Aquatic biomes include both freshwater and marine biomes and are characterized by salinity and light penetration, unlike terrestrial biomes where temperature and precipitation are the primary factors. When we talk about salinity, we generally refer to the saltiness of the water. However, technically, salinity measures the dissolved solutes in a solution. This has a profound effect on osmosis and water balance in organisms. Water depth affects the availability of light and also influences pressure and temperature, but our current focus is on light. The upper regions of a body of water are considered part of the photic zone, which is the depth of water that receives sufficient light to support photosynthesis. As demonstrated in the illustration here, the photic zone is relatively shallow compared to other zones. Below the photic zone is the aphotic zone, which receives little to no sunlight. The deepest parts of the aphotic zone are referred to as the abyssal zone, which are in perpetual darkness and have no light penetration. This condition has fascinating effects on the organisms living there.

Aside from depth, the cloudiness of the water, technically referred to as turbidity, can also affect light penetration. Turbidity results from suspended particles in the water, making it appear cloudy. For example, when two bodies of water converge, you can see a distinct line of high turbidity caused by the abundance of suspended particles. Turbidity reduces light's ability to penetrate the water. Furthermore, we sometimes use the term littoral zone for areas of a body of water that are close enough to the shore that sunlight can penetrate all the way to the bottom, supporting photosynthesis in aquatic plants. The pelagic zone constitutes the open water of the sea, which includes parts of the photic and aphotic zones and is not near the shore or ocean bottom. Below the pelagic zone lies the benthic zone, often referred to as the seabed, which includes the sediment and some sub-surface areas. The benthic zone is rich in detritus, consisting of debris or waste, usually from dead organic matter. This zone is a critical area where many organisms feed on and recycle this organic matter. Let's go ahead and flip the page.