Mercury stable isotopes in seabirds in the Ebro Delta (NE Iberian Peninsula): Inter-specific and temporal differences.

Mercury (Hg) is a global pollutant, which particularly affects aquatic ecosystems, both marine and freshwater. Top-predators depending on these environments, such as seabirds, are regarded as suitable bioindicators of Hg pollution. In the Ebro Delta (NE Iberian Peninsula), legacy Hg pollution from a chlor-alkali industry operating in Flix and located ca. 100 km upstream of the Ebro River mouth has been impacting the delta environment and the neighboring coastal area. Furthermore, levels of Hg in the biota of the Mediterranean Sea are known to be high compared to other marine areas. In this work we used a Hg stable isotopes approach in feathers to understand the processes leading to different Hg concentrations in three Laridae species breeding in sympatry in the area (Audouin's gull Ichthyaetus audouinii, black-headed gull Chroicocephalus ridibundus, common tern Sterna hirundo). These species have distinct trophic ecologies, exhibiting a differential use of marine resources and freshwater resources (i.e., rice paddies prey). Moreover, for Audouin's gull, in which in the Ebro Delta colony temporal differences in Hg levels were documented previously, we used Hg stable isotopes to understand the impact of anthropogenic activities on Hg levels in the colony over time. Hg stable isotopes differentiated the three Laridae species according to their trophic ecologies. Furthermore, for Audouin's gull we observed temporal variations in Hg isotopic signatures possibly owing to anthropogenic-derived pollution in the Ebro Delta. To the best of our knowledge this is the first time Hg stable isotopes have been reported in seabirds from the NW Mediterranean.

Temporal mercury dynamics throughout the rice cultivation season in the Ebro Delta (NE Spain): An integrative approach.

During the last few decades, inputs of mercury (Hg) to the environment from anthropogenic sources have increased. The Ebro Delta is an important area of rice production in the Iberian Peninsula. Given the industrial activity and its legacy pollution along the Ebro river, residues containing Hg have been transported throughout the Ebro Delta ecosystems. Rice paddies are regarded as propitious environments for Hg methylation and its subsequent incorporation to plants and rice paddies' food webs. We have analyzed how Hg dynamics change throughout the rice cultivation season in different compartments from the paddies' ecosystems: soil, water, rice plants and fauna. Furthermore, we assessed the effect of different agricultural practices (ecological vs. conventional) associated to various flooding patterns (wet vs. mild alternating wet and dry) to the Hg levels in rice fields. Finally, we have estimated the proportion of methylmercury (MeHg) to total mercury in a subset of samples, as MeHg is the most bioaccumulable toxic form for humans and wildlife. Overall, we observed varying degrees of mercury concentration over the rice cultivation season in the different compartments. We found that different agricultural practices and flooding patterns did not influence the THg levels observed in water, soil or plants. However, Hg concentrations in fauna samples seemed to be affected by hydroperiod and we also observed evidence of Hg biomagnification along the rice fields' aquatic food webs.

Understanding the role of ecological factors affecting mercury concentrations in the blue shark (Prionace glauca).

Human activities have increased environmental concentrations of pollutants in marine ecosystems, which can cause harmful effects on marine organisms. Top predators are particularly susceptible to bioaccumulation and biomagnification of pollutants through the food webs and are described as good sentinels for monitoring metal accumulation such mercury (Hg) in marine ecosystems. However, to be used as sentinels, it is important to understand the main ecological factors affecting the concentrations of pollutants in these organisms. In the present study, our main objective was to investigate the effect of body size, sex, trophic niche and geographic area on Hg concentrations in a top marine top predator, the blue shark (Prionace glauca). We analysed Hg in muscle samples from male and female blue sharks of different body sizes collected from the waters surrounding the Canary Islands and the South of Portugal, in the Atlantic Ocean, to waters of the north-western Mediterranean Sea. The results revealed that the sampling area was an important factor explaining Hg concentrations, showing higher values in the Mediterranean blue sharks. We also found a positive relationship between Hg concentrations and body size of blue sharks, indicating a bioaccumulation process of this pollutant in relation with body size. Moreover, we observed a relationship between Hg concentrations and δ13C values, a proxy of the use of inshore-offshore marine habitats. Individuals with depleted δ13C values that potentially foraged in offshore waters showed higher Hg values. Importantly, most of the analysed blue sharks presented Hg concentrations that exceeded the limits established by the European Union for human consumption.

