Is phytoplankton a producer
The phytoplankton is not a producer, but it captures sunlight through chlorophyll. This organism requires sunlight and nutrients like nitrate, phosphate and calcium in order to grow. For reference, go to http://earthobservatory.nasa.gov/Features/Phytoplankton/.
A producer of what? It is the bottom rung in the chain of life in the ocean. No plankton, no life, that simple.