Abstract
In marine ecosystems, carbon export is driven by particle flux which is modulated by aggregation, remineralization, and grazing processes. Zooplankton contribute to the sinking flux through the egestion of fast sinking fecal pellets but may also attenuate the flux by tearing apart phytoplankton aggregates into small pieces through swimming activity or direct ingestion. Freely suspended cells, artificial monospecific aggregates from two different diatom species (Chaetoceros neogracile and Skeletonema merinol) and natural aggregates of Melosira sp. were independently incubated with five different copepod species (Acadia clausi, Temora longicomis, Calanus helgolandicus, Euterpina acutifrons, and Calanus hyperboreus). During the grazing experiments initiated with free diatoms, E. acutifrons feeding activity evidenced by ingestion rates of 157 +/- 155 ng Chl a ind(-1) d(-1), induced a significant increase of S. marinoi aggregation. Transparent exopolymeric particles (TEP) production was only slightly boosted by the presence of grazers and turbulences created by swimming may be the main trigger of the aggregation processes. All copepods studied were able to graze on aggregates and quantitative estimates led to chlorophyll a ingestion rates (expressed in Chla a equivalent, i.e., the sum of chlorophyll a and pheopigments in their guts) ranging from 4 to 23 ng Chl a(eq) ind(-1) d(-1). The relation between equivalent spherical diameters (ESDs) and sinking velocities of the aggregates did not significantly change after grazing, suggesting that copepod grazing did not affect aggregate density as also shown by Si:C and C:N ratios. Three main trends in particle dynamics could be identified and further linked to the copepod feeding behavior and the size ratio between prey and predators: (1) Fragmentation of S. marinoi aggregates by the cruise feeder T. longicomis and of Melosira sp. aggregates by C. hyperboreus at prey to predator size ratios larger than 15; (2) no change of particle dynamics in the presence of the detritic cruise feeder E acutifrons; and finally (3) re-aggregation of C. neogracile and S. marinoi aggregates when the two filter feeders A. clausi and C. helgolandicus were grazing on aggregate at prey to predator size ratios lower than 10. Aggregation of freely suspended cells or small aggregates was facilitated by turbulence resulting from active swimming of small copepods. However, stronger turbulence created by larger cruise feeders copepods prevent aggregate formation and even made them vulnerable to breakage.
Highlights
In marine ecosystems, diatoms play a key role in the biological carbon pump (Jin et al, 2006; Tréguer et al, 2017)
Copepods actively grazed on S. marinoi (Experiments 1 and 2) but no grazing was measurable on C. neogracile (Experiment 3), i.e., phytoplankton growth exceeded grazing rates as evidenced by similar cell abundance
Our work suggested that the ability to predict the consequences of copepod activity on particle dynamic could necessitate better understanding of copepod functional traits
Summary
Diatoms play a key role in the biological carbon pump (Jin et al, 2006; Tréguer et al, 2017). Considering the balance between sinking and remineralization, only large and fast sinking particles formed in the mixed layer can reach the sequestration depth (Moriceau et al, 2007), i.e., 1000 m depth considering that an efficient carbon entrapment is longer than a thousand year (Passow and Carlson, 2012) This mechanistic view is confirmed by in situ profiles of particle fluxes (Guidi et al, 2007). Zooplankton grazers can strongly modulate the particle fluxes in the water column Organisms such as salps, appendicularians, and copepods are acknowledged to be important contributors to carbon export via the production of fast sinking fecal pellets resulting from grazing (Stemmann et al, 2002; Turner, 2002, 2015; Boyd and Trull, 2007; Stamieszkin et al, 2015; Lalande et al, 2016). Other zooplankton organisms, such as appendicularians, are major contributors to vertical particle carbon flux (Alldredge et al, 2005), via the production of cellulosic houses embedded with detritus or other plankton organisms (Gorsky et al, 1999; Vargas et al, 2002; Lombard and Kiørboe, 2010; Lombard et al, 2013a)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
