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The role of life cycle stages of diatoms in decoupling carbon and silica cycles in polar regions

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Citation:
Brichta, M. (2004): The role of life cycle stages of diatoms in decoupling carbon and silica cycles in polar regions , Oral presentation, SCAR Open Science Conference, 25-31 July, Bremen, Germany. .
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Abstract:

Life cycles of diatoms in polar regions comprise various stages in which the frustules of vegetative cells are discarded. These include gametangial stages during the sexual phase and the transition to stages such as resting spores/cells or winter forms.Diatoms from untreated samples from water, deep-water, sediment traps or surface sediment were identified, counted and measured. Biogenic silica content was analysed from water and sediment traps samples. Antarctic Peninsula (AP) water column and shelf surface sediment samples were taken from an autumn cruise (ANTXVIII/5b). Greenland Sea (GS) samples were taken from 3 years' sediment trap deployments, 2 autumn and 1 summer cruise during ARKTIEFII.Proboscia inermis (21%) and Corethron pennatum (17%) dominated biomass at the AP during an autumn bloom (2.5 mgChla/m3). Massive sexual reproduction was detected in both species besides the formation of winter stages in P. inermis. Male gametes and winter forms are liberated from vegetative frustules. Eucampia antarctica winter forms were discarding the vegetative theca and rudimentary valve and Odontella weissflogii was observed only as resting spores also discarding the vegetative remnant cell wall. The latter species were present in lower numbers. At the GS Rhizosolenia hebetata f. semispina, Proboscia eumorpha and Fragilariopsis cf. cylindriformis were the only important diatom species. The former two were passing through a sexual phase in which male gametangial frustules were being discarded. R. hebetata f. semispina auxospores undergo a further transformation into R. h. f. hiemalis which discards the rudimentary auxospore wall. Sinking events of empty halves of frustules and remnants were recorded in sediment traps (GS) and observed in the surface sediment samples (AP). The comparison of the specific size-frequency distributions of the width of empty halves found sedimented with that from intact cell walls in the water column was made for both regions. The matching of these size-frequency distributions, the observation of auxospores, gametangial cells and resting cells, releasing high amounts of empty cell walls, in conjunction with the analysis of BSi profiles and BSi measurements in the trapped material provides evidence that life cycles of these big diatom species can drive the silica flux.Analysis of the constituents of diatoms in the high latitudes areas studied led to the conclusion that a large portion of biogenic silica transported to depth is directly related to the remnants of diatom life cycle stages. Consideration of these processes can help to clarify the high Si:C ratios commonly found in high latitude sinking matter.

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