It's kind of interesting how the plants compete. In spring and early summer, azolla usually dominates. Once the water lettuce and water hyacinth get going, the azolla starts to die off and is largely replaced by duckweed. Eventually even the duckweed starts clearing out, and the water lettuce starts looking kind of malnourished even as it (along with the water hyacinth) takes over every available surface.

I'm not sure what resource the plants are competing for. Phosphorous maybe?