It is predicated that algal blooms will become an increasing problem under changing climatic conditions. This is particularly concerning for the potable water treatment industry since algogenic organic matter (AOM) in surface waters supplying water treatment works (WTWs) can cause a number of treatment issues. However, whilst previous studies have shown that AOM is distinct from terrigenous, humic-dominated organic matter, limited information exists relating to changes in the character of AOM during different algal growth phases. In this study, reservoir water containing dissolved organic carbon (DOC) dominated by humic material was enriched with nutrient medium to create an algal bloom. Over the course of the algal bloom, DOC was characterised using XAD-fractionation and UV absorbance measurements. In addition, the reactivity of DOC with chlorine both before and after XAD-fractionation was assessed using trihalomethane formation potential (THMFP) and bromine incorporation measurements to monitor whether THM yield and speciation varied between different growth phases. Characterisation of DOC during the algal bloom indicated a shift towards more hydrophilic, aliphatic (low specific UV absorbance; SUVA) DOC with the release of extracellular organic matter (EOM) and later intracellular organic matter (IOM) during cell lysis. XAD-fractionation results suggest that algae produce predominantly hydrophilic neutral (HPIN) DOC. In contrast to some existing research, our study shows a marked change in DOC reactivity over time with a reduction in standardised THMFP (STHMFP) and the initial rate of THM formation observed as the algal bloom progressed. However, bromine incorporation increased with culture age.