Giant phytoplankton revealed by in situ imaging

Abstract Deployment of an autonomous Zooglider in the western Mediterranean Sea revealed the presence of long phytoplankton chains attaining maximum lengths of 6.4–10.4 mm suspended in situ. Elongate chains included the diatoms Guinardia , Proboscia , two other diatom morphologies, and solitary filaments of the cyanobacterium Trichodesmium . Most elongate chains were distributed in the upper 100 m of the water column, overlapping the deep chlorophyll maximum, but some extended to depths of 400 m, suggesting occasional export into deep waters. Such elongate chains appear to be an unutilizable prey resource for most mesozooplankton grazers, including planktonic copepods and appendicularians. Elongate chains are likely to be widely distributed in the ocean, but their detection requires noninvasive measurement methods that do not disrupt the fluid environment or disturb suspended phytoplankton.