Fig. 6. Seasonal changes in malodor indicator compounds in manure slurries of the two-stage phototrophic lagoon system. (A) p-Cresol (2-methyl phenol) (,) and 4-ethyl phenol (•,). (B) Phenol (,) and indoles (indole + 3-methyl indole ,). Open symbols, two-stage first phototrophic lagoon (TS1PL); solid symbols, two-stage second phototrophic lagoon (TS2PL). Values are the averages of two samples for each lagoon, and the coefficient of variation over all sampling times was about 5%. A similar profile was observed for the single-stage phototrophic lagoon (SSPL) system (data not shown).