Patterns of dark respiration in aquatic systems

We used continuous measurements of dissolved oxygen (DO) in dark bottles to characterise patterns of the dark respiration rate (Rdark) for three marine phytoplankton monocultures and in natural-water samples from two marine coastal systems. Furthermore, patterns of ecosystem community respiration rate were determined from open-water changes in DO in a fjord and in a lake. We considered two models of Rdark to describe temporal changes in DO: constant Rdark and decreasing Rdark; increasing Rdark. In addition, the effect of incubation time on Rdark was investigated in bottle incubations. Constant Rdark was observed in short-term (12-h) bottle incubations in natural-water samples from two marine coastal systems. Declining Rdark was observed in marine phytoplankton cultures and open-water measurements in a lake. Increasing Rdark was observed in open-water measurements in a fjord, particularly during summer. Long-term (120-h) bottle incubations in natural-water samples showed an increase in Rdark after 48 and 72h. We show that the conventional expectation of constant rates of respiration in darkness is far from typical, because non-linear changes are common under both controlled experimental conditions, as well as for open-water measurements of ecosystem respiration.