1. Sea water composition Sea water has been defined as a weak solution of almost everything. Most of the ocean's salts were derived from gradual processes such the breaking up of the cooled igneous rocks of the Earth's crust by weathering and erosion, the wearing down of mountains, and the dissolving action of rains and streams which transported their mineral washings to the sea. Some of the ocean's salts have been dissolved from rocks and sediments below its floor. Other sources of salts include the solid and gaseous materials that escaped from the Earth's crust through volcanic vents or that originated in the atmosphere.

3. COMPARISON OF RIVER WATER AND SEA WATER COMPOSITION Average river Average sea River waterSea water Ionswater (mM/l) water (mM/l) ratio to Clratio to Cl HCO 3 − 0.862.385.3750.0044 SO 4 − 0.0728.20.431250.0517 Cl − 0.16545.011 Ca 2+ 0.3310.22.06250.0187 Mg 2+ 0.1553.20.93750.09761 Na + 0.23468.01.43750.8587 K + 0.0310.20.18750.0187 Salinity

5. THE DEFINITION OF UNESCO “The total amount of solid matter, expressed in grams per one kilogramme of sea water, when all the carbonates have been converted in oxides, the bromides and the iodides replaced with the chlorides and all the organic matter have completely been oxidized”.

6. MEASUREMENT METHODS Chemical method The classical method to assess the salinity chemically was defined by Knudsen. With this method we set the chlorine content of a given water volume; by means of it and through opportune numerical relations we finally obtain the salinity value. The chlorine ions Cl - (55%), that are easily achievable through the titration of AgNO 3, is used to determine the salinity of a sea water sample.

7. The relationship between the salinity and the chlorinity is expressed by: S ‰ = 1.80655 Cl ‰ Cl is the chlorinity of the sample, defined as the chlorides concentration, considering that bromides and iodides have been replaced by chlorides. The chlorinity is measurable by means of the titration and the salinity through replacement in the previous equation.

8. Physical method The electric conductivity is the most used parameter to assess the salinity by means of the physical methods. It is a measurement of the ions’ migration that depends on both the amount of the dissolved salts and on the temperature of the electrolyte at the moment of the measurement Pure water: It is not a good electricity conductor Sea water: It is a good electricity conductor In 1930 it has been observed that the electrical conductivity of sea water was proportional to its salinity.

9. An example of a conductivity cell, which is part of the multiprobe

10. Remember: Nowadays salinity is not more expressed as 35.6 ‰, which was linked to chlorinity but only as 35.6 (PSU = practical standard unit), because it derives from conducibility, and thus it is dimensionless

11. In coastal and transitional cold temperate environments salinity ranges are very large. Usually at the surface there is a relevant minimum in autumn, at the bottom salinity is more stable(data from the Gulf of Trieste) <-----climatology

12. Salinity in the lagoon is linked to both salty water intrusions via tidal motion and fresh water discharge from the land. In the figure mean salinity distribution in the Lagoon of Venice, higher near the mouths that connect the Lagoon to the sea

13. In the figure it is shown the main isohalines in the west part of the Grado and Marano Lagoon (northern Adriatic Sea) in the 90 ’ s

14. On the entire Grado and Marano Lagoon salinity ranges are very wide from almost fresh water values near the rivers ’ input to a marine value at the main mouths. There is also a strong east to west decreasing trend

15. Density The density of an object is the ratio of its mass to its volume. Density can also be expressed as specific gravity, which is the ratio of the density of a material to the density of water, where the density of water in SI units is 1000 kilogram per cubic meter. The density of seawater varies with temperature and salinity of the water. As temperature increases, density decreases. As salinity of the water increases, density also increases. Although the density of seawater varies at different points in the ocean, a good estimate of its density at the ocean's surface is 1025 kilogram per cubic meter. Its specific gravity is therefore 1.025 Link to a density calculator http://www.es.flinders.edu.au/~mattom/Utilities/density.html

16. The density of sea water depends upon three variables: temperature, salinity, and pressure. These are indicated by designating the density by the symbol ρ s, ϑ, p, but, when dealing with numerical values, space is saved by introducing σ s, ϑ, p which is defined in the following manner: σ s, ϑ, p = ( ρ s, ϑ, p -1) 1000 Thus, if ρ s, ϑ, p = 1.02575, σ s, ϑ, p = 25.75.

