Complex sentence: These organisms include drifting bacteria, archaea, algae, protozoa and animals that inhabit the pelagic zone of oceans, seas, or bodies of fresh water; that is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification.
Split into simpler sentences: These organisms include drifting bacteria, archaea, algae, protozoa and animals that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. Essentially, plankton are defined by their ecological niche rather than any phylogenetic or taxonomic classification.