Base-line variations in stable isotope values in an Arctic marine ecosystem: effects of carbon and nitrogen uptake by phytoplankton

Abstract

Stable isotope values are useful for elucidating C and N cycling and pathways in marine and aquatic ecosystems. Variations in the base-line isotope values, the d 13 C and d 15 N values of phytoplankton, put constraints on their usefulness as tracers for trophic interactions and sources of organic matter in food web studies, however. We investigated the C and N stable isotope values of suspended particulate organic matter in relation to uptake of total dissolved inorganic carbon and nitrate, chlorophyll a concentration and the isotope composition of dissolved inorganic carbon in an Arctic marine environment (northern Barents Sea) in order to improve the understanding of factors regulating the variation in stable isotope values at the base of the marine food web. The stable isotope values of water-column suspended particulate organic carbon (d 13 C org ) and nitrogen (d 15 N org ) varied from -28.3% to -20.2% and 2.9% to 8.3%, respectively, among stations sampled during spring and summer. d 13 C org was not linearly related to carbon uptake, but the values were on average 3% higher at stations in a late-bloom stage, characterised by higher carbon uptake compared to early-bloom stations. Accumulation of phytoplankton biomass had a strong impact on d 13 C org values, reflected in a positive relationship between d 13 C org and chlorophyll a concentration. d 15 N org was positively related to the percentage of nitrate taken up from initial (winter) concentrations. These results indicate a strong relationship between bloom progression and isotope composition of particulate organic C and N pools. Synoptic data on stable isotope compositions, nutrient concentrations and phytoplankton biomass therefore improve the interpretation of isotope values when these are compared across pools with different turnover times, such as phytoplankton and consumers or suspended and sedimentary organic matter.

Interpreting nitrogen stable isotopes in the study of migratory fishes in marine ecosystems

“…We analyzed potential sources of this variability by examining diets, oceanographic conditions and fish migration. Diet shifts with size do not account for the variability, but the utilization, differences in nutrient sources (e.g., denitrification vs. N 2 fixation) and the subsequent biological transformations of these nutrients (Altabet 2001, Sigman and Casciotti 2001, Tamelander et al 2009Graham et al 2010;Carlisle et al 2012). The δ 15 N and δ 13 C values of an organism alone provide little information about its ultimate trophic position, and it is essential that ratios are compared to an appropriate baseline (Post 2002;Borrell et al 2011).…”

An isotopic baseline (δ13C, δ15N) for fishes of Lake Winnipeg: Implications for investigating impacts of eutrophication and invasive species

“…The ratios of 13 C/ 12 C in dissolved inorganic carbon (bicarbonate) and primary production are conserved as energy is transferred through the food web, providing an indicator of energy source and habitat (Hobson et al, 2002;Tamelander et al, 2009). Conversely, the relative trophic position of aquatic species can be inferred based on stepwise trophic enrichment in tissue 15 N from the base of the food web (Hobson et al, 2002;Minagawa and Wada, 1984).…”

Spatial variability of carbon (δ13C) and nitrogen (δ15N) stable isotope ratios in an Arctic marine food web

“…In short, the baseline shifts appeared to explain most of the spatial variation in δ Baseline isotope values are influenced by the physical and biological properties of local currents (Pantoja et al 2002), terrestrial input (Carlier et al 2007) and species composition and bloom progression (Tamelander et al 2009). The combined effect of these direct and indirect factors on primary producers will propagate up the food web, influencing the isotopic signatures of all trophic levels, though with diminishing strength towards higher trophic levels.…”

“…showed that rising temperature can have a negative effect on δ Whether these factors cause the differences in δ 15 N values remains unknown, as other undocumented factors such as changes in the composition of primary production or seasonal progression could result in similar variation (Tamelander et al 2009). …”