The use of novel biomarkers to determine dietary mercury accumulation in nestling waterbirds

Mercury (Hg) depuration into growing feathers is a well-studied phenomenon in waterbirds. Although the kinetics of Hg excretion in relation to molt and diet has been studied extensively, the relationship between the individual nutritional condition of nestlings and dietary Hg accumulation has not been investigated. In the present study, a body-condition index (BCI) and nutritional condition index (NCI) for nestlings of two waterbird species occupying different trophic positions on the aquatic food web were determined and used to predict Hg accumulation through diet. Candidate models consisting of these indices and nestling age were compared using Akaike's information criterion corrected for small sample sizes. For both species, the top-performing model contained the sole parameter of nutritional condition index (NCI). The relationship between Hg and NCI was stronger in the species foraging higher on the trophic web, which experienced higher rates of Hg depuration into feathers. Models containing BCI could not be discounted (AICc?相似文献   

Trophic Transfer of Trace Elements in an Isotopically Constructed Food Chain From a Semi-enclosed Marine Coastal Area (Stagnone di Marsala, Sicily, Mediterranean)

Trace element accumulation is particularly important in coastal and transitional environments, which act as contaminant buffers between the continental and marine systems. We compared trace element transfer to the biota in two locations with different open-sea exposures in a semi-enclosed marine coastal area (Stagnone di Marsala, Sicily, Italy) using isotopically reconstructed food chains. Samples of sediment, macroalgae, seagrasses, invertebrates, fish, and bird feathers were sampled in July 2006 and analysed for stable carbon and nitrogen isotopes (δ¹³C, δ¹⁵N) and trace elements (arsenic [As], cadmium [Cd], total mercury [THg], and lead [Pb]). Trophic magnification factors were calculated through the relationships between trace elements and δ¹⁵N in consumers. As and Pb were greater in organic matter sources (sediments and primary producers), whereas Cd and THg were greater in bird feathers. At the food chain level, an insignificant trophic transfer was found for all elements, suggesting biodilution rather than biomagnification. Sediments were more contaminated in the location with lower open-sea exposure. Macroalgae and seagrasses overall mirrored the spatial pattern highlighted in sediments, whereas differences between the two locations became further decreased moving toward higher trophic levels, indicating that trophic transfer of sediment and macrophyte-bound trace elements to the coastal lagoon food chain may be of relatively minor importance.     

Mercury and Nitrogen Isotope in a Marine Species from a Tropical Coastal Food Web

The present study raised the hypothesis that the trophic status in a tropical coastal food web from southeastern Brazil can be measured by the relation between total mercury (THg) and nitrogen isotope (δ¹⁵N) in their components. The analysed species were grouped into six trophic positions: primary producer (phytoplankton), primary consumer (zooplankton), consumer 1 (omnivore shrimp), consumer 2 (pelagic carnivores represented by squid and fish species), consumer 3 (demersal carnivores represented by fish species) and consumer 4 (pelagic-demersal top carnivore represented by the fish Trichiurus lepturus). The values of THg, δ¹⁵N, and trophic level (TLv) increased significantly from primary producer toward top carnivore. Our data regarding trophic magnification (6.84) and biomagnification powers (0.25 for δ¹⁵N and 0.83 for TLv) indicated that Hg biomagnification throughout trophic positions is high in this tropical food web, which could be primarily related to the quality of the local water.     

Metals and polycyclic aromatic hydrocarbons in colonial waterbird eggs from Lake Athabasca and the Peace-Athabasca Delta, Canada

