1.3 Aquatic Biomes

Water biomes aren't defined by rainfall the way land biomes are. Instead they differ in a few key factors.

• Salinity: how much salt is dissolved in the water. Ocean water averages about 35 parts per thousand; fresh water has almost none. Brackish water is in between.
• Depth: sunlight only reaches the top layer of water, called the photic zone. Below it, in the aphotic zone, there isn't enough light for photosynthesis.
• Turbidity: how cloudy the water is. Cloudy water blocks light and lowers photosynthesis.
• Flow: fast streams are well mixed and high in oxygen; still water can become low in oxygen.
• Temperature and nutrients: cold water holds more dissolved oxygen, and nutrient-rich water supports more algae.

Rivers and streams carry water downhill, often with high oxygen near their source and more sediment and nutrients farther downstream. Lakes and ponds are standing bodies of fresh water. A lake has zones: the shallow littoral zone near shore, where rooted plants grow; the open, sunlit limnetic zone, where algae (phytoplankton) live; the deep, dark profundal zone; and the benthic zone along the bottom, where decomposers break down dead material.

Freshwater wetlands, like swamps, marshes and bogs, are areas where soil is saturated with water for at least part of the year. They store floodwater, filter pollutants and give habitat to many species. Rivers and lakes also matter to people as a major source of drinking water.

The open ocean covers most of Earth's surface. It has low productivity per square meter because nutrients are scarce in the sunlit surface, but because it's so huge, its algae produce a large share of the world's oxygen, roughly half of all the oxygen made by photosynthesis.

Coral reefs grow in warm, shallow, clear tropical water. Corals live in mutualism with tiny algae inside their tissues: the algae make food by photosynthesis and the coral gives them shelter and nutrients. That's why reefs need clear, sunlit water. Reefs have very high biodiversity.

Estuaries are where a river meets the ocean, so fresh and salt water mix into brackish water. Rivers bring in nutrients, which makes estuaries extremely productive and important nurseries for young fish and shellfish. Salt marshes are estuary wetlands with grasses, found in temperate areas. Mangrove swamps are their tropical version, with salt-tolerant trees whose tangled roots protect coasts from storm surges and erosion.

The intertidal zone is the strip of shore between high and low tide. Organisms there, like barnacles and mussels, must survive being covered by water and then exposed to air twice a day.

Productivity in water depends mainly on light and nutrients. Shallow coastal areas like estuaries, salt marshes, mangroves and reefs have both: sunlight reaches the bottom or near it, and nutrients wash in from land or are recycled quickly. The deep open ocean has plenty of water but little light below the surface and few nutrients near the top, so it is far less productive per unit area.