Aquatic Primary Production

The ecosystem concept has been particularly useful and extensivelyemployed in the study of aquatic primary productivity. The flowof energy through the system is an attractive area of investigationwhen it involves some process, but has a more restricted valuewhen units of biomass are simply converted to calories. Althoughwe are able to measure primary productivity in terms of thecarbon fixed, we are not yet able to measure the actual changein the oxidative state of the newly fixed carbon. The fate ofphotosynthate as food for higher trophic levels is thereforedependent upon a considerable array of biological and environmentalvariables. Primary productivity is considered in terms of itsevolution from measures of standing crop and yield, which havebeen gradually replaced by measures of rate of carbon uptakeor oxygen production, or by measure of nutrient loss, or bychange of CO₂ in the environment. Data from five lakes are usedto illustrate the evolutionary thread of eutrophication andthe great range in primary productivity to be expected on thebasis of either unit volume or unit surface area at differenttrophic states. Light and nutrients are important in limitingprimary productivity, and are contributing factors to the greatvariability which one may encounter within a given lake. Onlywith a sounder understanding of productivity at the base ofthe food-chain can we have any real hope of controlling theproductivity of aquatic environments for the benefit of man.