Effect of different rice farming practices on the bioavailability of mercury: A mesocosm experiment with common goldfish (Carassius auratus).

Wetlands such as rice paddies are important ecosystems that provide habitat for a numerous range of species but are considered as a major source of mercury in the most toxic form of methylmercury. An in situ mesocosm experiment was conducted during the rice cultivation season of 2018 in rice paddies from the Ebro Delta (NE Spain) to investigate the bioaccumulation of mercury in fish. Common goldfish (Carassius auratus) were exposed to three types of rice-fields subject to different agricultural management (conventional or ecological agriculture) and different hydric practices (wet, dry) and were sampled monthly. Total mercury concentration in fish increased over time for almost all the fields under study, but the mercury increase was particularly higher for fish exposed to ecological fields. We conducted δ15N and δ13C measurements to follow up fish diet changes within or among fields. The results have shown there were no variations in the diet of the fish over time within fields, although differences among management practices were observed, which could explain to some extent the THg variation in fish exposed to each of the agricultural practices. Results indicate that wetting and drying rice fields might be a greater source of mercury for fish and a potential hotspot for MeHg production enhanced by physicochemical parameters. Thus, mercury bioaccumulation in fish inhabiting rice paddies depend on several factors what, in turn, could lead to biomagnification across the food webs.

Rhamphotheca as a useful indicator of mercury in seabirds.

The evaluation of mercury (Hg) toxicity in wildlife species has prompted a search for sensitive indicators to accurately measure the body burden of Hg. Despite the extensive use of feathers as an indicator of Hg in birds, they do not appear to be an entirely suitable indicator of the extent of contamination in certain conditions and bird species since Hg levels are influenced by the growth period, timing of the last moult and migration. This research aimed to evaluate rhamphotheca as a potential indicator of environmental Hg concentrations in seabirds. We used culled yellow-legged gulls (Larus michahellis) (n = 20) and determined THg in rhamphotheca and feathers. We distinguished between upper and lower rhamphotheca, and divided each one into 16 equal portions along the culmen to analyse their THg content spatially. In each bird, THg was also determined in primary (P1) and secondary (S8) feathers and compared with rhamphotheca. The median (25th, 75th percentile) rhamphotheca Hg concentration was 13.44 (9.63, 17.46) µg/g, which was twofold higher than in the feathers 7.56 (4.88, 12.89) µg/g. Median THg in rhamphotheca was significantly higher (p < 0.05) in females 15.05 (10.35, 23.04) µg/g than in males 12.34 (8.57, 15.19) µg/g, whereas no differences (p > 0.05) were found in the feathers. No significant differences in Hg levels were found between upper and lower beak mandibles or along either. In contrast, significant differences in Hg concentrations were found between the P1 and S8 feathers (mean, 12.04 vs. 6.04 µg/g). No correlation was found between Hg content in rhamphotheca and feathers. Mercury levels in rhamphotheca exhibited stronger significant relationships with weight (R2 = 0.568), length (R2 = 0.524) and culmen (R2 = 0.347) than the levels in the feathers, which showed no correlation. Overall, our results suggest that rhamphotheca is a suitable tissue indicator for Hg monitoring in gulls; however, further studies are needed to extend our research to other bird species.

Long-term trace element assessment after a mine spill: Pollution persistence and bioaccumulation in the trophic web.

Trace elements can be toxic when they cannot be easily removed after entering an ecosystem, so a long-term assessment is fundamental to guide ecosystem restoration after catastrophic pollution. In 1998, a pyrite mining accident in Aznalcóllar (south-western Spain) spilled toxic waste over a large area of the Guadiamar river basin, where, after restoration tasks, the Guadiamar Green Corridor was established. Eight years after the mine accident (2005-2006), the ground-dwelling insectivorous lizard Psammodromus algirus registered high trace-element levels within the study area compared to specimens from a nearby unpolluted control site. In 2017, 20 years after the accident, we repeated the sampling for this lizard species and also quantified trace elements in vegetation as well as in arthropod samples in order to identify remnant trace-element accumulation with the aim of assessing the transfer of these elements through the trophic web. We found remnant trace-element contamination in organisms of the polluted site compared to those from the unpolluted site. All trace-element concentrations were higher in arthropods than in plants, suggesting these compounds bioaccumulate through the trophic web. Lizards from the polluted areas had higher As, Cd, and Hg concentrations than did individuals from the unpolluted area. Lizard abundance between sampling periods (2005-06 and 2017) did not vary in unpolluted transects but strongly declined at polluted ones. By contrast, the Normalized Difference Vegetation Index indicated that in the study period, the vegetation was similar at the two sampling sites. These results suggest that, 20 years after the accident, the trace-element pollution could be the cause of a severe demographic decline of the lizard in the polluted area.

