Fjord Water Circulation Patterns and Dysoxic/Anoxic Conditions in a Mediterranean Semi-Enclosed Embayment in the Amvrakikos Gulf, Greece
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Estuarine, Coastal and Shelf Science 88 (2010) 473e481 Contents lists available at ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss Fjord water circulation patterns and dysoxic/anoxic conditions in a Mediterranean semi-enclosed embayment in the Amvrakikos Gulf, Greece George Ferentinos a,*, George Papatheodorou a, Maria Geraga a, Margarita Iatrou a, Elias Fakiris a, Dimitris Christodoulou a, Evagelos Dimitriou b, Constantin Koutsikopoulos c a Laboratory of Marine Geology and Physical Oceanography, Department of Geology, University of Patras, Rio Patras 26 500, Greece b Prefecture of Aitoloakarnania, 30200 Messolonghi, Greece c Department of Biology, University of Patras, Rio Patras, Greece article info abstract Article history: Oceanographic research in the Amvrakikos Gulf in Western Greece, a semi-enclosed embayment isolated Received 12 October 2009 from the Ionian Sea by a narrow, shallow sill, has shown that it is characterised by a fjord-like ocean- Accepted 11 May 2010 ographic regime. The Gulf is characterised by a well-stratified two layer structure in the water column Available online 21 May 2010 made up of a surface layer and a bottom layer that are separated by a strong pycnocline. At the entrance over the sill, there is a brackish water outflow in the surface water and a saline water inflow in the near- keywords: bed region. This morphology and water circulation pattern makes the Amvrakikos Gulf the only Medi- oxygen depletion terranean Sea fjord. The investigations have also shown that the surface layer is well oxygenated, Fjord water circulation fi Amvrakikos gulf whereas in the pycnocline, the dissolved oxygen (DO) declines sharply and nally attains a value of zero, Greece thus dividing the water column into oxic, dysoxic and anoxic environments. At the dysoxic/anoxic Mediterranean sea interface, at a depth of approximately 35 m, a sharp redox cline develops with Eh values between 0 and 120 mV occurring above and values between 0 and À250 mV occurring below, where oxic and anoxic biochemical processes prevail, respectively. On the seafloor underneath the anoxic waters, a black silt layer and a white mat cover resembling Beggiatoa-like cells are formed. The dysoxic/anoxic conditions appeared during the last 20 to 30 years and have been caused by the excessive use of fertilisers, the increase in animal stocks, intensive fish farming and domestic effluents. The inflicted dysoxia/anoxia has resulted in habitat loss on the seafloor over an area that makes up just over 50% of the total Gulf area and approximately 28% of the total water volume. Furthermore, anoxia is also considered to have been responsible for the sudden fish mortality which occurred in aquaculture rafts in the Gulf in February 2008. Therefore, anoxic conditions can be consid- ered to be a potential hazard to the ecosystem and to the present thriving fishing and mariculture industry in the Gulf. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction characterised by high anthropogenic inputs from urban, industrial and agricultural activities. Therefore, they are very vulnerable to Coastal embayments are highly diverse environments with pollution, and some of these areas tend to become either dysoxic or a varying spectrum of physical isolation from the open sea and anoxic. Since 1969, the number of areas in the marine environment water circulation patterns. These embayments mainly include characterised by these conditions has spread exponentially around estuaries, lagoons, rias and fjords. the world, which has serious consequences for ecosystem func- Coastal embayments are characterised by high productivity and tioning (Diaz and Rosenberg, 2008). are very important nursery grounds for several commercial fish The purpose of the present paper is to examine the oceano- species (Valiela, 1991). The contribution of these areas to the overall graphic regime, the water circulation patterns, the water oxygen fish and shellfish landings is considerable. Both extensive and efficiency and the prevailing environmental seafloor conditions in intensive aquaculture activities are also carried out in these pro- one of the largest semi-enclosed embayments in the Mediterra- tected areas (Valiela, 1991). However, coastal embayments are nean Sea: the Amvrakikos Gulf in the Ionian Sea, Northwestern Greece (Fig. 1a,c). The Amvrakikos Gulf is well known for shrimp fi * Corresponding author. shing and for an extensive aquaculture industry that has devel- E-mail address: [email protected] (G. Ferentinos). oped over the last 20 years. 