Effectiveness of various counting methods in detecting viable phytoplankton

Abstract

Comparisons of counts from 14C 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. © 1978 Taylor & Francis Group, LLC.