Continental and atmospheric forcing of near-coastal ecosystem dynamics in the eastern Mediterranean Sea

Observations of the eastern Mediterranean Sea – carried out by means of combined optical, infrared and microwave remote sensing techniques – provide classical examples of continental and atmospheric forcing of near-coastal ecosystem dynamics. Although the Levantine Basin is known for its oligotrophic character, bio-optical markers – derived from satellite data collected in the last decade – dusplay a stronger and stronger signal at several environmental hotspots. Along the southern coastline, the river Nile plume has by far the largest impact on the ecosystem, providing a constant source of nutrients of continental origin. Although such impact was much reduced by the Aswan High Dam, significant concentrations of water constituents still characterize the marine area in front of the delta. To the east, the Egyptian-Israeli-Lebanese coastal area appears as one of the main sites for anomalous algal blooming. Turbulent diffusion from this area, downstream from the Nile delta, with respect to the prevailing cyclonic circulation of the Levantine Basin, generates sizeable plumes extending offshore. This new mechanism for bio-chemical enrichment of the eastern Mediterranean, supplying nutrients of coastal origin to the primary producers that sustain the food web in this otherwise oligotrophic region – nutrients that originate from the ever increasing use of fertilizers for agriculture, as well as from the proliferation of untreated sewage outfalls, due to population growth along the coast – has been suggested as the reason for the recent recovery of local fisheries, which had collapsed in the mid 1960’s after the Nile damming. In the northern near-coastal zone, instead, atmospheric forcing appears to be the main factor driving ecosystem dynamics. Northerly winds blowing along the axis of the Aegean Sea funnel through the straits between the Islands of Crete and Rhodes, generating a (quasi) permanent vortex pair. This comprises the anticyclonic Ierapetra Gyre, to the south-west of the straits, and the cyclonic Rhodes Gyre, to the north-east. In the first case, the convergence in the surface layer is linked to the accretion and downwelling of warm, nutrient-poor waters; in the second, the divergence due to Ekman pumping is linked to the upwelling of colder, nutrient-rich waters, and to the main algal blooming events of the Levantine Basin.