Influence of diet in the accumulation of organochlorine compounds in herons breeding in remote riverine environments

Chemosphere 145 (2016) 438e444

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Influence of diet in the accumulation of organochlorine compounds in herons breeding in remote riverine environments David Huertas a, Joan O. Grimalt a, *, Lluis Jover b, Carola Sanpera c a

Department of Environmental Chemistry, IDAEA-CSIC, Jordi Girona 18-26, 08034 Barcelona, Catalonia, Spain Department of Public Health, Faculty of Medicine, University of Barcelona, Casanova 143, 08036 Barcelona, Catalonia, Spain c Department of Animal Biology, Faculty of Biology, University of Barcelona, Av. Diagonal 645, 08028 Barcelona, Catalonia, Spain b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Organochlorine compounds (OCs) accumulate more in egrets feeding on aquatic preys.
Lower OC accumulation is found in egrets that feed on aquatic and terrestrial preys.
OCs of low biomagnification potential do not show selective accumulation in egrets.
The diet differences between species are confirmed with the d13C and d15N ratios.
p,p’-DDT in cattle egret is maybe still showing influences of past agriculture use.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 August 2015 Received in revised form 23 November 2015 Accepted 26 November 2015 Available online 13 December 2015

The composition of organochlorine compounds (OCs), pentachlorobenzene (PeCB), hexachlorobenzene (HCB), hexachlorocyclohexanes (HCHs), DDTs and polychlorobiphenyls (PCBs), has been analyzed in eggs from cattle egret (Bubulcus ibis) and little egret (Egretta garzetta), two species of herons (family Ardeidae), nesting at the same remote riverine environment (Aiguabarreig, Ebro River). These two species were selected to evaluate the importance of diet in the accumulation of OCs. Cattle egret essentially feeds on dry grassy habitats and follow cattle or other large animals whereas little egret feeds on fish, amphibians and crustaceans captured in shallow waters. The d15N and d13C isotopic composition of the sampled eggs was studied and the results were consistent with these species feeding habits. In both species, the compounds accumulated the most were the less volatile and more lipophilic, e.g. PCB congeners of higher chlorination, DDT and metabolites. The distinct foraging species preferences were reflected in significant higher concentrations in little egret than cattle egret of all pollutant groups analysed. These differences were statistically significant for DDTs and PCBs (p < 0.015 and p < 0.047, respectively), e.g. the p,p’-DDE and PCB concentrations were 6 and 4.5 times higher, respectively, in the former than the latter. This strong contrast indicates that in remote environments aquatic riverine ecosystems are more efficient OC reservoirs than the terrestrial ecosystem. © 2015 Elsevier Ltd. All rights reserved.

Handling Editor: Caroline Gaus Keywords: Herons Organochlorine compounds Egrets Diet Riverine systems Stable isotope composition

* Corresponding author. E-mail address: [email protected] (J.O. Grimalt). http://dx.doi.org/10.1016/j.chemosphere.2015.11.101 0045-6535/© 2015 Elsevier Ltd. All rights reserved.

