Estuarine and Coastal Marine Science (1973), I, 195-202

Some Physical and Chemical Properties of the Gulf of Corinth

James J. Anderson and Eddy C. Carmacka Department of Oceanography, University of Washington SeattEe, W~h~~gt~ 98195~ U.S.A.

received 12 Mar-ch 1973

The Gulf of Corinth is a silled embayment opening into the Ionian Sea on the west and through the Corinth Canal to the Aegean Sea on the east. Winter cooling of the nearshore waters results in convective overturn and renewal of the deep water of the basin. Renewal at the intermediate depths occurs when Ionian Sea water enters the basin over the sill. The basin acts as a slight nutrient trap.

Introduction

The Gulf of Corinth is a basin between the mainland of Greece and the peninsula Pelopon- nesus (Figure I). Its west end opens to the Ionian Sea over a sill of approximately 60 m. The east end of the basin is connected to the Aegean Sea through the Corinth Canal. The canal is a minimum of 8 m deep and 21 m wide. The maximum depths in the gulf are over 800 m. The purpose of this paper is to describe briefly the water properties in the gulf and postulate the processes controlling them.

Data source

Oceanographic observations were obtained from the National Oceanographic Data Center in

Washington, D.C. and climatic data from the Smithsonian Miscellaneous Collections (194.4.) and the Environmental Science Services Administration (1969, 1970).

The first observations within the gulf were collected by the Danish ship Thor at 38”ro’N and z~“33’E on 16 August rgro. On 20 and 21 May 1930 the Danish again occupied the gulf at 38”05’N, 22O42’E and 3S015’N, 22’20’E. The University of Washington vessel Thomas G. Thompson occupied a station at 38”14*6’N and 22”31*3’E on 17 February rg7o and six months later on g August an Israeli ship occupied stations at 38”r7’N, 22*19*0’E and 38”08mo’N, 22”45*0’E. Temperature, salinity and oxygen were observed at all stations but nitrate, phosphate, silicate and nitrite were observed only at the rg7o stations.

In the Ionian Sea to the west of the Gulf of Corinth 21 stations, within 30 nautical miles of 38”oo’N and 20°30’E, have been used for comparison with the water within the gulf. Observations were made by Denmark in 1930, France in 1956, Italy in 1959, Italy and Russia in 1960, America in x962, Italy in rg66 and Israel in 1970. The data spans the months March through September. All the stations have temperature and salinity data, most have oxygen observations and four stations have nitrate, nitrite, phosphate and silicate observations.

“Present address: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, Calif. 92037, U.S.A.

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Some properties of the Gulf of Corinth I97

Observations in the Gulf of Corinth

Observations in February and August 1970 illustrate the summer and winter variations of the chemical and physical parameters in the Gulf of Corinth (Figures 2, 3, 4 and 5).

The temperature in the Gulf (Figure z) varies seasonally above 200 m but below 200 m it is relatively constant with maximum variations of less than 0.3 “C for any set of data and most are within 0.1 “C. The deep water temperature in February was less than the August temperatures by about 0.05 “C. In the years for which data are available, the temperature of the bottom water ranged from 12.82 to 13-y. “C.

Temperature (“Cl

Figure 2. Temperature profile in the Gulf of Corinth for 17 February 1970 (0)

and 9 August 1970 (A).

In August the temperature in the upper 200 m increased exponentially from just below 14 “C at 200 m to above 25 “C at the surface. The air temperature for that time was 27 “C. In February the surface temperature was just above 14.3 “C and the air temperature IO “C. A temperature inversion layer was evident between IOO and 175 m with a maximum of 14’59 “C at 150 m.

The salinity profiles (Figure 3) for August and February 1970 differ by a nearly uniform 0.04%~ in the deep part of the basin. The salinity of the deep water in August was $X*41 to 38.42x0, while in February it varied between 38.37 and 38*38%,. The salinity in August 1910 was the highest observed, with values between 38.56 and 3%58x,, and in May 1930 the salinity below 200 m ranged between 38.46 and 38059%~. There seems to be no trend in the deep water salinity gradient.

