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 δ13C and δ15N 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 (δ13Corg) and nitrogen (δ15Norg) varied from −28.3‰ to −20.2‰ and 2.9‰ to 8.3‰, respectively, among stations sampled during spring and summer. δ13Corg 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 δ13Corg values, reflected in a positive relationship between δ13Corg and chlorophyll a concentration. δ15Norg 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.