Size distribution in ultraphytoplankton: a comparative analysis of counting methods

Area of Science:

• Marine microbiology
• Phytoplankton ecology
• Oceanography

Background:

• Accurate quantification of ultraphytoplankton is crucial for understanding marine ecosystem dynamics.
• Traditional size fractionation methods like sequential and separate filtration may introduce inaccuracies.

Purpose of the Study:

• To compare the effectiveness of different ultraphytoplankton size fractionation methods.
• To assess the accuracy of sequential filtration, separate filtration, and direct observation after DAPI staining.

Main Methods:

• Analysis of water samples from the Cretan Sea at three depth layers.
• Sequential filtration using 5, 1, and 0.2 micrometer pore size filters.
• Separate filtration using 5 & 0.2 micrometer and 1 & 0.2 micrometer filters.
• Direct filtration on 0.2 micrometer filters after DAPI staining and microscopic observation.

Main Results:

• Direct observation after DAPI staining yielded significantly higher total and group-specific (flagellates, cyanobacteria) abundances.
• Sequential and separate filtration methods underestimated total abundance by 25-50%.
• Size fraction estimates were imprecise to unreliable across different groups and size ranges.

Conclusions:

• Direct observation after DAPI staining provides more informative ultraphytoplankton estimates compared to filtration methods.
• Sequential and separate filtration methods have significant limitations and likely overestimate ultraphytoplankton abundance.
• Current methods for phytoplankton size fractionation may require re-evaluation for improved accuracy.