Impact of appendicularians on detritus and export fluxes: a model approach at Dyfamed  site

So far, the role of appendicularians in the biogeochemical cycling of organic
matter has been largely overlooked. Appendicularians represent only a fraction of
total mesozooplankton biomass, however these ubiquitous zooplankters have very
high filtration and growth rates compared to copepods, and produce numerous
fecal pellets and filtering houses contributing to export production by aggregating
small marine particles. To study their quantitative impact on biogeochemical flux,
we have included this group in the biogeochemical flux model, using a recently
developed ecophysiological model. One-dimensional annual simulations of the
pelagic ecosystem including appendicularians were conducted with realistic surface
forcing for the year 2000, using data from the DyFAMed open ocean station. The
appendicularian grazing impact was generally low, but appendicularians increased
detritus production by 8% and export production by 55% compared to a simulation
without appendicularians. Therefore, current biogeochemical models
lacking appendicularians probably under, or misestimate the detritus and export
production by omitting the pathway from small-sized plankton to fast sinking detritus.
Detritus production and export rates are 60% lower than the estimates from
mesotrophic sites, showing that appendicularians’ role is lower but still significant
in oligotrophic environments. The simulated annual export at 200 m exceeds sediment
trap values by 44%, suggesting an intense degradation during the sinking of
appendicularian detritus, supported by observations made at other sites. Thus,
degradation and grazing of appendicularian detritus need better quantification if
we are to accurately assess the role of appendicularia in export flux.