Compared to terrestrial plants, autotrophs in aquatic ecosystems have a lower biomass because of the high rate of population turnover.
Production is defined as the amount of biomass synthesized per unit of time per unit area. Looking at terrestrial versus aquatic systems, there is little difference in the shape of the pyramids. Production by autotrophs is always greater than that of the herbivores, which is greater than that of the carnivores. However, when talking about biomass (the amount of living matter in a unit area or volume of habitat), these two systems are quite different. If we consider a terrestrial ecosystem, the biomass of plants and trees is far greater than the biomass of deer, moose, and other herbivores. In turn, the biomass of the carnivores that feed on these herbivores is smaller still. The sample biomass in terrestrial systems is a fairly close approximation of annual production, since terrestrial plants are generally larger, have fewer generations per year, and produce new tissue at a slower rate.
By contrast, aquatic systems project an inverse biomass pyramid. The production of aquatic primary producers can exceed their mean biomass by over 100 times, as compared to terrestrial systems. Remember that biomass is a static measurement (the mass at any given time), but infers nothing about how many days accumulation it represents. Tiny organisms, such as phytoplankton, tend to have high rates of reproduction and population turnover, thereby increasing production. However, since they are efficiently grazed, their biomass remains low.