Plankton Biomass Calculator

This calculator turns a microscope cell count from a small subsample into a full estimate of plankton density and carbon biomass for the water you actually sampled. The first step scales up: it multiplies the cells you counted by the ratio of total concentrated sample volume to subsample volume examined, giving the total cells present in the whole sample. Dividing that by the volume of seawater originally filtered through the net yields cell density in cells per liter (and, dividing by 1000, cells per milliliter) — the standard units used in oceanographic reporting. Biomass conversion then applies a two-step chain common in plankton ecology: average cell biovolume in cubic micrometers is multiplied by a carbon-to-biovolume conversion factor (defaulting to 0.11 pg C/µm³, the typical value for diatoms; dinoflagellates run closer to 0.22) to get carbon mass per cell, which is then scaled by cell density and unit-converted from picograms to micrograms.

The resulting µg C/L figure is numerically identical to mg C/m³, a unit equivalence worth knowing since both appear in the literature. A rough chlorophyll-a estimate is also derived by dividing carbon biomass by 50, the commonly cited carbon-to-chlorophyll ratio for phytoplankton — useful as a sanity check but not a substitute for direct chlorophyll extraction. Accuracy depends heavily on getting the subsample truly representative and on choosing a cell-volume estimate that matches the actual taxa present, since biovolume can vary by orders of magnitude between diatoms and dinoflagellates.