Thermaikos Gulf Coastal System, NW Aegean Sea: an overview of water/sediment fluxes in relation to air-land-ocean interactions and human activities

This study presents an overview of the Holocene formation and evolution of the coastal system of Thermaikos Gulf (NW Aegean Sea). The system is divide d into the terrestrial sub-system and the oceanic sub-system; the former re presents 90%, while the latter includes only 10% of the total area. This pa rticular coastal zone includes the second most important socio-economic are a of Greece and in the southern Balkans, the Thessaloniki region; this is i n terms of population concentration(> 1 million people), industry, agricult ure, aquaculture, trade and services. The geomorphology of the coastal zone is controlled by sediment inputs, nearshore water circulation, and the lev el of wave activity. The large quantities of sediments (with yields > 500 t onnes/km(2) per year), delivered annually by the main rivers (Axios, Aliakm on, Pinios, and Gallikos) and other seasonal streams are responsible for th e general progradation of the coastline and the formation of the Holocene s edimentary cover over the seabed of the Gulf. Changes to the coastline can be identified on macro- and meso-time scales; the former include the evolut ion of the deltaic plains (at >1 km(2)/year), while the latter incorporates seasonal changes along sections of the coastline (e.g. sandy spits), mostl y due to the anthropogenic activities. The overall water circulation patter n in Thermaikos Gulf is characterised by northerly water movement, from the central and eastern part of the Gulf; this is compensated by southerly mov ement along its western part. The prevailing climate (winds and pressure sy stems) appears to control the surface water circulation, while near-bed cur rent measurements reveal a general moderate (<15 cm/s) southerly flow, i.e. offshore, towards the deep water Sporades Basin. Waves approaching from so utherly directions play also a role in controlling the shoreline configurat ion. Various human activities within the coastal system place considerable pressure on the natural evolution of the coastal zone ecosystem, Thus, the construction of darns along the routes of the main rivers has reduced drama tically the water/sediment fluxes; this caused, for example, retreat of the deltaic coastlines and seawater intrusion into the groundwater aquifers. S imilarly, pollution and/or eutrophication of the nearshore marine environme nt have resulted from the inputs of industrial wastes, urban untreated sewa ge, and agricultural activities on the coastal plains. This effect is demon strated by high levels of pollutants, nutrients, and by the increased conce ntrations of non-residual trace-metals within the surficial sediments. Fina lly, climatic changes associated with a potential rise in sea level (i.e. 3 0-50 cm) will threaten a substantial part of the low-lying lands of Thermai kos Gulf. Thus, systematic and thorough monitoring is needed in order to pr otect the coastal ecosystem; this will ensure its sustainable development a nd successful management, in relation to present and future socio-economic activities and climatic changes. (C) 2000 Elsevier Science B.V. All rights reserved.