Much light gets reflected at the surface

Seawater has a freezing point of about -1.8 °C (28.8 °F). When its temperature becomes low enough, ice crystals form on the surface. These break into small pieces and coalesce into flat discs that form a thick suspension known as frazil. In calm conditions this freezes into a thin flat sheet known as nilas, which thickens as new ice forms on its underside. In more turbulent seas, frazil crystals join together into flat discs known as pancakes. These slide under each other and coalesce to form floes. In the process of freezing, salt water and air are trapped in the interstices between the ice crystals. Nilas may have a salinity of 12–15 ppt, but by the time the sea ice is one year old, this falls to 4–6 ppt. The air remains and may support an ice ecosystem which includes viruses, bacteria, protozoa, algae and fungi.[19]

The amount of oxygen found in seawater depends primarily on the plants growing in it. These are mainly algae, including phytoplankton, but also include some vascular plants such as seagrasses. In daylight the photosynthetic activity of these plants produces oxygen, which dissolves in the seawater where it is used by marine animals. At night, photosynthesis stops, and the amount of dissolved oxygen declines. In the deep sea, where insufficient light penetrates for plants to grow, there is very little dissolved oxygen. In its absence, organic material is broken down by anaerobic bacteria producing hydrogen sulphide.[20]

Seawater is slightly alkaline and during historic times has had a pH of about 8.2. More recently, increased amounts of carbon dioxide in the atmosphere have resulted in more of it dissolving in the ocean forming carbonic acid and have lowered this pH level to 8.1. The pH is expected to reach 7.7 by the year 2100, an increase of 320 percent in acidity in a century.[21] One important element for the formation of skeletal material in marine animals is calcium, but this is easily precipitated out in the form of calcium carbonate as the sea becomes more acidic.[22] This is likely to have profound effects on certain planktonic marine organisms because their ability to form shells will be reduced. These include snail-like molluscs known as pteropods, single-celled algae called coccolithophorids and foraminifera.[21] All of these are important parts of the food chain and a diminution in their numbers will have significant consequences.[21] In tropical regions, corals are likely to be severely affected by a lack of calcium and this will adversely impact other reef dwellers.[21]

The amount of light that penetrates the sea depends on the angle of the sun, the weather conditions and the turbidity of the water. Much light gets reflected at the surface, and red light gets absorbed in the top few metres. Yellow and green light reach greater depths, and blue and violet light may penetrate as deep as 1,000 metres (3,300 ft) under ideal conditions. There is insufficient light for photosynthesis and plant growth beyond a depth of about 200 metres (660 ft).[23] The Blue Grotto at Capri demonstrates this absorption of short wave light. As it passes through the water, red light is selectively absorbed, leaving the cavern bathed in a blue glow emanating from the large underwater entrance.