Mercury Trophic Transfer in a Eutrophic Lake: The Importance of Habitat‐specific Foraging

Abstract

Abstract. Mercury (Hg) trophic transfer and bioaccumulation in fish from a mineimpacted, eutrophic lake were examined in relation to foraging habitat, trophic position, and size. Diet analysis indicated that there were clear ontogenetic shifts in foraging habitats and trophic position. Pelagic diet decreased and benthic diet increased with increasing fish length in bluegill, black crappie, inland silverside, and largemouth bass, whereas there was no shift for prickly sculpin or threadfin shad. Stable carbon isotope values (d 13 C) were inversely related to the proportion of pelagic prey items in the diet, but there was no clear relationship with benthic foraging. There were distinct differences between pelagic and benthic prey basal d13 C values, with a range of approximately À28ø in pelagic zooplankton to approximately À20ø in benthic caddisflies. Profundal prey such as chironomid larvae had intermediate d13 C values of approximately À24ø, reflecting the influence of pelagic detrital subsidies and suppressing the propagation of the benthic carbon isotope signal up the food chain. Fish total mercury (TotHg) concentrations varied with habitat-specific foraging, trophic position, and size; however, the relationships differed among species and ages. When controlling for the effects of species, length, and trophic position, TotHg and d 13 C were positively correlated, indicating that Hg trophic transfer is linked to benthic foraging. When examined on a species-specific basis, TotHg was positively correlated with d 13 C only for bluegill, largemouth bass, and threadfin shad. However, diet-based multiple regression analyses suggested that TotHg also increased with benthic foraging for inland silverside and black crappie. In both species, benthic prey items were dominated by chironomid larvae, explaining the discrepancy with d 13 C. These results illustrate the importance of foraging habitat to Hg bioaccumulation and indicate that pelagic carbon can strongly subsidize the basal energy sources of benthic organisms.

Seasonal Variations in the Use of Profundal Habitat among Freshwater Fishes in Lake Norsjø, Southern Norway, and Subsequent Effects on Fish Mercury Concentrations

“…The distributions of Tot-Hg for the sampled fish species appeared to be influenced by habitat [8,11,40], trophic position [5][6][7][8]10] and age distributions [8,11]. Due to the similar, profundal habitat, the average Tot-Hg-concentrations (ww.)…”

“…where R represents the ratio of the heavier isotope 13 C or 15 N to the lighter 12 C or 14 N [8,40]. As standard material, Pee Dee belemnite limestone was used to calculate δ 13 C [50], and atmospheric nitrogen for δ 15 N.…”

“…for A. charr (0.24 ± 0.21 ppm), E. smelt (0.22 ± 0.08 ppm) and whitefish (0.20 ± 0.09 ppm) did not differ substantially. However, A. charr exhibited the largest range, highest values and lowest median Tot-Hg-concentrations (Table 1), which was likely caused by a catch of mainly small and young fish from a species with the highest potential to accumulate Tot-Hg due to high maximum age [8,11,67], a profundal diet consumed all year [8,11,40,60], and the highest average δ 15 N [5][6][7][8]10]. For the standard dataset of explanatory variables, A. charr exhibited intermediate predicted Tot-Hg-concentrations (0.12-0.18 ppm ww.…”

“…Methylated Hg is an environmental pollutant of concern in aquatic environments [1][2][3][4], as it is accumulated in biota, and concentrations rise in accordance with trophic position [5][6][7][8][9][10]. Fish and fish-eating wildlife often have toxic concentrations of total Hg (Tot-Hg) as a result [7].…”

“…Fish and fish-eating wildlife often have toxic concentrations of total Hg (Tot-Hg) as a result [7]. In addition to trophic position, Hg-concentrations in fish are well documented to increase with increasing age [8,11] and length [8,[11][12][13][14]. Contrarily, increasing weight at the same length or age results in lower Hg-concentrations, either by somatic growth dilution (SGD) [11,[15][16][17][18][19][20][21], or by further concentrating Hg during starvation [22].…”