High Trophic Niche Overlap between a Native and Invasive Mink Does Not Drive Trophic Displacement of the Native Mink during an Invasion Process.

The pressure elicited by invasive species on native species significantly increases with the increase of the overlap of their ecological niches. Still, the specific mechanisms of the trophic displacement of native species during the invasion process are unclear. The effects of the invasive American mink (Neovison vison) on the critically endangered European mink (Mustela lutreola) was assessed by analyses of diet and niche overlap during the invasion process. To do this, the isotopic composition (δ13C and δ15N) of both species of mink and their four main types of prey was analysed. Significant trophic overlap between the native European mink and invasive American mink was found when they coexisted in sympatry. Furthermore, both mink species were characterised by significant individual variation in diet and no obvious change in diet of the native species in response to the arrival of the introduced species was observed. High niche overlap registered between both species in sympatry with no displacement in diet of the native mink in response to the arrival of the invasive mink is expected to have important consequences for the viability and conservation of the native mink populations, as it suggests high competitive pressure.

Following up mercury pollution in the Ebro Delta (NE Spain): Audouin's gull fledglings as model organisms to elucidate anthropogenic impacts on the environment.

As top-predators in marine ecosystems, seabirds are regarded as appropriate bioindicator species for a variety of contaminants. Mercury (Hg) is a global pollutant, which can biomagnify along marine and freshwater food webs. Therefore, mercury body burden in seabirds, such as gulls, will integrate information about pollution in the environment. In the Ebro Delta (NE Spain), legacy mercury pollution from a chlor-alkali industry located ca. 100 km upstream of the Ebro river mouth has been affecting the delta environment. We have analyzed a 15-year temporal series (2004-2019) of Hg in birds from a breeding colony of Audouin's gull (Ichthyaetus audouinii) in the Ebro Delta to understand how fluctuations in Hg levels are coupled to human activities in the industrial area in the upstream region of the river. Stable isotopic signatures of C and N (δ13Cbulk and δ15Nbulk) are determined to characterize the trophic ecology of the species. Since only δ13Cbulk but not δ15Nbulk was associated with THg levels, we used compound-specific stable nitrogen isotope analysis of amino acids (AA-CSIA) to evaluate the causes of variation in δ15Nbulk to further investigate the idea of a decoupling of δ15Nbulk and THg over time. We found Audouin's gull to be sensitive to Hg variations in the environment due to anthropogenic changes and to be a good indicator species for this contaminant in the Ebro Delta.

Relating trophic ecology and Hg species contamination in a resident opportunistic seabird of the Bay of Biscay.

Methylmercury (MeHg) is the most bioavailable and toxic form of the globally distributed pollutant Hg. Organisms of higher trophic levels living in aquatic ecosystems have potentially higher concentrations of MeHg. In this work, we analysed both MeHg and inorganic Mercury (Hg(II)) concentrations from dorsal feathers of chicks from ten colonies of Yellow-legged Gull (Larus michahellis) in the south-eastern part of the Bay of Biscay. Overall, we detected a high mean MeHg concentration that, however, differed among colonies. Additionally, based on stable isotopes analysis (δ13C and δ15N) and conducting General Linear Mixed Models, we found that chicks which were mostly/mainly fed with prey of marine origin had higher levels of MeHg. We propose Yellow-legged Gull as a reliable biomonitor for Hg species, as it is easy for sampling and in compliance with the Minamata convention on Mercury.

Telomere shortening rate predicts species life span.

Telomere shortening to a critical length can trigger aging and shorter life spans in mice and humans by a mechanism that involves induction of a persistent DNA damage response at chromosome ends and loss of cellular viability. However, whether telomere length is a universal determinant of species longevity is not known. To determine whether telomere shortening can be a single parameter to predict species longevities, here we measured in parallel the telomere length of a wide variety of species (birds and mammals) with very different life spans and body sizes, including mouse (Mus musculus), goat (Capra hircus), Audouin's gull (Larus audouinii), reindeer (Rangifer tarandus), griffon vulture (Gyps fulvus), bottlenose dolphin (Tursiops truncatus), American flamingo (Phoenicopterus ruber), and Sumatran elephant (Elephas maximus sumatranus). We found that the telomere shortening rate, but not the initial telomere length alone, is a powerful predictor of species life span. These results support the notion that critical telomere shortening and the consequent onset of telomeric DNA damage and cellular senescence are a general determinant of species life span.