In 2009, aquatic bird eggs from a variety of species were collected from three sites in northern Alberta, Canada. Two sites were located in receiving waters of the Athabasca River, which drains the oil sands industrial region north of Fort McMurray, Alberta. The third site, located on the Peace River, was remote from the influence of the Athabasca River. Levels of mercury, arsenic, and polycyclic aromatic hydrocarbons (PAHs) were measured in the eggs along with nitrogen stable isotopes (δ1?N) as an indicator of bird trophic position. Levels of As and PAHs in eggs were low, whereas Hg was measureable in all samples. Egg Hg levels increased with δ1?N values (a proxy of food web trophic position); however, some eggs exhibited Hg levels greater than expected based on trophic position. These eggs were from sites in receiving waters of the Athabasca River, namely, Mamawi Lake and Egg Island. Levels of Hg in egg pools were correlated with naphthalene levels, perhaps indicating a common source of contamination. Temporal comparison of Hg levels in California gull (Larus californicus) eggs from the Lake Athabasca colony indicated that egg Hg burdens increased 40% from 1977 to 2009. More research is required to evaluate temporal trends in levels of environmental contaminants and to identify sources.     

Assessment of Trace Elements and Stable Isotopes of Three Ardeid Species at Birama Swamp, Cuba

The Birama Swamp is the second largest wetland in the Caribbean region and it is inhabited by large populations of waterbirds. Here we report, for the first time, the foraging ecology and pollutant levels of three Ardeidae species: Cattle egret (Bubulcus ibis), Snowy egret (Egretta thula), and Tricolored heron (E. tricolor) breeding in this wetland using stable-isotope (δ ¹⁵N and δ ¹³C) and trace elements [mercury (Hg), lead (Pb), and selenium (Se)] analysis of chick feathers. Our results showed that individuals from all species occupied similar trophic levels. However, we found significant differences for δ ¹³C, with the highest values in cattle egret indicating its use of terrestrial habitats and a generalist and opportunist behavior. No significant differences were found for Pb among species. Yet, Hg levels were greater and similar in tricolored heron and snowy egret than in cattle egret, which was associated with their greater use of aquatic environments. Snowy egret had the lowest values of Se differing significantly with the other two species suggesting a different relative use of prey type. Modeling log-Hg concentration in relation to δ ¹⁵N and δ ¹³C showed an independent and significant relationship among species but without interaction with species level indicating that within a particular species, higher Hg levels were associated with higher δ ¹⁵N values. There was no interaction between δ ¹⁵N and δ ¹³C in the general linear models for Se and Pb in all species. We found an association between δ ¹⁵N and species in Pb for snowy egret. The foraging habitat use of these species and the low levels of pollutants, which are lower than in other similar habitats in other areas of the world, indicated that there is not risk of negative effects in juvenile birds of the Birama Swamp colony that may impair their survival. Our results can be used as a baseline to achieve management regulations.     

An Ecologically Framed Mercury Survey of Finfish of the Lower Chesapeake Bay

Total mercury (THg) and methylmercury (MHg) concentrations and determents of mercury (Hg) accumulation were examined for muscle tissues of 10 finfish from the lower Chesapeake Bay (LCB) and its tributaries. There was no suggestion of potential human harm from Hg due to LCB fish consumption: None of the sampled fish had THg concentrations approaching the United States Environmental Protection Agency human health screening value. Hg concentrations in different fish species generally increased with the increasing stable isotope of nitrogen 15 (δ¹⁵N) but not the stable isotope of carbon 13 (δ¹³C), thus suggesting that trophic position but not dietary carbon source is a dominant determinant. An MHg biomagnification model was built to estimate a food web magnification factor of approximately 10-fold increase per trophic level. Based on otolith strontium-to-calcium ratios, Atlantic croaker inhabiting less saline waters might accumulate more Hg than those inhabiting more saline waters. The SAS mixed procedure identified significant positive intraspecies relationships between MHg concentration and δ¹³C for summer flounder, weakfish, American eel, Atlantic croaker, and spot.     

Mercury Concentrations in the Fish Community from Indrawati River,Nepal

This study quantified concentrations of mercury (Hg) and its trophic transfer along the fish community in the Indrawati River, Nepal. Stable isotope ratios of nitrogen (δ¹⁵N) and carbon (δ¹³C), complemented by stomach contents data were used to assess the food web structure and trophic transfer of Hg in 54 fishes; 43 Shizothorax richardsonii and 11 Barilius spp. [B. bendelisis (1), B. vagra (3) and B. barila (7)]. Sixty-one muscle samples (including six replicates) were used for the analysis of total mercury (THg) and stable isotopes. Mean THg concentrations in B. spp. and the more common species S. richardsonii was observed to be 218.23 (ng/g, ww) and 90.82 (ng/g, ww), respectively. THg versus total length in both S. richardsonii and B. spp. showed a decreasing tendency with an increase in age. Regression of logTHg versus δ¹⁵N among the fish species showed a significant positive correlation only in S. richardsonii indicating biomagnification along the trophic level in this species.     

