Abstract
The trophic position concept is central in system ecology, and in this study, trophic position (TP) estimates from stable-isotopes and an Ecopath mass-balance food web model for the Barents Sea were compared. Two alternative models for estimating TP from stable isotopes, with fixed or scaled trophic fractionation were applied. The mass-balance model was parametrized and balanced for year 2000, was comprised of 108 functional groups (Gs), and was based on biomass and diet data largely based on predator stomach data. Literature search for the Barents Sea Large Marine Ecosystem revealed 93 sources with stable isotope data (δ15N values) for 83 FGs, and 25 of the publications had trophic position estimated from nitrogen stable isotopes. Trophic positions estimated from the mass-balance model ranged to 5.1 TP and were highly correlated with group mean δ15N values, and also highly correlated with the original literature estimates of trophic positions from stable isotopes. On average, TP from the mass-balance model was 0.1 TP higher than the original literature TP estimates (TPSIR) from stable isotopes. A trophic enrichment factor (TEF) was estimated assuming fixed fractionation and minimizing differences between trophic positions from Ecopath and TP predicted from δ15N values assuming a baseline value for δ15N calculated for pelagic particulate organic matter at a baseline TP of 1.0. The estimated TEF of 3.0‰ was lower than the most commonly used TEF of 3.4 and 3.8‰ in the literature. The pelagic whales and pelagic invertebrates functional groups tended to have higher trophic positions from Ecopath than from stable isotopes while benthic invertebrate functional groups tended to show an opposite pattern. Trophic positions calculated using the scaled trophic fractionation approach resulted in lower TP than from Ecopath for intermediate TPs and also a larger TP range in the BS. It is concluded that TPs estimated from δ15N values using a linear model compared better to the Ecopath model than the TPs from scaled fractionation approach.
Highlights
Following the introduction of integer trophic levels by Lindeman (1942) and fractional trophic levels by Odum and Heald (1975), the use of trophic levels has developed and it has become a conceptual pillar in ecosystem analysis
Many of the δ15N values had not been used to calculate TPSIR values in the original sources and 759 TPSIR values from 65 functional groups were published in the original sources
The material included a total of 87 δ15N values from samples of sediment organic matter (Supplementary Table 3)
Summary
Following the introduction of integer trophic levels by Lindeman (1942) and fractional trophic levels by Odum and Heald (1975), the use of trophic levels has developed and it has become a conceptual pillar in ecosystem analysis. Fractional trophic levels have been termed trophic positions (TP) in the literature (Odum and Heald, 1975; Vander Zanden and Rasmussen, 2001; Hussey et al, 2014a). TP is included in the theoretical basis and calculation of many ecosystem metrics and indicators such as trophic efficiency, transfer efficiency and omnivory index (Shannon et al, 2014). TP is an important predictor of trophic enrichment of pollutants in food webs (Hop et al, 2002; Jæger et al, 2009). Trophic position estimates may be useful for evaluation of fisheries exploitation and management strategies, and it is important to evaluate methods used to estimate trophic positions and factors affecting uncertainty in these estimates
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