# Presentation: 657674

## Accumulation and Fate of Mercury in an Everglades Aquatic Food Web

**Hg levels in Gambusia did not follow a consistent pattern with hydroperiod.**
**Wild and caged fish results were similar.**
**Little difference in diets between hydroperiods.**
**Hg levels in fish across the landscape seem to be mediated by site characteristics, including availability and temperature.**

**Conclusions:**
**Most Everglades fishes were omnivorous; few species were strict herbivores or piscivores.**
**Hg levels varied within and among taxa.**
**Trophic positions estimated from gut and stable-isotope analyses were similar.**
**Hg bioaccumulation was positively correlated with the trophic positions (diet) of aquatic animals.**

***Accumulation and Fate of Mercury in an Everglades Aquatic Food Web***

_**Acknowledgements**_
- Support provided by:  Florida Dept. of Environmental Protection, Dr. Tom Atkeson; Florida International University; U. S. National Park Service; USGS-Biological Resources Division

**Results and Discussion:**
- Most Everglades fishes have generalistic diets. Large native species feed mainly on invertebrates and fishes; smaller native fishes are primarily omnivorous. Few species are strict herbivores/detritivores (Table1, Figure 2).
- The trophic scores/positions of the fishes based on prey volumes from guts agreed well with the ordination patterns from Nonmetric-multidimensional scaling (NMDS) (Figure 3) (Stress = 0.153, r2  = 0.887).

- The trophic scores and total mercury levels of the fishes were positively correlated (Spearman’s r = 0.706, P< 0.001, n=39), as were those of the invertebrates (Pearson’s 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) (Figure 4).
- Nitrogen and carbon stable-isotope signatures were measured for aquatic animals and plants at Site 1-L (Figure 5).  Those of invertebrates were highly correlated with Hg levels (Spearman’s r= 0.745, P = 0.02, n=10)(Figure 6a, Table 2).  Fishes had the same result (Pearson's r = 0.806, P < 0.001, n=21) (Figure 6b, Table 2).

**Results and Discussion:**
- Mean concentrations varied from 38.4 ng g-1 in 3-S, June 1997 to 265.07 ng g-1  at 1-L, Spring, 1996. Hydroperiod significantly affected Hg concentrations and explained the greatest amount of variation in the model, but there were interaction effects among marsh hydroperiod, fish size, site, and time-of year (Figure 8).
- Uptake rates in caged fish ranged from 0.25-3.61 ng g-1d-1, and interactions occurred among time-of-year, site and hydroperiod.  The patterns of uptake were similar for wild and caged *Gambusia* (Figures 8 and 9).

- Survival normally exceeded 80%.
- Growth varied with time of year, site and hydroperiod, but greatest growth occurred in the short-hydroperiod marshes.
- The results of these studies suggest that dietary bioaccumulation determines mercury in Everglades aquatic animals, and that, although hydroperiod plays a role in mercury uptake, its effect varies with season and specific location (Figure 10).

**Methods:**
- Each month, a minimum of 30 specimens of wild fish were collected from each site for total Hg analysis.
- As a control, long- and short-hydroperiod stock mosquitofish were captive-raised at ENP to be stocked into field cages.
- 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 (Figure 7).
- At the trial’s 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.

- Uptake rates were generally greatest at Site 1 (North SRS), and lowest at Site 3 (TS).
- At site 2, fish from 2-L usually had higher Hg uptake rates than at 2-S, but the effect of hydroperiod was generally reversed at the other two locations.