In an underwater ecosystem, important abiotic factors include salinity, temperature, depth, amount of dissolved gases, substrate type, pH, currents and light availability. Of all of these, light is the most important since producers like algae rely on the presence of light in order to capture solar energy and convert it to the chemical energy found in organic compounds like glucose. This is the basis of the underwater ecosystem's food chain. Water absorbs light and as the depth increases, eventually, there will be a depth without any light present. This is the aphotic zone. In the ocean, the photic zone reaches 650 feet below the surface and below this is the aphotic zone. Since light doesn't penetrate below this area, photosynthetic organisms won't live there either. Therefore, organisms in the greatest depths rely on particles of organic matter that sink from the surface for nutrients, and some can carry out chemosynthesis to meet their energy needs. The thermal properties of water are important because of the stratification seen in freshwater ecosystems--ice floats above the more dense liquid water below which keeps organisms alive in cold months. In marine systems, thermal energy absorbed by the sea helps to moderate the climate and weather globally. However, although light and heat energy are extremely important, a change in salinity, temperature, pH or any of the other factors listed can wreak havoc on the delicate balance operating in an ecosystem between the living (biotic factors) and their non-living environment. An example is coral bleaching. Due to warmer temperatures in the ocean, many coral lose their algae endosymbionts that they rely on. This causes their tissues to turn white and adds stress to the coral. They may eventually recover or possibly die. They are an important part of the ecosystems in warm tropical seas.