Trull and Armand, DSR-II, in press

This paper demonstrates that the 13C content of Southern Ocean

phytoplankton increases with their size, and that comparison of the

size-fractionated phytoplankton 13C contents with the 13C enrichment which

accompanies seasonal DIC depletion can be used to identify the size of

phytoplankton which contribute most to seasonal carbon export (in this

case, as is no surprise, large diatoms). The variation of 13C with size

also offers potential for the evaluation of particle aggregation processes.

Insights into Southern Ocean carbon export from

the d13C of particles and dissolved inorganic carbon during the SOIREE

iron release experiment

T.W. Trull₁ and L. Armand₂

1. Antarctic CRC, Univ. of Tasmania, Hobart, 7001, Australia

(Tom.Trull@utas.edu.au)

2. IASOS, Univ. of Tasmania, Hobart, 7001, Australia

(Leanne.Armand@utas.edu.au)

Deep-Sea Research II, SOIREE special volume

ABSTRACT

13C contents of total dissolved inorganic carbon (d13C-DIC) and particulate

organic matter (d13Corg) were determined to examine the factors influencing

phytoplankton 13C contents and carbon export from the SOIREE iron-induced

algal bloom. Suspended particles sieved into 200, 70, 20, 5, and 1 micron

(µ) size classes displayed an extremely large range in d13Corg of 8‰.

d13Corg values increased from -28‰ for the 1-5µ class to a maximum of -20‰

for the 20-70µ class, which was dominated by the large pennate diatom

Fragilariopsis kerguelensis. Larger particles (70-200µ and >200µ) had

similar d13Corg to the smaller (1-5 and 5-20µ) particles, reflecting both

the presence of long narrow Thalassiothrix antarctica diatoms and

zooplankton which grazed on small phytoplankton. Comparison of results

inside and outside the bloom identified cell surface/volume ratio (mainly

reflecting cell size) as the dominant control of phytoplankton d13Corg,

with subsidiary roles for growth rate and seawater [CO2]aq.

The SOIREE iron fertilization provoked an increase in the proportion of

large (>20m) diatoms. This increased the d13Corg of the bulk suspended

particles within the mixed layer, but there was minimal increase in the

d13Corg of sub-surface suspended particles and negigible change in the

d13Corg of particles obtained with sediment traps suspended below the

bloom. This suggests that there was no increase in carbon export over the

~13 day observation period. However, comparison to d13Corg results from

previous voyages, and to vertical changes in d13C-DIC, suggests that large

diatoms control carbon export from the Antarctic Zone over the summer

growth season. This result must be viewed with great caution as it is

based on very sparse data and involves several assumptions.