Open Ocean -- The Pelagic Zone

• Moving away from the coast, the availability of nutrients decreases rapidly.
• Nutrients become a limiting factor for life.
• The open ocean is sometimes referred to as a “marine desert” due to the relative lack of life here.
• Animals found here must be able to travel great distances to find food.

The Photic Zone

• Sunlight can penetrate the ocean to various depths depending on water clarity, sun angle, and cloud coverage.
• Generally, there is enough light for photosynthesis only in the top 200 meters (600 or so feet) of the ocean.
• Complete darkness generally exists below 1000 meters (3300 feet)

Deep Ocean: The Abyssal Zone

• The deep ocean is the least explored region of the ocean.
• Cold water, extreme pressure, and complete darkness are constant here.
• Nearly 90% of the volume of the oceans is in the deep ocean.
• Very few manned vehicles can dive into this region.
• The biology of organisms here is among the least understood on Earth.

• Also known as “marine snow”.
• A whale carcass can feed scavengers for as long as two years.

• Freshwater ecosystems have a very low salinity level.
• Include lakes, ponds, rivers, streams, and inland wetlands.

• The littoral zone is near the shore and contains shallow, sunlit waters. There is high biological diversity in this zone due to the presence of photosynthetic plants and algae.
• The photic zone, also called the euphotic or limnetic zone, is the part of a lake or ocean where the rate of photosynthesis is greater than the rate of respiration by phytoplankton. Most of the photosynthesis in the lake occurs here, producing the majority of the food and oxygen.
• Below the euphotic zone is the Profundal Zone or Hypolimnion (cold water region, also called the Aphotic zone). The profundal zone is located below the thermocline where the sunlight does not penetrate. Again, the size of this zone depends on the age and water clarity of the pond or lake. The profundal zone typically has lower fish populations because of the lack of oxygen during many parts of the year.
• The benthic zone is the bottom of the lake; the organisms found there depend on the depth at that area. The benthic zone is the pond or lakes digestive system. This is where bacteria decompose organic matter from dead algae, aquatic plants, and fish and animal waste. The benthic zone increases as the pond or lake ages.

• Oligotrophic lakes are generally very clear, deep, and cold. The lake substrate is typically firm and sandy. Nutrient levels are low, so the lake generally does not support large populations of aquatic plants, animals, or algae. The fish that occur in oligotrophic lakes are often low in abundance, but large in size. Many oligotrophic lakes divide into two layers in the summer, a condition known as stratification. The lower layer, called the hypolimnion, is cold and supports cold-water specialist fishes, like lake trout and cisco. These species require cold temperatures and high oxygen levels, so they remain in the lake’s lower level throughout the summer.
• Mesotrophic lakes contain moderate amounts of nutrients, and contain healthy, diverse populations of aquatic plants, algae, and fish. Occasional algae blooms may occur. If the lake is deep enough to stratify, the hypolimnion often becomes low in oxygen by the end of summer, and may result in some phosphorus release from the sediments.
• Eutrophic lakes are high in nutrients and contain large populations of aquatic plants, algae, and fish. The lake substrate is typically soft and mucky. The aquatic plants and algae often grow to nuisance levels, and the fish species are generally tolerant of warm temperatures and low oxygen conditions. Common fish species include carp, bullheads, and bluegills. If the lake is deep enough to stratify, the hypolimnion is usually very low in oxygen by mid-summer. This results in a release of phosphorus from the sediments, which can fuel algae blooms.
• Hyper-eutrophic lakes are very high in nutrients, and often exhibit large algae blooms, which may include dangerous levels of blue-green algae. Fish communities in hyper-eutrophic lakes are dominated by carp and other species that can tolerate warm temperatures and low oxygen conditions. Most hypereutrophic lakes are small impoundments of streams, and fed by large watersheds composed of urban and/or agricultural land uses.

Rivers, Streams and Creeks

• Oxygen levels decrease, but nutrient levels rise.

• The water continues to warm, oxygen levels decrease. The nutrients continue to increase.

• Freshwater mixes with saltwater, forming brackish water.