Emmanuel Laurenceau – PhD confirmation seminar
Date: Friday, 15 June 2012, 3:30 PM
Venue: IMAS seminar room
Abstract:
The estimation of the carbon export efficiency associated with naturally iron-fertilised spring blooms occurring in the vicinity of Antarctic and Subantarctic islands is a key question regarding the importance of the Southern ocean in the climate system and its sensitivity to climate change. The settling rate of marine particles plays a major role in carbon export efficiency but the processes involved are complex and hardly predictable from simple parameterisations.
During the second Kerguelen Ocean and Plateau compared Study (KEOPS2) conducted in October – November 2011, we sampled seawater at 6 stations in high phytoplankton biomass layers (30 and 80 meters) and incubated it in rolling tanks to form organic aggregates by physical aggregation via differential settling. Sinking velocity and physico-chemical measurements completed by microscopic studies were conducted individually on 66 aggregates to assess a large range of parameters.
Surprisingly, the sinking velocity (U) showed a slight negative correlation to size yielding U (m day-1) = 227 (ESD, mm)-0.902, r2 = 0.23, suggesting that physical aggregation may not accelerate carbon export. Microscopic studies revealed positive correlations between the sinking velocity and the average cell length () of diatoms composing the aggregates, with separate relationships for two distinct groups of sampling stations: U1 (m day-1) = 2.72 , r2 = 0.72 and U2 (m day-1) = 0.453 , r2=0.90. Preliminary investigations show that time since bloom initiation combined with total incubation duration is a possible explanation of the difference observed between group 1 and 2, suggesting a role for cell senescence.
Our results confirm that sinking velocity is a complex function of physical, chemical and ecological parameters and emphasize the importance of phytoplankton community structure to carbon export efficiency.