Seasonal changes in background levels of deuterium and oxygen-18 proves water drinking by harp seals, which affects the use of the DLW method

Abstract

The aim of the study was to monitor seasonal changes in stable isotopes of pool freshwater, and harp seal (Phoca groenlandica) body water, and to study if these potential seasonal changes might bias results of using the doubly labeled water (DLW) method when measuring energy expenditure in animals with access to fresh water. Seasonal changes in the background levels of deuterium and oxygen-18 in the body water of four captive harp seals and in the freshwater pool in which they were kept, were measured over a time period of one year. The seals were offered daily amounts of capelin and kept under a seasonal photoperiod of 69° N. Large seasonal variations of deuterium and oxygen-18 in the pool water was measured, and the isotope abundance in body water showed similar seasonal changes as the pool water. This shows that the seals were continuously equilibrating with surrounding water due to significant daily water drinking. Variations in background levels of deuterium and oxygen-18 in freshwater sources may be due to seasonal changes in physical processes such as precipitation and evaporation that may cause fractionation of isotopes. Rapid and abrupt changes in the background levels of deuterium and oxygen-18 may complicate calculation of energy expenditure by use of the DLW method. It is therefore strongly recommended to perform analysis of seasonal changes in background levels of isotopes before the DLW method is applied on (free-ranging) animals, and to use a control group in order to correct for changes in background levels.