Calculation of Daily Average Photosynthesis

Abstract

It is important to integrate net photosynthesis over depth and time to obtain accurate estimates of daily average net photosynthetic rates. Three different methods for calculating daily average net photosynthesis in a water column as a function of incident light are evaluated. Systematic errors are introduced in two of these methods by failing to accommodate the logarithmic extinction of light in one case and by failing to accommodate the variability of incident light within a day in the other case. The severity of the error increases as the amount of incident light received in a day increases. The magnitude of the error can be significant, e.g., comparable to oxygen deficits associated with large point discharges of oxygen—demanding substances, in deep turbid rivers enriched in phytoplankton. These sources of error should be avoided in modeling productive rivers, lakes, and estuaries.