Dissolved oxygen and nutrients distribution in eastern and western Mediterranean in summer 1999 (Trans Mediterranean Cruise)

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1 8ο Πανελλήνιο Συμποσιο Ωκεανογραφίας & Αλιείας Dissolved oxygen and nutrients distribution in eastern and western Mediterranean in summer 1999 (Trans Mediterranean Cruise) Evangelia Krasakopoulou & Ekaterini Souvermezoglou Hellenic Centre for Marine Research, 46.7 km Athens-Sounio Avenue, Anavyssos Abstract In June 1999, the Trans Mediterranean Cruise (TMC) based on an almost longitudinal transect along the Mediterranean was conducted. Sampling stations were selected to represent the main basins and/or physical features of the Mediterranean Sea and were 7 stations in the eastern Mediterranean and 5 in the western Mediterranean basin. In surface waters of both basins the oxygen, nitrate and phosphate concentrations are almost similar, but in the deeper layers the concentrations and the shape of the vertical distribution differ considerably. In general, the phosphate and nitrate content in the deep part of the eastern basin is considerably less than at the same depth in the western basin. The western Mediterranean stations are characterised by a distinct oxygen minimum layer formed at mid depths ( m) followed by maximum nutrients concentrations. The N:P ratio for the entire Mediterranean basin far exceeds the Redfield ratio suggesting an overall system P deficiency relative to N. The higher N:P ratio of the eastern Mediterranean shows that is strongly P limited and that across the entire Mediterranean basin the degree of P limitation increases from west to east. Keywords: oxygen; nutrients; N:P ratio; Mediterranean Διαλυμeνο οξυγoνο και θρεπτικa στην ανατολικh και δυτικh Μεσoγειο το καλοκαiρι του 1999 (Διαμεσογειακoς Πλoας) Ευαγγελία Κρασακοπούλου & Αικατερίνη Σουβερμέζογλου Ελληνικό Κέντρο Θαλασσίων Ερευνών, 46.7 χλμ Λεωφ. Αθηνών-Σουνίου, Ανάβυσσος Περίληψη Τον Ιούνιο 1999 πραγματοποιήθηκε ένας διαμεσογειακός πλόας σχεδόν κατά μήκος της λεκάνης της Μεσογείου. Οι σταθμοί δειγματοληψίας (7 στην ανατολική Μεσόγειο και 5 στη δυτική) επιλέχθηκαν ώστε να είναι αντιπροσωπευτικοί των διαφόρων υπο-λεκανών ή/και των υδρογραφικών χαρακτηριστικών της Μεσογείου. Αν και οι συγκεντρώσεις οξυγόνου, νιτρικών και φωσφορικών είναι παρόμοιες στα επιφανειακά νερά ανατολικής και δυτικής Μεσογείου στα βαθύτερα στρώματα οι συγκεντρώσεις και η μορφή της κατακόρυφης κατανομής διαφοροποιούναι σημαντικά. Γενικά οι συγκεντρώσεις φωσφορικών και νιτρικών στα βαθειά νερά της ανατολικής λεκάνης υπολείπονται σημαντικά των συγκεντρώσεων σε αντίστοιχα βάθη της δυτικής λεκάνης. Η δυτική λεκάνη χαρακτηρίζεται από ένα ευδιάκριτο στρώμα με ελάχιστες συγκεντρώσεις οξυγόνου και μέγιστες συγκεντρώσεις θρεπτικών. O λόγος Ν:Ρ τόσο για όλη τη λεκάνη της Μεσογείου όσο και για τις δύο υπολεκάνες είναι αρκετά μεγαλύτερος από την αναλογία Redfield υποδεικνύοντας ότι στο σύστημα υπάρχει ένδεια Ρ σε σχέση με το Ν, η οποία μάλιστα είναι εντονότερη στην ανατολική Μεσόγειο. Λέξεις κλειδιά: οξυγόνο; θρεπτικά; λόγος Ν:P; Μεσόγειος

Introduction The general depletion of nutrients in the Mediterranean Sea compared with the nutrient content of the adjacent Atlantic waters, as well as their west to east decrease in concentrations

