An apparatus for cultivation of phytoplankton and other autotrophic aquatic species in an aquatic medium in one embodiment comprises a plurality of liquid-impermeable transparent baffles, each forming an elongated trough-like enclosure, depending from a transparent top chamber, each baffle in communication with the top chamber, and two end walls for enclosing the top chamber and baffles. Introduction of water into the baffles sinks the apparatus in the liquid medium to the top of the baffles, leaving the top chamber above the surface of the medium. Light to which the top panel is exposed passes through the top chamber, the water in the baffles, and the baffles to encourage growth of selected microscopic organisms in the surrounding medium.