Abstract
The influence of seawater carbon dioxide (CO2) concentration on the size distribution of suspended particles (2–60μm) 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 (~190μatm CO2), present (~370μatm CO2) and future (~700μatm 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 (<4μm) 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.
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