Flow variability and longitudinal characteristics of organic carbon in the Lachlan River, Australia

Abstract. Heterotrophic organic-carbon cycling is a major source of energy to aquatic food webs, yet there are few studies into patterns of heterotrophic productivity in large lowland rivers. The Lachlan River experienced a period of extreme flow variability from September 2010 to February 2011; for example, daily discharge (MLday1) at one site reached.22 times its 10-year average. Heterotrophic cycling of dissolved organic carbon (DOC) and particulate organic carbon (POC) were assessed over this period at six sites on the Lachlan River. Concentrations of total organic carbon (TOC) ranged from 7 to 30mgL1, of which the majority was in dissolved form. Concentration of DOC was positively correlated with daily discharge. Biochemical oxygen demand of TOC over 5 days (BOD5) showed significant variability, ranging from 0.6 to 6.6mg O2 L 1. BOD5 did not appear related to discharge, but instead to a range of other factors, including regulation via weirs, lateral and longitudinal factors. Partitioning of DOC and POC showed that POC had an influence on BOD5 comparable to DOC. This is relevant to environmental-flow management in the Lachlan River, the Murray–Darling Basin and rivers generally, by showing that flow variability influences a fundamental ecosystem characteristic, namely organic carbon