1. Water is much more than a passive medium in which life exists. Its unique chemistry with hydrogen bonds and high specific heat allow it to store latent heat and moderate global temperatures. Its movements transport heat energy from the equatorial region poleward into both hemispheres. Water is able to dissolve many compounds, including important nutrients.
2. Light. Most marine food chains begin with photosynthetic single-celled organisms (the phytoplankters) which are affected by daily and seasonal changes in light intensity and duration. Sunlight penetrates transparent water, but different wavelengths are absorbed at different depths. The longest wave lengths (reds and oranges) are absorbed in the first 50 ft of the water column. Most other wavelengths are absorbed in the next 130 ft. The shortest wavelengths (blues and violet) penetrate the deepest, making the sea appear blue on a sunny day. (Actual depths vary with the clarity or murkiness of the water. The clearer the water the deeper light can reach.
When only one percent of the sunlight received at the sea surface remains, photosynthesis is only sufficient to maintain life. No growth or reproduction can occur. The depth at which this occurs is called the compensation zone. It represents the bottom of the euphotic zone. Below 3,000 ft there is no light.
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3. Pressure. On average, at sea level, the weight of the atmosphere exerts 14.7 lbs/in2 of pressure or 1 atmosphere. In the ocean, due to the added weight of water, pressure increases 1 atmosphere for every 33 ft of depth. On the deep sea floor pressure may reach more than 500 atmospheres.
4. Dissolved gases:
a. Oxygen, a product of photosynthesis, is greatest at the surface where the sea water is in contact with the atmosphere. The colder the water, the more oxygen it can contain.
b. Carbon dioxide is absorbed from the atmosphere, a process which may slow global warming. Its levels, however, may be lower in the euphotic zone due its use by algae and bacteria during photosynthesis.
c. Nitrogen must be fixed into nitrates before used by most marine algae. This accomplished by other microorganisms, such as cyanobacteria. Nitrogen is a major limiting factor in the sea.
5. Nutrients. Macronutrients in marine ecosystems include carbon, nitrogen, phosphorus, silicon, sulfur, potassium, and sodium. Micronutrients include iron, zinc, copper, manganese and some vitamins. Nitrogen is the most common limiting factor for algal growth; phosphorus the second most common.
6. Temperature varies with depth and with latitude. Infrared wavelengths (heat energy) of sunlight are absorbed in the top 3 ft of the water column. Waves mix the warmed water into the top 30 ft, the surface layer. Below this layer is a transition zone wherein temperature decreases rapidly with depth. This is the thermocline. Below the thermocline is the deep zone, where temperature decreases only slightly with increasing depth. Most seawater in the deep zone stays at about 37° F all year. Colder water (33° to 35.5° F) may be found at or near the seafloor.
space Due to its salt content, which acts as an antifreeze, sea water does not freeze until 28.5° F. As the freezing point is approached, the density of water decreases. Very cold water rises to the surface, where ice forms.
7. Salinity refers to the amount of dissolved matter (salts) in seawater. The Average is 35 ppt. Salinity varies geographically according to precipitation, discharge from rivers, and evaporation (a function of temperature). The formation of ice increases salinity in the unfrozen water. Generally salinity is highest in the surface layer, below which is a transition zone known as the halocline. Below the halocline, salinity remains fairly constant.
8. Density is a function of temperature and salinity. Warmer water is less dense than cooler water and floats upon it. Freshwater is less dense than seawater and so floats atop it. The lowest density usually occurs in the surface layer since particles tend to sink and the wataer is warmer than that below. The transition zone in which there is a rapid increase in salinity with depth is called the pycnocline. The pycnocline is a barrier to the exchange of nutrients between layers, but also helps keep phytoplankton in the sunlit waters near the surface. Wave action helps return sinking nutrients and phytoplankters to the surface.
9. Waves. A wave is actually just energy moving toward shore. The water molecules do not move laterally, but rotate up and down in circular orbits. Each orbit sets water below into motion to form a vertical chain of ever smaller orbits with less energy than the one directly above. The chain extends downward to a depth 1.2 times the wave height. Below that level, water is unaffected. Waves are generated by wind action.
As a wave approaches the shore, the friction created when orbits contact the sea bottom slows the base of the wave. The crest then gets ahead of the base and crashes. This creates a breaker, landward of which lies the surf zone. The energy remaining in the wave after the formation of a breaker raises the water level and thrusts the water upon the shore. Sand, gravels, shells, and other abrasive particles are picked up as the water moves onto land.
10. Tides are the product of the gravitational pull f the sun and, especially, the moon on the oceans. The shape and orientation of coastlines and their seafloors determine the frequency and the tidal range at a particular place. Most coasts experience two high tides and two low tides each day, but a few ̶ such as along the Gulf of California ̶ have only one high tide and one low tide per day.
