Abstract. The PEACETIME cruise (May–June 2017) was a basin-scale
survey covering the Provencal, Algerian, Tyrrhenian, and Ionian basins during
the post-spring bloom period and was dedicated to tracking the impact of
Saharan 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 in
the Algerian Basin (during 5 d at the so-called “FAST long-duration
station”) as well as the study of a latter response to another dust event in the Tyrrhenian
Basin (by sampling from 5 to 12 d after the deposition). This
paper documents the structural and functional patterns of the zooplankton
component during this survey, including their responses to these two dust
events. The mesozooplankton were sampled at 12 stations using nets with
two different mesh sizes (100 and 200 µm) that were mounted on a Bongo frame for vertical
hauls within the depth layer from 0 to 300 m. The Algerian and Tyrrhenian basins were found to be quite similar in terms of
hydrological and biological variables, which clearly differentiated them
from the northern Provencal Basin and the eastern Ionian Basin. In general,
total mesozooplankton showed reduced variations in abundance and biomass
values over the whole area, with a noticeable contribution from the small size
fraction (<500 µm) 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 the
mesozooplankton size spectrum at all stations. The taxonomic structure was
dominated by copepods, mainly cyclopoid and calanoid copepods, and was completed by
appendicularians, ostracods, and chaetognaths. Zooplankton taxa assemblages,
analyzed using multivariate analysis and rank frequency diagrams, slightly
differed between basins, which is in agreement with recently proposed Mediterranean
regional patterns. However, the strongest changes in the zooplankton community were linked to the abovementioned
dust deposition events. A synoptic analysis of the two dust events observed
in the Tyrrhenian and Algerian basins, based on the rank frequency
diagrams and a derived index proposed by Mouillot and Lepretre (2000),
delivered a conceptual model of a virtual time series of the zooplankton
community responses after a dust deposition event. The initial phase before
the deposition event (state 0) was dominated by small-sized cells consumed by
their typical zooplankton filter feeders (small copepods and
appendicularians). The disturbed phase during the first 5 d
following the deposition event (state 1) then induced a strong increase in
filter feeders and grazers of larger cells as well as the progressive attraction of
carnivorous species, leading to a sharp increase in the zooplankton
distribution index. Afterward, this index progressively decreased from day 5
to 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 zooplankton
community after a dust event. To our knowledge, PEACETIME is the first in situ study that has allowed for the observation
of mesozooplankton responses before and soon after natural Saharan dust
depositions. The change in the rank frequency diagrams of the zooplankton
taxonomic structure is an interesting tool to highlight short-term responses
of zooplankton to episodic dust deposition events. Obviously dust-stimulated
pelagic productivity impacts up to mesozooplankton in terms of strong but
short changes in taxa assemblages and trophic structure, with potential
implications for oligotrophic systems such as the Mediterranean Sea.
Interaction of sinking behaviour of Saharan dust and lithogenic and biogenic fluxes in the Canary Basin