Important part of pelagic environment is its upper layer, shallower than 150 m which contains the biggest biomass of primary producers – phytoplankton (photic zone). Phytoplankton develops in regular annual cycles which depend on certain physical, chemical and biological parameters. The organic matter in the sea is represented by carbon compounds. Organic matter in the sea can originate from other ecosystems (alochtonous) or it can be produced by primary producers (autochtonous). The one that is assimilated in epipelagic zone is vital for all marine ecosystems, especially for those on the bottom of the ocean. Pelagic system has mechanisms for recycling and vertical export of organic matter. It is conducted by aggregation and sedimentation of organic matter or through different food webs. Aggregation of organic metter and its sinking is taking place after large diatom blooms, and it's also called „marine snow“. „Marine snow“ is created by sticky diatom exopolimers which act as centers of aggregation. Faecal pellets of zooplankton are also very important way of vertical transport. They contain non-digested organic matter. Microbial decomposers recycle dissolved organic matter and return it into food web.