Marine: The Open Ocean
Most of our planet is covered in saltwater, home to several major biomes. The ocean itself divides into five zones that mirror lake environments. The intertidal zone teems with nutrients and biodiversity, while the coastal zone (similar to littoral) remains shallow and biodiverse.
The euphotic zone gets plenty of sunlight but has fewer nutrients. Below this, the bathyal zone receives minimal light, resulting in low productivity. At the deepest levels, the abyssal zone exists in permanent darkness with virtually no productivity except near hydrothermal vents.
In warm, shallow seas, coral reefs create underwater oases despite nutrient-poor water. Each coral consists of thousands of tiny polyps living in symbiosis with algae. During daylight, the algae provide nutrients through photosynthesis, while at night, the polyp's mouth captures drifting food—creating environments with remarkably high productivity and biodiversity.
Big Picture: Across all aquatic biomes, the key factors determining productivity and biodiversity are light availability, water depth, temperature, dissolved oxygen content, and nutrient availability. Understanding these helps you understand life in water!