Introduction The lagoon of Mesolonghi-Etoliko is located on the west coast of Greece between latitude 38°15′ to 38°30′ north and longitude 21°05′ to 21°35′ east, covering an area of approximately 16,000 hectares (Figure 1). Klisova lagoon located between latitude 38°18′–38°22′ and longitude 20°26′–21°29′, is a part of the complex lagoonal area of Mesolonghi-Etoliko constituting its east extremity, covering an area of 2,500 hectares. West Klisova (about 1,900 ha) and East Klisova (about 600 ha) are today two artificially separated parts of what was once a unique lagoon. The purpose for the separation (made in late 60’s) was to facilitate removal to the sea, of Mesolonghi city sewage treatment plant effluent that empties in East Klisova. Mesolonghi-Etoliko lagoon is of great importance for fisheries but is heavily affected in recent years from human activities. Klisova lagoon in particular, accepting sewage treatment plant’s effluent, garbage (as city’s dump serves a part of W. Klisova) and irrigation water, is strongly affected and continuously modificated by these human activities that year after year are intensified. The main cause for such an environmental pressure is the continuously expanding adjacent city of Mesolonghi. The aim of this study was to gain an inside com- prehensive view in Klisova lagoon’s pollution situation. Sampling started on April 1993 and ended on May 1994. The selected monitoring practice (sampling frequency every 15 days at least), made possible to check the present state of pollution and to gather informations about the year round changes of crucial water parameters. These parameters are of two categories concerning their usefulness. The first category comprises all these that can give informa- tion about pollution load (B.O.D, oxygen, nutrient salts etc.). The second comprises 3 crucial parame- ters of paramount importance for giving the lagoon its special character comparing with sea and govern the inward-outward movements of fish (a fact that constitutes the basis for effective lagoonal fish resource exploitation). These are temperature, and conductivity-salinity that were monitored at sampling stations more frequently (every 7 days instead of 15 for all the other). It is well known that lagoonal water temperature as compared to seawater, follows more closely and reach equilibrium faster with air temperature. The cause for this is their big GeoJournal 41.1: 15–23.  1997 (January) Kluwer Academic Publishers. Printed in the Netherlands. A one year water monitoring study of Klisova lagoon (Mesolonghi, W. Greece) Hotos, George N.; Avramidou, Despina E., Department of Aquaculture and Fisheries, Technological Educational Institute (T.E.I) of Mesolonghi, Mesolonghi, Greece Received 8 September 1995; accepted 14 August 1996 Abstract: During a one year study in Klisova lagoon of Mesolonghi, it was found that between West Klisova and East Klisova there exist great differences in the degree of pollution owing to eutrophication, as expressed mainly by BOD and primary productivity measurements. East Klisova showed higher values for both these parameters all year round. In West Klisova there exist also some places with bad water circulation conditions, exhibiting extreme variations in temperature-salinity, anoxic conditions and high BOD. In East Klisova salinity is much lower than West Klisova because of a constant inflow of fresh water from the insufficiently functioning sewage water treatment plant of Mesolonghi town and from irrigation water pumping stations. In the sea around Klisova lagoon, there were also occasionally measured high values for BOD, ammonia and nutrient ions indicating a rather polluted environment. Considering temperature it became possible to create regression equations between water and air temperature. This was the result of a very frequent sampling, showing high regression coefficients (> 0.8) having thus a predictive value for use in lagoonal fisheries management.