The afternoon depression in primary productivity in a high rate oxidation pond (HROP)

Algae (mainly Euglena sp.) from a high rate oxidation pond (HROP) were used for studying the afternoon depression in primary productivity. The phenomenon was observed on the same date by laboratory measurements of photosynthesis and respiration (oxygen evolution method) as well as by in situ determinations of ¹⁴C incorporation. The following values of α were calculated: morning, 0.014 μmol O₂/mg Chi a/min//mol quanta.m².s; afternoon, 0.008. Assuming a constant k_c of 0.006 m²/mg Chi a we found the quantum requirement (ø^-1) in the morning sample to be considerably lower than in the afternoon sample (surface: morning 44 mol quanta/mol O₂; afternoon, 71). Besides this reduction in photosynthetic efficiency the afternoon sample also exhibited reduced light saturated photosynthetic rates (P_max) and enhanced dark respiration rates. The combination of these three effects led to considerably lower areal primary productivity in the HROP in the afternoon. We suggest that this phenomenon is brought about by carbon limitation and cell overloading by photosynthetic products.