Large clean mesocosms and simulated dust deposition: a new methodology to investigate responses of marine oligotrophic ecosystems to atmospheric inputs

Abstract

Intense Saharan dust deposition occurs over large oligotrophic areas in the Mediterranean Sea and in the Tropical Atlantic, and its impact on the biogeochemical functioning of such oligotrophic ecosystems needs to be understood. However, due to the logistical difficulties of investigating 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 setup 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 studies of 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 collected in a source area and by performing subsequent appropriate physico-chemical treatments in the laboratory, including an eventual processing by simulated cloud water. The comparison of the physico-chemical characteristics of produced dust analogues with the literature confirms that our experimental simulations are representative of dust, aging during atmospheric transport, and subsequent deposition to the Mediterranean. Second, we demonstrate the feasibility in coastal areas to installing, in situ, a series of large (6 × 52 m<sup>3</sup>) mesocosms without perturbing the local ecosystem. The setup, containing no metallic parts and with the least possible induced perturbation during the sampling sequence, provides an approach for 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 three mesocosms serving as controls (CONTROLS-Meso = no addition) and three mesocosms seeded with the same amount of Saharan dust (DUST-Meso = 10 g m<sup>−2</sup> of sprayed dust). A large panel of biogeochemical parameters was measured at 0.1 m, at 5 m and 10 m in all of the mesocosms and at a selected site outside the mesocosms before seeding and at regular intervals afterward. Statistical analyses of the results show that data from three mesocosms that received the same treatment are highly reproducible (variability < 30%) and that there is no significant difference between data obtained from CONTROLS-Meso and data obtained outside the mesocosms. <br><br> This paper demonstrates that the methodology developed in the DUNE project is suitable to quantifying and parameterizing the impact of atmospheric chemical forcing in a low-nutrient, low-chlorophyll (LNLC) ecosystem. Such large mesocosms can be considered as 1-D ecosystems so that the parameterization obtained from these experiments can be integrated into ecosystem models

Dust deposition: iron source or sink? A case study

“…The temporal evolution of Chl is also consistent with the observation of a doubling at the end of the experiment (Guieu , 2011;Guieu, 2009). However, the observed Chl concentrations reached 0.2 mg m −3 already 48 h after the addition and remained at this level until the end of the experiment.…”

“…Guieu et al (2010) mentioned that a fraction of particles could have been lost during exchange of the traps or adhered to the conical bottom of mesocosms. The sticking of particles on the mesocosm walls and in the conical part of the mesocosms could play a role for both reduced concentration in the water column and lower sedimentation.…”

“…The mesocosms were cylindrical with a diameter of 2.3 m and a volume of 52 m 3 , enclosing an upper water layer of 15 m. Details of the mesocosm construction have been described in Guieu et al (2010). After deployment and closing of the mesocosms, the initial conditions of the experiment were measured.…”

Sources and mixing state of summertime background aerosol in the north-western Mediterranean basin

“…This indicates that mineral dust was not characterised well by the ATOFMS. Indeed, the dust particle mass size distribution is mainly in the > PM 2.5 fraction, but submicron particles dominate the dust particle number size distribution (see Gomes et al, 1990;Guieu et al, 2010) as confirmed by in situ and column-integrated particle size distribution measurements during the campaign .…”

Response of the Eastern Mediterranean Microbial Ecosystem to Dust and Dust Affected by Acid Processing in the Atmosphere

“…The proxy dust was resuspended using a custom-made resuspension chamber and the PM10 was separated for subsequent use. Previous studies have used similar procedures for experimental work with Saharan dust (Lafon et al, 2006;Guieu et al, 2010;Shi et al, 2011). Within the text we use the term PM10 dust or simply dust for this size fractionated proxy.…”