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Appendicularian houses fate and role in carbon sedimentation and nutrition of zooplankton

Final Report Summary - HOUSES FATE (Appendicularian houses fate and role in carbon sedimentation and nutrition of zooplankton)

The objectives of this project were to understand:
1) the fate of appendicularians houses,
2) it impact on the carbon cycle and
3) nutrition of zooplankton organisms on these houses.
Nearly all the major objectives have been attained and additional research objectives have been added to the research plan.

A full set of experiments allowed showing that houses significantly shrink after been discarded and lost 60 % of it size after one hour and 87 % five days with no influence of the appendicularian size. This deflation is linked to a 2-3x increase of the sedimentation rate of the house and a 20-60 % mass loss of the particle during the first hour notably due to a backflushing of the house filter by the strong deflation process. During this first hour the house release a visible trail of small particles that may be used by potential house consumers to locate it. After the first hour, only the effect of bacterial degradation remains visible. These results have great impact on our understanding on how marine snow will change properties during it sedimentation in the ocean interior. This objective has been fully accomplished.

Experiments on house consumption by different species of copepods have been conducted. These experiments showed that all these species effectively consumes appendicularian houses and that their faecal pellets production depends on the density of houses. Knowing that these species only consumes a small part of houses, it is likely that this increased feeding with denser house concentration is related to ability of copepods to find particles, graze on it and finally quit the house. Additional observations on aggregates formed by different type of material showed the different copepods species produced more faecal pellets on appendicularian houses, highlighting that discarded houses can be a major source of feeding for zooplankton. This objective has been fully accomplished.

Experiments have been done on copepods to observe their behaviour when faced to sinking appendicularian houses. These observations allowed to characterise how copepods detects sinking discarded houses and to separate and quantify the influence of different detection mechanisms (mechanic or detection of chemical clues). These experiments also showed that copepods, using chemical clues can detects sinking houses from distances > 4 cm which, when applied to in situ observations, critically increase their potential degradation pressure on aggregates. These observations also allowed estimating for the first time critical parameters used in various models describing the action of zooplankton on degradation processes. Then this objective has been fully accomplished.

In combination to those objectives, preliminary observations had raised some new perspective of this work that will have a high impact in the scientific community.

1) Our observations on houses sedimentation rates had raised the question of the effect of ballast materials (coccolithophorids, Saharian dusts, diatoms) on the sedimentation of these houses. An experiment on this subject have been planned to investigate this effect in spring 2010. Our results for the first times quantified the effect of different ballast materials on the sinking properties of appendicularians houses and faecal pellets. This objective has been fully accomplished.

2) Preliminary observations had shown that, whereas considered as non selective feeders, appendicularians can select the food which will be ingested. A set experiment had been launched in summer-fall 2009. This experiment showed that appendicularian actively select their food on the size of food particles, but also based on chemical indications on toxicity and nutrient depletion of the algal strain. This objective has been fully accomplished.