The slope of the simple linear regression between log10 transformed mercury (Hg) concentration and stable nitrogen isotope values (δ(15)N), hereafter called trophic magnification slope (TMS), from several trophic levels in a food web can represent the overall degree of Hg biomagnification. We compiled data from 69 studies that determined total Hg (THg) or methyl Hg (MeHg) TMS values in 205 aquatic food webs worldwide. Hg TMS values were compared against physicochemical and biological factors hypothesized to affect Hg biomagnification in aquatic systems. Food webs ranged across 1.7 ± 0.7 (mean ± SD) and 1.8 ± 0.8 trophic levels (calculated using δ(15)N from baseline to top predator) for THg and MeHg, respectively. The average trophic level (based on δ(15)N) of the upper-trophic-level organisms in the food web was 3.7 ± 0.8 and 3.8 ± 0.8 for THg and MeHg food webs, respectively. For MeHg, the mean TMS value was 0.24 ± 0.08 but varied from 0.08 to 0.53 and was, on average, 1.5 times higher than that for THg with a mean of 0.16 ± 0.11 (range: -0.19 to 0.48). Both THg and MeHg TMS values were significantly and positively correlated with latitude. TMS values in freshwater sites increased with dissolved organic carbon and decreased with total phosphorus and atmospheric Hg deposition. Results suggest that Hg biomagnification through food webs is highest in cold and low productivity systems; however, much of the among-system variability in TMS values remains unexplained. We identify critical data gaps and provide recommendations for future studies that would improve our understanding of global Hg biomagnification.

We examined the effects of fish predation on emergent insect-mediated methyl mercury (MeHg) flux across a gradient of MeHg contamination in experimental ponds. Emergent insects were collected from ponds with (n = 5) and without fish (n = 5) over a six week period using floating emergence traps. We found that the potential for MeHg flux increased with Hg contamination levels of the ponds but that the realized MeHg flux of individual insect taxa was determined by fish presence. Fish acted as size-selective predators and reduced MeHg flux by suppressing emergence of large insect taxa (dragonflies and damselflies) but not small insect taxa (chironomids and microcaddisflies). MeHg flux by small insect taxa was correlated with concentrations of MeHg in terrestrial spiders along the shorelines of the study ponds, demonstrating for the first time the cross-system transport of MeHg by emergent insects to a terrestrial spider.

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.

Mercury (Hg) contamination of small human-made ponds and surrounding terrestrial communities may be 1 of the largest unstudied Hg-pollution problems in the United States. Humans have built millions of small ponds in the Great Plains of the United States, and these ponds have become contaminated with atmospherically deposited mercury. In aquatic ecosystems, less toxic forms of Hg deposited from the atmosphere are converted to highly toxic methylmercury (MeHg). Methylmercury is incorporated into the aquatic food web and then can be transferred to terrestrial food webs via emergent aquatic insects. The authors present a conceptual model that describes the movement of MeHg produced in aquatic ecosystems to terrestrial consumers via insects emerging from small human-made ponds. The authors hypothesize that pond permanence and the level of Hg contamination of the food web control this emergent insect-mediated flux of MeHg. The highest insect-mediated flux of MeHg is predicted to be from fishless semipermanent ponds with food webs that are highly contaminated with MeHg. Further development and testing of the conceptual model presented in the present column, particularly in the context of a changing climate, will require research at the regional, watershed, and pond scales.

Most studies examining the influence of ecological characteristics of fish on Hg concentration in fish tissues have focused on a few variables and been conducted in northern ecosystems. We examined how total length (TL), age, food-web position (estimated using delta13C and delta15N), and habitat were related to total Hg concentrations in 10 species of fish from Caddo Lake, a subtropical reservoir located on the border of Texas and Louisiana, USA. We observed biomagnification in the Caddo Lake fish assemblage, and the enrichment factors (the slope of the relationship between lamda15N and total Hg concentration) in the two habitats were 0.19 and 0.24, similar to those found in other studies. Although trophic position was the best predictor of total Hg concentration between species, age and TL were the best predictors of total Hg concentration within species. Unlike studies conducted in deep lakes, lamda13C values of fish tissue, a measure of the extent to which fish feed in food webs based on pelagic or littoral primary production, was not a good predictor of total Hg concentration in Caddo Lake fish. Total Hg concentrations in fish were elevated in forested-wetland habitats relative to open-water habitats. Data collected in the present study indicate that more Hg likely was available for incorporation into the base of the food web in the forested-wetland habitat than in the open-water habitat. Our results help to clarify the relationship between ecological characteristics of fish and Hg concentration in fish tissue and can be used by researchers as well as public and environmental health officials when designing Hg monitoring studies.

