The impact of recycling of organic carbon on the stable carbon isotopic composition of dissolved inorganic carbon in a stratified marine system (Kyllaren fjord, Norway)

A negative carbon isotope shift in sedimentary organic carbon deposited in stratified marine and lacustrine systems has often been inferred to be a consequence of the process of recycling of respired and, therefore, C-13-depleted, dissolved inorganic carbon (DIC) formed from mineralization of descending organic matter. To study this process, we measured delta C-DIC and delta C-13 values of particulate organic carbon (POC) over an annual cycle in the permanently stratified Kyllaren fjord in Norway. A notable accumulation of respired DIC below the chemocline was evident from the substantially C-13-depleted DIC (ca. -19%). Especially in autumn to early spring, respired DIC from the deep anoxic water is mixed into the oxygenated surface water and the calculated respired DIC contribution to the total DIC pool was up to similar to 40% in early spring in the upper 2 m of the water column. At 4 m-depth, just below the chemocline, the respired DIC contribution reaches ca. 90% of the total DIC pool. Assimilation of the respired DIC seems to exert only a small effect on delta C-13(POC), which has an average delta C-13 value of -24%. The measured photoautotrophic fractionation (epsilon(p)) was low (< 10 parts per thousand) during the majority of the year. This is likely responsible for reducing the apparent impact of recycling of respired DIC on delta C-13(POC). However, in June 2002, photoautotrophic use of the 13C-depleted DIC is obvious from a C-13-depletion of POC (-33.7 parts per thousand) derived from a bloom of the protist Euglena sp. (c) 2005 Elsevier Ltd. All rights reserved.