Abstract
Abstract. The influence of seawater carbon dioxide (CO2) concentration on the size distribution of suspended particles (2–60 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 μatm CO2), present (~370 μatm 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 Micromonas spp. increased with CO2, or showed maximum abundance at present day CO2 (i.e. Emiliania huxleyi). 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.
Highlights
The increase in atmospheric CO2 concentration since the beginning of industrialisation, associated risks of ocean acidification, and the potential consequences for marine carbon cycling and global climate have recently gathered attention beyond purely scientific interest
Following the development of the phytoplankton bloom, Chla increased exponentially in each of the mesocosms until a maximum value was reached between day 9 and day 13 of the experiment (Fig. 1a)
The aim of this study was to test the hypothesis that CO2 concentration can affect the size distribution of cells during the course of a phytoplankton bloom
Summary
The increase in atmospheric CO2 concentration since the beginning of industrialisation, associated risks of ocean acidification, and the potential consequences for marine carbon cycling and global climate have recently gathered attention beyond purely scientific interest. Since the middle of the 18th century, the atmospheric concentration of CO2 has increased rapidly from 280 μatm to 366 μatm in 1998, and several future scenarios predict a further increase to 750 μatm in 2100 (IPCC scenario IS92a) (Houghton et al, 2001). The seawater carbonate chemistry has responded noticeably, with a decrease from preindustrial surface ocean pH of 8.25 down to 8.08 presently. Modelling studies predict a further reduction of pH by 0.7 up to the year 2300, which would be more than experienced by marine life for the last 300 000 years (Caldeira and Wickett, 2003)
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