Zooplankton Preservative
How Biomass is Calculated
Zooplankton biomass is expressed as dry weight — the mass of an organism after all water has been removed, typically in micrograms (µg) per litre of water sampled. Rather than physically drying and weighing each specimen, dry weight is estimated mathematically using length-weight regressions: taxon-specific equations that convert a body length measurement directly into an estimated dry weight.
At IdentaZoop, body lengths are measured from a video monitor connected to a digital microscope camera, with digital calipers linked to the processing computer via an Opto-RS232 cable. As each individual is examined under the microscope, it is assigned a ZEBRA2 species code and its body length is recorded — logging every measurement directly to that specimen. Once processing is complete, the raw ZEBRA2 output (.dbf files) is transferred to a second computer. There, custom Python scripts automate the population of client-specific Excel spreadsheets and apply the appropriate length-weight regression to each measurement, generating a dry weight estimate per individual. The accuracy of the resulting biomass data depends directly on which regressions are applied — and those regressions were established using formalin-preserved specimens.
Formalin vs. Ethanol: Effect on Biomass Accuracy
Species identification, density counts, and community composition data are unaffected by preservation method — ethanol-preserved crustacean zooplankton are morphologically suitable for species-level identification.
Biomass is a different matter. The length-weight regressions used to convert body measurements into weight estimates were established using formalin-preserved specimens. Formalin has long been the standard for this work because it fixes tissue proteins and preserves body dimensions reliably. Ethanol acts as a solvent — it dehydrates tissue and leaches body substances, causing significant dry weight loss that is not accounted for in formalin-based regressions. Applying those regressions to ethanol-preserved specimens therefore systematically overestimates biomass.
For ethanol-preserved samples where biomass is specifically required, processing can proceed — but clients are made fully aware of this limitation before committing to that approach. Where ethanol-specific length-weight regressions exist for the target species, those can be incorporated instead.
If accurate biomass is a project deliverable, formalin preservation is strongly recommended.
Preparing Formalin-Preserved Samples
The standard preservative for freshwater crustacean zooplankton is 2–5% formaldehyde — equivalent to 5.4–13.5% formalin. These two terms are frequently confused: formaldehyde is the chemical compound (HCHO); formalin is a saturated aqueous solution of formaldehyde gas, specifically 37–40% formaldehyde by weight. This means that 37% formaldehyde and 100% formalin describe the same solution. Confusing the two when preparing preservative leads to serious errors in concentration.
The preservative should be buffered with sodium borate or hexamine to prevent acidification over time, which can dissolve zooplankton carapaces. Glycerol or sucrose is added to stabilize specimen morphology and further protect carapace integrity during long-term storage. Samples submitted to IdentaZoop should be preserved at 2–5% formaldehyde using a buffered and sweetened solution.
Under Canadian WHMIS regulations, formaldehyde is classified as a confirmed human carcinogen. Preparation and handling require fume hood ventilation, appropriate respirators, and chemical-resistant gloves. Refer to the current MSDS for formalin for full handling requirements before preparing preservative in the field or lab.