2 Introduction The general depletion of nutrients in the Mediterranean Sea compared with the nutrient content of the adjacent Atlantic waters, as well as their west to east decrease in concentrations along the different basins, is well known (Mc Gill, 1965). The recent observations confirm the general depletion of nutrients compared with other parts of the world s oceans (Souvermezoglou, 1989). There is a limited supply to its surface waters from both its deeper layers and external sources (i.e. Atlantic inflow, riverine discharges and atmospheric input) but the principal reason for this nutrient deficiency is related to its hydrology and circulation as a concentration basin. In June 1999, a cruise was conducted, based on an almost longitudinal transect along the Mediterranean Sea, from the eastern Levantine to the Balearic Sea, so-called Trans Mediterranean Cruise (TMC). The sampling stations were selected to represent the main basins and/or physical features of the Mediterranean Sea (e.g. Rhodes gyre, Levantine basin, Cretan cyclonic gyre, S. Ionian basin, Sicily Strait, Algerian basin). A main objective was to study the spatial variability in the distribution of dissolved oxygen and nutrients relative to the hydrography. 8ο Πανελλήνιο Συμποσιο Ωκεανογραφίας & Αλιείας Methodology This study was carried out during the Trans Mediterranean cruise (7 June - 2 July 1999) on the RV Aegaeo in the framework of the EC MATER project. Water samples were collected from 7 stations in the eastern Mediterranean and 5 in the western Mediterranean Sea (Fig. 1). Water samples were collected with 24 Niskin 10-litre bottles mounted on a General Oceanics rosette equipped with a Sea-Bird SBE-911+ CTD probe. For the dissolved oxygen determination samples taken from the Niskin bottles were analysed immediately after collection with the Winkler method as modified by Carpenter (1965a and 1965b). Samples for the determination of nutrients were collected in 100 ml polyethylene bottles and kept continuously under deep freeze (-20 C), until their analysis in the laboratory by a Bran+Luebbe II autoanalyser. The methods described by Bendschneider et al. (1952) and Stickland and Parsons (1968) for nitrite and nitrate were employed. Phosphate were measured by a Perkin Elmer Lambda 2S UV/ VIS Spectrometer, according to the method of Murphy and Riley (1962). Results and Discussion Dissolved Oxygen and nutrients distribution Figure 1. Location of the stations sampled during the Trans Mediterranean cruise (summer 1999).

3 8ο Πανελλήνιο Συμποσιο Ωκεανογραφίας & Αλιείας 3 Dis.Oxygen (ml/l) Nitrate (µmol/l) Phosphate (µmol/l) Depth (m) stations E4 (west. Med.) 3 (east. Med.) Figure 2. Vertical distribution of dissolved oxygen, nitrate and phosphate at selected stations in the eastern ( ) and western (;)Mediterranean sea during Trans Mediterranean cruise (summer 1999). Near the surface oxygen values are of comparable magnitude throughout both eastern and western basins, being slightly higher in the western one. Concentrations rise sharply below the surface and reach a shallow maximum in about m depth, attributed to the enhanced photosynthetic activity as well as to lower temperature. Below about 200 m depth oxygen concentrations decrease vertically but they exhibit significantly different shape in the two Mediterranean basins (Fig. 2). The western Mediterranean stations are characterised by a distinct oxygen minimum layer formed at mid depths ( m), which corresponds to the LIW horizon. Oxygen concentrations in this layer ranged between 3.74 ml/l (Sta. 8) and 4.09 ml/l (Sta. 10). The Levantine Intermediate Water (LIW), which originates in the eastern Mediterranean, flows in the western basin at intermediate depths (~ m) and as it is an aged water mass is characterised by an increased oxygen consumption associated with high nutrient regeneration. Below the oxygen minimum layer, oxygen concentration increases towards the bottom at both basins being higher in the western one, probably attributed to more effective ventilation during winter (Statford et al., 1998). A striking feature in the vertical distribution of oxygen in the western basin is the relative increase of the oxygen content detected in the deep layer of the stations 10 and E4, indicating the presence of newly formed dense water (Bethoux et al., 2002). During the summer stratified period nitrate and phosphate are below or near the detection limit in the surface waters of both basins and increase with depth much faster in the western basin than in the eastern one, inducing very strong concentration gradients at mid-depths (Fig. 2). In general, the phosphate and nitrate content in the deep part of the eastern basin is considerably less than at the same depth in the western basin. Maximum nutrients concentrations are recorded in the oxygen minimum layer of station 8, attaining µmol/l and 9.25 µmol/l for phosphate and nitrate respectively. In the eastern basin the concentrations in the m layer do not exceed µmol/l for phosphate and 5.45 µmol/l for nitrate. In the deeper than 1000m part of the water column nutrient concentrations exhibit weak variability which is indicated by low standard deviations: deep mean nitrate and phosphate concentrations are respectively 6.67±0.96 and 0.350±0.029 µmol/l in the western basin and 3.97±0.47 and 0.158±0.035 µmol/l in the eastern basin.

