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. The PEACETIME cruise (May–June 2017) was a basin-scalesurvey covering the Provencal, Algerian, Tyrrhenian, and Ionian basins duringthe post-spring bloom period and was dedicated to tracking the impact ofSaharan dust deposition events on the Mediterranean Sea pelagic ecosystem.Two such events occurred during this period, and the cruise strategy allowed for the study of the initial phase of the ecosystem response to one dust event inthe Algerian Basin (during 5 d at the so-called “FAST long-durationstation”) as well as the study of a latter response to another dust event in the TyrrhenianBasin (by sampling from 5 to 12 d after the deposition). Thispaper documents the structural and functional patterns of the zooplanktoncomponent during this survey, including their responses to these two dustevents. The mesozooplankton were sampled at 12 stations using nets withtwo different mesh sizes (100 and 200  µ m) that were mounted on a Bongo frame for verticalhauls within the depth layer from 0 to 300 m. The Algerian and Tyrrhenian basins were found to be quite similar in terms ofhydrological and biological variables, which clearly differentiated themfrom the northern Provencal Basin and the eastern Ionian Basin. In general,total mesozooplankton showed reduced variations in abundance and biomassvalues over the whole area, with a noticeable contribution from the small sizefraction ( µ m) of up to 50 % with respect to abundance and 25 % with respect to biomass. This small size fraction makes a significant contribution (15 %–21 %) to the mesozooplankton fluxes (carbon demand, grazing pressure,respiration, and excretion), which is estimated using allometric relationships to themesozooplankton size spectrum at all stations. The taxonomic structure wasdominated by copepods, mainly cyclopoid and calanoid copepods, and was completed byappendicularians, ostracods, and chaetognaths. Zooplankton taxa assemblages,analyzed using multivariate analysis and rank frequency diagrams, slightlydiffered between basins, which is in agreement with recently proposed Mediterraneanregional patterns. However, the strongest changes in the zooplankton community were linked to the abovementioneddust deposition events. A synoptic analysis of the two dust events observedin the Tyrrhenian and Algerian basins, based on the rank frequencydiagrams and a derived index proposed by Mouillot and Lepretre (2000),delivered a conceptual model of a virtual time series of the zooplanktoncommunity responses after a dust deposition event. The initial phase beforethe deposition event (state 0) was dominated by small-sized cells consumed bytheir typical zooplankton filter feeders (small copepods andappendicularians). The disturbed phase during the first 5 dfollowing the deposition event (state 1) then induced a strong increase infilter feeders and grazers of larger cells as well as the progressive attraction ofcarnivorous species, leading to a sharp increase in the zooplanktondistribution index. Afterward, this index progressively decreased from day 5to day 12 following the event, highlighting a diversification of the community (state 2). A 3-week delay was estimated for the index to return to its initial value,potentially indicating the recovery time of a Mediterranean zooplanktoncommunity after a dust event. To our knowledge, PEACETIME is the first in situ study that has allowed for the observationof mesozooplankton responses before and soon after natural Saharan dustdepositions. The change in the rank frequency diagrams of the zooplanktontaxonomic structure is an interesting tool to highlight short-term responsesof zooplankton to episodic dust deposition events. Obviously dust-stimulatedpelagic productivity impacts up to mesozooplankton in terms of strong butshort changes in taxa assemblages and trophic structure, with potentialimplications for oligotrophic systems such as the Mediterranean Sea.

biogeosciences

Structure and function of epipelagic mesozooplankton and their response to dust deposition events during the spring PEACETIME cruise in the Mediterranean Sea