Effects of Phytoplankton Growth Phase on Settling Properties of Marine Aggregates

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.

Copepod Grazing Influences Diatom Aggregation and Particle Dynamics

“…In a recent study investigating copepod grazing behavior on aggregated particles, Koski et al (2017) demonstrated that both harpacticoida and poecilostomatoida copepods were able to feed on aggregates and could thus attenuate the particle carbon flux. Aggregation processes and dynamics are increasingly understood via the combination of laboratory experiments and models (Beauvais et al, 2006;Passow and De La Rocha, 2006;Gärdes et al, 2011;Jackson, 2015;Prairie et al, 2019), mesocosm experiments (Alldredge et al, 1995;Passow and Alldredge, 1995b;Svensen et al, 2001Svensen et al, , 2002Moriceau et al, 2018;Cisternas-Novoa et al, 2019), and in situ observations (Lampitt et al, 2010;Laurenceau-Cornec et al, 2015;Nowald et al, 2015;Giering et al, 2017;Cavan et al, 2018;Bach et al, 2019). In the meantime, studies focusing on disaggregation processes due to remineralization or zooplankton activity (Goldthwait et al, 2004;Taucher et al, 2018) remain limited despite their importance in providing new insights to better understand particle export in the mesopelagic zone.…”

The effect of phytoplankton properties on the ingestion of marine snow by Calanus pacificus

“…Prieto et al (2001) suggested that copepods could trigger a higher production of TEP when interacting with diatoms, creating a potential positive feedback loop in nutritional production. Furthermore, production of TEP varies during different stages of the phytoplankton growth curve (Prairie et al 2019), and for different phytoplankton species (Passow 2002), providing a possible explanation for the variation in ingestion rate we observed.…”

“…Both size and shape of marine snow aggregates can vary based on the phytoplankton that are present Wilkinson 1990, Li andLogan 1995). In addition, aggregate density can depend on the growth phase of the phytoplankton cultures (Prairie et al 2019), potentially explaining our qualitative observations of the changes in the apparent compactness of S. maranoi aggregates between growth phases ( Figure 6). One difference between the two species used in our experiments is that T. weissfloggi does not form chains as commonly as S. maranoi, which is a smaller and highly chain-forming diatom.…”

“…Even if the zooplankton are not directly feeding on the aggregates, fragmentation of the particles can occur when zooplankton interact with them (Dilling and Alldredge 2000, Kiørboe 2001, Goldthwait et al 2004, Kiko et al 2017. This fragmentation will result in changes in the size and density of the marine snow particles, which will then alter their sinking rate (Gibbs 1985, Prairie et al 2019, thus changing the efficiency of the biological pump. In addition, zooplankton foraging on marine snow repackages marine snow aggregates into dense fecal pellets (Turner andFerrante 1979, Shanks andEdmonson 1990), which generally sink at faster rates then marine snow (Bruland and Silver 1981).…”

“…Phytoplankton, specifically diatoms, also develop different physiological characteristics with age (De Troch et al 2012), and previous studies have shown that zooplankton can demonstrate a food preference based on phytoplankton growth phase Hay 2006, Barofsky 2009). Given this, ingestion of marine snow may also depend on the age of the phytoplankton from which they are formed, particularly since Prairie et al (2019) showed that phytoplankton growth phase affected the TEP production and density of marine snow. Despite this, no study has specifically looked at the effect of phytoplankton growth phase on the ingestion of marine snow aggregates by zooplankton, although observed copepod foraging on aged marine snow from the ocean.…”