Eddies and Chlorophyll-a in the Mediterranean Sea

By Eleni Livanou and Stella Psarra (Institute of Oceanography, Hellenic Centre for Marine Research)

The Mediterranean Sea is one of the most nutrient-poor (oligotrophic) marine regions worldwide. Whereby an east-west gradient of diminishing macronutrients (nitrogen and phosphorus), as well as Chlorophyll-a and primary production can be observed. Availability of nutrients is determined by circulation, which is very complex in the Eastern Mediterranean Sea (EMS). Although in-situ measurements are scarce across the EMS, satellites are able to measure both circulation patterns as well as Chl-a concentrations, thus capturing the complex oceanography and biology of this marine system.

The Mediterranean Sea is a semi-enclosed concentration-type oceanic basin dominated by an anti-estuarine circulation (less dense fresher water flows on top of more saline water, generating a current that is strongest and warmest near the surface) where low salinity Atlantic Water (AW) enters the basin through the Straits of Gibraltar and flows eastward. Part of this AW passes through the straits of Sicily, driving the surface circulation of the Eastern Mediterranean Sea (EMS), which consists of the Ionian and Levantine sub-basins. The AW flows in on the surface/subsurface layer while more saline water of Levantine origin flows out of the basin at the subsurface layer (~150–400m). Owed to its anti-estuarine circulation, the Mediterranean Sea consists one of the most oligotrophic marine regions worldwide, characterised by a west-east gradient of increasing oligotrophy in terms of macronutrients (nitrogen and phosphorus), Chlorophyll-a (Chl-a) concentration and primary production (Figure 1). Thus, the EMS is characterised as ultra-oligotrophic with extremely low inorganic nutrient concentrations in the euphotic zone (top layer of the water with enough sunlight for photosynthesis to occur) resulting in very low values of Chl-a (<0.05mg per cubic meter) especially in the southeast part of the basin (Figure 1).

EMS circulation is complex with numerous mesoscale structures, such as permanent or semi-permanent eddies. Some well-characterised structures are evident when plotting the CMEMS estimated Mean Dynamic Topography for the 1993-2012 period (Figure 2).

These mesoscale eddies play a fundamental role in phytoplankton dynamics since they favour vertical exchange of water masses thus transporting nutrients by upwelling and sinking processes and having a strong impact on biological activity in the surface waters of the EMS.

High-resolution satellite data sets of altimetry and ocean colour are extremely useful in examining the effect of mesoscale structures on surface Chl-a in the EMS, where in-situ measurements are scarce.

In Figure 3 monthly means of surface geostrophic velocities are overlaid on the respective Chl-a concentrations for the EMS, for selected months, representing each season, during 2020 (February, April, August, November). On February 2020, when the winter mixing is totally established, an increase in Chl-a concentration in the wider area of the EMS is observed. In April and later in August 2020, Chl-a concentration decreases as a result of the onset and culmination of thermal stratification, respectively, and the resulting exhaustion of nutrients in the euphotic zone. During the onset of thermal stratification (April 2020) the decrease of Chl-a is first observed at the southern part of the basin while during August 2020 the entire basin exhibits extremely low values of Chl-a. However, in the center of the Rhodes Cyclone, where vertical mixing causes the shallowing of the nutricline, increased Chl-a concentration is sustained locally, compared to the surrounding area.

This 'hotspot' of phytoplankton productivity in the centre of the gyre is evident and persistent throughout the year, even during August when stratification is most intense. Pelops Anticyclone, as the Rhodes cyclone, is also a permanent structure in the EMS and it is easily spotted throughout the selected months. However, the effect of this gyre on Chl-a concentration is not as pronounced as the effect of the Rhodes Gyre.

The Ierapetra Anticyclone forms during summer-early fall (Figure 3, August 2020) and it is a consequence of the Etesian winds that pass through the mountains of eastern Crete during this period. Nevertheless, both the Ierapetra and Pelops anticyclones do not produce a distinct signature on the Chl-a concentration and, in general, low Chl-a concentrations, representative of the oligotrophy of the EMS, are observed in these structures. Finally, the effect of the Cretan Cyclone is evident in August 2020, when a small area with a moderate increase of Chl-a in the center of the gyre, as compared to the surrounding area, can be observed.

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