Dietary Exposure to Mercury in Nestling Wood Storks (Mycteria americana) in Georgia

The United States breeding population of the wood stork (Mycteria americana) was listed as endangered in 1984; the U.S. Fish and Wildlife Service has listed the determination of potential contaminant hazards as a task important to the recovery of the species. Storks nest in colonies and adult storks forage over wide areas and return with prey items to feed to nestlings. The most recently consumed prey items are usually regurgitated by nestlings when they are handled. In 1995, over 200 food items from four wood stork colonies were collected while banding nestlings between 20 and 40 days old, and total mercury determined in individual food items. Two colonies were located on the Atlantic coast and two in inland areas. Overall dietary mercury concentration for each colony was calculated based on the biomass of each type of food item within the diet. Freshwater prey items had higher mercury concentrations than saltwater items most frequently chosen by storks. Daily Hg doses were estimated for a range of nestling sizes and daily food consumption rates. Risk factors were calculated based on two lowest observable adverse effect concentrations (LOAECs) from previous avian mercury ingestion studies. Based on one LOAEC, all four colonies in this study were at risk of sublethal effects due to mercury in the diet of nestling wood storks. More extensive studies of sublethal effects in avian species from mercury contamination are necessary for more accurate risk assessment. Received: 16 June 1997/Accepted: 17 September 1997     

Ontogenetic patterns in bluefish (Pomatomus saltatrix) feeding ecology and the effect on mercury biomagnification

In this study, bluefish (Pomatomus saltatrix; age 0-7, n?=?632) and their prey (forage fish, macroinvertebrates, zooplankton; n?=?2,005) were collected from the Narragansett Bay estuary (RI, USA), and total Hg concentration was measured in white muscle and whole-body tissues, respectively. Bluefish Hg concentrations were analyzed relative to fish length, prey Hg content, and ontogenetic shifts in habitat use and foraging ecology, the latter assessed using stomach content analysis (n?=?711) and stable nitrogen (δ(15)N) and carbon (δ(13)C) isotope measurements (n?=?360). Diet and δ(13)C analysis showed that age 0 bluefish consumed both benthic and pelagic prey (silversides, sand shrimp, planktonic crustaceans; δ(13)C?=?-?16.52‰), whereas age 1?+ bluefish fed almost exclusively on pelagic forage fish (Atlantic menhaden, herring; δ(13)C?=?-?17.33‰). Bluefish total Hg concentrations were significantly correlated with length (mean Hg?=?0.041 and 0.254?ppm wet wt for age 0 and age 1?+ bluefish, respectively). Furthermore, Hg biomagnification rates were maximal during bluefish early life stages and decelerated over time, resulting in relatively high Hg concentrations in age 0 fish. Rapid Hg accumulation in age 0 bluefish is attributed to these individuals occupying a comparable trophic level to age 1?+ bluefish (δ(15)N?=?15.58 and 16.09‰; trophic level?=?3.55 and 3.71 for age 0 and age 1?+ bluefish, respectively), as well as juveniles having greater standardized consumption rates of Hg-contaminated prey. Finally, bluefish larger than 30?cm total length consistently had Hg levels above the U.S. Environmental Protection Agency criterion of 0.3?ppm. As such, frequent consumption of bluefish could pose a human health risk, and preferentially consuming smaller bluefish may be an inadequate strategy for minimizing human dietary exposure to Hg.     

