Here's why:

* High Nutrient Input: Estuaries receive a constant influx of nutrients from both freshwater rivers and the saltwater ocean. These nutrients fuel the growth of phytoplankton, the base of the food web.

* Sunlight Penetration: Estuaries often have shallow waters, allowing sunlight to penetrate and support photosynthesis by phytoplankton.

* High Biodiversity: The unique mix of freshwater and saltwater creates a diverse habitat, supporting a wide array of species, from fish and shellfish to birds and mammals.

While other aquatic biomes, like coral reefs, are incredibly rich in biodiversity, estuaries are often considered the most productive because of their exceptional nutrient availability and high rates of primary production.