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Joel Trexler (Florida International University); Ronald Jones
1. Examine trophic relationships in the Everglades fish community to identify patterns of resource use by a temperate fauna in a subtropical, seasonal wetland. 2. Group species into trophic groups based on volumetric contributions of prey items and corroborate with stable-isotope analysis. 3. Measure total Hg in water, soils, plants, invertebrates, and fishes. 4. Correlate Hg levels with trophic positions of invertebrates and fishes based on gut and stable-isotope analyses.
This study was proposed to answer a series of questions posed by a multi-agency task force convened to address the mercury problem in the Everglades. These questions included understanding the extent of contamination in aquatic biota, the pathways by which Hg moves through the aquatic food web, and the processes that affect bioaccumulation.
Trexler, J. C.
Winemiller, K. O.; Loftus, W. F.; Akin, S.
The full article is available via journal subscription or single article purchase. The abstract may be viewed on the website below by selecting (abstract)..
1. Fishes collected from three habitats at high and low water in a long-hydroperiod wetland at Site 1-L in Shark Slough, Everglades National Park. Only high-water samples used in Hg correlations. 2. Approximately 4,000 specimens of 26 native fishes from 1977-1981, supplemented by 6 introduced species during 1995-1997, collected with rotenone, electrofishing, nets, and angling. 3. Small size of food items required the pooling of like taxa from each size-class of fish for volumetric analysis.Volumes measured to 0.001ml with a blood-sedimentation tube. 4. For HgTotal all samples collected with clean techniques. 5. Invertebrate samples were composited, while large fish were homogenized and subsampled. 6. Whole-body samples of animals were digested with acid in sealed ampules in an autoclave. 7. Total Hg measured with atomic-fluorescence spectrometer, after generating callibration curves and including Standard Reference Materials as checks for each run.
Trophic Classification 1. Fish trophic scores were calculated by summing the products of the dietary contributions of the 34 prey types and their trophic levels 2. Scores were used to group fishes into five discrete trophic classes (guilds): 1 = Mainly Herbivorous, >50% plant material; 2 = Omnivorous, 25-50% plant material; 3 = Omnivorous, <25% Plant material; 4 = Omnivorous, mainly animal prey; 5 = Predominantly carnivorous, fish and decapods
The trophic scores and total mercury levels of the fishes were positively correlated (Spearmanís r = 0.706, P< 0.001, n=39), as were those of the invertebrates (Pearsonís r = 0.749, p < 0.001). There were significant differences in square-root-transformed total Hg concentrations among the species of fishes (ANOVA, F31,515 = 52.035, P < 0.001). The five classes differed significantly in mean Hg levels (ANOVA, F, 4, 24 = 9.17, P < 0.0005)
Nitrogen and carbon stable-isotope signatures were measured for aquatic animals and plants at Site 1-L
The second part of the project was based on the following rationale:
1. Differences in fish and invertebrate densities between short- and long-hydroperiod marshes may result from food-web differences. 2. Differing food resources may result in differences in mercury accumulation. 3. If Gambusia experience different levels of Hg, chronic exposure may result in tolerant genotypes. Transplanting of fish across hydroperiods may show differences in survival or growth.
The objectives were to:
Measure total Hg levels in wild Gambusia from 3 pairs of short- and long- hydroperiod marshes across seasons ). Estimate Hg concentrations, Hg uptake, growth rates, and survival of caged Gambusia from those marshes to compare to wild-fish patterns. Examine diet differences among hydroperiods from gut and stable-isotope analyses with relation to Hg patterns.
The methods for collecting data were:
1. Each month, a minimum of 30 specimens of wild fish were collected from each site for total Hg analysis. 2. As a control, long- and short-hydroperiod stock mosquitofish were captive-raised at ENP to be stocked into field cages. 3. Possible Hg-tolerance differences in the populations were examined by transplanting mosquitofish originating from short-hydroperiod stocks into cages at long-hydroperiod locations, and visa versa. Five cages at each site were stocked with progeny from the same hydroperiod type, and 5 alternate cages stocked with progeny from the other category
At the trialís end, stocked caged fish were recovered and wild fish were collected from the same site. All were measured and weighed (for growth analysis) and analyzed for Hg
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