Abstract
Abstract. Together with T–S properties, particle abundance in situ measurements are useful to discriminate water masses and derive circulation patterns. In the upper layers of the Ionian Sea, the fresher Atlantic Waters (AW) recently crossing the Sicily Channel meet the resident and saltier AW, which circulated cyclonically in the eastern basin and modified after evaporation and eventually cooling. In May 2017, during the PEACETIME cruise, fluorescence and particle abundance sampled at high resolution revealed unexpected heterogeneity in the central Ionian Sea. Surface salinity measurements, together with altimetry-derived and hull-mounted acoustic Doppler current profiler (ADCP) currents, describe a zonal pathway of AW entering the Ionian Sea, consistent with the so-called cyclonic mode in the North Ionian Gyre. The ION-Tr transect, located between 19–20∘ E at approximately 36∘ N, turned out to be at the crossroads of three water masses, mostly coming from the west, north and an isolated anticyclonic eddy northeast of ION-Tr. Using Lagrangian numerical simulations, we suggest that the contrast in particle loads along ION-Tr originates from particles transported from these three different water masses. Waters from the west, identified as AW carried by a strong southwestward jet, were moderate in particle load, probably originating from the Sicily Channel. The water mass from the north, carrying abundant particles, probably originated in the northern Ionian Sea, or further away from the south Adriatic Sea. Waters from the eddy, depleted in particles and chl a, may originate from south of the Peloponnese, where the Pelops eddy forms. The central Ionian Sea hence appears as a mosaic area, where waters of contrasted biological history meet. This contrast is particularly clear in spring, when blooming and non-blooming areas co-occur. Interpreting the complex dynamics of physical–biogeochemical coupling from discrete measurements made at isolated stations at sea is a challenge. The combination of multiparametric in situ measurements at high resolution with remote sensing and Lagrangian modeling appears as one adequate way to address this challenge.
Highlights
The study of particle abundance, size structure, and dynamics is a key to understand the marine ecosystem, from primary and secondary production to export and remineralization all the way through food-web dynamics (McDonnell et al, 2015; Giering et al, 2020)
Located in the center of the Mediterranean Sea, the Ionian Sea is a crossroad where three different surface water masses meet: (i) recent Atlantic water (AW) flowing from the Sicily Channel, (ii) older Atlantic Waters (AW) flowing from the east after its cyclonic circulation in the Levantine basin and (iii) fresher
North of 36◦ N, the similarity of ST7 and SAV water mass properties (Fig. 6) supports a single origin of water masses in the northern Ionian Sea. This distribution of AW in the Ionian Sea is consistent with a cyclonic mode of the so-called North Ionian Gyre (NIG, Gacicet al., 2010) that characterizes the circulation in the Ionian Sea north of ca. 35.5◦ N
Summary
The study of particle abundance, size structure, and dynamics is a key to understand the marine ecosystem, from primary and secondary production to export and remineralization all the way through food-web dynamics (McDonnell et al, 2015; Giering et al, 2020). From west to east and north to south, the Mediterranean Sea harbors areas of very distinct productivity, going from eutrophic (e.g., in the Alboran Sea) to ultra-oligotrophic (e.g., in the Levantine basin; Moutin et al, 2017). This heterogeneity is further enhanced by the complex general and mesoscale circulation that favors exchanges between these contrasted areas, leading to a fine-scale spatial heterogeneity in the distribution of biogeochemical properties and particles (Karageorgis et al, 2008; Durrieu de Madron et al, 1992; Rousselet et al, 2019). The mode of the NIG has a strong influence on the dispersal of water masses and properties in the Ionian basin, impacting its productivity (Lavigne et al, 2018)
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