Above 200 m the salinity changes are similar to the temperature changes. In August 1970, the salinity was at a maximum at the surface, presumably because of summer evaporation, and there was a minimum near 50 m, presumably because of the advection of Ionian Seawater into the basin or perhaps a result of previous high precipitation. In February 1970 the salinity was uniform down to IOO m with a maximum between 125 and 175 m, corresponding to the temperature maximum.

198 ‘J.J: Anderson & E. C. Carmack

In the surface layers temperature is the primary density-controlling variable in the Gulf of Corinth, so seasonal changes in sigma-t values are opposite to those of temperature. The deep water sigma-t varies slightly but significantly with the season. In February 1970 sigma-t was 28.97 at the bottom and 28.92 at 200 m. In August 1970 it was 28.99 at the bottom and 28.91 at 200 m. This is an increase of 0.02 sigma-t units between February and August arising from a corresponding 0.04%~ increase in the salinity. The bottom water sigma-t was 29.22 in August 1910 and 29.08 in May 1930.

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-I Figure 3. Salinity profile in the Gulf of Corinth for 17 February 1970 (0) and 9 August (A).

The oxygen distribution (Figure 4) changes seasonally throughout the entire water column. The surface is saturated in both winter and summer with a slight supersaturation just below the surface. In the bottom 200 m of the water column there was a significant increase in the oxygen concentration (0.070 mg-atom/liter) between February and August, corresponding to a change in saturation of 71 to 85%. During all the observations except in February 1970 there was an oxygen minimum between 200 and 500 m, resulting in a per cent oxygen saturation minimum. In 1910 and 1930 this minimum was only a few per cent, but in August 1970 the oxygen minimum was only 77% saturated compared with 90% in February

1970. The bottom water silicate concentration (Figure 5) decreased by 9 pg-atom/liter between

February and August 1970, corresponding to the increase in oxygen during that period. The silicate profiles between IOO and 500 m are similar with a slight decrease of silicate in the waters above IOO m in the summer.

Phosphate concentrations in the gulf are generally less than 0.5 pg-atom/liter. The data are scattered but there is a tendency for phosphate to increase with depth. There were approxi- mately 0.35 and 0.4 yg-atom/liter of phosphate near the bottom in the winter and summer, The average nitrate concentration was around IO pg-atom/liter and there was no uniform trend with depth.

Only from December through March are the air temperatures less than the deep water temperatures of the basin. The average air temperature in December is near I I “C, it is 9 “C

Some properties of the Gulf of Corilzth 199

in January and February and near II “C in March. April and November temperatures are above 15 “C, with summer temperatures over 27 “C. December is the wettest month with around 70 mm of precipitation with the total rainfall decreasing to typical values of 5z,47 and 30 mm for the months of January, February and March. In 1910 the ‘temperatures were 2 “C above normal in February and 2 “C below normal in March. Precipitation was near twice the normal for all winter months except December, which had 27 mm of precipitation. The 1930 winter was normal with respect to temperature, while February of that year was wet with 127 mm of precipitation and March was dry with 12 mm. The winter of 1970 was warm and wet with temperatures around 2 “C above normal all winter and precipitation values of 362, I++, 122 and 31 mm for the months December through March.

1 600,

Oxygen (mg-atom/liter)

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; 800 I r’ I’U I

Figure J.. Oxygen profile in the Gulf of Corinth for 17 February 1970 (0) and 9 hl@lSt 1970 (A).

Ionian Sea observations and comparisons with the Gulf of Corinth

Seasonal changes in the Ionian Sea directly west of the mouth of the Gulf of Corinth are similar to those in the gulf but there are significant differences. Below 200 m the Ionian Sea is significantly warmer and more saline than the gulf (Figure 6). The most significant conclusion that can be drawn from the T-S diagrams (Figure 6) is that the deep water of the gulf cannot be formed from the mixing of Ionian Sea water along isopycnals.