D. Huertas et al. / Chemosphere 145 (2016) 438e444

1. Introduction Aquatic ecosystems are crucial environments during the breeding season of a great number of birds which obtain protection and feeding opportunities between reedbeds, bushes, trees or mangroves. Unfortunately, birds living in these environments accumulate organochlorine compounds (OCs) which may compromise reproduction and species survival. Thus, p,p’-DDE has been described to generate deleterious effects such as low eggshell thickness, embryo mortality and chick malformations (Aurigi et al., 2000; De Luca-Abbot et al., 2001). In birds of prey, decreases in eggshell thickness and reproduction impairment have been observed for p,p’-DDE egg concentrations above 2 mg/g ww (Pain et al., 1999). In some species of herons, reproduction disturbances, mainly non-viable eggs, and chick mortality have been observed for 2 and 3.5 mg/g ww (Thomas and Anthony, 1999; McEwen et al., 1984). The effects of polychlorobiphenyls (PCBs) and other organochlorine compounds are difficult to assess independently from those of DDTs. The lowest observable effect concentration (LOEC) for total PCBs in cormorants for reproductive success was observed to be 3.5 mg/g ww (Tillitt et al., 1992) and 30 mg/g ww were correlated with embryo mortality (Sakellarides et al., 2006; Verreault et al., 2006; Erikstad et al., 2011). The specific properties of these pollutants, namely their chemical stability and lipophilicity, has led to their widespread occurrence in all continental ecosystems, including the most remote sites (Grimalt et al., 2001). These properties also involve increasing accumulation along the food chain and highest concentrations in the high predators (Catalan et al., 2004). Bird predators may therefore be used as sentinel organisms of the pollution burden of the food webs that sustain them. Attempts to identify specific food web effects have been rarely addressed. Comparison of concentrations of OCs between top bird predators may be confounded by metabolic differences between species or differences in the concentrations of OCs in the environments where they live, among other aspects. This topic has been addressed in the present study by comparison of the OC concentrations in cattle egret (Bulbucus ibis) and little egret (Egretta garsetta), two species of herons (family Ardeaide), living in one remote riverine site, Aiguabarreig (Ebro River, 41230 5900 N, 0190 3800 E). This site is a freshwater swamp containing shallow waters and has a high ecological value as breeding zone for migratory species. The location is full of islands surrounded by slow waters and extended riparian forests that make it especially suitable for waterbirds that nest in aquatic environments. It is formed at the confluence of Segre and Cinca, two tributaries of the Ebro River (Fig. 1). The river discharges into the Mediterranean Sea forming a large Delta that has been catalogued as an UNESCO wildlife and bird reserve (Pastor et al., 2004). Fig. 1 shows some key areas from the Ebro River containing heron colonies. Cattle egret is usually found in woodlands near lakes or rivers, in swamps or in small inlands, sharing space with other wetland birds. Unlike most other herons, it feeds in relatively dry grassy habitats, often accompanying cattle or other large mammals, since it catches insects, spiders, earthworms and small vertebrates disturbed by these animals (Telfair and Raymond, 2006). Little egret prefers platforms of sticks in trees or shrubs and sometimes reed beds or bamboo groves (BirdLife International, 2011). This species commonly share breeding zones with cattle egret but it preferably eats fish, amphibians and crustaceans which are captured in shallow waters nearby the nesting zone. In Catalonia, both species are found in irrigated herbaceous crops and wetlands located between 0 and 200 m above sea level (ICO, 2011a and b).

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The stable isotope signatures of carbon (d13C) and nitrogen (d15N) were studied to evaluate assimilated, and not only ingested, diet. These signatures have been extensively used in studies of waterbird communities (Hobson et al., 2000; Ramírez et al., 2011; Rodríguez et al., 2013). d15N reflects the trophic level, with consumer signatures being higher than in their prey whereas information on the carbon sources entering into the food webs can be obtained from d13C (Hobson, 1999). The two selected heron species live in the same remote environment under the influence of long-range OC transport. Comparison of the OC concentrations in their eggs offers an opportunity for identification of specific food web effects on the accumulation of OCs in these top predators. This approach is particularly important in studies from southern Europe given the limited information on pesticide and PCB contamination of Ardeidae in this world region (Focardi et al., 1988; De Cruz et al., 1997; Sakellarides et al., 2006). 2. Material and methods 2.1. Sampling Eighteen eggs were collected during the spring season of 2006 at Aiguabarreig. Half belonged to cattle egret and half to little egret. Eggs were collected in different nests in each colony. The smallest egg was selected in each laying. They were labelled and kept refrigerated during transport to the laboratory where they were stored frozen (20  C) until analysis. Egg content was separated from the eggshell, weighed, and placed into a glass container for freeze-drying. Freeze-dried samples were homogenized and two sample aliquots were used, one for OC analysis and the other for stable isotope determinations. 2.2. Isotope analysis The samples for isotope analysis were extracted with methanol and chloroform. Lean sub-samples (ca. 0.36 mg) were placed in tin buckets and crimped for combustion. The instrumental determinations were carried out by elemental analysis-isotope ratio mass spectrometry using a Thermo Finnigan Flash 1112 elemental analyser coupled to a Delta isotope ratio mass spectrometer via a CONFLO III interface. The stable isotope ratios were expressed in conventional notation as parts per thousand (‰) following the equation: dX ¼ [(R sample/R standard) - 1]*1000, where X is 15N or 13C and R is the corresponding 15N/14N or 13C/12C ratio. The standards for 15N and 13 C were atmospheric nitrogen and Pee Dee Belemnite, respectively. Precision and accuracy for the d13C and d15N measurements were 0.1‰ and 0.3‰, respectively. The laboratory used the following international standards that were run every 12 samples: IAEA CH7 (87% of C), IAEA CH6 (42% of C) and USGS 24 (100% of C) for 13C and IAEA N1 and IAEA N2 (with 21% of N) and IAEA NO3 (13.8% of N) for 15N. 2.3. Sample preparation and clean up for OC analysis The individual eggs samples (1.5 g) were ground with activated sodium sulphate until a fine powder was obtained. The powder mixtures were then introduced into previously cleaned cellulose cartridges (24 h in Soxhlet). These mixtures were Soxhlet-extracted with 100 mL of n-hexaneedichloromethane (4:1 v/v) for 18 h. At this step, 1,2,4,5-tetrabromobenzene (TBB) and PCB-200 were added as recovery standards. The lipid content of all samples was determined gravimetrically from an aliquot of the extract. The rest of the extract was concentrated under vacuum to 2 mL and 2 mL of sulphuric acid were added. After vigorous stirring in a Vortex-mixer

