Light-dependent oxygen cycling measured by an oxygen-18 isotope dilution technique

Abstract

A new 18O isotope dilution technique is presented and examples are given of the determination of true gross oxygen evolution (photosynthesis) and oxygen uptake (respiration) in plankton samples from estuarine surface waters and in the seagrass Zostera marina. The technique involves enriching dissolved oxygen with 36O2, and measuring changes in isotope enrichment and oxygen concentrations with time. It is relatively simple to apply, and less expensive than previously used 18O methods. Oxygen uptake rates of plankton samples incubated under various light conditions ranged from 0.75 to 10 times that in darkness. Gross oxygen evolution rates determined by isotope dilution were from 0 to 80% higher than gross oxygen evolution rates calculated from parallel light and dark bottle experiments and ranged between 180 and 800 .mu.M O2 (.mu.M chl a)-1 h-1. Ratios of gross oxygen uptake to gross oxygen evolution were between 0.12 and 0.67 and depended on both light intensity and sampling time. Occasionally, net oxygen evolution and isotope dilution occurred in darkened bottles, suggesting the existence of a non-photosynthetic process that produces new oxygen. In Z. marina, oxygen uptake at saturatig light intensities was 4.7 times the rate in darknesss. The data indicate the existence of significant, but variable, light stimulation of oxygen consumption in estuarine waters and higher rates of gross oxygen production than measured by the light and dark bottle technique.