Effects of CO₂ on particle size distribution and phytoplankton abundance during a mesocosm bloom experiment (PeECE II)

• Main Authors: M. Schartau, B. Delille, R. Bellerby, U. Riebesell, K. G. Schulz, A. Engel
• Format: Article
• Language: English
• Published: Copernicus Publications 2008-04-01
• Series: Biogeosciences
• Online Access: http://www.biogeosciences.net/5/509/2008/bg-5-509-2008.pdf

The influence of seawater carbon dioxide (CO₂) concentration on the size distribution of suspended particles (2&ndash;60 μm) and on phytoplankton abundance was investigated during a mesocosm experiment at the large scale facility (LFS) in Bergen, Norway, in the frame of the Pelagic Ecosystem CO₂ Enrichment study (PeECE II). In nine outdoor enclosures the partial pressure of CO₂ in seawater was modified by an aeration system to simulate past (~190 &mu;atm CO₂), present (~370 &mu;atm CO₂) and future (~700 μatm CO₂) CO₂ conditions in triplicates. Due to the initial addition of inorganic nutrients, phytoplankton blooms developed in all mesocosms and were monitored over a period of 19 days. Seawater samples were collected daily for analysing the abundance of suspended particles and phytoplankton with the Coulter Counter and with Flow Cytometry, respectively. During the bloom period, the abundance of small particles (<4 μm) significantly increased at past, and decreased at future CO₂ levels. At that time, a direct relationship between the total-surface-to-total-volume ratio of suspended particles and DIC concentration was determined for all mesocosms. Significant changes with respect to the CO₂ treatment were also observed in the phytoplankton community structure. While some populations such as diatoms seemed to be insensitive to the CO₂ treatment, others like <i>Micromonas spp</i>. increased with CO₂, or showed maximum abundance at present day CO₂ (i.e. <i>Emiliania huxleyi</i>). The strongest response to CO₂ was observed in the abundance of small autotrophic nano-plankton that strongly increased during the bloom in the past CO₂ mesocosms. Together, changes in particle size distribution and phytoplankton community indicate a complex interplay between the ability of the cells to physiologically respond to changes in CO₂ and size selection. Size of cells is of general importance for a variety of processes in marine systems such as diffusion-limited uptake of substrates, resource allocation, predator-prey interaction, and gravitational settling. The observed changes in particle size distribution are therefore discussed with respect to biogeochemical cycling and ecosystem functioning.