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Fig. 1. Map of the Catalan region of Ebro River including the sampled site.

(2 min) the mixture was centrifuged to remove any foam in the interface and the sulphuric acid layer was discarded. This clean-up step was repeated until a colourless transparent n-hexane layer (2 mL) was obtained (3e5 times). The final sulphuric acid mixture was re-extracted with n-hexane (2  2 mL) and all n-hexane solutions were combined and concentrated by vacuum rotary evaporation (20  C, 20 Torr) to small volumes (ca. 300 mL). The solutions were then transferred to vials and evaporated just to dryness under a gentle stream of nitrogen. The cleaned extract was re-dissolved in 50 ml of isooctane for instrumental analysis. A solution of PCB 142 was added as internal standard prior to injection.

2.5. Quality assurance A blank was made for each batch of analysed samples. In each sample the standard recoveries (TBB and PCB 200) were calculated. Recoveries of the spiked standards into samples ranged between 75 ± 12% and 95 ± 11%, respectively. Residue concentrations were reported on a wet weight basis and were adjusted for recovery. Limits of detection and quantification were determined from the average instrumental signal of the blanks plus three and five times the standard deviation, respectively. The detection limits of all compounds analysed by GC-ECD ranged between 0.02 and 1.2 ng/g.

2.4. Instrumental analysis

2.6. Statistical procedures

Pentachlorobenzene (PeCB), hexachlorobenzene (HCB), hexachlorocyclohexanes (HCHs; a-, b-, g- and d-isomers), DDTs (o,p’DDT, p,p’-DDT, o,p’-DDE, p,p’-DDE, o,p’-DDD and p,p’-DDD) and PCBs (congeners 28, 52, 101, 118, 138, 153 and 180) were analyzed in a HewlettePackard gas chromatograph Model HP-5890 equipped with an electron-capture detector and an HP-7673-A autosampler. The separation was achieved with a 60 m  0.25 mm I.D. DB-5 capillary column (J&W Scientific, Folsom, CA, USA) coated with 5% phenyl methyl polysiloxane (film thickness 0.25 mm). The oven temperature was programmed from 90  C (holding time 2 min) to 130  C at 15  C/min and finally to 290  C at 4  C/min, keeping the final temperature for 10 min. Injector and detector temperatures were 280  C and 320  C, respectively. Injection was performed in the splitless mode, keeping the split valve closed for 35 s. Helium was the carrier gas (1.5 mL/min) and nitrogen was used as make-up gas for the detector (60 mL/min). Structural confirmations were performed by negative ion chemical ionization mass spectrometry. A GC system from Agilent Technologies 6890A coupled to a MS detector 5973N was used. The system was equipped with a 60 m  0.25 mm I.D. DB-5 column (J&W Scientific) coated with 5% phenyl methyl polysiloxane (film thickness 0.25 mm). Helium was used as carrier gas at a flow of 1.0 mL/min. Ammonia was used as reagent gas at a flow of 1.75 mL/ min. The temperature program started at 90  C (holding time 2 min), then increased to 150  C at 10 C/min and to 310  C at 4  C/ min with a final holding time of 20 min. Injector, ion source and transfer line temperatures were 250, 176 and 280  C, respectively. The dwell time was 50 ms/channel.

