An optical detection system for the study of fine scale vertical displacement of microalgae in an artificial water column

A proper knowledge of the vertical organization of the phytoplankton is of fundamental importance for our understanding of the functions of pelagic ec osystems. Essential in this context is the existence of vertical gradients in environmental parameters. However, little is known about how the fine ve rtical structures of phytoplankton species are formed and maintained. In si tu study of phytoplankton is biased by the fact that submersing instruments can disturb or even destroy the fine vertical gradients in species composi tion and/or cell numbers. We have designed and constructed an optical instr umentation system by which fine-scale vertical displacements of microalgae can be studied in an artificial water column without influencing fine physi cal, chemical, and biological structures of the water column. This enables us to find out more about the fine-scale behavioral responses of microalgae to vertical gradients in environmental parameters. We describe the main sy stem, present some test results, and conclude that our optical system is ab le to reveal fine-scale vertical displacements of microalgae in an artifici al water column and that the system can detect differences in cell densitie s down to 100 cells.ml(-1).