This review has shown that while measuring primary productivity is a cornerstone of aquatic science, the raw data must be interpreted within a complex ecological framework. The numbers generated by these methods are not endpoints but rather starting points for understanding the intricate flow of energy through an ecosystem. Applying productivity data directly to simple ecological models, such as the trophic-dynamic concept of energy flow introduced by Lindeman (1942), is complicated by several factors.

Key challenges that disrupt simple, linear energy flow models include:

• The diversion of a significant portion of fixed carbon through extracellular products released by phytoplankton, which enters a bacterial pathway rather than the primary grazing food chain.
• The substantial energy inputs from allochthonous material—organic matter originating outside the aquatic system—which can be a major food source for consumers.
• The role of chemosynthesis as an additional, and often unmeasured, form of primary production that contributes to the ecosystem’s energy base.

Furthermore, a portion of primary production must be reserved for the maintenance of community structure, an energy cost sometimes referred to as “information.” There is indirect evidence that grazers may only have access to production in excess of this maintenance threshold. Consequently, a small increase in primary production can provide a relatively larger increase in the food supply available to grazers and the fish that consume them.

A sound understanding of the processes controlling primary productivity is not merely of academic interest; it is of “unquestioned importance for survival.” A clear grasp of these fundamental processes is essential for managing aquatic ecosystems, whether the goal is to increase global food production or to reverse the detrimental effects of eutrophication. Only by combining robust measurement with a sophisticated ecological understanding can we hope to manage the productivity of aquatic environments for the benefit of mankind.