Tentative study of flow patterns in the North Aegean Sea using NOAA-AVHRR images and 2D model simulation

A statistical technique for image processing, the maximum cross correlation (MCC) method, was utilized on sequences of NOAA-AVHRR thermal data in order to explore the surface advective current dynamics at the discharge region of the Hellespont in the North Aegean Sea. A 2D numerical flow model was also used in order to simulate the barotropic flow pattern of the surface water layer. The model was forced with diurnal wind fields obtained for the same period as the satellite infrared images. The currents (magnitude and direction) derived from the two methods compare satisfactorily despite the fact that some model simplifications were made.     

Environmental considerations for design of the Athens sea outfall,Saronikos Gulf,greece

The pollution problems in Saronikos Gulf arise mainly from the discharge of about 600,000 m³/day of untreated wastewater into the northern part of this gulf. To alleviate the situation, a new major treatment and sea disposal system is under construction. Its design included various physical oceanographic and marine geological surveys. This article summarizes their results and assesses their impact on the unique design features of the outfall and diffusers.During summer, the stratification at the site of discharge is sufficiently strong to prevent the submerged sewage field from surfacing. This strong summer stratification coupled with the virtual absence of upwelling and the presence of a well-organized current field at the level of effluent trapping (about 40 m) ensures the offshore advection of the diluted submerged wastefield, via either the dominant cyclonic or the rarer anticyclonic circulatory patterns. The sea floor morphology is smooth (except near the Psyttalia coastline where poorly sorted coarse biogenic sands are steeply inclined), and the bottom is covered by an unconsolidated sedimentary layer with a thickness ranging from 10 to 167 cm and consisting in the western part of silty sand and in the eastern part of sandy silt. The seismic data indicated the absence of active faults, valleys, and channels. Below the surficial sediment cover, two seismic units were discerned, which may be correlated with the Quaternary and the Triassic outcrops on the island. The marine geological results indicated that rather minor technical work will be needed for the emplacement of the outfall system.     

Field data analysis and application of a complex water column biogeochemical model in different areas of a semi-enclosed basin: towards the development of an ecosystem management tool

The Pagasitikos gulf ecosystem is studied through the analysis of experimental field data acquired during several monitoring projects and the application of a complex biogeochemical model. The gulf was separated into three different parts (internal, top central-external, bottom central-external) according to the patterns exhibited by the key ecosystem indicators. Unlike other semi-enclosed gulfs Pagasitikos can be characterised as meso-oligotrophic undergoing periods of P or N limitation. Although the signal of nutrient inputs is not very clear in the field data, their importance is assessed through simulation. Increased phosphate concentrations either due to mixing or due to anthropogenic activities can result in phytoplanktonic blooms with significant contribution by diatoms. The effect of hydrodynamic patterns on primary production has been demonstrated through ecosystem modeling indicating that due to long stratification periods, all nutrients released through the benthic regeneration are trapped in the deeper layers, developing a microbial food web. However when the thermocline erodes nutrients find their way up in the upper layers of the euphotic zone and the system turns into more classical type with primary producers growing significantly faster.     

Assessing Marine Ecosystem Response to Nutrient Inputs

The response of the Pagasitikos Gulf to enrichment caused by run-off fertilizers and the development and evolution of harmful algal blooms is investigated through ecosystem modelling. A standard generic complex model has been developed to describe the ecosystem processes of Pagasitikos and has been validated with in situ data. Additionally external nutrient fluxes have been assimilated and incorporated into the ecosystem dynamics. The investigation of spatial effects due to nutrient enrichment is investigated along a North–South transect. When externally forced the model successfully assimilates the external river inputs producing nutrient and chlorophyll-a concentrations, which are in good agreement with the in situ data. The nutrient inputs result in a more stable ecosystem at the north part of the Gulf and in the development of eutrophic conditions. The changes in the ecosystem functioning with emphasis on the nutrient cycling, the increase of primary production, and the modes of operation are investigated and discussed.