Mercury Biomagnification in the Aquaculture Pond Ecosystem in the Pearl River Delta

This is the first study to investigate the rate of mercury (Hg) biomagnification in the aquaculture pond ecosystem of the Pearl River Delta (PRD), China, by analyzing total mercury (THg) and methyl mercury (MeHg) concentrations in various species of fish at different trophic levels (TLs). Species representing a gradient of trophic positions in the aquaculture pond food chains were chosen for analyzing THg and MeHg concentrations. In this study, there were two kinds of the aquaculture pond food chains: (1) omnivorous (fish feeds, zooplankton, grass carp [Ctenopharyngodon idellus], and bighead carp [Aristichthys nobilis]) and (2) predatory (zooplankton, mud carp [Cirrhina molitorella], and mandarin fish [Siniperca kneri]). Bighead carp and mandarin fish had the highest MeHg and THg concentrations, i.e., an order of magnitude higher than other species, in their respective food chains. More than 90% of the THg concentrations detected in bighead carp, mandarin fish, and mud carp were in the methylated form. In this study, %MeHg increased with TLs and MeHg concentrations, reflecting that MeHg is the dominant chemical species of Hg accumulated in higher concentrations in biota, especially biota associated with higher TLs in the food chains. The trophic magnification factors were 2.32 and 2.60 for MeHg and 1.94 and 2.03 for THg in omnivorous and predatory food chains, respectively, in PRD. Hg concentrations in fish tissue correlated to Hg levels in the ambient environment, and sediment seemed to be the major source for Hg accumulated in fish. In addition, feeding habit also affected Hg accumulation in different fish species. Four significant linear relationships were obtained between log-THg and δ¹⁵N and between log-MeHg and δ¹⁵N. The slope of the regression equations, as biomagnification power, was smaller in magnitude compared with those reported for temperate and arctic marine and freshwater ecosystems, indicating that THg and MeHg biomagnifications were lower in this PRD subtropical aquaculture pond ecosystem. This was probably due to low Hg bioavailability at lower TLs as well as individual feeding behavior of fish.     

Cadmium and Lead Exposure Associated with Reduced Growth Rates, Poorer Fledging Success of Little Blue Heron Chicks (Egretta caerulea) in South Louisiana Wetlands

Persistent contaminants in ecosystems are often monitored via organisms that bioaccumulate the pollutant of interest. Several published studies have used colonial wading birds to assess contamination by heavy metals in aquatic ecosystems. Using a nonlethal sampling regimen, this study identified chick feathers and guano as tissues that best reflect local contamination. Both cadmium and lead were detected in food samples (0.3–4.3 μg/g Cd, 0.8–9.3 μg/g Pb), guano (0.2–3.2 μg/g Cd, 1.0–9.5 μg/g Pb), and feathers (0.6–25.4 μg/g Cd, 1.2–16.9 μg/g Pb) from little blue heron chicks at levels similar to other studies. Chicks exposed to cadmium had significantly slower growth rates than nonexposed chicks, and exposure to lead was correlated with increased nestling mortality. The appropriate scale of comparison necessary to demonstrate these effects and the usefulness of wading birds as bioindicators are discussed. Received: 22 September 1998/Accepted: 24 April 1999     

Mercury Concentrations and Se:Hg Molar Ratios in Flyingfish (<Emphasis Type="Italic">Exocoetus volitans</Emphasis>) and Squid (<Emphasis Type="Italic">Uroteuthis chinensis</Emphasis>)

We measured total mercury (Hg) and selenium (Se) concentrations as well as stable nitrogen (N) isotopic composition in flyingfish and squid muscle tissues from the eastern Indian Ocean and western South China Sea. The results showed that the mean Hg concentration in squid muscle from the western South China Sea was lower than that in the eastern Indian Ocean. The Hg concentrations in flyingfish and squid muscle samples were positively correlated with organism size (length and weight) and δ¹⁵N in all the study areas. Furthermore, we found a negative correlation between Se and Hg in molar content of flyingfish and squid muscle from the western South China Sea. The Se:Hg molar ratio was significantly negative correlated with fish weight and δ¹⁵N, suggesting that the Se:Hg molar ratio decreases with the increase of fish size and trophic level in the food web.     

