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 μm 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.

Importance quantitative de diff�rentes classes de taille phytoplanctoniques en milieu lacustre eutrophe

“…D'une part, les cellules de tres petite taille ne sont pas toujours denombrees au microscope inverse en raison, soit de leur destruction lors de la fixation des echantillons au lugol, soit de leur non-sedimentation dans les chambres de comptage, soit enfin, en raison de leur petite taille . De nombreuses etudes ont d'ailleurs mis en evidence une mauvaise estimation de la fraction 'nanoplanctonique' par cette methode, aussi bien en lacs ou reservoirs (Paerl, 1978 ;Boswell et al, 1980 ;Amblard, 1983 ;Elser et al, 1986) qu'en mer ou milieu estuarien (Mel'Nikov, 1977 ;Sinclair et al, 1979 ;Booth et al, 1982 ;Davis & Sieburth, 1982 ;Caron, 1983 ;Reid, 1983 ;Hewes et al, 1984). D'autre part, mais la 1'explication est sans doute de portee plus limitee, les teneurs cellulaires en chlorophylle a du `nanoplancton' sont sensiblement plus elevees que celles des cellules phytoplanctoniques de grande taille (Paasche, 1960 ;Eppley & Sloane, 1966 ;Munawar et al, 1978 ;Malone et al, 1979 ;Elser et al ., 1986).…”

Composition and biomass of phytoplankton assemblages in coastal Antarctic waters: a comparison of chemotaxonomic and microscopic analyses

“…According to our previous discussion, these flagellates could belong mostly to chrysophytes and prymnesiophytes, taxa that showed disagreement in the estimation by microscopy and chemotaxonomy, and in lower proportion to chlorophytes and cryptophytes that showed a high level of agreement among C and chl a biomass estimates. In general, light microscopy is considered inaccurate for the study of small forms, in particular phytoflagellates, due to problems in sample processing as well as cell enumeration and sizing (Paerl 1978, Hewes et al 1984, Kuosa 1988, Booth 1993, Bidigare et al 1996, Schlüter et al 2000. However, the good agreement found between the microscopic and pigment analysis for cryptophytes and chlorophytes allows us to state that phytoflagellate cells can be well preserved in iodine Lugol's fixed samples, and their enumeration and sizing can be accurately done using sedimentation chambers and an ordinary light microscope.…”

Seasonal changes in phytoplankton densities in four North Island lakes, 1973–74

“…The nanoplankton (1-5 ^m diameter) were largely missed in counts made from samples preserved in Lugol's iodine; they are easily confused with detrital particles (Paerl 1978).…”