Seawater Chemistry - Atoms What are atoms? 1. Atoms are the fundamental units of matter. 2. They are composed of smaller parts: a. Nucleus i. Protons (positively charged) ii. Neutrons (have no charge) b. Electrons The different components of atoms determine the type and “behavior” of the elements…

Seawater Chemistry - Atoms

Seawater Chemistry - Molecules What are molecules? The smallest unit of a substance made up of two or more atoms bound to each other. Molecules can form by both ionic as well as covalent bonds…

Seawater Chemistry - Molecules In case of ionic bonds, atoms first either loose or gain an electron, and become either positively or negatively charged. Note that only certain types of atoms can do this. The ability of an atom to become positively or negatively charged (i.e. become an ion), as well as the level of the charge, depends on the characteristics of the atoms, particularly the number of electrons on their outer shells… Once ions are formed, positive and negative charges attract, creating individual molecules. In case of molecules formed by ionic bonds, many of these molecules can also come together to form more organized crystal structures / salts… Ionic Bonds:

Seawater Chemistry - Molecules Covalent Bonds: In case of covalent bonds, atoms share electrons on their outermost shells/orbits. This creates a physically more stable condition. Again, the ability to form covalent bonds, the number of the bonds, etc., depends on various characteristics of the atoms. Remember that covalent bonds are very strong…

Seawater Chemistry - Water Water is a polar molecule (part of the molecule is slightly positively charged and the other side is slightly negatively Charged). The polarity and ability to form hydrogen bonds gives water some very special characteristics, many of which are essential for maintaining life on earth as we know it… Water is a very special molecule, important to all biological systems. This polarity allows for formation of weak bonds between water (and other charged) molecules… these bonds are referred to as hydrogen bonds.

Seawater Chemistry – Water and Temperature Molecular structure of water changes as a function of temperature. In ice, hydrogen bonds maintain a crystal form made up of hexagonal structures. When ice melts, heat is absorbed by hydrogen bonds and they begin to break. There is no actual increase in the temperature. When all the ice has melted, additional heat causes temperature to rise, such that some molecules break free and evaporate… at 100 degrees Celsius, almost all hydrogen bonds are broken and water boils.

Seawater Chemistry - Water The hydrogen bonds formed by water molecules also give water a certain “stickiness” (very scientific eh?)… we therefore say that water has high cohesion. Cohesion on the surface of the water creates surface tension (resistance to being broken). These characteristics allow water to be carried to the top of trees, or for some organisms to float, swim, or “walk” on water.

Seawater Chemistry - Water Water can be a great solvent for both salts and certain biological molecules… How do you think this works? Water as solvent of salts Water as solvent for organic molecules

Seawater Chemistry So how is seawater different from “fresh-water”? It’s got salts in it!! On average, seawater has about 35 grams of salts per liter of water (i.e. salinity of 35 parts per thousand).

Seawater Chemistry What are the sources of the salts? 1. Physical and chemical breakdown of rocks 2. Hydrothermal vents and volcanoes Water runoffs from land are typically rich in positive ions whereas volcanoes and hydro- thermal vents are good sources of negative ions… Although this is evident very close to riverheads and hydro- thermal vents, in general the ocean water is very well mixed and no differences are seen… i.e. minerals and ions in the ocean maintain a constant ratio.

Seawater Chemistry Final notes on seawater salts… Although there is some variation in the salinity of seawater from one location to the next (30-40 parts per thousand), in general, open ocean water has relatively constant salinity (~34-36 ppt). Larger variations and fluctuations can and do occur in coastal areas… Although the total salts in the seawater may vary, the relative ratio of the different salts remain relatively constant. This is referred to as the “rule of constant proportions”. The density (weight/volume) of seawater is affected by its salt content, where water with higher salt content will have higher density than water with lower salt content. Note though that other factors such as temperature also play major roles in determining seawater density, and all the variable would need to be taken into account if one needs to determine the density of a mass of seawater…

Seawater Chemistry - Temperature Oceanic surface water temperatures are typically warmer in lower latitudes and colder in higher latitudes… In this satellite picture, red and orange represent warmer areas and the blue and purple represent colder regions…

Seawater Chemistry - Temperature Seawater temperature also varies as a function of depth…

Seawater Chemistry - Temperature The temperature profile associated with depth is not however the same in all Latitudes…

Seawater Chemistry - Temperature Depth/temperature profiles for seawater vary as a function of latitude… Note that the most distinct temperature layering and thermoclines are seen closer to the equator, and that there is very little temperature variation with depth as one approaches the higher latitudes.

Seawater Chemistry - Temperature In mid and low latitudes (temperate and tropical) one can observe a distinct permanent thermocline, whereas in high latitudes (polar) this thermocline is not observed as clearly. In case of the mid-latitudes, one can also observe a seasonal thermocline within the mixed layer of oceanic water…

Seawater Chemistry - Density Seawater density though is a dual function of both temperature and salinity.

Seawater Chemistry - Density Oceanic pycnoclines are also most apparent in equatorial regions and are almost non- existent in higher latitudes.

Seawater Chemistry - Pressure With increased depth, organisms living in the ocean need to adjust to the changes in pressure associated with the weight of the seawater above them. Remember that in some places, ocean depth can reach 10,000 meters or more!! More on this when we discuss deep ocean organisms… There is an increase of one atmospheric unit of pressure for every 10 meter increase in depth.

Seawater Chemistry - Light Although water is “transparent” light (electromagnetic radiation) can not penetrate very deep into the ocean. Light is essential not only for organisms in the surface to see each other (consider implications for predator-prey interactions), but also for photosynthesis and primary production within the ocean. The part of the ocean where light penetrates is referred to as the photic zone. In the clearest oceans, the photic zone may extend as much as 600 meters, but typically the depth is 100 meters or less (much less in coastal waters)…. (The area bellow the photic zone is referred to as the aphotic zone). Almost all of the photosynthesis in the ocean occurs in the top 100 meters; typically down to the depth where about 1% of light penetrates. Light is reduced as a function of depth because of 1. Scattering – light being bounced around water molecules, particles, organics, etc.; and 2. Absorption – light is absorbed by the molecules of water and is converted to heat… Why do you think there is a difference between light depth in coastal versus open oceans? How about between one coast and the next?

Seawater Chemistry - Light Light is made up of a range of wavelengths of electromagnetic radiation. The different wavelengths all travel different distances into the water depths… Note that blue light travels the furthest… This is why we typically see the ocean as blue. How do you think different organisms would look like at different depths?

Seawater Chemistry – Measuring Light Complicated equipment and light meters can be used to determine the amount of light penetrating the ocean, but for many cases, a secchi disc can give us all the information we need. More on light in the ocean later…

Seawater Chemistry – Dissolved Gases Check out the board!!