0272-7714/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2010.05.006 474 G. Ferentinos et al. / Estuarine, Coastal and Shelf Science 88 (2010) 473e481 Fig. 1. (a) Bathymetric map of the Amvrakikos Gulf, the location of sampling stations and the location of the fish farms; (b) Bathymetric map of the Preveza Straits, which connects the Amvrakikos Gulf to the Ionian Sea. The location of the sampling stations and the position of the salinity and density sections (ABCD) along the Straits are shown in Fig. 5; and (c) Location of the study area in Western Greece. This research was triggered by the sudden massive mortality of rock to the south and east, and to the north by the delta plain of the fish in aquaculture rafts in the north eastern part of the Gulf Arachthos and Louros rivers and associated lagoons (Fig. 1a). (Fig. 1a), which occurred in February 2008, during an extremely The Gulf was formed in the Middle Quaternary period cold, wet and windy spell (Fisheries Commission). The study was (Anastasakis et al., 2007). During the isotope stage, MIS3 and MIS2 carried out 1 1/2 years after this event. (ca 50e11 ka BP), when the sea level was lower than 55 m in relation to its present position, the western part of the Gulf 1.1. Regional setting emerged, whereas the eastern part was occupied by a lake (Kapsimalis et al., 2005). The marine transgression took place at The Amvrakikos Gulf, a semi-enclosed embayment in North- approximately 11 ka BP and the Gulf attained its present shape at western Greece, is approximately 35 km long and 6e15 km wide approximately 4 ka BP (Kapsimalis et al., 2005). Therefore, it can be (Fig. 1a). Its maximum depth is 65 m, and it is separated from the said that the evolution of the Gulf was similar to that of the Baltic open Ionian Sea by a beachebarrier complex. The Gulf and the Sea and the Black Sea. Ionian Sea are connected through a narrow, elongated channel, the The oceanographic regime at the entrance to the Gulf is char- Preveza Straits, that is approximately 6 km long and ranges in width acterised by a semi-diurnal tide with an average tidal range of only from 0.8 to 2 km (Fig. 1b). The channel at its entrance is approxi- 5 cm and a maximum recorded range of 25 cm (Variagin, 1972). mately 2 km wide and gradually narrows, reaching a width of Inside the Gulf, a low energy wave regime prevails, due to the approximately 0.8 km at the mid-channel (Fig. 1b). The sill depth limited fetch of approximately 35 km (Poulos et al.,1998). Two large over this distance is between 2 and 10 m, including a man-made rivers, the Louros and Arachthos discharge approximately navigational channel. Past the middle, the channel gradually 609 Â 105 m3 and 2002 Â 106 m3, respectively, per year (Therianos, widens reaching a width of approximately 2.5 km, and the sill 1974). Since 1980, two dams control the run-off of the Arachthos deepens to approximately 20 m (Fig. 1b). The Gulf is bounded by River. The Gulf has a positive water balance, the water input from G. Ferentinos et al. / Estuarine, Coastal and Shelf Science 88 (2010) 473e481 475 rivers run-off is 2063 Â 106 m3 and the precipitation is approxi- surface layer is homogenous, with a temperature between 29 and mately 1200 mm/year (Hellenic Meteorological Service), whereas 30 C and salinity between 32.5 and 33. The bottom layer is also the output due to evaporation is approximately 1180 mm/year homogenous, with a temperature of between 15 and 16 C and (Public Power Corporation S.A.) salinity between 37 and 39. The thermocline develops between 8 The water column is highly stratified during the year with a high and 16 m, whereas the halocline is sharp and develops between 8 salinity gradient between 5 and 10 m deep (Friligos and Koussouris, and 12 m. In the eastern and northern parts of the Gulf at the upper- 1977; Friligos et al., 1997). The surficial water layer in the summer most part of the surface layer between depths of 0 and 3 m, a low (July) has a salinity ranging from 31 to 33 ppt and a temperature salinity layer develops that appears to be associated with the plume ranging from 23.5 to 27 C. The bottom water layer has salinities of of the Arachthos River. The southern and western parts of the Gulf between 37 and 38 ppt (Piper et al., 1982) and temperatures of are not affected directly by the rivers discharge. between 17.3 and 19.0 C. The Gulf is characterised by high primary A similar oceanographic regime prevails in winter in the Gulf. productivity with phytoplankton densities between 7.5 Â 105 and Measurements of temperature and salinity in the water column 2.0 Â 107 cells/l (Gotsis Skreta et al., 2000) and high levels of carried out by Poulos et al. (1998) near the mouths of the Arachthos eutrophication with average yearly concentrations of PO4-P, NH4-N and Louros rivers showed the presence of three layers: (1) a surfi- and NO3-N of approximately 0.4, 0.45 and 2.2 mg at/l, respectively cial layer that was approximately 3e4 m thick with salinity of (Friligos et al., 1997).