Abstract
Marine snow aggregates often dominate carbon export from the surface layer to the deep ocean. Therefore, understanding the formation and properties of aggregates is essential to the study of the biological pump. Previous studies have observed a relationship between phytoplankton growth phase and the production of transparent exopolymer particles (TEP), the sticky particles secreted by phytoplankton that act as the glue during aggregate formation. In this experimental study, we aim to determine the effect of phytoplankton growth phase on properties related to aggregate settling. Cultures of the diatom Thalassiosira weissflogii were grown to four different growth phases and incubated in rotating cylindrical tanks to form aggregates. Aggregate excess density and delayed settling time through a sharp density gradient were quantified for the aggregates that were formed, and relative TEP concentration was measured for cultures before aggregate formation. Compared to the first growth phase, later phytoplankton growth phases were found to have higher relative TEP concentration and aggregates with lower excess densities and longer delayed settling times. These findings may suggest that, although particle concentrations are higher at later stages of phytoplankton blooms, aggregates may be less dense and sink slower, thus affecting carbon export.
Highlights
Vertical carbon flux via the biological carbon pump removes more than 10 billion tons of carbon from the surface ocean every year, playing a major role in biogeochemical cycling and regulating global climate [1]
Previous studies have observed a relationship between phytoplankton growth phase and the production of transparent exopolymer particles (TEP), the sticky particles secreted by phytoplankton that act as the glue during aggregate formation
Aggregate excess density and delayed settling time through a sharp density gradient were quantified for the aggregates that were formed, and relative TEP concentration was measured for cultures before aggregate formation
Summary
Vertical carbon flux via the biological carbon pump removes more than 10 billion tons of carbon from the surface ocean every year, playing a major role in biogeochemical cycling and regulating global climate [1]. Other experiments with Thalasiosira weissflogii found that TEP production may depend on growth rate [18] These studies, in conjunction with others that have found higher sticking efficiency during aggregation in later growth phases [10,19], have demonstrated a clear link between phytoplankton growth phase and aggregate formation. It is still not understood how phytoplankton growth phase affects the properties of the aggregates that are formed. Less dense aggregates are more likely to have reduced sinking velocities as they pass through sharp density gradients, a phenomenon referred to as delayed settling [21,22], allowing thin layers of aggregates to form [23,24], which will further affect carbon remineralization and export
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