MEPS 173:127-138 (1998)  -  doi:10.3354/meps173127

Phytoplankton carbon isotope fractionation during a diatom spring bloom in a Norwegian fjord

H. Kukert*, U. Riebesell**

Alfred Wegener Institute for Polar and Marine Research, Postfach 12 01 61, D-27515 Bremerhaven, Germany

ABSTRACT: The stable carbon isotope composition (δ13C) of particulate organic carbon (POC) was measured in 3 size fractions (POCtotal, POC>20 µm, POC<20 µm) during a phytoplankton spring bloom dominated by the diatom Skeletonema costatum in Lindåspollene, a land-locked fjord in southern Norway. In addition to standard parameters for characterizing the phytoplankton bloom (chlorophyll, nutrient, and POC concentrations, and species composition), simultaneous measurements of δ13C of dissolved inorganic carbon (DIC), total alkalinity and DIC concentration were obtained to determine temporal trends in dissolved carbon dioxide concentration and in carbon isotope fractionation (ε p) of the POC size fractions. The carbon isotope composition of the >20 µm size fraction, which was dominated by diatoms, was ca 2o/oo heavier than that of the <20 µm fraction, which was mainly composed of flagellates. δ13C of both size fractions increased by about 3o/oo over the course of the bloom. A 5o/oo increase in δ13C-PO Ctotal during the bloom resulted partly from a shift in the phytoplankton community from a flagellate- to a diatom-dominated one. Carbon isotope fractionation of all fractions decreased with declining CO2(aq) concentration (14 to >6 µmol l-1). A positive correlation between ε p and [CO2(aq)] in the diatom size fraction was obtained for the period of exponential growth. Deviation from this correlation occurred after the peak in cell density and chlorphyll a (chl a) concentration, when POC still continued to increase, and may be related to changing phytoplankton growth rates or to possible effects of nutrient (nitrate) limitation on ε p. Comparison of these results with those of previous field studies shows that, while an inverse relationship is consistently observed between ε p and the ratio of instantaneous growth rate and CO2 concentration {µi/[CO2(aq)]}, considerable scatter exists in this relationship. While this scatter may have partly resulted from inconsistencies between the different studies in estimating phytoplankton growth rate, it could also reflect that factors other than growth rate and CO2 concentration significantly contribute to determining isotope fractionation by marine phytoplankton in the natural environment.

KEY WORDS: δ13C · Isotope fractionation · CO2 · Phytoplankton · Diatom bloom

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