Algae
Algae is difficult to define, but it is a vital part of a swamp ecosystem. While it is found all over the world, in every sort of aquatic ecosystem, it is very common in swamps.
Most algaes float in the water, but some are attached to rocks or other plants. Algae can vary its pigment to adapt to the amount of sunlight it gets, which is helpful in the murky waters of a swamp. It can also reproduce sexually or asexually, meaning it does not die off easily, and there is an abundance of algae in swamps.
Algae is a major producer in swamps, perhaps the most important of the producers. It is mostly small fish that consume the algae, but because small fish are often eaten by bigger fish, amphibians, or mammals, the algae produces energy that is carried up for most of the food chain.