A negative carbon isotope shift in sedimentar
y organic carbon deposited in stratified marine and lacustrine s
ystems has often been inferred to be a consequence of the process of rec
ycling of respired and, therefore,
13C-depleted, dissolved inorganic carbon (DIC) formed from mineralization of descending organic matter. To stud
y this process, we measured
δ13C
DIC and
δ13C values of particulate organic carbon (POC) over an annual c
ycle in the permanentl
y stratified K
yllaren fjord in Norwa
y. A notable accumulation of respired DIC below the chemocline was evident from the substantiall
y 13C-depleted DIC (ca. −19‰). Especiall
y in autumn to earl
y spring, respired DIC from the deep anoxic water is mixed into the ox
ygenated surface water and the calculated respired DIC contribution to the total DIC pool was up to
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40 % in earl
y 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 onl
y a small effect on
δ13C
POC, which has an average
δ13C value of −24‰. The measured photoautotrophic fractionation (
εp) was low (<10‰) during the majorit
y of the
year. This is likel
y responsible for reducing the apparent impact of rec
ycling of respired DIC on
δ13C
POC. However, in June 2002, photoautotrophic use of the
13C-depleted DIC is obvious from a
13C-depletion of POC (−33.7‰) derived from a bloom of the protist
Euglena sp.