What type of organism is primarily responsible for producing organic matter in aquatic ecosystems?

Phytoplankton are the primary producers in aquatic ecosystems, playing a crucial role in the process of photosynthesis. These microscopic organisms, which are predominantly found in the upper layers of oceans and freshwater bodies, convert sunlight and carbon dioxide into organic matter, forming the base of the aquatic food web. They are responsible for a significant percentage of the world's oxygen production and are critical in sustaining marine life.

In contrast, zooplankton are primarily consumers that feed on phytoplankton and other small organisms, while nekton encompasses larger, swimming animals like fish and marine mammals that rely on both phytoplankton and zooplankton for food. Benthos, on the other hand, refers to organisms living on or in the sediment at the bottom of aquatic environments, engaging mainly in decomposition and recycling of organic material but not in primary production.