PRESERVATION OF MARINE PLANKTONIC CILIATES - LOSSES AND CELL SHRINKAGE DURING FIXATION

For enumeration of microzooplankton (20-200 mum size fraction), includ ing planktonic ciliates, water samples are usually fixed and preserved , then concentrated by sedimentation in Utermohl chambers and examined with an inverted microscope. However, losses of ciliates may occur du ring fixation and handling, and fixation may shrink cells. Estimates o f abundance with several commonly used fixatives were compared in samp les from the North Atlantic and subarctic Pacific Oceans and in sample s from cultures. Buffered formaldehyde has the advantage that it allow s epifluorescence microscopy to be used, but, on average, ciliate coun ts from the North Atlantic were 56 % higher (95 % CI 30 to 82 %) in sa mples fixed with 10 % acid Lugol's solution than in samples fixed with 2 % formaldehyde. Ciliate counts from the subarctic Pacific were 23 t o 49 % higher in samples fixed with 10 or 20 % acid Lugol's solution t han in samples fixed with 5 % acid Lugol's solution. Fixation with 10 or 20 % acid Lugol's solution results in significantly higher cell cou nts than fixation with formaldehyde or dilute acid Lugol's solution, b ut shrinks cells severely and often distorts their morphology. Bouin's solution yields cell counts that are usually similar to those with 10 % acid Lugol's solution, but with less shrinkage. No single fixation method is ideal for all purposes; fixative-specific and assemblage- or taxon-specific correction factors are necessary for accurate estimate s of cell numbers and cell volumes/biomass.