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

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 δ(15)N and δ(13)C 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.

Organochlorine compounds in Purple Heron eggs (Ardea purpurea) nesting in sites located around a chlor-alkali plant (Ebro River).

Eggs of Purple Heron (Ardea purpurea) were collected from three sampled sites inside the Ebro River basin in years 2006 and 2007. These sites were located besides (Flix), upstream (Aiguabarreig) and downstream (Delta) a chlor-alkali plant. Organochlorine compounds (OCs) such as dichlorodiphenyltrichloroethanes (DDTs), polychlorobiphenyls (PCBs), polychlorostyrenes (PCSs), hexachlorobenzene (HCB), pentachlorobenzene (PeCB) and hexachlorocyclohexanes (HCHs) were analysed to assess what are the accumulation patterns of these compounds in aquatic migratory birds breeding in the area of influence of the emissions from this industrial installation. Comparison of the egg concentrations between the three sites show higher concentrations of compounds manufactured in the past in the factory (PCBs, p,p'-DDT) or by-products of OC synthesis (HCB, PeCB and PCSs) in Flix than in Aiguabarreig reflecting a clear influence from the emissions of the chlor-alkali plant. The eggs collected in the Ebro Delta showed higher concentrations of total DDTs (mainly p,p'-DDE) than in the reference site (Aiguabarreig) which could reflect past applications of this insecticide in the area for agriculture. In contrast, HCHs were found in higher concentrations in the Delta and Aiguabarreig than in the Flix Reservoir. These compounds have been used as insecticides in agriculture and were not manufactured in the chlor-alkali plant. The present results show that despite Purple Herons are migratory birds, the food web transfer of OCs during the breeding season is sufficient for the accumulation of these compounds in the eggs, leading to statistically significant concentration differences between sites. These differences are consistent with the emissions of these pollutants from industrial or agricultural sources to the aquatic environments. Some of the p,p'-DDE concentrations observed in the area nearby the chlor-alkali plant are above the threshold effects for reproductive impairment.

Perfluoroalkyl and polyfluoroalkyl substances in entire clutches of Audouin's gulls from the Ebro Delta.

The aim of the present study was to determine the distribution of per- and polyfluoroalkyl substances (PFASs) in three-egg clutches of Audouin's gull (Larus audouinii) breeding in Ebro Delta's colony according to the laying order (a, b and c eggs). Five PFASs were analyzed in 30 eggs (yolk and albumen separately), corresponding to 10 three-egg clutches. Carbon and nitrogen stable isotopes were measured as dietary tracers. PFASs were not detected in albumen. In egg yolks, perfluorooctane sulfonate (PFOS) was the main compound detected followed by perfluorononanoic acid (PFNA), perfluorohexane sulfonate (PFHS) and perfluorooctanoic acid (PFOA). Perfluorooctane sulfonate (PFBS) was not detected. Mean ΣPFASs for a-eggs was of 236±57 ng g(-1) yolk wet weight (ww), for b-eggs was of 140±56 ng g(-1) yolk ww and for c-eggs, 133±54 ng g(-1) yolk ww. PFOS concentration decreased according to the laying order of the eggs, showing significant differences between consecutive eggs. In addition, significant correlation (rs2=0.7-0.9) was observed for PFOS concentration within the eggs from the same clutch. No relationship was found between PFOS levels and stable isotopes signatures. Capsule: In Audouin gull's eggs, PFOS was the main PFASs detected and its concentration decreased according to the laying sequence.

Ten years after the prestige oil spill: seabird trophic ecology as indicator of long-term effects on the coastal marine ecosystem.

