Large clean mesocosms and simulated dust deposition: a new methodology to investigate responses of marine oligotrophic ecosystems to atmospheric inputs
Authors: Guieu C. , Dulac F. , Desboeufs K. , Wagener T. , Pulido-Villena E. , Grisoni J.-M. , Louis F. , Ridame C. , Blain S. , Brunet C. , Bon Nguyen E. , Tran S. , Labiadh M. , Dominici J.-M. 
Published: April 2010

Abstract. Intense Saharan dust deposition occurs over large oligotrophic areas in the Mediterranean Sea and the Tropical Atlantic and its impact on the biogeochemical functioning of such oligotrophic ecosystems needs to be understood. However, due to the logistical difficulties to investigate in-situ natural dust events and due to the inherent limitations of microcosm laboratory experiments, new experimental approaches need to be developed. In this paper, we present a new experimental set up based on large clean mesocoms deployed in the frame of the DUNE (a DUst experiment in a low-Nutrient, low-chlorophyll Ecosystem) project. We demonstrate that these tools are highly relevant and provide a powerful new strategy to in situ study the response of an oligotrophic ecosystem to chemical forcing by atmospheric deposition of African dust. First, we describe how to cope with the large amount of dust aerosol needed to conduct the seeding experiments, by producing an analogue from soil collection in a source area and performing subsequent appropriate physico-chemical treatment in the laboratory including an eventual processing by simulated cloud water. The comparison of physico-chemical characteristics of produced dust analogues with the literature confirms that our experimental simulations are representative of dust, ageing during atmospheric transport, and subsequent deposition to the Mediterranean. Second, we demonstrate the feasibility in coastal area to installing in situ, a series of large (6×52 m3) mesocosms without perturbing the local ecosystem. All the setup, containing no metallic part and with as less as possible induced perturbation during the sampling sequence, allows working with the required conditions for biogeochemical studies in oligotrophic environments where nutrient and micronutrients are at nano- or subnano-molar levels. Two distinct "seeding experiments" were conducted by deploying mesocosms in triplicates: three mesocosms serving as controls (CONTROLS-Meso = no addition) and three mesocosms being seeded with the same amount of Saharan dust (DUST-Meso = 10 g m2 of sprayed dust). A large panel of biogeochemical parameters was measured at 0.1 m, at 5 m and 10 m in all the mesocosms and at a selected site outside the mesocosms, before the seeding and at regular intervals after. Statistical analyses of the results show that data from triplicate mesocosms are highly reproducible (variability <30% among triplicates) and that there is no significant difference between data obtained from CONTROLS-Meso and from outside the mesocosms. This paper demonstrates that the methodology developed in the DUNE project is suitable to quantify and parameterize the impact of atmospheric chemical forcing to a low-nutrient, low-chlorophyll (LNLC) ecosystem. Such large mesocosms can be considered as 1D ecosystem so that the parameterization obtained from those experiments can indeed be integrated into ecosystem models.