ABSTRACT: The stable carbon isotope composition (d¹³C) of particulate organic carbon (POC) was measured in 3 size fractions (POC_total, 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 d¹³C 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 (e_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. d¹³C of both size fractions increased by about 3o/oo over the course of the bloom. A 5o/oo increase in d¹³C-PO C_total 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 e_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 e_p. Comparison of these results with those of previous field studies shows that, while an inverse relationship is consistently observed between e_p and the ratio of instantaneous growth rate and CO₂ concentration {µ_i/[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: d¹³C · Isotope fractionation · CO₂ · Phytoplankton · Diatom bloom

Published in MEPS Vol. 173 (1998) on November 12
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1998