17. Note that as salinity increases freezing point decreases: salty water freezes at - 2 °C

18. Example of T-S diagram in the Gulf of Trieste.

19. Climatology of the Gulf of Trieste

20. Dissolved gasses in seawater The gasses dissolved in sea water are in constant equilibrium with the atmosphere but their relative concentrations depend on each gas' solubility, which depends also on salinity and temperature. As salinity increases, the amount of gas dissolved decreases because more water molecules are immobilised by the salt ion. As water temperature increases, the increased mobility of gas molecules makes them escape from the water, thereby reducing the amount of gas dissolved. Inert gases like nitrogen and argon do not take part in the processes of life and are thus not affected by plant and animal life. But non-conservative gases like oxygen and carbondioxide are influenced by sea life. Plants reduce the concentration of carbondioxide in the presence of sunlight, whereas animals do the opposite in either light or darkness.

21. Solubility of Oxygen in the water (table a) as function of partial pressure in each of the two compartments (atmosphere - water). The same for Nitrogen (table b). Usually gas concentration are expressed as % saturation = 100G G 1 a b where G is the observed concentration of the gas and G 1 is the saturation value corresponding to the temperature and salinity. Remember: N can be considered as a Conservative element in the ocean although it enters the biological cycles.

22. Temperature0°12°24° Chlorinity (‰) O 2 N 2 CO 2 O 2 N 2 CO 2 O 2 N 2 CO 2 ml/Lml/Lml/Lml/Lml/Lml/Lml/Lml/L ml/L 049.24 23.00171536.7517.80111829.3814.63782 1640.1 15.02148930.611.5698024.89.36695 2038.0 14.21143829.110.9994723.68.96677 As temperature increases solubility decreases as well as salinity increases solubility decreases. This means that diluted cold water can absorb more gasses.

24. Sampling Water is collected at discrete depths along the water column for oxygen, nutrient, chlorophyll and other biological analysis by means of sampling bottles. Sampling in the water column strategy mostly depends on column depth. If depth is around 20 m usually sampling depths are at least 4 (surface, 5m, 10m, near the bottom). Alternatively one can choose optical depths on the basis of light profile. If the water column is very shallow one or two depths can be sampled. Order of parameters sampled always starts with oxygen, which has to be sampled immediately, without forming bubbles, and total alcalinity (CO 2 ). Then dissolved organic compounds and microbiological parameters, followed by chlorophyll, phytoplankton, etc.

25. Dissolved Oxygen Analysis First devised in 1889, the Winkler method is considered the "gold standard" for measuring the concentration of dissolved oxygen in a sample of water. Through a series of chemical reactions, the O 2 combines with iodine to form a golden yellow chemical. Therefore each oxygen molecule is associated with an iodine molecule, and we can measure oxygen by measuring the iodine. When the iodine is neutralized by the addition of sodium thiosulfate, the golden color disappears, and we can determine how much iodine (hence oxygen) was in the sample. Once the water sample is collected, it is important to "fix" the sample immediately. Phytoplankton, bacteria, and other organisms in the sample can quickly change the oxygen content of the sample through photosynthesis and respiration.

28. In the past a burette was used and the point of color change was detected by eyes. Now we use this automatic device (Titrino)

29. Oxygen and CO 2 concentrations are strongly controlled by both photosynthesis and by the reverse process: respiration

30. Anoxia

31. Generally at 2 mgL -1 of Oxygen marine organisms flee from the zone (if they can!) at 1 mg L -1 who couldn ’ t escape dies.

32. Oxygen in the Gulf of Trieste: Usually oversaturated, occasionally at the bottom under-saturated, usually at the end of summer.

33. The top map is an aerial view of the Chesapeake with the head of the Bay on the left and the mouth on the right. The bottom diagram is a transect of the Bay with surface waters at the top. The red and orange areas indicate low dissolved oxygen conditions inhospitable to most species living in the Bay. Data gathered from bi-weekly monitoring cruises shows the spatial extent of low dissolved oxygen levels in the Bay’s mainstream.