Effectiveness of various counting methods in detecting viable phytoplankton

Abstract Comparisons of counts from 14 C microautoradiographs of photosynthetically active natural phytoplankton populations with counts from Utermöhl or membrane‐filter methods often reveal significant discrepancies in biomass estimates; the viability of armoured species such as diatoms cannot be determined effectively by the latter two methods. Furthermore, nanoplankton < 5 μ m in diameter are difficult to distinguish from detrital material, especially when Utermöhl methods are used at routine magnifications. Considering that nanoplankton commonly account for between 20% and 50% of total phytoplankton biomass in various lakes, current methods tend to overlook a major segment of the primary producers. In overestimating viable diatoms, dinoflagellates, and armoured green algae by as much as 50% and underestimating nanoplankton by a similar magnitude, conventional counting methods may be inadequate to describe true phytoplankton biomass. These discrepancies can be discovered and corrected by applying microautoradiography.