R. Gallisai

Saharan dust deposition may affect phytoplankton growth in the mediterranean sea at ecological time scales

The surface waters of the Mediterranean Sea are extremely poor in the nutrients necessary for plankton growth. At the same time, the Mediterranean Sea borders with the largest and most active desert areas in the world and the atmosphere over the basin is subject to frequent injections of mineral dust particles. We describe statistical correlations between dust deposition over the Mediterranean Sea and surface chlorophyll concentrations at ecological time scales. Aerosol deposition of Saharan origin may explain 1 to 10% (average 5%) of seasonally detrended chlorophyll variability in the low nutrient-low chlorophyll Mediterranean. Most of the statistically significant correlations are positive with main effects in spring over the Eastern and Central Mediterranean, conforming to a view of dust events fueling needed nutrients to the planktonic community. Some areas show negative effects of dust deposition on chlorophyll, coinciding with regions under a large influence of aerosols from European origin. The influence of dust deposition on chlorophyll dynamics may become larger in future scenarios of increased aridity and shallowing of the mixed layer.

Horizontal and Vertical Distributions of Transparent Exopolymer Particles (TEP) in the NW Mediterranean Sea Are Linked to Chlorophyll a and O2 Variability

Transparent Exopolymer Particles (TEP) are relevant in particle and carbon fluxes in the ocean, and have economic impact in the desalination industry affecting reverse osmosis membrane fouling. However, general models of their occurrence and dynamics are not yet possible because of the poorly known co-variations with other physical and biological variables. Here, we describe TEP distributions in the NW Mediterranean Sea during late spring 2012, along perpendicular and parallel transects to the Catalan coast. The stations in the parallel transect were sampled at the surface, while the stations in the perpendicular transect were sampled from the surface to the bathypelagic, including the bottom nepheloid layers. We also followed the short-term TEP dynamics along a 2-day cycle in offshore waters. TEP concentrations in the area ranged from 4.9 to 122.8 and averaged 31.4 ± 12.0 μg XG eq L−1. The distribution of TEP measured in transects parallel to the Catalan Coast correlated those of chlorophyll a (Chla) in May but not in June, when higher TEP-values with respect to Chla were observed. TEP horizontal variability in epipelagic waters from the coast to the open sea also correlated to that of Chla, O2 (that we interpret as a proxy of primary production) and bacterial production (BP). In contrast, the TEP vertical distributions in epipelagic waters were uncoupled from those of Chla, as TEP maxima were located above the deep chlorophyll maxima. The vertical distribution of TEP in the epipelagic zone was correlated with O2 and BP, suggesting combined phytoplankton (through primary production) and bacterial (through carbon reprocessing) TEP sources. However, no clear temporal patterns arose during the 2-day cycle. In meso- and bathypelagic waters, where phytoplanktonic sources are minor, TEP concentrations (10.1 ± 4.3 μg XG eq l−1) were half those in the epipelagic, but we observed relative TEP increments coinciding with the presence of nepheloid layers. These TEP increases were not paralleled by increases in particulate organic carbon, indicating that TEP are likely to act as aggregating agents of the mostly inorganic particles present in these bottom nepheloid layers.

Large Saharan dust storms: Implications for chlorophyll dynamics in the Mediterranean Sea

We investigate the large (LDE) and very large (VLDE) Saharan dust deposition events that occurred between 2000 and 2007 and their short-term impact on the dynamics of marine phytoplankton in the Mediterranean Sea. A total of 153 LDE were identified unevenly distributed over the years. Events were more frequent during winter, in the Eastern Mediterranean, and autumn, when they affected both the Western and the Central Mediterranean. Most of the 31 VLDE occurred during winter and autumn in the Central Mediterranean. The dynamics of chlorophyll after VLDE were studied as a proxy for phytoplankton response to atmospheric dust. A significant response of chlorophyll to dust addition was evident; this appeared to be especially true for the Western Mediterranean where a chlorophyll increases of up to 345% was recorded, whereas in the Central Mediterranean it was up to 146% and in the Eastern Mediterranean up to 121%. Chlorophyll response behavior was quite heterogeneous probably as a result of the uniqueness of each VLDE, the differences between Mediterranean areas, the community structure of phytoplankton and the interaction between bacteria and phytoplankton for new resources. An eastward decreasing trend in chlorophyll response was observed, which is in accordance with the relative importance of bacterial activity with respect to phytoplankton. The increase in mineral aerosols with increased aridity in the region together with the decrease in the depth of the mixed layer of the oceans should boost the importance of aerosols fueling marine production.