Final answer:
The primary source of primary production in estuaries primarily comes from phytoplankton.
Step-by-step explanation:
In estuaries, most primary production comes from phytoplankton, which use photosynthesis to create organic compounds and form the base of the food web in these nutrient-rich environments.
Phytoplankton are tiny photosynthetic organisms that inhabit the upper layers of water bodies and are responsible for converting sunlight into organic compounds via photosynthesis. They form the basis of aquatic food webs, providing food for a variety of marine organisms, including zooplankton and small fishes.
Estuaries are dynamic environments where freshwater mixes with saltwater, creating a nutrient-rich habitat ideal for phytoplankton growth. Other producers like seaweeds and chemosynthetic bacteria also contribute to primary production, but to a lesser degree compared to phytoplankton. Factors such as well-oxygenated water, stable temperature, and the presence of silt contribute to the high productivity found in the neritic zone of estuaries.