Structure and functioning of epipelagic mesozooplankton and response to dust events during the spring PEACETIME cruisein the Mediterranean Sea

Abstract. The PEACETIME cruise (May–June 2017) was a basin scale study mainly dedicated to the study of different planktonic trophic regimes in the Algerian, Tyrrhenian and Ionian basins and, in particular, focusing on areas impacted by Saharan dust deposition. This paper presents the structural and functioning patterns of the zooplankton component during this survey, including their responses to two major dust events in the Algerian and Tyrrhenian basins. The mesozooplankon was sampled at 12 stations by combining nets with 2 mesh sizes (100 and 200 µm) mounted on a bongo frame for vertical hauls within the upper 300 meter layer. In this general post-bloom situation, total mesozooplankton showed reduced variations in abundance and biomass over the whole area, with a noticeable contribution of the small size fraction (< 500 µm) of up to 50 % in abundance and 25 % in biomass. The taxonomic structure was dominated by copepods, mainly cyclopoids and calanoids, and completed by appendicularians, ostracods and chaetognaths. Distinct zooplankton taxa assemblages in the three main regions were in agreement with recently proposed regional patterns for the Mediterranean Basin, although the assemblages found in the western Ionian stations presented a closer analogy with those of the Tyrrhenian basin than with those of the Ionian basin., probably due to Atlantic water influence. Zooplankton carbon demand, grazing pressure, respiration and excretion rates were estimated using allometric relationships to the mesozooplankton size-spectrum. On average, the daily zooplankton consumption potentially represents 15 % of the phytoplankton stock, almost the whole of the primary production, with a narrow range of variations, and its excretion contributes roughly one quarter of the N and P requirements of phytoplankton production. The small size fractions make a significant contribution to these mesozooplankton estimated fluxes. Whereas in the Algerian basin (long station FAST), the initial impact on the pelagic ecosystem of a tracked dust deposition was studied, the survey of the southern Tyrrhenian basin occurred almost a week after another dust event. The changes in mesozooplankton taxonomic structure appear to be a relevant indicator to study this response, with an initial phase with no real dominance of taxa, then a disturbed state of the community with strong dominance of certain herbivorous taxa and the appearance of carnivorous species, and finally a recovery state towards a more stable system with diversification of the community. To our knowledge, PEACETIME is the first in situ study allowing observation of mesozooplankton responses before and soon after natural Saharan dust depositions. The changes in rank-frequency diagrams of the zooplankton taxonomic structure are an interesting index to highlight short-term responses of zooplankton to episodic dust deposition events.The PEACETIME cruise (May-June 2017) was a basin scale study mainly dedicated to the study of different 10 planktonic trophic regimes in the Algerian, Tyrrhenian and Ionian basins and, in particular, focusing on areas impacted by Saharan dust deposition. This paper presents the structural and functioning patterns of the zooplankton component during this survey, including their responses to two major dust events in the Algerian and Tyrrhenian basins. The mesozooplankon was sampled at 12 stations by combining nets with 2 mesh sizes (100 and 200 µm) mounted on a bongo frame for vertical hauls within the upper 300 meter layer. In this general post-bloom situation, total mesozooplankton showed reduced 15 variations in abundance and biomass over the whole area, with a noticeable contribution of the small size fraction (< 500 µm) of up to 50 % in abundance and 25 % in biomass. The taxonomic structure was dominated by copepods, mainly cyclopoids and calanoids, and completed by appendicularians, ostracods and chaetognaths. Distinct zooplankton taxa assemblages in the three main regions were in agreement with recently proposed regional patterns for the Mediterranean Basin, although the assemblages found in the western Ionian stations presented a closer analogy with those of the Tyrrhenian 20 basin than with those of the Ionian basin., probably due to Atlantic water influence. Zooplankton carbon demand, grazing pressure, respiration and excretion rates were estimated using allometric relationships to the mesozooplankton size-spectrum. On average, the daily zooplankton consumption potentially represents 15 % of the phytoplankton stock, almost the whole of the primary production, with a narrow range of variations, and its excretion contributes roughly one quarter of the N and P requirements of phytoplankton production. The small size fractions make a 25 significant contribution to these mesozooplankton estimated fluxes. Whereas in the Algerian basin (long station FAST), the initial impact on the pelagic ecosystem of a tracked dust deposition was studied, the survey of the southern Tyrrhenian basin occurred almost a week after another dust event. The changes in mesozooplankton taxonomic structure appear to be a relevant indicator to study this response, with an initial phase with no real dominance of taxa, then a disturbed state of the community with strong dominance of certain herbivorous taxa and the 30 appearance of carnivorous species, and finally a recovery state towards a more stable system with diversification of th