How are you measuring "Nutrients"? Your analysis is probably correct, dead bacteria become nutrients. Also, in a 24 hour period your bacteria will be dividing very quickly. It is possible that besides dying and releasing nutrients, they are metabolizing the nutrients you are providing into a form that is better detected by your methods.
Which nutrients are you interested on? Cause depending on the nutrients, the method would change. Using a spectrophotometric method you always need to be careful cause you can have a similar molecule that absorves in the same wavelength as your of interest... Maybe HPLC or other chromatographic method would be better.