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PEACETIME minicosms test experiment

Climate reactors to study marine biogeochemical cycles at the ocean-atmosphere interface:
a test experiment before the PEACETIME cruise

Coordinated  by Cécile Guieu & Frédéric Gazeau, LOV (Laboratoire d’Océanographie de Villefranche)

20th June – 1st July 2016, Villefranche sur Mer, France

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Impact of ocean-atmosphere exchanges (such as atmospheric deposition, ocean acidification) on biogeochemical cycles have been intensively studied over the past decade at LOV. This required to develop several experimental setups such as the large pelagic mesocosms that were successfully deployed during several projects (DUNE (ANR), MEDSEA (EU) and VAHINE (LEFE-CYBER)). These devices are accurate tools to study processes in coastal area where they can be anchored. In order to conduct similar process studies on board R:Vs, we recently developed a small version of these devices that can be either used in the laboratory or embarked at sea.

The containers (300 L made in HDPE (H 1.09 m, diameter 0.68 m, area: 0.36 m2)) have a conical base where a sediment trap can be adapted. As for the mesocosms, the technical specifications implied that no metal parts can be used (in order to have no contamination and be able to follow trace elements during the course of the experiment). It also implied to simulate the lighting and disturbance of the study area. The monitoring of environmental parameters during the experiment (pH, temperature, light) has to be ensured.

We developed eight reactors in the frame of the MISTRALS / MERMEX program. We conducted a test-experience in July 2016 involving ten participants including colleagues from the LOV, OOV, MIO and LAMP laboratories.

Four reactors (2 controls and 2 with desert dust addition) were tested on the natural assemblage of the Bay of Villefranche-sur-mer during four days. Results show that no contamination has been detected and chemical and biological measurements were reproducible between duplicates. Aerosol emission measurements were also successfully performed.

The goal of the experience was also to highlight the items requiring improvement: mainly we need to work of some waterproof issues in some parts, to improve the integrated sampling system and to design a new piece for sampling air emissions.

This is currently being done in order that our eight devices can be embarked during the PEACETIME campaign (May 2017 on board R/V Pourquoi Pas?). During the campaign, a minimum of 3 experiments will be conducted to study the impact of dust deposition on the natural assemblage according current and future pH and temperature conditions.

Lighting installation on the lids (Jean-Michel Grisoni (OOV) and one of his student assembling the 8 lids).
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Different cleaning stages of the reactors with soap and then acid

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Filling the reactors (6 x 300 L) with the water pumped at 150 meters from the edge of the pontoon in the Bay of Villefranche. The large tank is used to homogenise the filling of the 6 reactors. 4 “clean” teflon pumps are used for the different steps.

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In top view, the main filling and pumping of the intermediate tank
with the help from Elvira Pulido Villena and her PhD Student Kahina Djaoudi from MIO 

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Meanwhile inside the cold room, the reactors are  filling up and Fredéric Gazeau controls the light sensors

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Seeding phase with Saharan dust: a solution containing the dust is sprayed at the surface to mimic a rain.

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Sampling step: using a peristaltic pump, we initiate the water arrival and then by gravity the samples are either collected unfiltered (biological parameters) or directly filtered online (chemical parameters). On the right, the sediment trap at the base of the reactor is changed every 24 hours. At the end of the experiment, the remaining water inside the reactors was entirely emptied through a sieve of 180 micrometre for zooplankton analysis by Zooscan.

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