lesson
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If you drink a large bottle of water right before stepping on a scale, your weight instantly jumps by two pounds—even though you haven't grown a single new cell.
In ecology, we face this exact puzzle when tracking how much energy and matter move through food webs.
Wet Mass vs. Dry Mass
Biomass is the total mass of organic material in an organism. We can measure it as wet mass (the organism weighed alive, including water) or dry mass (the mass after all water has been removed).
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Wet mass is quick and does not harm the organism, but water content changes constantly based on recent rainfall, humidity, drinking, or transpiration.
Because water contains no organic energy, two plants with identical wet masses could contain completely different amounts of biological tissue.
How do ecologists remove all that fluctuating water without accidentally destroying the living tissue they want to measure?
Measuring Dry Mass
To find dry mass, scientists place a biological sample into a low-temperature drying oven, typically set between 80∘C and 100∘C, to evaporate the water.
The sample is weighed repeatedly at regular intervals until the reading stops decreasing—a state known as reaching a constant mass.
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A common mistake is turning the oven up to extreme temperatures like 200∘C to speed up the process.