MEPS 184:31-41 (1999)  -  doi:10.3354/meps184031

ABSTRACT: Stable carbon isotope fractionation (ε _p) of 7 marine phytoplankton species grown in different irradiance cycles was measured under nutrient-replete conditions at a high light intensity in batch cultures. Compared to experiments under continuous light, all species exhibited a significantly higher instantaneous growth rate (μ_i), defined as the rate of carbon fixation during the photoperiod, when cultivated at 12:12 h, 16:8 h, or 18:6 h light:dark (L/D) cycles. Isotopic fractionation by the diatoms Skeletonema costatum, Asterionella glacialis, Thalassiosira punctigera, and Coscinodiscus wailesii (Group I) was 4 to 6o/oo lower in a 16:8 h L/D cycle than under continuous light, which we attribute to differences in μ_i. In contrast, ε _p in Phaeodactylum tricornutum, Thalassiosira weissflogii, and in the dinoflagellate Scrippsiella trochoidea (Group II) was largely insensitive to daylength-related differences in instantaneous growth rate. Since other studies have reported growth-rate dependent fractionation under N-limited conditions in P. tricornutum, μ_i-related effects on fractionation apparently depend on the factor controlling growth rate. We suggest that a general relationship between ε_i and μ_i/[CO_2,aq] may not exist. For 1 species of each group we tested the effect of variable CO₂ concentration, [CO_2,aq], on isotopic fractionation. A decrease in [CO_2,aq] from ca 26 to 3 µmol kg^-1 caused a decrease in ε _p by less than 3o/oo. This indicates that variation in μ_i in response to changes in daylength has a similar or even greater effect on isotopic fractionation than [CO_2,aq] in some of the species tested. In both groups ε _p tended to be higher in smaller species at comparable growth rates. In 24 and 48 h time series the algal cells became progressively enriched in ¹³C during the day and the first hours of the dark period, followed by ¹³C depletion in the 2 h before beginning of the following light period. The daily amplitude of the algal isotopic composition (δ¹³C), however, was <=1.5o/oo, which demonstrates that diurnal variation in δ¹³C is relatively small.

KEY WORDS: Isotope fractionation · ¹³C discrimination · Growth rate · CO₂ · Paleobarometer · Daylength · Diurnal variation · Diatoms · Phytoplankton