In situ camera observations reveal major role of zooplankton in modulating marine snow formation during an upwelling-induced plankton bloom

• Published in: Progress in Oceanography
• Main Authors: Taucher, J, Stange, P, Alguero-Muniz, M, Bach, LT, Nauendorf, A, Kolzenburg, R, Budenbender, J, Riebesell, U
• Format: Article in Journal/Newspaper
• Language: English
• Published: Pergamon-Elsevier Science Ltd 2018
• Subjects: Earth Sciences; Oceanography; Biological Oceanography; North Atlantic
• Online Access: https://doi.org/10.1016/j.pocean.2018.01.004; http://ecite.utas.edu.au/133671

Particle aggregation and the consequent formation of marine snow alter important properties of biogenic particles(size, sinking rate, degradability), thus playing a key role in controlling the vertical flux of organic matterto the deep ocean. However, there are still large uncertainties about rates and mechanisms of particle aggregation,as well as the role of plankton community structure in modifying biomass transfer from small particlesto large fast-sinking aggregates. Here we present data from a high-resolution underwater camera system that we used to observe particle sizedistributions and formation of marine snow (aggregates>0.5 mm) over the course of a 9-week in situ mesocosmexperiment in the Eastern Subtropical North Atlantic. After an oligotrophic phase of almost 4 weeks, addition ofnutrient-rich deep water (650 m) initiated the development of a pronounced diatom bloom and the subsequentformation of large marine snow aggregates in all 8 mesocosms. We observed a substantial time lag between thepeaks of chlorophyll a and marine snow biovolume of 912 days, which is much longer than previously reportedand indicates a marked temporal decoupling of phytoplankton growth and marine snow formation during ourstudy. Despite this time lag, our observations revealed substantial transfer of biomass from small particle sizes(single phytoplankton cells and chains) to marine snow aggregates of up to 2.5mm diameter (ESD), with most ofthe biovolume being contained in the 0.51mm size range. Notably, the abundance and community compositionof mesozooplankton had a substantial influence on the temporal development of particle size spectra and formationof marine snow aggregates: While higher copepod abundances were related to reduced aggregate formationand biomass transfer towards larger particle sizes, the presence of appendicularia and doliolids enhancedformation of large marine snow. Furthermore, we combined in situ particle size distributions with measurements of particle sinking velocity tocompute instantaneous (potential) vertical mass flux. However, somewhat surprisingly, we did not find a coherentrelationship between our computed flux and measured vertical mass flux (collected by sediment traps in15m depth). Although the onset of measured vertical flux roughly coincided with the emergence of marinesnow, we found substantial variability in mass flux among mesocosms that was not related to marine snownumbers, and was instead presumably driven by zooplankton-mediated alteration of sinking biomass and exportof small particles (fecal pellets). Altogether, our findings highlight the role of zooplankton community composition and feeding interactionson particle size spectra and formation of marine snow aggregates, with important implications for our understandingof particle aggregation and vertical flux of organic matter in the ocean.