Average plant biomass is the average weight of several samples of fresh, live aquatic plants growing in one square meter of a lake's area. This measurement is taken separately for each category of plants: submersed, floating-leaved, and emersed. Measurements of average plant biomass are commonly used to help assess a waterbody's overall biological productivity and to assess the abundance of the different categories of aquatic plants present in a waterbody.

When LAKEWATCH staff measures aquatic plant biomass, they use the following procedure:

Emergent plants are collected to calculate "average plant biomass."

• Between 10 and 30 evenly-spaced sampling locations are chosen around the waterbody;

• At each sampling location, sampling is performed in three zones -- the emergent plant zone, the submersed plant zone, and the floating-leaved plant zone;

• At each site, a square frame (one-quarter of a square meter in area) is tossed onto the water and all above ground aquatic plant material that falls inside the frame is harvested;

  

• Harvested plants are whirled around in a net bag to spin off excess water. The plants are then weighed on a scale that measures kilograms or grams;

• To calculate the biomass that would be in one square meter, the weight is multiplied times 4; and

• Values from all the sites for each category of aquatic platns are averaged separately.

Submersed plants and floating-leaved plants are collected. Once collected they are spun to shake off excess water before being weighed.

The recorded weights are referred to as wet weights, because the plants are not allowed to dry out internally (like dried flowers). Plant biomass is also refered to as fresh plant weight and is reported by Florida LAKEWATCH in units of kilograms wet weight per square meter (abbreviated kg wet wt/m² ).

Some professionals weigh plants that have been allowed to dry internally. Others include the weight of plant roots in their biomass measurement. LAKEWATCH only uses the wet weight of above-ground plant biomass because no expensive equipment is required and the results are just as useful for LAKEWATCH purposes.