Mean values, standard deviations and variability ranges (minimum and maximum) were calculated for all organochlorine compounds. Box plots were used for description of compound distributions, including medians, percentiles and outliers. The UManneWhitney test was used for examining the differences between species in relation to OC concentrations. Groups with identical number of samples (n ¼ 9) were compared. The results were considered significant at p < 0.05. 3. Results and discussion 3.1. General quantitative composition Eggs are commonly used to assess OC exposure in birds (De Luca-Abbott et al., 2001). Collection of these samples is logistically simple and minimizes stress and disturbance of the individuals. Previous DDE and PCB studies on black crowned nightheron eggs showed that within-clutch variability was smaller (12e17%) than inter-clutch variability (83e88%) (Custer et al., 1990). These pollutants are conveyed directly from the mother to the eggs via lipid transfer and the egg OC concentrations provide estimates of the degree of pollution of the adult females (Aurigi et al., 2000). The organic pollutants present in the eggs are transferred during the breeding season. They therefore provide a matrix formed during the same breeding time period for all individuals. This is an advantage for comparative purposes, including inter-species differences, because it provides a normalized material generated in

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the same ecosystem. The OC concentrations found in the two heron species are summarized in Table 1. In both cases, the compounds with lower volatility and higher lipophilicity are those found in higher levels. Higher concentrations were found for total DDTs and PCBs than HCB and HCHs which is in agreement with previous studies (Connell et al., 2003; Sanpera et al., 2003) and shows the biomagnification potential of these compounds. 3.2. Pentachlorobenzene, hexachlorobenzene, hexachlorocyclohexanes The PeCB concentrations were just above detection limits. The mean concentrations of HCB and sum of HCHs were between 2 and 4 ng/g. The distributions of HCHs were predominated by b- and gisomers, which was consistent with observations from other studies (Albanis et al., 1996). The HCH concentrations in cattle egret were similar to those observed in studies of the same species in South-Africa (0.78 ng/g and 0.82 ng/g, respectively; Bouwman et al., 2008, Table 2). In Asia, HCHs were extensively used in agriculture and the observed concentrations were about one order of magnitude higher than those of the present study, e.g. 44 ng/g (China, Dong et al., 2004), 24e65 ng/g in wetlands from Pujab (Malik et al., 2011) or 3e32 ng/g in Pakistan (Sanpera et al., 2003). Concerning little egret, total HCH concentrations of 17 ng/g were described in Hong Kong (Connell et al., 2003) which were higher than in the Aiguabarreig specimens. 3.3. DDTs The distributions of DDT metabolites were similar in both species (Fig. 2). pp'-DDE was largely dominant and p,p0 -DDT was the second major metabolite. The strong predominance of p,p’-DDE

Table 1 Concentrations (mean and range, ng/g wet weight) of organochlorine compounds, lipid content and stable isotope composition in eggs of Cattle Egret (B. ibis) and Little Egret (E. garzetta) from the sampled location (Aiguabarreig). Compounds

Cattle egret N ¼ 9

Little egret N ¼ 9

PeCB HCB

0.2 (nd-0.3) 1.9 (0.6e3.4)

0.3 (nd-0.5) 2.8 (0.5e7.8)

a-HCH b-HCH g-HCH d-HCH Sum HCHs

0.2 1.1 1.2 0.3 2.5

0.2 1.9 1.3 0.5 3.9

o,p’-DDE p,p’-DDE o,p’-DDD p,p’-DDD o,p’-DDT p,p’-DDT Sum DDTs

0.2 (nd-1.1) 42.5 (9.6e120) 0.6 (nd-0.6) 0.3 (nd-0.7) 0.3 (nd-0.5) 2.7 (nd-8.8) 45 (18e120)

(nd-0.8) (0.7e2.0) (nd-6.3) (nd-1.1) (0.9e7.0)

PCB 28 0.5 (nd-1.1) PCB 52 0.1 (nd-0.3) PCB 101 0.5 (nd-1.2) PCB 118 2.8 (nd-11) PCB 138 13 (1.2e67) PCB 153 21 (3.0e99) PCB 180 13 (2.1e58) Sum PCBs 51 (6.9e240) % Lipids 9.0 (4.0e18) Isotopic composition (‰) d15N 12.1 (9.8; 19.5) d13C 21.6 (28.12; 18.97)

(nd-1.1) (0.3e6.4) (nd-3.1) (nd-1.5) (1.1e12)

1.0 (nd-6.3) 260 (6.1e850) 1.3 (nd-2.9) 1.7 (nd-4.1) 0.7 (nd-3.0) 2.5 (nd-4.9) 270 (6.1e860) 1.8 (nd-5.1) 0.4 (nd-0.9) 1.5 (nd-3.3) 19 (nd-53) 62 (1.4e210) 96 (2.6e280) 53 (1.5e140) 230 (6.6e690) 5.6 (3.4e9.4) 17.5 (10.3; 20.8) 25.0 (28.32; 20.43)

