Photosynthesis/respiration ratios in aquatic microcosms under arsenic stress

Abstract

The ratio of net photosynthesis (P) to total ecosystem respiration (R) usually declines when an aquatic ecosystem is subjected to various types of stress. P/R ratios were measured in 12 80-l microcosms containing water, sediment, and entire biotic communities (dominated by Elodea sp.) from a shallow pond. P and R were estimated from changes in dissolved O₂ concentrations during the day and night, respectively. After 10 weeks, the microcosms were stressed by the addition of sodium arsenate at concentrations of 0, 0.066, 11.5, and 143 ppm (as As). P/R ranged from 1.0 to 1.4 in all microcosms before arsenate was added. Under stress, both P and R declined in the 11.5 and 143 ppm microcosms, with negative net photosynthesis (i.e., decreases in dissolved O₂ during the day) observed on several occasions. P/R in these microcosms fell to zero or below, returning to l.0 after three weeks. P/R remained above l.0 in the 0.066 ppm and control microcosms. The P/R response could be used for screening suspected hazardous substances in microcosms, as well as for monitoring natural ecosystems.