ABSTRACT
Phototrophic picoplankton, or picophytoplankton, was a little known part of the marine and freshwater phytoplankton prior to their discovery as an abundant and ubiquitous component of many plankton communities. Epifluorescence microscopy has evolved into a relatively routine method for quantifying the abundance and biomass of phototrophic picoplankton in both marine and fresh waters. By utilizing the natural autofluorescence of chlorophyll and phycobilin photosynthetic pigments, phototrophic cells were easily identified among the heterotrophic picoplankton. This chapter describes the most common methods used to handle and enumerate picophytoplankton by autofluorescence microscopy. It discusses many of the common pitfalls and technical refinemenis. Different combinations of excitation filters and dichroic mirrors are used to isolate specific wavebands of light and excite the photopigments characteristic of different classes of phytoplankton. Polycarbonate membrane or aluminum oxide filters are normally used for picoplankton enumeration because cells can be concentrated into a flat field of view for easy microscopic analysis.