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indicates that the inputs of this insecticide into these species can generally be explained by old DDT uses. The concentrations of total DDTs in the cattle and little egret eggs were 45 ng/g and 270 ng/g, respectively (Table 1; Fig. 3). The concentrations in cattle egret eggs were higher than those found in other studies from other remote areas of Punjab and South Africa, 15 ng/g (Malik et al., 2011) and 24 ng/g (Bouwman et al., 2008), respectively, but lower than in China, 150 ng/g (Dong et al., 2004) (Table 2). In little egret eggs, total DDT concentrations were higher than those described in Italy, 150 ng/g (Fasola et al., 1998), similar to those described in Punjab and China (140e560 ng/g; Malik et al., 2011; Dong et al., 2004) and much lower than those described in Hong Kong and the Danube Delta (1200-14,000 ng/g; Aurigi et al., 2000; Connell et al., 2003). 3.4. Polychlorobiphenyls The PCB congeners showed similar distributions in both species (Fig. 4). The lower volatile congeners, PCB-153, PCB-180 and PCB138 (in order of abundance), were dominant in all samples examined. In each sample the sum of these three compounds represented between 80 and 100% of the total PCBs. The mean percentages of each congener were practically the same when comparing between species. PCB-153 was the most abundant, 42% and 40% of total PCBs in cattle egret and little egret, respectively. The proportions of PCB-180 and PCB-138 were 27% and 23% in cattle egret and 25% and 24% in little egret, respectively. Similar PCB distributions were described in egrets from South Africa (Bouwman et al., 2008) and Greece (Antoniadou et al., 2007). The concentrations of total PCBs were 51 ng/g and 230 ng/g in cattle and little egrets, respectively. In little egret they were higher than those reported in Greece and Italy, 18 ng/g (Antoniadou et al., 2007) and 77 ng/g (Fasola et al., 1998), respectively. They were in the same range of variation as those reported in Punjab and China, 1e800 ng/g (Dong et al., 2004; Malik et al., 2011) and lower than those found in Hong Kong and the Danube Delta (630e960 ng/g, Aurigi et al., 2000; Connell et al., 2003) (Table 2). 3.5. Differences between species The average d13C ratios of the eggs from cattle egret and little egret samples, 21.6 and 25.0‰, respectively, (Table 1) were significantly different (p < 0.02). Higher isotope values of the former were consistent with a higher proportion of dietary terrestrial sources (O'Leary, 1988; Tolosa et al., 1999) which, in turn, agreed with the known dietary habits of these species. The average d15N ratios of these two species, 12.2 and 17.5‰, respectively, (Table 1) were also significantly different (p < 0.006). The difference may reflect that little egret was situated in a higher trophic level than cattle egret (Minegawa and Wada, 1984) but it could also reflect that the basal isotopic composition was higher in the aquatic than in the terrestrial habitats. In any case, this significantly different isotopic composition corresponded to distinct food sources of each egret species in the area of study. The diet of cattle egret had a much stronger contribution of terrestrial sources than the diet of little egret. Average concentrations of the most volatile compounds, PeCB, HCB and HCHs did not show significant differences between the two species (Table 1; Fig. 3). As mentioned above, these volatile compounds were in low concentrations in all egg samples, showing low biomagnification in herons. Conversely, total PCBs and DDTs showed significant differences between species (ManneWhitney U test: p < 0.047 and p < 0.015, respectively). The mean concentrations of total DDTs and total PCBs were about 6 and 4.5-fold higher in little egret than in cattle egret,

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Table 2 Summay of the mean concentrations of organochlorine compounds (ng/g wet weight) in cattle egret and little egret eggs from this study and other studies. Species

PeCB

HCB

Sum HCHs

Sum DDTs

Sum PCBs

Site

Reference

Cattle egret Little egret Cattle egret Little egret Cattle egret Cattle egret Cattle egret Little egret Little egret Little egret Little egret

0.2 0.3 e e e e e e e e e

1.9 2.8 e e 0.8 e e e e e e

2.5 3.9 24e65 e 0.8 44 3e32 17 e e e

45 270 15 560 24 150 e 1200 150 e 14000

51 230 e e 5.7 800 1 630 77 18 960

Ebro River

Present study

Pujab

Malik et al., 2011

South Africa China Pakistan Hong Kong Italy Greece Danube Delta

Bouwman et al., 2008 Dong et al., 2004 Sanpera et al., 2003 Connell et al., 2003 Fasola et al., 1998 Antoniadou et al., 2007 Aurigi et al., 2000