The oxygen profile in the Ionian Sea generally coincides with the Gulf of Corinth profile throughout the year, with a significant difference in the variability of the deep oxygen concentrations in the two regions. At 800 m in the Ionian Sea the oxygen concentration is 0.380 mg-atom/liter with maximum variations of o-010 mg-atom/liter as determined from 21 stations. The average concentration of oxygen in the deep water of the gulf is 0.414 mg- atom/liter (including all data) but the variations in the gulf are up to 0.045 mg-atom/liter. In the Ionian Sea there is no minimum in the percent saturation curve.

The nutrient concentrations in the Ionian Sea are lower than those of the Gulf of Corinth. Integrating the nutrient profiles in the water column between o and 800 m for the Gulf of

200 J.J. Anderson &3 E. ‘(. Carmack

Corinth and the Ionian Sea allows a rough estimate of the nutrient mass differences in these two areas. Comparing three Corinth stations and four Ionian Sea stations, there were roughly three times as much silicate and one and a half times as much phosphate and nitrate in the Gulf of Corinth.

Shcate [pg atom/liter)

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Figure 5. Silicate profile in the Gulf of Corinth for 17 February 1970 (0) and 9 August 1970 (A).

/ I /I I I 38.0 38 2 384 38 6 30.8 ;

Salinity (%o)

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Figure 6. Shows T-S curves for February in the Gulf of Corinth and the Ionian Sea directly west of the gulf. The curve labeled TT-072 is for the gulf on 17 February 1970 at 38'14.6'N, 0~2~31~3% and the curve labeled A-6211 is for 17 February 1962 at 38”oo’N, 020°11*5’E.

Some properties of the Gulf of Corinth 201

Discussion The increase in oxygen between February and August 1970 (Figure 4) could only arise from the advection of oxygenated water from the near surface region into the bottom zoo m of the basin. The large variation in the bottom oxygen concentration in the basin confirms that the renewal process was not limited to 1970.

The source of the water that renews the deep water of the basin can be inferred from the T-S diagram (Figure 6). It is evident that deep water renewal over the sill from the Ionian Sea is not possible because this would raise the temperature of the bottom water by I or z “C and increase the salinity by more than 0*2%,. The TT-072 curve (Figure 6) shows that the water between 250 and 800 m in the gulf is formed by the cooling of the surface water by I “C followed by sinking into the deep part of the basin.

An important feature in February 1970 was the temperature and salinity maximum between 125 and 175 m. This maximum arose from the advection of Ionian Sea water over the sill into the gulf. From the T-S diagram (Figure 6) it appears that water at ISO m inside the gulf is a mixture of 20% Ionian Sea water and 80% Corinth surface layer water. The surface layer of the gulf has a sigma-t of 27'73 and the Ionian water 28.77 at 125 m. The resulting mixture, which is centered at 150 m inside the gulf, has a sigma-t of 28.75. Processes have been observed in Saanich Inlet on Vancouver Island (Anderson & Devol, 1973), that can account for intrusions of water over a sill. Such processes seem to occur in Gulf of Corinth, with the new water remaining at an intermediate level in the basin.

From the meteorological data it is evident that deep water can form from December to March, because during this time the air temperature is less than the bottom temperature of the gulf. Precipitation is greatest in December and decreases through March, thus the combination of temperature and precipitation make February and March the most favorable months for convective overturn.

A correlation between the winter air temperatures and the deep water temperatures in the summer is evident. The lowest deep water temperatures occurred in 1910 and the coldest air temperatures occurred in March and January of that year. The next coldest winter was in 1930, while during the winter of 1970 air temperatures were 2 “C warmer and deep water was o-5 “C warmer than the winter of 1910. The correlation with salinity and precipitation is not so well established. The deep water salinities were highest in 1910 but 1930 was the driest winter. In 1970 the salinities were o-2%, less than in 1910 while the winter precipitations were approximately the same except for December 1970, when there were 362 mm of rain compared to 27 mm in 1910.