Methylmercury (MeHg) is an environmental contaminant that can have adverse effects on wildlife. Because MeHg is produced by bacteria in aquatic ecosystems, studies of MeHg contamination of food webs historically have focused on aquatic organisms. However, recent studies have shown that terrestrial organisms such as songbirds can be contaminated with MeHg by feeding on MeHg-contaminated spiders. In the present study, the authors examined the risk that MeHg-contaminated terrestrial long-jawed orb weaver spiders (Tetragnatha sp.) pose to songbirds at Caddo Lake (Texas/Louisiana, USA). Methylmercury concentrations in spiders were significantly different in river, wetland, and open-water habitats. The authors calculated spider-based wildlife values (the minimum spider MeHg concentrations causing physiologically significant doses in consumers) to assess exposure risks for arachnivorous birds. Methylmercury concentrations in spiders exceeded wildlife values for Carolina chickadee (Poecile carolinensis) nestlings, with the highest risk in the river habitat. The present study indicates that MeHg concentrations in terrestrial spiders vary with habitat and can pose a threat to small-bodied nestling birds that consume large amounts of spiders at Caddo Lake. This MeHg threat to songbirds may not be unique to Caddo Lake and may extend throughout the southeastern United States.

Estuarine fish may remain as residents in areas with favorable conditions for feeding and refuge, but become mobile and transient where conditions are less favorable. We developed a new approach involving sulfur stable isotope (d 34 S) distributions in fish muscle tissue to track residents and transients across estuarine salinity zones. Salinity tracking was based on d 34 S contrasts between freshwater and marine waters. This 5-yr study of two Louisiana estuaries showed that riverine and upper-estuarine fish consistently had low d 34 S values (25% to +5%) in contrast to fish from the lower, more marine portion of estuaries that had higher d 34 S values (11-17%). Residents were identified using tests of normality within community-level d 34 S distributions, and conceptually were considered animals permanently residing at the sampling station but also any animals present from nearby areas with similar salinities. Transients had d 34 S values atypical of both the location of capture and the local salinity regime. Results showed good resolution of fish movement at small 0.1-4-km scales for low-salinity (, 2) upper-estuary stations, and good detection of long-range migrants from the upper estuary into the lower estuary at a coarser spatial scale of 10-30 km. On an average basis, O of the estuarine fish fauna was resident and M transient. Transients were a minority in most species but commonly included small as well as large fish. This novel d 34 S approach may be a general technique for evaluating fish residency and movement across salinity zones in estuaries.Station and habitat evaluations play an important role in conservation and restoration decisions. In estuaries with a typical mix of marsh and open water areas spread over a freshwater to marine salinity gradient of 0 to 35, it is often unclear which sites and habitats are most important for fish productivity and biodiversity. Both refuge and feeding opportunities are aspects of estuarine habitats and stations important to fish (Boesch and Turner 1984), and higher fish abundances are often taken as evidence that sites are important or essential habitat (Minello et al. 2003). Further concerns center around how habitats support growth or decrease predation risk, but these aspects are harder to measure and evaluate (Beck et al. 2001). A more recent approach to habitat evaluation is to use chemical markers such as stable isotopes and trace metals to determine which habitats have relatively high importance for fish, especially habitats that have important nursery or productivity functions (Thorrold et al. 2002;Wissel and Fry 2005). Residency is potentially an additional way to assess habitat quality, with low residency and high transience likely indicating suboptimal habitats (Hoffman et al. 2007). The advantages residents may have include better avoiding local predation and better accessing local nutritious foods. The chemical marker approach can be strong for studying residency (Fry and Ewel 2003), and identifying stations where fish isotope chemistry indic...

We examined the biomass-dependent effects of common carp (Cyprinus carpio) on water quality in 10 ponds at the Eagle Mountain Fish Hatchery, Fort Worth, Texas, USA. Ponds contained 0-465 kg ha )1 of common carp. We measured limnological variables at weekly intervals for four weeks in early summer, after which ponds were drained and the biomass of fish and macrophytes was determined. Common carp biomass was significantly positively correlated with chlorophyll a, total phosphorus, total nitrogen, and Keratella spp. density and negatively correlated to bushy pondweed (Najas guadalupensis) biomass. In addition, we combined our data with data from comparable studies to develop more robust regression models that predict the biomass-dependent effects of common carp on water quality variables across a wide range of systems.