Rapid utilisation of storm water-derived dissolved organic carbon and its fractions in an urban lake

Hypoxia occurred in the Torrens Lake, South Australia, after a 16-mm rainfall which discharged high concentrations of dissolved organic carbon (DOC) into the Lake. This work explores the hypothesis that hypoxia was correlated with the bioavailability of DOC. Carbonaceous biological oxygen demand and a decrease in DOC from samples collected immediately after stormwater was discharged into the Lake confirmed the presence of an active aerobic microbial community. In addition, the inlet and outlet of the Lake were monitored over a 10-day period. Dissolved oxygen fell from 10 to <3 mg L-1 within 44 h as the DOC increased and then decreased. A similar pattern occurred at the outlet after a lag of 20 h. At the inlet, because minimal mixing with the Lake water occurred, the rapid decrease of DOC and its fractions was interpreted as a function of aerobic microbial activity. Aquatic humic substances (AHS) were metabolised fastest, followed by hydrophilic acids (HiA) and hydrophilic neutrals (HiN). The warm nature of Mediterranean water bodies during summer may make them more susceptible to hypoxia as the frequency of extreme droughts allow accumulation of leaf litter and other debris that is subsequently mobilised by episodic floods.