H. Kukert et U. Riebesell, PHYTOPLANKTON CARBON-ISOTOPE FRACTIONATION DURING A DIATOM SPRING BLOOM IN A NORWEGIAN FJORD, Marine ecology. Progress series, 173, 1998, pp. 127-137
The stable carbon isotope composition (delta(13)C) Of particulate orga
nic carbon (POC) was measured in 3 size fractions (POCtotal, POC>20 mu
m, POC<20 mu m) during a phytoplankton spring bloom dominated by the
diatom Skeletonema costatum in Lindaspollene, a land-locked fjord in s
outhern Norway. In addition to standard parameters for characterizing
the phytoplankton bloom (chlorophyll, nutrient, and POC concentrations
, and species composition), simultaneous measurements of delta(13)C Of
dissolved inorganic carbon (DIC), total alkalinity and DIC concentrat
ion were obtained to determine temporal trends in dissolved carbon dio
xide concentration and in carbon isotope fractionation (epsilon(p)) of
the POC size fractions. The carbon isotope composition of the >20 mu
m size fraction, which was dominated by diatoms, was ca 2 parts per th
ousand heavier than that of the <20 mu m fraction, which was mainly co
mposed of flagellates. delta(13)C Of both size fractions increased by
about 3 parts per thousand over the course of the bloom. A 5 parts per
thousand increase in delta(13)C-POCtotal during the bloom resulted pa
rtly 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 mu mol l(-1)
). A positive correlation between epsilon(p) and [CO2(aq)] in the diat
om size fraction was obtained for the period of exponential growth. De
viation from this correlation occurred after the peak in cell density
and chlorphyll a (chl a) concentration, when POC still continued to in
crease, and may be related to changing phytoplankton growth rates or t
o possible effects of nutrient (nitrate) limitation on epsilon(p). Com
parison of these results with those of previous field studies shows th
at, while an inverse relationship is consistently observed between eps
ilon(p) and the ratio of instantaneous growth rate and CO2 concentrati
on {mu(i)/[CO2(aq)]}, considerable scatter exists in this relationship
. While this scatter may have partly resulted from inconsistencies bet
ween the different studies in estimating phytoplankton growth rate, it
could also reflect that factors other than growth rate and CO2 concen
tration significantly contribute to determining isotope fractionation
by marine phytoplankton in the natural environment.