<P> Biomass is the mass of living biological organisms in a given area or ecosystem at a given time . Biomass can refer to species biomass, which is the mass of one or more species, or to community biomass, which is the mass of all species in the community . It can include microorganisms, plants or animals . The mass can be expressed as the average mass per unit area, or as the total mass in the community . </P> <P> How biomass is measured depends on why it is being measured . Sometimes, the biomass is regarded as the natural mass of organisms in situ, just as they are . For example, in a salmon fishery, the salmon biomass might be regarded as the total wet weight the salmon would have if they were taken out of the water . In other contexts, biomass can be measured in terms of the dried organic mass, so perhaps only 30% of the actual weight might count, the rest being water . For other purposes, only biological tissues count, and teeth, bones and shells are excluded . In some applications, biomass is measured as the mass of organically bound carbon (C) that is present . </P>

What is biomass and how does it change between trophic levels