Major oil spills can have long-term impacts since oil pollution does not only result in acute mortality of marine organisms, but also affects productivity levels, predator-prey dynamics, and damages habitats that support marine communities. However, despite the conservation implications of oil accidents, the monitoring and assessment of its lasting impacts still remains a difficult and daunting task. Here, we used European shags to evaluate the overall, lasting effects of the Prestige oil spill (2002) on the affected marine ecosystem. Using δ ¹5N and Hg analysis, we trace temporal changes in feeding ecology potentially related to alterations of the food web due to the spill. Using climatic and oceanic data, we also investigate the influence of North Atlantic Oscillation (NAO) index, the sea surface temperature (SST) and the chlorophyll a (Chl a) on the observed changes. Analysis of δ ¹5N and Hg concentrations revealed that after the Prestige oil spill, shag chicks abruptly switched their trophic level from a diet based on a high percentage of demersal-benthic fish to a higher proportion of pelagic/semi-pelagic species. There was no evidence that Chl a, SST and NAO reflected any particular changes or severity in environmental conditions for any year or season that may explain the sudden change observed in trophic level. Thus, this study highlighted an impact on the marine food web for at least three years. Our results provide the best evidence to date of the long-term consequences of the Prestige oil spill. They also show how, regardless of wider oceanographic variability, lasting impacts on predator-prey dynamics can be assessed using biochemical markers. This is particularly useful if larger scale and longer term monitoring of all trophic levels is unfeasible due to limited funding or high ecosystem complexity.

Trophic ecology of a resident Yellow-legged Gull (Larus michahellis) population in the Bay of Biscay.

Stable isotopes analyses (SIAs) are an efficient tool to obtain a general insight into the diet of generalist consumers, such as the Yellow-legged Gull (Larus michahellis). Here we analysed δ(13)C, δ(15)N and δ(34)S values in feathers of chicks and adults, and used Bayesian triple-isotope mixing models to reconstruct the diet of a Yellow-legged Gull population breeding in the southeastern Bay of Biscay. Questions to test were (1) whether adults and chicks rely on different feeding resources during breeding period; (2) whether there is a seasonal foraging effect involving a higher proportion of refuse food in winter compared to summer, and (3) the magnitude of the annual variation in diet. Prey consumption differed between colonies, among years, and also varied slightly between seasons, and this was mainly due to a differential use of prey of marine origin. However, diet did not differ between age classes. These results suggest a relatively monotonous diet with only slight variations from year to year, seasonally and at a local geographic scale.

Seabird feathers as monitors of the levels and persistence of heavy metal pollution after the Prestige oil spill.

We measured heavy metal concentrations in yellow-legged gulls (n = 196) and European shags (n = 189) in order to assess the temporal pattern of contaminant exposure following the Prestige oil spill in November 2002. We analysed Pb, Cu, Zn, Cr, Ni and V levels in chick feathers sampled at four colonies during seven post-spill years (2003-2009), and compared results with pre-spill levels obtained from feathers of juvenile shag corpses (grown in spring/summer 2002). Following the Prestige wreck, Cu (4.3-10 µg g(-1)) and Pb concentrations (1.0-1.4 µg g(-1)) were, respectively, between two and five times higher than pre-spill levels (1.5-3.6 and 0.1-0.4 µg g(-1)), but returned to previous background concentrations after three years. Our study highlights the suitability of chick feathers of seabirds for assessing the impact of oil spills on heavy metal contamination, and provides the best evidence to date on the persistence of oil pollution after the Prestige incident.

Tracing waterbird exposure to total mercury and selenium: a case study at the solar saltworks of Thyna (sfax, Tunisia).

Saltworks have emerged as important alternative/complementary feeding habitats for avifauna. However, the consequences of such habitat shifts in terms of changes in exposure to contaminants are poorly understood. We evaluated the exposure of the waterbird community breeding at the saltworks of Thyna (Tunisia) to total Hg (THg) and Se according to their differential use of saltworks dietary resources, as revealed by δ¹³C and δ¹5N values in their eggs (included species [n] -sorted according to increasing reliance on saltworks resources: Yellow-legged Gull Larus michahellis [12], Common Tern Sterna hirundo [12], Slender-billed Gull Larus genei [15], Little Egret Egretta garzetta [20], and Pied Avocet Recurvirostra avosetta [22]). Concentrations of THg and Se were under the threshold points for deleterious effects. Egg THg concentrations significantly decreased as the dietary contribution of saltworks resources increased (mean: 3.23, 1.66, 0.76, 0.4, and 0.27 µg/g dw, respectively). Conversely, egg Se concentrations did not vary according to foraging habitats (2.49, 2.96, 2.61, 3.27, and 1.5 µg/g dw, respectively). Tracing waterbird exposure to THg and Se at saltworks was feasible through the use stable isotopic assays of eggs. Birds using saltworks are not exposed to higher concentrations of THg and Se than in adjacent marine habitats.

Evaluating cleansing effects on trace elements and stable isotope values in feathers of oiled birds.

