Water – The Liquid of Life

Shape and Structure of Water Molecule In the covalent bonds of a water molecule, the electrons are not being shared equally between the oxygen and the hydrogen. This unequal sharing of electrons makes water a polar molecule where the oxygen has a slightly negative charge and the hydrogen(s) have a slightly positive charge

Weak hydrogen bonds are formed between adjacent water molecules due to the difference in charges of the oxygen and hydrogen. Hydrogen bonds are stronger the lower the temperature and are broken as water goes from liquid to gas.

Properties Cohesion – water molecules “stick” together due to the weak hydrogen bonds between them Surface tension – water has the highest surface tension of all liquids. Cohesion of water molecules creates a thin “skin” and a strong boundary between air and water. Surface tension is highest at colder temperatures. Surface tension is critical for sea-surface organisms. Adhesion – water molecules “stick” to other substances

High boiling point (100°C) and high freezing point (0°C) causes most water to exist as a liquid at earth’s surface temperatures. Contrast: Ethanol BP 78°C and FP -114°C

Heat capacity – ability to resist rapid temperature changes (amount of energy required to change temperature). Heat capacity of pure water is 1 cal/g/°C, which is among the highest of liquids. Contrast: grain alcohol 0.23 cal/g/°C and copper 0.09 cal/g/°C Affects the earth’s climate and helps to maintain stable temperatures in the aquatic environments. Circulating water also transfers heat from warmer regions to colder areas and vice versa, helping to maintain the earth’s overall conditions.

Viscosity – the “thickness” of a fluid Viscosity – the “thickness” of a fluid. Viscosity is higher at colder temperatures How viscous the fluid is will effect the amount of energy organisms use to move through the fluid. Density – mass per volume of the substance. The density of pure water is 1 g/mL (g/cc, g/cm3) Salinity effects the density of water. The higher the salinity the higher the density. When solutes are dissolved in water the mass increases, but the volume remains the same...increasing the density. Average density of seawater is 1.02-1.03 g/cm3

Temperature also effects density Temperature also effects density. The colder the water is the more dense it becomes (water molecules are closer together and take up less space, therefore decreasing volume, but mass stays the same). Pure water is most dense at 4°C, if you continue to drop the temperature below that the molecules begin to spread out and the density rises…at 0°C the density of ice is 0.917 g/cm3

Variations in both temperature and salinity occur mostly at the surface and along coasts. This means that the density of the water in coastal and surface waters are more variable and leads to mixing Cold water is more dense and will sink…warm water rises Saltier water is more dense and will sink…..fresher water is less dense rises Changes in both salinity and temperature will cause water to flow and mix

Salinity – is the total amount of dissolved solids in water Salinity – is the total amount of dissolved solids in water. Salinity is expressed in parts per thousand (‰). The average salinity of the oceans is 35 ‰, which means there are 35 grams of dissolved salts in 1000 mL of water. Brackish water is a term used for water with salinities as low as 0.6 ‰ up to 30 ‰ Estuaries, mouth of rivers, etc. Salinity will vary the most at the surface where evaporation and precipitation influence the area, as well as near coastal regions where run-off from land will cause the salinity to decrease

Colligative properties of seawater – properties of a liquid that are altered by the presence of a solute Raised boiling point Decreased freezing temperature Ability to create osmotic pressure Electrically conductive Decreased heat capacity Slowed evaporation Strength of the colligative properties depends on the quantity of solute Natural freshwater has some solutes so can have colligative properties to some degree

Major Elements in Seawater The majority of seawater is WATER (965 grams of H2O in 1000 grams of seawater at average salinity) Chloride (Cl-) - 55.03% Sodium (Na+) - 30.59% Sulfate (SO4-2) - 7.68 % Magnesium (Mg+2) - 3.68 % Calcium (Ca+2) - 1.18% Potassium (K+) - 1.11% Bicarbonate (HCO3-) - 0.41% Other elements - 0.32%

Principle of Constant Proportions – no matter how much the salinity varies the proportions of the major elements/compounds do not change. Scientists can use this knowledge to measure one element and then calculate the total salinity using the proportion of that element. Most common element used for determining salinity using this method is chlorinity, since it has the greatest proportion.

Methods of Measuring Salinity Salinometer – measure the electrical conductivity Refractometer – measures the refraction angle light as it passes through the sample of seawater (saltwater bends the light more than freshwater) Hydrometer – measures the density of the sample CTD sensor – takes information on conductivity, temperature, and depth for analysis

Where do salts come from? Input Weathering River discharge Volcanic eruptions Hydrothermal vents Output Sea spray Biological processes – organisms take in certain elements to build skeletons, etc. Chemical processes – certain chemical reactions may form precipitates or deposits

pH of Seawater Average pH of seawater is 8.0, but varies with depth. pH is higher at the surface (8.2-8.4) and decreases with depth. The primary factor that effects pH is the amount of carbon dioxide that binds with water to form carbonic acid (H2CO3). At the surface CO2 is taken in by photosynthetic organisms, so the pH is higher. As you go deeper in the water column, photosynthetic organisms cannot survive (no light) so organisms that produce CO2 dominate, and the pH goes down (around 7.8).

Temperature Profile The average temperature of the oceans is about 3.5°C. The surface temperature fluctuates with heat from the sun (season and latitude) but the deep ocean remains constant. Thermocline – region of the water column where temperature changes rapidly with very little change in depth.

Salinity Profile The average salinity is 35 ‰ (PPT, or parts per thousand) The surface salinity will vary due to evaporation and precipitation and in coastal regions due to run-off Halocline – the region where salinity changes rapidly with depth

Density Profile Density increases with depth going from 1.025 g/cm3 to 1.028 g/cm3 Pycnocline – region where density changes rapidly with depth

Oxygen Profile Oxygen will be higher at the surface due to the large numbers of photosynthetic organisms producing O2 during photosynthesis. Oxygen drops significantly below the photic zone because there is no photosynthesis, yet lots of cellular respiration (use of O2 by organisms). This is referred to as the OMZ – oxygen minimum zone.

Carbon Dioxide Profile Carbon dioxide will be lower in the surface water due to photosynthetic organisms taking it in for photosynthesis. Carbon dioxide levels rise below the photic zone due to fact that there are no plants/algae consuming it, yet numerous organisms producing it through cellular respiration. Carbon dioxide levels are usually inverse of oxygen levels (Cellular respiration is opposite reaction to photosynthesis). When oxygen is high, carbon dioxide is low and vice versa.

Pressure Pressure exerted by the atmosphere at sea-level is 1 ATM (14.7 psi or 1013 mbar) For every 10 m (33 ft) you descend (in seawater) you add 1 ATM Pressure (ATM) = depth (m) +1 10