Trophic level determines levels of brominated flame-retardants in coastal herring gulls

Liver concentrations of eight polybrominated diphenyl ethers (ΣPBDEs: sum of brominated diphenyl ethers [BDE]-28, -47, -99, -100, -153, -154, -183, and -209) ranged from 135 to 985 ng g⁻¹ lipid weight (lw) in coastal herring gulls (Larus argentatus) from the marine Hvaler Archipelago (The Glomma River Estuary), Norway. Hexabromocyclododecane (HBCD) concentrations ranged from 10 to 698 ng g⁻¹ lw. High range in δ¹³C indicates that gulls were subject to a diversity of carbon sources, likely reflecting their mixed feeding on terrestrial and marine organisms, or diversity of autochthonous and allochthonous (watershed) energy sources at the bases of their marine/estuarial food chains. Inverse relationships of HBCD, and to somewhat lesser extent of BDE-209, with δ¹³C values suggest higher abundance of these compounds in the land-derived energy-sources of the gulls. Inverse relationships of BDE-99, BDE-183 and BDE-209 with δ¹⁵N suggest that trophic relationships affect bioaccumulation of these compounds in the herring gulls, with greater bioaccumulation from lower trophic level prey species. This may be because these PBDE congeners are subject of debromination in higher trophic levels prey species of the gulls (e.g., teleost fish). Levels of BDE-209 (up to 95 ng/g lipid) of these herring gulls from 1998 were in the higher range reported in European birds, and not matched by other reports in North Sea seabirds. The present study suggests that the currently used brominated flame-retardants (BFRs), BDE-209 and HBCD relate to changing nutrient allocation in the herring gulls, and represent a risk to seabirds exploiting near-shore and estuary ecosystems.     

Model Estimates Bioaccumulation of Total PCBs,Dioxin–Furan TEQs,and Total Mercury in Mink Liver Based on Concentrations in Lake Ontario Water

We used stable isotope analysis and a bioaccumulation model to estimate concentrations of total polychlorinated biphenyls (PCBs), dioxin–furan toxic equivalents (TEQs), and total mercury (Hg) in mink and to compare predicted ranges with their chemical concentrations in mink liver (PCB, TEQ) and brain (Hg). Actual concentrations were within predicted bounds for total PCB, dioxin–furan TEQ, and Hg except in two cases (lowest PCB and highest Hg) which were very close to predicted bounds. Based on ¹⁵N analysis, the trophic level of mink ranged from 3.4 to 3.9. Animals at the upper end of the range were exposed to Lake Ontario water and its food web, whereas those at the lower end were captured at inland locations. Because of the complexity of wetland (an important habitat for mink in this study) food webs with pelagic, littoral, and terrestrial carbon sources and overlapping ¹³C signatures, whether the origins of mink diets were aquatic or terrestrial could not be determined. We have established a nondestructive biomonitoring tool to reasonably estimate concentrations of total PCBs, TEQs and total Hg in mink tissues as concentrations of these chemicals change in their water supply.     

Mercury Accumulation in Sharks From the Coastal Waters of Southwest Florida

As large long-lived predators, sharks are particularly vulnerable to exposure to methylmercury biomagnified through the marine food web. Accordingly, nonlethal means were used to collect tissues for determining mercury (Hg) concentrations and stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N) from a total of 69 sharks, comprising 7 species, caught off Southwest Florida from May 2010 through June 2013. Species included blacknose (Carcharhinus acronotus), blacktip (C. limbatus), bull (C. leucas), great hammerhead (Sphyrna mokarran), lemon (Negaprion brevirostris), sharpnose (Rhizoprionodon terraenovae), and tiger sharks (Galeocerdo cuvier). The sharks contained Hg concentrations in their muscle tissues ranging from 0.19 mg/kg (wet-weight basis) in a tiger shark to 4.52 mg/kg in a blacktip shark. Individual differences in total length and δ¹³C explained much of the intraspecific variation in Hg concentrations in blacknose, blacktip, and sharpnose sharks, but similar patterns were not evident for Hg and δ¹⁵N. Interspecific differences in Hg concentration were evident with greater concentrations in slower-growing, mature blacktip sharks and lower concentrations in faster-growing, young tiger sharks than other species. These results are consistent with previous studies reporting age-dependent growth rate can be an important determinant of intraspecific and interspecific patterns in Hg accumulation. The Hg concentrations observed in these sharks, in particular the blacktip shark, also suggested that Hg may pose a threat to shark health and fitness.     