To 17 February of 1970 there appeared to be no significant increment of new water in the deep region of the basin as inferred from the oxygen and nutrient profiles. In August it was evident that new water had entered the deep region, thus the major deep water renewal probably occurred after 17 February. This is reasonable and in line with the meteorological data because the precipitation significantly decreases in March while the temperature is still low enough to form bottom water.

The lack of correlation between the precipitation and deep water salinity values might be because of the injection of Ionian Sea water into the basin, which could offset the salinity decrease of the surface layer caused by winter precipitation. With the onset of cold winter air, convection begins and results in the thickening of the surface layer. This process was evident in February 1970, when the mixed layer extended to IOO m. This convective mixing would eventually include part of the salinity maximum layer and increase the salinity of the surface layer, which would eventually become dense enough to replace the bottom water.

202 J. J. Anderson & E. C. Cavmack

Thus the salinity of the deep water of the basin may reflect the combined effects of winter precipitation and the introduction of high salinity Ionian Sea water to an intermediate level in the basin, while the temperature of the deep water is related primarily to the winter air temperature regime.

There are inadequate data to show whether or not deep water overturns each winter but using the oxygen concentrations observed in the deep water of the basin and oxygen con- sumption rates from similar types of environments we can postulate that overturn occurred during the winters of 1910, 1930, 1969 and 1970. Using an oxygen consumption rate of IO ng-atom/liter/hour, as measured for the deep water of Saanich Inlet, on Vancouver Island, B.C., Canada (Anderson & Devol, 1973), we compute that in six months the time between winter overturn and August, the oxygen saturation of the deep water would have decreased by approximately 9%. In August 1970 the deep water of the basin was 85% saturated. Thus the water in the deep layers would be 93% saturated just after overturn in late February or March 1970. On 17 February, 1970 the deep water was 71% saturated and if this reflects 12 months of respiration, the newly renewed water in 1969 would be 89% saturated. Thus it seems that the newly overturned water is near 90% saturation for any given year and the same water would be near 70% saturation just before renewal the next year. Correspondingly, if renewal did not occur one year, then the saturation of the deep water would drop below 70% to as low as 50% by the next winter. Since such low values were not observed in the basin, it appears that renewal has occurred in the winters of agog-1910,

1929-30, 1968-69 and 1969-70.

In the spring, summer and autumn the density of the deep water of the gulf will decrease by the diffusion of salt into the surface layer and heat into the deep layer. The effectiveness of these transfers and the year-to-year variability in the density of the surface water in the winter will be the factors determining whether or not convection occurs throughout the entire water column each winter.

The Gulf of Corinth acts as a nutrient trap because of the bathymetry that restricts deep water exchange with the Ionian Sea, although accumulation of nutrients is relatively small compared to that in anoxic basins. Phosphate and nitrate are increased by a factor of 1.5 and silicate by 3. The accumulations are no larger because of the low productivity and nutrient concentration in the surface layers, the absence of strong vertical isolation of the deep waters of the basin and the winter convective overturn of the water column.

Acknowledgements

The authors wish to thank Dr F. A. Richards and A. H. Devol for their critical reviews of the manuscript. We are especially indebted to Dr F. A. Richards for providing the impetus to this study. This research was supported by the National Science Foundation under Grant GA-24875 and by Office of Naval Research Contract N-OOOI~-~~-A-OIO~-OOI~. This is Contribution No. 731 from the Department of Oceanography, University of Washington.

References

Anderson, J. J. & Devol, A. H. (1973) Deep water renewal in Saanich Inlet, an intermittently an&c basin. Estuarine and Coastal Marine Science I, I-IO.

Smithsonian Miscellaneous Collections (I 944) World Weather Records 79, 530-53 I ; 90, 220-221.

U.S. Department of Commerce, Environmental Sciences Services Administration, Environmental Data Service (1969, 1970). Monthly Climatic Data for the World 22, I ; 12; 23, 1-8.