Massive shelf dense water flow influences plankton community structure and particle transport over long distance

Fabrizio Bernardi Aubry,Lucia Bongiorni,Stefania Finotto,Francesco Acri,Jacopo Chiggiato,Sandro Carniel,Francesco Marcello Falcieri,Alfredo Boldrin,Gian Marco Luna,Mauro Sclavo,Elisa Camatti

doi:10.1038/s41598-018-22569-2

Fabrizio Bernardi Aubry, Lucia Bongiorni + Show 9 more

Open Access

https://doi.org/10.1038/s41598-018-22569-2

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Abstract

Dense waters (DW) formation in shelf areas and their cascading off the shelf break play a major role in ventilating deep waters, thus potentially affecting ecosystem functioning and biogeochemical cycles. However, whether DW flow across shelves may affect the composition and structure of plankton communities down to the seafloor and the particles transport over long distances has not been fully investigated. Following the 2012 north Adriatic Sea cold outbreak, DW masses were intercepted at ca. 460 km south the area of origin and compared to resident ones in term of plankton biomass partitioning (pico to micro size) and phytoplankton species composition. Results indicated a relatively higher contribution of heterotrophs in DW than in deep resident water masses, probably as result of DW-mediated advection of fresh organic matter available to consumers. DWs showed unusual high abundances of Skeletonema sp., a diatom that bloomed in the north Adriatic during DW formation. The Lagrangian numerical model set up on this diatom confirmed that DW flow could be an important mechanism for plankton/particles export to deep waters. We conclude that the predicted climate-induced variability in DW formation events could have the potential to affect the ecosystem functioning of the deeper part of the Mediterranean basin, even at significant distance from generation sites.

Highlights

The open-ocean dense water convection has long been acknowledged as substantial player in the global thermohaline circulation, while only recently the role of dense water formation over the shelf and cascading off the shelf break[1,2] has been recognized
The New Adriatic Deep Water was found near the bottom at two stations (367 and 600 m). This water mass is relatively richer in dissolved oxygen than the resident one being generated by recent cascading of Northern Adriatic Dense Water (NAdDW) turbulently mixed with Modified Levantine Intermediate Water (MLIW) along the descent
While in the surface water masses, the presence of low nutrients can be the direct consequence of intense nutrients uptake by phytoplankton, in deep waters, the relatively higher nutrients concentrations coupled with higher heterotrophs and autotrophs (H/A) ratios could suggest the prevalence of remineralization processes mediated mainly by the heterotrophic picoplankton community

Summary

Introduction

The open-ocean dense water convection has long been acknowledged as substantial player in the global thermohaline circulation, while only recently the role of dense water formation over the shelf and cascading off the shelf break[1,2] has been recognized. Within the Mediterranean, the northern Adriatic Sea is one of the three generation sites of dense shelf water formation along with the Gulf of Lion and the northern Aegean Sea[17] In this area, the severe heat loss and evaporation occurring during cold outbreaks associated to the Bora (a cold and dry north-easterly wind) densify the resident water mass producing the so-called Northern Adriatic Dense Water (NAdDW)[18,19,20]. The cold air outbreak (CAO), associated to a severe cold and dry flow of northeasterlies[25] and the very limited discharge of the Po River in the preceding fall, caused the formation of exceptional dense shelf water[2,22] Following this event, an interdisciplinary, multiplatform, dedicated cruise (ODW 2012) was organized and water samples were collected in the southern Adriatic at about 460 km from the area of origin where the main cascading events take place. The partitioning of plankton biomass and the phytoplankton species composition of different water masses were compared to answer the following questions: i) does the flow of DW affect the biomass and relative contribution of the different plankton size classes in resident water masses? ii) are the phytoplankton diversity and community structure affected by DW advection? iii) could the dense water flow actively transport small particles from the NAdDW generation area to the cascading site? iv) are the high concentrations of Skeletonema sp. (up to 40% of phytoplankton abundance) found in dense water samples during the 2012 event connected with a phytoplankton bloom in the NA during the generation event?

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