Episodic deep vertical mixing in the Rhodes gyre (Mediterranean Sea) triggers an exceptional mesozooplankton outburst

Abstract

The Northwest Levantine Basin of the Eastern Mediterranean Sea has been historically considered as the source region of the Levantine Intermediate Water of the Mediterranean, which is formed in the permanent Rhodes cyclonic gyre. In late winter-early spring 1992, an unusual deep mixing of the water column down to 2000 dbar was observed. This event, caused by synergy of extreme meteorological winter conditions and hydrodynamical regime, led for first time to deep water formation; the new Levantine deep water mass replaced and/or mixed locally with the deep water mass of Adriatic origin, which dominated since the beginning of the 20th century. The hydrological characteristics of the newly formed water were different from the old ones, which historically occupied the deep and bottom layers of the entire Eastern Mediterranean. Since then, similar extreme event has not been reported. Mesozooplankton samples were collected in the 0–300 m layer of the Rhodes gyre, the Cretan Sea and the Cretan Passage. Zooplankton abundance was found three to fivefold higher in the 0–50 m layer of the Rhodes gyre than in that of the adjacent areas. This increase should be related to the water deep mixing and the resulted uplifting of nutrients and phytoplankton increment. In the core of the gyre and down to 300 m, the community was characterized by the abundance of Subeucalanus monachus adults, late copepodids and nauplii (almost 50 % of copepods in the 0–50 m layer). The formation of the new Levantine deep water in combination with the Eastern Mediterranean Transient event that was in progress, revealed a new hydrological regime in the NW Levantine Sea compared to the traditional one. The observed very deep convection supported an exceptional mesozooplankton outburst in a cyclonic gyre surrounded by the very oligotrophic Eastern Mediterranean waters.