Photoinhibition and the diurnal variation of phytoplankton photosynthesis—I. Development of a photosynthesis—irradiance model from studies of in situ responses

Abstract

Diurnal series of fluorescence and photosynthesis assays were conducted in high altitude (3803 m), tropical (16°), Lake Titicaca (Peru/Bolivia). Near-surface diurnal thermoclines formed on typical days of high photon flux density (PFD, ˜ 2000 μE m −2 s −1 ). In the depth range of diurnal stratification profiles of in vivo fluorescence, both without (F a and with (F b DCMU, exhibited a mean decrease of 64% from morning to mid-day, but little change (mean increase of 1.5%) through the afternoon. Three times during the day surface, mid-depth (3–5 m) and deep (15–20 m) phytoplankton samples were incubated with H 14 CO 3− under short (<2 h) exposures to a range of in situ PFDs. Comparison of phytoplankton in different samples (ANOVA) showed identical photosynthetic response in sunrise (isothermal) samples but a significant drop in surface and mid-depth photosynthesis at all PFDs during times of diurnal stratification. Similarly, both low-light (α) and light-saturated ( P2max photosynthetic parameters were lower in mid-day surface samples compared to deep samples. In addition, previously photoinhibited samples had a higher threshold intensity for photoinhibition, IT . These results, together with diurnal time series of fluorescence from in situ incubations, demonstrate that recovery from extended episodes of photoinhibition during diurnal stratification is slower than suggested by previous observations in vitro . Photosynthesis by near-surface phytoplankton is different in light increasing up to IT than light decreasing from IT . This effect can be modeled by reducing α and Pmax as a function of the maximum photoinhibiting PFD in the diurnal light history.