Feathers of seabirds are widely used as a nondestructive tissue for pollution monitoring of trace elements, as well as convenient samples for trophic ecology studies by means of stable isotope analysis (SIA). Nevertheless, feathers can be occasionally impregnated with oil from deliberate ship discharges and from massive oil spill accidents. The feather structure makes them effective traps for particles and are subject to external contamination. It is unknown to what extent the oil adhered to feathers can change trace element concentrations or stable isotope signatures. This study has two primary objectives: (1) to assess if there are differences between trace element concentrations and stable isotope signatures of oiled and clean feathers, and (2) to determine if the cleansing of oiled feathers using commonly applied techniques such as sodium hydroxide (NaOH) washes in combination with an organic solvent (hexane) is more effective than using NaOH alone. In order to do this, we analysed trace elements (Se, Hg, Pb, Cu and Zn) and stable isotopes (delta(13)C and delta(15)N) of individual feathers of yellow-legged gulls (Larus michahellis) which were affected by the 2002 Prestige oil spill in Galicia (NW Spain). Two sets of feathers were analysed, one group were oil-free (Control group) and the other had oil adhered to its surface (Oiled group). We expected to find differences between control and oiled feathers when cleaning exclusively with NaOH and no differences when using hexane. Our results did not show significant differences between Control and Oiled groups as a consequence of the cleansing method used. Unexpectedly, the additional cleansing with hexane resulted in decreasing selenium concentrations and increasing zinc and delta(15)N values in all groups of feathers.

Assessing the effects of the Prestige oil spill on the European shag (Phalacrocorax aristotelis): trace elements and stable isotopes.

The Prestige oil spill resulted in the mortality of several seabird species on the Atlantic NW coast of Spain. Shag casualties were particularly relevant, since populations are resident in the area the whole year round and because of several features which make them highly vulnerable to environmental hazards. Ecological catastrophes give us the opportunity of collecting samples which, otherwise, would be difficult to obtain. We examine the potential of shag corpses as bioindicators of inorganic pollution and the possible factors of variability, such as biological traits (sex, age) or nutrition status. We determined trace elements (Hg, Se, Cr, Pb, Zn and Cu) and isotopic signatures (15N, 13C) in soft tissues (muscle, liver) and in primary feathers formed at different times (before and after the Prestige) in individuals of known sex and age, collected at the time of the Prestige disaster. These were compared with data from another group of shags trapped accidentally in fishing gear in 2005. Our results did not seem to be affected by sex or age on any of the analysed variables. The higher nitrogen isotopic signatures in the soft tissues of the Prestige shags may be related to the nutrition stress caused by a poorer body condition, which is also reflected in increasing levels of some metals in the liver. This isotopic enrichment was also observed in newly forming feathers when compared to the old ones. On the other hand, the lower delta15N and Hg values in shag feathers from 2005 point to a shift in feeding resources to prey of lower trophic levels. We found that feather features (being an inert tissue and having a conservative composition), if combined with careful dating and chemical analysis, offer a very useful tool to evaluate temporal and spatial changes in seabird ecology in relation to pollution events.

Use of fatty acids to explain variability of organochlorine concentrations in eggs and plasma of common terns (Sterna hirundo).

We have studied the breeding parameters, organochlorine compounds (OCs) concentrations, and fatty acid (FA) composition of egg yolks (n = 47) and plasma (n = 90) of common terns (Sterna hirundo) from two colonies (Banya and Fangar) in the Ebro delta, NE Spain. Terns from Banya tend to have smaller clutch size and lower hatching success than terns from Fangar. p,p'-DDE and polychlorinated biphenyls (PCBs) concentrations were almost 2-fold higher in yolks from Banya in 1998 than from Fangar in 1999, and the percentage of n-6 PUFA was positively correlated with these contaminants. Differences between samplings in OCs concentrations in plasma were less evident, and were affected by breeding chronology. The highest OCs concentrations in yolks from Banya may be explained by two processes involving the increased deposition of n-6 PUFA: (1) higher mobilization of endogenous fat due to lower food resources, or (2) differences in the diet between colonies. Birds from Banya may have been feeding at a higher degree on discards of trawling fisheries composed of demersal and benthic fish species that are more exposed to contaminants from sediment and have lower n-3/n-6 ratio, whereas birds from Fangar would feed mostly on pelagic species of small clupeiformes that are less polluted and have higher n-3/n-6 ratios.