Trace Elements and Carbon and Nitrogen Stable Isotopes in Organisms from a Tropical Coastal Lagoon

Trace elements (Fe, Mn, Al, Zn, Cr, Cu, Ni, Pb, Cd, Hg, and As) and stable isotope ratios (δ¹³C and δ¹⁵N) were analyzed in sediments, invertebrates, and fishes from a tropical coastal lagoon influenced by iron ore mining and processing activities to assess the differences in trace element accumulation patterns among species and to investigate relations with trophic levels of the organisms involved. Overall significant negative relations between trophic level (given by ¹⁵N) and trace element concentrations in gastropods and crustaceans showed differences in internal controls of trace element accumulation among the species of different trophic positions, leading to trace element dilution. Generally, no significant relation between δ¹⁵N and trace element concentrations was observed among fish species, probably due to omnivory in a number of species as well as fast growth. Trace element accumulation was observed in the fish tissues, with higher levels of most trace elements found in liver compared with muscle and gill. Levels of Fe, Mn, Al, and Hg in invertebrates, and Fe and Cu in fish livers, were comparable with levels in organisms and tissues from other contaminated areas. Trace element levels in fish muscle were below the international safety baseline standards for human consumption.     

Using feathers to assess risk of mercury and selenium to bald eagle reproduction in the Great Lakes region

Mercury (Hg) and selenium (Se) concentrations were determined in feathers of nestling and adult bald eagles (Haliaeetus leucocephalus) in the Great Lakes region, 1985–1989. Relationships between concentrations and two measures of reproduction, productivity and nesting success, were examined. Hg and Se were detected in all feathers analyzed. A maximum concentration of 66 mg/kg Hg was found in adult feathers in the upper peninsula of Michigan. The geometric means of Hg in adult feathers for sample areas were: interior lower peninsula of Michigan, 21 mg/kg; interior upper peninsula of Michigan, 21 mg/kg; Lake Superior, 22 mg/kg; Lakes Michigan and Huron, 20 mg/kg; and Lake Erie, 13 mg/kg. The geometric means of Hg in nestling feathers for sample areas were: interior lower peninsula of Michigan, 8.8 mg/kg; interior upper peninsula of Michigan, 8.1 mg/kg; Lake Superior, 8.7 mg/kg; Lakes Michigan and Huron, 8.0 mg/kg; Lake Erie, 3.7 mg/kg; and Voyageurs National Park, 20 mg/kg. Se concentrations were not significantly different across regions or between adult and nestling feathers, and Se concentrations ranged from 0.8 to 3.2 mg/kg.No significant relationships between adult or nestling feather concentrations and either measure of reproduction, productivity or nesting success, was found for either Hg or Se. It was concluded that neither Hg nor Se concentrations are currently affecting bald eagle reproduction in the Great Lakes region.     

Temporal trends of mercury, cesium, potassium, selenium, and thallium in Arctic char (Salvelinus alpinus) from Lake Hazen, Nunavut, Canada: effects of trophic position, size, and age

