Evidence for aggregation and export of cyanobacteria and nano-eukaryotes from the Sargasso Sea euphotic zone

摘要

Pico-plankton and nano-plankton are generally thought to represent anegligible fraction of the total particulate organic carbon (POC) exportflux in oligotrophic gyres due to their small size, slow individual sinkingrates, and tight grazer control that leads to high rates of recycling in theeuphotic zone. Based upon recent inverse modeling and network analysishowever, it has been hypothesized that pico-plankton, including thecyanobacteria Synechococcus and Prochlorococcus, and nano-plankton contribute significantly to POC export,via formation and gravitational settling of aggregates and/or consumption ofthose aggregates by mesozooplankton, in proportion to their contribution tonet primary production. This study presents total suspended particulate(>0.7 μm) and particle size-fractionated (10–20 μm, 20–53 μm, >53 μm) pigment concentrations from within and below the euphoticzone in the oligotrophic subtropical North Atlantic, collected using Niskinbottles and large volume in-situ pumps, respectively. Results show the indicatorpigments for Synechococcus, Prochlorococcus and nano-eukaryotes are; (1) found at depths down to 500 m,and; (2) essentially constant, relative to the sum of all indicator pigments,across particle size fractions ranging from 10 μm to >53 μm.Based upon the presence of chlorophyll precursor and degradation pigments,and that in situ pumps do not effectively sample fecal pellets, it is concludedthat these pigments were redistributed to deeper waters on larger, morerapidly sinking aggregates likely by gravitational settling and/orconvective mixing. Using available pigment and ancillary data from thesecruises, these Synechococcus, Prochlorococcus and nano-plankton derived aggregates are estimated tocontribute 2–13% (5 ± 4%), 1–20% (5 ± 7%), and6–43% (23 ± 14%) of the total sediment trap POC flux measured onthe same cruises, respectively. Furthermore, nano-eukaryotes contributeequally to POC export and autotrophic biomass, while cyanobacteriacontributions to POC export are one-tenth of their contribution toautotrophic biomass. These field observations provide direct evidence thatpico- and nano-plankton represent a significant contribution to the totalPOC export via formation of aggregates in this oligotrophic ocean gyre. Wesuggest that aggregate formation and fate should be included in ecosystemmodels, particularly as oligotrophic regions are hypothesized to expand inareal extent with warming and increased stratification in the future.