Abstract
The effect of episodicity of Saharan dust deposition on the pelagic microbial food web was studied in the oligotrophic Eastern Mediterranean by means of a mesocosm experiment in May 2014. Two different treatments in triplicates (addition of natural Saharan dust in a single-strong pulse or in three smaller consecutive doses of the same total quantity), and three unamended controls were employed; chemical and biological parameters were measured during a 10-day experiment. Temporal changes in primary (PP) and bacterial (BP) production, chlorophyll a (Chla) concentration and heterotrophic bacteria, Synechococcus and mesozooplankton abundance were studied. The results suggested that the auto- and hetero-trophic components of the food web (at least the prokaryotes) were enhanced by the dust addition (and by the nitrogen and phosphorus added through dust). Furthermore, a 1-day delay was observed for PP, BP and Chla increases when dust was added in three daily doses; however, the maximal values attained were similar in the two treatments. Although the effect was evident in the first osmotrophic level (phytoplankton and bacteria), it was lost further up the food web, masked under the impact of grazing exerted by predators such as heterotrophic flagellates, ciliates and dinoflagellates. This was partly proved by two dilution experiments. This study demonstrates the important role of atmospheric deposition and protist grazing when evaluating the effect on oligotrophic systems characterised by increased numbers of trophic levels.
Highlights
Atmospheric deposition is an important source of macro-nutrients and micro-trace metals to the ocean (Jickells et al, 2005; Mahowald et al, 2008)
Characterization of Dust Particles Used in the Mesocosm Experiment
SA (Single Addition) represented a strong dust deposition event of 3 g m−2 day−1 while in terms of nutrient content, Total Dissolved Nitrogen (TDN) and Total Phosphorus (TP), it corresponds to daily deposition fluxes of
Summary
Atmospheric deposition is an important source of macro-nutrients and micro-trace metals to the ocean (Jickells et al, 2005; Mahowald et al, 2008). In high-nutrient low-chlorophyll (HNLC) areas, dust deposition provides iron to the ocean surface and stimulates phytoplankton growth rate and ocean productivity (Martin, 1990; Jickells et al, 2005; Boyd et al, 2007). The low nutrient concentrations, caused by the lack of large rivers in the eastern basin and deteriorated by the Mediterranean anti-estuarine circulation, together with the nitrate:phosphate (N:P) ratio in the deep layers of ca. A Lagrangian experiment in the Levantine Sea suggested bacterioplankton limitation by phosphorus (P) and phytoplankton co-limitation by nitrogen (N) & P in the summer (Krom et al, 2005b; Pitta et al, 2005; Thingstad et al, 2005; Zohary et al, 2005). N-limitation of primary production (Lagaria et al, 2011) and N & P co-limitation of heterotrophic prokaryote growth (Tanaka et al, 2011) have been found during summer time in the Levantine Basin during microcosm experiments, the ratio N:P in both dissolved and particulate organic fractions suggested P starvation
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