Aquatic depositional environments where anoxic conditions extend from bottom waters into the photic zone occurred frequently during the geologic past. Periods of photic-zone anoxia (PZA) can be recognized by the presence of specific lipids produced exclusively by the green sulphur bacteria (Chlorobiaceae) that are preserved in the sedimentary record. Chlorobiaceae perform anoxygenic photosynthesis that requires light penetration into H₂S-saturated waters. Sediment samples integrate paleoenvironmental conditions over time intervals ranging from, at best, a few decades to more commonly several millennia. Photic-zone anoxia recognized in paleoenvironmental analyses is thus defined as episodic, with no further interpretation on the duration and persistence of the anoxic events. We provide here a high-resolution, multiproxy chemostratigraphy study for the Toarcian Posidonia Black Shale from the Dotternhausen section in SW-Germany, in order to assess duration and seasonal fluctuation of photic-zone anoxia based on the relative abundance of derivatives of Chlorobiaceae lipids. We assess the variability in the degree and persistence of photic-zone anoxia using an aryl isoprenoid ratio (AIR) obtained by calculating the proportion of the short-chain C_13–17 versus the intermediate-chain C_18–22 aryl isoprenoids. Higher relative abundance of the short-chain analogues is interpreted as indicating more intensive aerobic degradation of aryl isoprenoids. AIR varies between values of 0.5, indicative of persistent PZA phases, and 3.0 for short-termed episodic PZA events. Absolute concentration of aryl isoprenoids is negatively correlated with the AIR, which is in agreement with a progressive diagenetic breakdown of aryl isoprenoids. The AIR decreases are validated using independent palecological and geochemical redox indicators for the Posidonia Shale in SW-Germany.