A method includes identifying a chlorophyll concentration/optical density (CCpOD) value using a chlorophyll concentration measurement of an autotroph culture and an optical density measurement of the autotroph culture. The method also includes identifying a change in the autotroph culture using the CCpOD value. The change in the autotroph culture can be identified by determining whether the CCpOD value falls outside upper and lower control limits. The upper and lower control limits can be identified using a specified number of previously-determined CCpOD values, which can be calculated when the autotroph culture is in a known healthy state. Multiple CCpOD values can be calculated, and an alarm can be generated if a specified number of the CCpOD values (such as one or more) fall outside the upper and lower control limits.