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The role of the Dardanelles outflow on the The role of the Dardanelles outflow on the biogeochemicalbiogeochemical

features of the North Aegean waters.features of the North Aegean waters.

Christina Zeri and Helen KaberiChristina Zeri and Helen Kaberi

Hellenic Centre for Marine ResearchInstitute of Oceanography

P.O. Box 712, Anavyssos 19013, Greece

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Although both the Black sea and the Mediterranean face similar environmental problems they have remarkably different physical and biogeochemical characteristics.

The Black Sea in particular has been seriously damaged during recent decades especially along the north west shelf where the Danube outflow represents 77% of the total discharge and is the main source of nutrients and pollutants

A two layer flow dominates the water exchange pattern in the two straits and the Sea of Marmara.The current flowing from the Black Sea to the Mediterranean has long been known to sailors and was described by the Apolonios in the third century B.C. The deep counter current was discovered in 1680 by Marsigli.

Over the past decade major advances have been made in the understanding of the water circulation and mixing patterns, of the biogeochemical fluxes of various substances and of the trophic conditions in each sea, through national and EU funded proiects such as MATER, KEYCOP, INTERREG Ietc.

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6000m

5000m

4000m

3000m

2000m

1000m

500m

20°E 25°E 30°E 35°E 40°E

35°N

40°N

45°N

Oc

ea

n D

ata

Vie

w

Region of interest Black Sea

LevantineIonian

Sam othraki Plateau

Lem

nos

Pla

teau

Evros river

Nes

tos

ri

ver

Str

ymo

n r

iver

Dardanelles outflow

Greece

TurkeyNorth

Aegean T

rough

3 8

3 9

4 0

4 1

2 3 2 4 2 5 2 6 2 7

Sea ofMarm ara

North Aegean Sea

Lem nos isl.

0

500

1000

1500

2000

(m )

Recent research has shown that

the biogeochemistry of the North

Aegean depends largely on the

characteristics of the

Dardanelles outflow.anelles

occur in late spring and summer

The river runoff contibutes approx. 8km3y-1 and the surface brakish water outflowing from the Dadanelles straits vary between 100 and 1200km3y-1.

Maximum river flow occurs from February to May whilst the largest outflow from the Dardanelles occur in late spring and summer

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The freshwater inputs, in combination with the seabed morphology, form a specific water column structure in the area. Three major water bodies can be identified:

Black Sea Water (BSW) originating from the Dardanelles Straits, with low salinity (<30 psu) and low temperature values (8-22C); this is detectable as a surface water body (0-50 m). This distinct surface water mass is modified by mixing with warm and saline waters of Levantine origin.

Levantine Intermediate Water (LIW) (28.0< < 29.2) occupies the whole area, at depths greater than 50 m and less than ~ 400 m.

Below 400 m depth, water circulation is restricted; this is due to the bottom morphology and the various basins are filled with saline dense waters ( >29.2), referred to as North Aegean Deep Water (NaDW).Deep water formation has not taken place since 1993

The deep water formation depends not only on atmospheric conditions but also on the hydrological characteristics of the surface layer of BSW which are directly related to the magnitude of the Dardanelles outflow

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M arch 97Sept. 97

32.0 34.0 36.0 38.0 40.0

Salinity (psu)

8.0

12.0

16.0

20.0

24.0

Te

mp

era

ture

(°C

)BSW (surface w aters)

LIW(m iddle w aters)

D W(bottom w aters)BSW (surface w aters)

Salinity vs Temperature in North Aegean waters

Three water masses are distinguishable

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The nutrient exchanges at the two straits follow the water exchange pattern, so that the inputs from the Marmara to the Aegean are 4-5 times the exports via the underflow, indicating a net mass transfer from the Black Sea towards the Mediterranean .

It is of great interest, that although inorganic nutrients are supplied to the Black Sea via rivers or the atmosphere, the increased inputs to the northern Aegean via the Dardanelles outflow concern the organic forms of nitrogen (DON) and phosphorus (DOP).

The dissolved inorganic forms of nitrogen and phosphorus fluxes imported to the North Aegean through the Dardanelles are less important than it was believed in the past.

The processes controlling transformations of nutrients from inorganic to organic forms are not yet clearly understood.

The Eastern Mediterranean has been characterised one of the most oligotrophic areas of the world. Nevertheless, the recent scientific results have shown that within the Eastern Mediterranean basin, a slight oligotrophy gradient exists along the N-S direction, with the North Aegean waters being relatively more productive than the South Aegean waters.

