The role of mesozooplankton grazing in the biogeochemical cycle of silicon in the Southern Ocean

Abstract

The role of copepod grazing, particularly of :i:Calanus simillimus:/i: and :i:Rhincalanus gigas:/i:, in the biogeochemical cycles of silicon (Si) and carbon (C) in the Antarctic Circumpolar Current (ACC) of the Southern Ocean is investigated. The two grazers show differences in feeding behavior before and in response to a diatom bloom stimulated by :i:in situ:/i: iron fertilization. The continuously high feeding activity of :i:C. simillimus:/i: on diatoms is conducive to enhance the export of primary produced C and Si. The grazing impact of this key species is high enough to influence population dynamics in the microplankton communities of the ACC. In the pre-bloom situation, :i:R. gigas:/i: fulfils most of its carbon requirement through grazing on detritus and thereby effectively reduces the vertical fecal flux produced by :i:C. simillimus:/i:. It is proposed that a Copepod-Retention-System for organic material is put in place by the copepod community under High Nutrient Low Chlorophyll (HNLC) conditions. Prey switching by :i:R. gigas:/i: from detritus to diatoms in the bloom situation lifts the grazing check on the detritus flux and enables loss of particulate C and Si from the surface layer with fast sinking fecal pellets. Results from dissolution experiments indicate that the enclosure of biogenic silica (BSi) in copepod fecal pellets prevents the dissolution of diatom frustules. Diatoms submitted to grazing of copepods and krill (:i:Euphausia superba:/i:) dissolved 4 to 26 times slower than un-grazed controls. :br:Estimates of C ingestion from :i:in vitro:/i: incubations and from gut fluorescence measurements are compared to respiratory carbon needs of copepods. Methods yield generally similar estimates for :i:C. simillimus:/i: whereas methods differ strongly for :i:R. gigas:/i: in the pre-bloom situation, reflecting the above mentioned differences in feeding behavior.