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

Tom Olk,Tobias Karlsson,Asle Økelsrud,Espen Lydersen

doi:10.3390/environments3040029

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https://doi.org/10.3390/environments3040029

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Journal: Environments	Publication Date: Nov 11, 2016
Citations: 5	License type: CC BY 4.0

Affiliation: University of South-Eastern Norway

Abstract

This study is based on monthly sampling of fish from grates mounted at an industrial water intake, located at a depth of 50 m in Lake Norsjø (Southern Norway) during the year 2014, to investigate seasonal variations in the use of the profundal habitat and subsequent variations in total Hg-concentrations in profundal fish. Data on various fish present in a cold and dark hypolimnion of a large, deep, dimictic lake within the upper temperate zone of the Northern Hemisphere are rare. While predominant species such as A. charr (Salvelinus alpinus) and E. smelt (Osmerus eperlanus) were continuously present in this habitat, whitefish (Coregonus lavaretus) occupied this habitat primarily during wintertime, while other common species like brown trout (Salmo trutta), perch (Perca fluviatilis) and northern pike (Esox lucius) were almost absent. Besides stomach analyses (diet) and biometry, stable isotope analyses (δ15N and δ13C) and total mercury (Tot-Hg) analyses were carried out on the caught fish. The δ13C signature and stomach analyses revealed a combined profundal-pelagic diet for all three species, A. charr with the most profundal-based diet. Length was the strongest predictor for Hg in whitefish and A. charr, while age was the strongest explanatory variable for Hg in E. smelt. A. charr was the only species exhibiting seasonal variation in Hg, highest during winter and spring.

Highlights

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]
The δ15 N range of 6.62‰ indicates an individual variation in trophic position by almost two trophic levels (Λ = 1.95) within the group of A. charr analysed, assuming a δ15 N enrichment by 3.4‰ per trophic level (Λ), as estimated by Minagawa and Wada and Post [36,38]
total Hg (Tot-Hg)-concentrations in fish increased with length and age in the profundal zone, while a less depleted δ13 C signature, and lower weight, corrected for length, resulted in higher

Summary

Introduction

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]. Stable isotope ratio analyses of carbon (δ13 C = C13 /C12 ) and nitrogen (δ15 N = N15 /N14 ) are a highly valuable tool to trace the energy flow (δ13 C) and trophic position (δ15 N) in food webs [34,35,36], as the different isotopes have different abilities to form chemical bonds [37]. This means that molecules containing the heavier isotope are more stable, while molecules containing the lighter isotope are more

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Conclusion