1.3 Aquatic Biomes

Freshwater biomes vary by depth and flow

Freshwater biomes include lakes/ponds (lentic) and rivers/streams (lotic). Lakes have zones: the littoral zone (shallow, plant-rich edge), limnetic zone (open surface water with sunlight), profundal zone (deep, low light), and benthic zone (bottom sediments). Rivers and streams show changes from fast, cold, oxygen-rich headwaters to slower, warmer, sediment-heavy waters downstream.

Marine biomes are defined by light and depth

Oceans divide into zones: the sunlit photic zone (supports photosynthesis) and the dark aphotic zone below. Horizontally, the intertidal zone is exposed at low tide, the neritic zone covers the continental shelf, and the oceanic zone is open ocean. Coral reefs and estuaries (where fresh and salt water mix) are highly productive marine ecosystems.

Salinity and nutrients shape productivity

Salinity (salt concentration) distinguishes freshwater (<1%), marine (~3.5%), and brackish (estuaries) systems. Turbidity (water clarity) and nutrient levels affect light penetration and productivity. Upwelling zones, where cold nutrient-rich water rises, and estuaries are among the most productive aquatic ecosystems due to abundant nutrients and light.

Key terms

Littoral zone

Shallow, nutrient-rich edge of a lake where rooted aquatic plants grow.

Photic zone

Upper ocean layer that receives enough sunlight for photosynthesis to occur.

Estuary

A highly productive ecosystem where freshwater from rivers mixes with salty ocean water.

Upwelling

The rising of cold, nutrient-rich deep ocean water to the surface, boosting productivity.

Lentic vs. lotic

Lentic = still water (lakes, ponds); lotic = flowing water (rivers, streams).

Try an AP-style question

Why are estuaries so productive?

1. They receive no sediment from rivers
2. They are deep, cold, and dark
3. They have extremely high salinity
4. River nutrients mix in shallow, sunlit water