Ecosystem Indicators of Water Quality Part II. Oxygen Production and Oxygen Demand

Abstract

Seasonal transitions from oxygen production to oxygen demand at coastal aquaculture sites in southwestern New Brunswick (SWNB) can be defined in terms of the production-respiration (P/R) ratio. During the summer, when P/R is greater than 1, an autotrophic ecosystem is in place, and dissolved oxygen (DO) in surface waters remains above thresholds for optimal fish growth. During the fall and winter, when P/R is less than 1, a heterotrophic system is in effect, and DO can decrease to below threshold. Photochemical decomposition may act as a seasonal link, contributing to the onset of net oxygen demand by facilitating the breakdown of terrestrial and marine organic carbon in the fall. The overall carbon load is not a useful index of the bioreactive material that creates oxygen demand. Instead, bacterial number and chlorophyll concentration (expressed in terms of a bacteria:chlorophyll ratio) may be better indicators of the seasonal regulation of oxygen dynamics by the ecosystem. Low DO is the cumulative effect of sustained oxygen demand; a seasonal change in P/R from greater to less than 1 is an early warning of this demand. The application of both indices (P/R and DO) in tandem can be used to develop ecosystem-sensitive plans for the management of water quality at aquaculture sites.