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La Trobe University Faculty of Science, Technology and Engineering Murray Darling Freshwater Research CentreMDFRC item.The possibility of using non-photochemical chlorophyll a fluorescence quenching analysis to indicate the prior light climate and to estimate the growth rate of A. circinalis cultures grown under a range of light regimes was examined. It was found that the degree of non-photochemical quenching measured following 9 - 12 h of darkness, qN (dark), was higher in cultures grown at a high total daily light dose (TDLD). This parameter was able to track large daily changes in TDLD, even in cultures exposed to diel light/dark cycles. The level of qN (dark) was influenced by changes to both the minimum (FO') and maximum (FM') levels of fluorescence. The relative change in FO' after dark incubation increased with growth irradiance, whereas the relative change in FM' after dark incubation saturated at moderate growth irradiances, showing a close relationship with cell growth. We observed that the relative change in FM' after dark incubation could distinguish between A. circinalis receiving the same TDLD but having different growth rates due to a difference in light regime. It is suggested that these fluorescence parameters may be suitable for estimating the TDLD and gross growth rate of A. circinalis in field environments, and therefore could be used to determine relative importance of light and loss factors in the formation and decline of cyanobacterial blooms.