Arctic char (Salvelinus alpinus L.), the top predator in High Arctic lakes, often is used as a bioindicator of Hg contamination in Arctic aquatic ecosystems. The present study investigated effects of trophic position, size, and age of Arctic char in Lake Hazen, the largest lake in the Canadian High Arctic (81 degrees 50'N, 70 degrees 25'W), on Hg bioaccumulation. In addition, several essential (Se, K) and nonessential elements (Tl, Cs) in char muscle tissue were examined to compare their behavior to that of Hg. Trophic position of Arctic char was identified by stable isotope (delta15N) signature. Temporal trends of Hg from seven sampling campaigns over a 16-year period (1990-2006) were investigated for the overall data and for one trophic class. Concentrations of Hg were not correlated with age but were positively related to fork length and trophic position. Large char with greater delta15N signatures (> 12 per thousand) had larger Hg concentrations (0.09-1.63 microg/g wet wt) than small char with smaller delta15N signatures (< 12 per thousand, 0.03-0.32 microg/g wet wt), indicating that Hg concentrations increased with trophic position. Nonessential Cs and Tl showed relationships to age, length, and trophic position similar to those of Hg, indicating their potential to bioaccumulate and biomagnify. Essential Se and K did not show these relationships. Concentrations of Hg were adjusted using delta15N, leading to less within-year variability and a more consistent temporal trend. The delta15N-adjusted trend showed no decline of Hg in Arctic char from Lake Hazen (1990-2006) in the overall data set and in the small morphotype. Trends for the same period before the adjustment were not significant for the overall data set, but a slight decrease was apparent in the small morphotype. The results confirm the need to consider trophic position and fish size when monitoring temporal trends of Hg, particularly for species with different morphotypes.     

Mercury concentrations are low in commercial fish species of Lake Ziway, Ethiopia, but stable isotope data indicated biomagnification

Stable isotope ratios of nitrogen (δ¹⁵N) and carbon (δ¹³C), complemented by stomach content data, were used to assess the food web structure and trophic transfer of mercury (Hg) in four commercial fish species of Lake Ziway, Ethiopia: Nile tilapia (Oreochromis niloticus), catfish (Clarias gariepinus), Tilapia zillii, and golden carp (Carassius auratus). Total mercury (THg in mg kg⁻¹, ww) concentrations were low, with mean values of 0.033, 0.034, 0.025, and 0.011, in C. gariepinus, T. zillii, C. auratus, and O. niloticus, respectively. The relationships between mercury concentrations against total length (TL) and total weight (TW) were positive and significant in T. zillii, C. auratus, and C. gariepinus (P<0.01), but not in O. niloticus, which even showed a decreasing tendency with increase in TL and TW. Regression of log THg vs. δ¹⁵N among all fish species showed a significant correlation, indicating that mercury is biomagnifying along the food web of Lake Ziway. Isotope ratios indicated that C. gariepinus occupied the highest trophic level of the food web of Lake Ziway; but contained similar THg concentrations as T. zillii, which is located at a lower trophic level, probably due to a faster growth rate of C. gariepinus, and thus an example of biodilution.     

Mercury speciation and biomagnification in the food web of Caddo Lake, Texas and Louisiana, USA, a subtropical freshwater ecosystem

We studied the biomagnification of total mercury and methylmercury in a subtropical freshwater lake, Caddo Lake, Texas and Louisiana, USA. The present study is unique in that it not only included invertebrates (seven species) and fish (six species) but also an amphibian (one species), reptiles (three species), and mammals (three species). Nonfish vertebrates such as those included in the present study are often not included in assessments of trophic transfer of Hg. Mean trophic position (determined using stable isotopes of nitrogen) ranged from 2.0 (indicative of a primary consumer) to 3.8 (indicative of a tertiary consumer). Mean total Hg concentrations ranged from 36 to 3,292 ng/g dry weight in muscle and whole body and from 150 to 30,171 ng/g dry weight in liver. Most of the Hg in muscle and whole-body tissue was found as methylmercury, and at least 50% of the Hg found in liver was in the inorganic form (with the exception of largemouth bass, Micropterus salmoides). Mercury concentrations were positively correlated with trophic position, indicating that biomagnification occurs in the food web of Caddo Lake. The food web magnification factors (FWMFs; slope of the relationship between mean Hg concentration and trophic position) for both total Hg and methylmercury were similar to those observed in other studies. Because most of the total Hg in consumers was methylmercury, the FWMF for methylmercury was not significantly different from the FWMF for total Hg. Some vertebrates examined in the present study had low Hg concentrations in their tissues similar to those observed in invertebrates, whereas others had concentrations of Hg in their tissues that in previous studies have been associated with negative health consequences in fish.