4 4 8ο Πανελλήνιο Συμποσιο Ωκεανογραφίας & Αλιείας Nitrite+Nitrate vs Phosphate ratio One of the most exceptional characteristics of the Mediterranean Sea, in addition to the general depletion of nutrients, is the high N:P ratio, higher than the Redfield ratio of 16 found in most of the world oceans. The cause for the high N:P ratio is not clear and various theories have been put forward to explain it (Kress at al., 2001; Moutin & Raimbault, 2002 and references therein). Model II linear regression of N (nitrite+nitrate) vs P (phosphate) was conducted using all available data pairs (Fig. 3), as well as using the data corresponding to the eastern and western basins (Table 1). The N:P ratio for the entire Mediterranean basin (20.55) far exceeds the Redfield ratio suggesting an overall system P deficiency relative to N. For the eastern Mediterranean the N vs P correlation has a slope of 25.25, higher than the slope of the western Mediterranean and in consistency to the N:P ratios found previously in the area (Krom et al., 1991; Kress & Herut, 2001). Nitrite+Nitrate (µmol/l) N = P R 2 =0.898 n = 200 eastern Med. western Med. N = P N = 16 P (Redfield) Phosphate (µmol/l) Figure 3. Nitrite+Nitrate vs phosphate in the eastern (,) and western (+) Mediterranean sea during Trans Mediterranean cruise (summer 1999). In addition, the results show that the eastern Mediterranean is strongly P limited and that across the entire Mediterranean basin the degree of P limitation increases from west to east. However, it is interesting to note that the intercept of the regression line, that represents the nitrate concentration when phosphate reach zero, has a value of in the eastern basin (Table 1), indicating N limitation due to the slight excess of P over N during that period. Conclusions Nitrate and phosphate concentrations at middepths and at depths >1000m are higher in the western than in the eastern part of the Mediterranean confirming thus the statement of McGill (1965) on the west to east decline of nutrient concentrations in the Mediterranean Sea. The N:P ratios calculated for the entire basin as well as for the eastern and western basins separately suggest that the Mediterranean sea is strongly P limited and that across the entire basin the degree of P limitation increases from west to east. References Bendschneider, K. and Robinson, R.J., A new spectrophotometric method for the determination of nitrite in sea water. Journal of Marine Research, XI, 1, BETHOUX, J.P., Durrieu de Madron, X., Nyffeler, F. and Talliez, D., Deep water in the western Mediterranean: peculiar 1999 and 2000 characteristics, shelf formation hypothesis, variability since 1970 and geochemical inferences. Journal of Marine Systems, 33-34, Carpenter, J.H., 1965(a). The accuracy of the Winkler method for the dissolved oxygen analysis. Limnology and Oceanography, 10, Carpenter, J.H., 1965(b). The Chesapeake Bay Institute technique for dissolved oxygen

5 8ο Πανελλήνιο Συμποσιο Ωκεανογραφίας & Αλιείας Table 1. Model II linear regression results of N vs P for the whole data set and for the eastern and western Mediterranean basins during summer all data eastern Med. western Med. Slope Intercept n R method. Limnology and Oceanography, 10, Kress, N. and Herut, B., Spatial and seasonal evolution of dissolved oxygen and nutrients in the Southern Levantine Basin (Eastern Mediterranean Sea): chemical characterization of the water masses and inferences on the N:P ratios. Deep Sea Research I, 48, Krom, M.D., Kress, N., Brenner, S. and Gordon, L.I., Phosphorus limitation of primary productivity in the Eastern Mediterranean Sea. Limnology and Oceanography, 36(3), McGill, D.A., The relative supplies of phosphate, nitrate and silicate in the Mediterranean Sea. Comm. Int. Explor. Sci. Mer Medit. Rapp. P.V. Reunions, 18, Moutin, T. and Raimbault, P., Primary production, carbon export and nutrients availability in western and eastern Mediterranean Sea in early summer 1996 (MINOS cruise). Journal of Marine Systems 33-34, Murphy, J. and Riley, J.P., A modified solution method for determination of phosphate in natural waters. Analytica Chimica Acta, 27, Souvermezoglou, E., Distribution of nutrients and oxygen in the Eastern Mediterranean Sea. Proceedings of the UNESCO/ IOC Second Scientific Workshop, Trieste, Italy, POEM Sci. Repts. 3, Cambridge, Massachusetts, USA, Strickland, J.D.H. and Parsons, T.R, A practical handbook of sea water analysis. Bull. Fish. Res. Bd. Canada, 167, 310p. Statford, K., Williams, R.G. and Drakopoulos, P.G., Estimating climatological age, from a model-derived oxygen-age relationship in the Mediterranean. Journal of Marine Systems 18,

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