The significantly higher total organic carbon (TOC) values observed in the low salinity upper layer of the North Aegean further confirm the above

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KA1

KA2

KA3

KA4KA5KA6

KA7

KA8

KA9

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

Depth (m)

Lem nos isl.

Dardanelle

s str.

Im vros isl.

Sam othraki is l.

0 20 40 60 80 100 120

D istance [m ]

-300

-250

-200

-150

-100

-50

Dep

th [

m]

KA-9 KA-8 KA-7 KA-6 KA-1KA-2KA-4

Septem ber 1999S igm a-Theta

20.0 21.0 22.0 23.0 24.0 25.0 26.0 27.0 28.0 29.0

0 10 20 30 40 50 60-300

-250

-200

-150

-100

-50

23.5 24.0 24.5 25.0 25.5 26.0 26.5 27.0 27.5 28.0 28.5 29.0

M arch-April 2000S igm a-Theta

KA-6 KA-1KA-2KA-4 KA-3

0 20 40 60 80 100 120

D istance [m ]

-300

-250

-200

-150

-100

-50

Dep

th [

m]

KA-9 KA-8 KA-6 KA-1KA-2KA-4

Septem ber 1999TO C [μm ol/L ]

50 75 100 125 150 175 200 225 250

0 10 20 30 40 50 60-300

-250

-200

-150

-100

-50

70 80 90 100 110 120 130

M arch-April 2000TO C [μm ol/L ]

KA-6 KA-1KA-2KA-4 KA-3

North-South section at the Lemnos plateaux.

Vertical distribution of density (sigma-theta) and total organic carbon (TOC)

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It is well documented that studied areas receive increased loads of pollutants from terrestrial sources; In addition, pollutant transport in an inter-basin scale is a subject that receives interest, especially in the Black Sea – North Aegean system.

Little is known about the dissolved trace metal concentrations in the Dardanelles Straits and the Sea of Marmara. However, it is known that the surface waters of the Black Sea, which outflow to the North Aegean, are relatively enriched in trace metals.

In the North Aegean, the water outflow from the Dardanelles Straits dominates over the local riverine inputs. BSW acts as a source of metals in the North Aegean Sea.

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0.00

1.00

2.00

3.00

4.00

WINTER SPRING SUMMER

Cu (nM)

0.00

2.00

4.00

6.00

8.00

WINTER SPRING SUMMER 0.00

5.00

10.00

15.00

WINTER SPRING SUMMER

Ni (nM) Mn (nM)

0.000

0.050

0.100

0.150

WINTER SPRING SUMMER

σ-θ<28.0 28.0<σ-θ<29.2 σ-θ>29.2

Cd

(nM)

Dissolved trace metal concentrations

in the three water masses of the North Aegean

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Winter

R2 = 0.3y = -0.38x + 16.71

0

2

4

6

29 31 33 35 37 39

Salinity (psu)

Cu

(n

M)

Summer

R2 = 0.4y = -0.74x + 33.00

0

5

10

15

29 31 33 35 37 39

Salinity (psu)

Ni (

nM

)

Summer

R2 = 0.2y = -1.63x + 70.62

0

10

20

30

29 31 33 35 37 39

Salinity (psu)

Mn

(n

M)

Since the surface layer is a mixture of riverine water, BSW and LIW, trace metal relationships with salinity will show to what extent water mixing affects their distribution. In Figure trace metal concentrations are plotted against salinity in the surface ( <28) mixed layer.

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The relationship of figure suggests that waters with low salinity and high metal concentration, such as river water and BSW, mix conservatively with high salinity – low metal concentration waters of Levantine origin.

These relationships are in accordance with the hydrography of the area: BSW, rich in trace metals, enters the North Aegean from the east and mixes, as it flows northward or southward, with LIW poor in trace metals coming from the south. Riverine waters, with high metal contents, enter the system from the north and meet the already modified (BSW + LIW) water mass moving northwards.

Whether other pollutants such as organic compounds behave near conservatively during the mixing process of Black Sea waters with the Levantine ones has not been investigated so far.

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The information gathered so far, concerning the impact of the Dardanelles outflow

on nutrient availability and pollutant transport in the North Aegean Sea, has

revealed important questions that remain unanswered..

•Why is the input of inorganic nutrients limited, although a considerable

volume of waters originating from the highly eutrophic Black Sea, enter the

North Aegean?

• What are the biogeochemical processes that control the alterations from the

inorganic to the organic forms of nutrients?

•Do pollutant loads from the Black Sea reach the North Aegean unmodified?

The answers to the above questions will fill important knowledge gaps, crucial for

managing future human impact on the Mediterranean Sea.

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THANK YOU!!