Fig. 2. Box-plot diagram for the concentrations of DDT and its metabolites (ng/g ww) in eggs of cattle egret and little egret collected in the Aiguabarreig site in 2006. Horizontal lines show median values, boxes are interquartilic ranges; whiskers correspond to non-outlier ranges.

respectively (Fig. 3). These differences were also reflected in the concentrations of individual PCB congeners (Fig. 4). Six of the 7 congeners analysed, PCB-28, PCB-101, PCB-118, PCB-138, PCB-153 and PCB-180, showed significantly higher concentrations in little egret than in cattle egret (p < 0.023; p < 0.018; p < 0.011; p < 0.047; p < 0.047 and p < 0.047, respectively). Concerning DDTs, significantly higher concentrations of p,p’-DDE and p,p’-DDD were found in little than in cattle egret (p < 0.015 and p < 0.005, respectively) (Fig. 2). o,p’-DDE, o,p’-DDD and o,p’-DDT were present in low amounts and, despite they generally showed higher concentrations in little egret (Fig. 2), the differences were not significant. The significantly higher concentrations of p,p’DDE, p,p’-DDD and PCBs in little egret were also consistent with the high biomagnification potential of these compounds indicating higher accumulation in the species with a full aquatic diet than in the species feeding on terrestrial preys. p,p’-DDT was the only compound showing a distinct behaviour between the two species. The average concentrations of these compounds were similar in both of them (Table 1, Fig. 2). This

Fig. 3. Box-plot diagram for the concentrations of organochlorine compounds (ng/g ww) in eggs of cattle egret and little egret collected in the Aiguabarreig site in 2006. Horizontal lines show median values, boxes are interquartilic ranges; whiskers correspond to non-outlier ranges.

distinct distribution cannot be explained by volatility or lipophilicity differences when comparing the properties of p,p’-DDT, p,p’-DDD and p,p’-DDE. Conversely, the dissimilar behaviour of p,p’-DDT could reflect specific inputs associated to its use as insecticide. Cattle egrets feed on insects and small terrestrial reptiles that can easily be exposed to higher concentrations of pollutants used in agriculture. Thus, residues of past agricultural DDT applications in nearby areas could still have a specific impact on terrestrial organisms. In the case of this specific input, it could equilibrate the effects resulting from the general trend of higher OC bioaccumulation in the species feeding on aquatic organisms.

4. Conclusions Comparison of the concentrations of OCs in eggs from little egret and cattle egret shows that pp’-DDE, pp’-DDD and PCBs are significantly found in higher levels in the former than the latter species. That is, the one feeding on aquatic preys shows higher concentrations of these compounds than that feeding on terrestrial preys. These diet differences are also reflected in the d13C

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References

Fig. 4. Box-plot diagram for the concentrations of PCB congeners (ng/g ww) in eggs of cattle egret and little egret collected in the Aiguabarreig site in 2006. Horizontal lines show median values, boxes are interquartilic ranges; whiskers correspond to nonoutlier ranges.

composition of the eggs from these two species. Cattle egret shows significantly more positive ratios, which is consistent with the more terrestrial diet. Significant differences are also observed in the d15N composition whose more negative ratios in little egret are consistent with feeding in a higher trophic chain position than cattle egret or, in any case, from different sources. The OCs showing this difference are those of low volatility and high lipophilicity which involves high biomagnification potential. The compounds with low biomagnification capacity, e.g. PeCB, HCB and HCHs, do not show selective enrichments between the two species. p,p’-DDT constitute an exception since it is present in similar concentrations in both species despite its low volatility and high lipophilicity. The agricultural use of this insecticide may have involved a specific impact of this compound in terrestrial organisms which are preys of cattle egret that could equilibrate the general trend of higher OC bioaccumulation in the species feeding on aquatic organisms.

Acknowledgements This research was performed under the guidance and the indications of Prof. Xavier Ruiz who passed away. All coauthors dedicate this paper to his memory. The authors thank the support of the staff from the Natural Reserve of Sebes for their assessment during sampling. Egg collection and field data recording were performed by Javier Cotín and Manolo García. Eggs were collected under licence of the Departament de Medi Ambient (Catalan Government) and Parc Natural del Delta de l’Ebre. D. Huertas thanks the financial support received from the Spanish Ministry of Science and Education for a PhD grant (FPU, AP2007-01824). Financial support is acknowledged from the project GRACCIE (CTM2014-59111-REDC) financed by the Spanish Ministry of Economy and Competitiveness.

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