 ± 4.31 12.68 13.20 10.97 11.30 10.15 ± 4.69 12.63N I 1 1 13 1 1 1 1 13 I

BIG SISTER I. DOE 12.31 3.97 4.44 ± I.(J2 8.03 8.00 4.92 8.09 6.37 ± 2.12 7.92 3.95PCB 44.52 16.61 21.1 I ± 3.99 32.65 24.86 17.23 28.02 23.19 ± 5.10 26.56 16.83N 1 1 13 I I I 1 13 1 1

GULLI. DOE 3.95 6.11 5.03 ± 2.92 8.22 12.44 8.49 11.03 12.24 ± 7.43 4.84 6.10PCB 16.58 21.66 20.76 ± 9.68 31.73 37.69 23.27 39.48 38.48 ± 18.06 20.01 22.74N I I 13 I 1 1 I 12 1 I