PHOTOINHIBITION OF TEMPERATE LAKE PHYTOPLANKTON BY NEAR‐SURFACE IRRADIANCE: EVIDENCE FROM VERTICAL PROFILES AND FIELD EXPERIMENTS

To evaluate the in situ occurrence of phytoplankton photoinhibition, the light‐mediated depression of chlorophyll in vivo fluorescence (IVF) and of the cellular fluorescence capacity (CFC) of phytoplankton was determined in three southeastern United States reservoirs. Vertical profiles of a fluorescence depression index (FDI) and of the CFC for reservoir phytoplankton showed that near‐surface photoinhibition of fluorescence properties occurred in association with high surface irradiance and weak vertical mixing of the water column. To characterize the time scales of photochemical and photosynthetic responses to and recovery from exposure to supraoptimal light intensity, phytoplankton IVF responses and ¹⁴C‐fixation rates were measured infield experiments. Phytoplankton chlorophyll IVF, CFC, and photosynthetic ¹⁴C fixation were rapidly (20–40 min) depressed when reservoir phytoplankton were exposed to surface irradiances (1700–2000 μE·m⁻²·s⁻¹). Light‐mediated increases in the FDI declined rapidly (20–40 min) to pre‐exposure levels during a subsequent low‐light (75–200 μE·m⁻²·s⁻¹) period, but CFC and ¹⁴C fixation recovered more slowly (>40 min). Exposure of reservoir phytoplankton to a light‐intensity gradient revealed both intensity and time thresholds for IVF and CFC depression. Phytoplankton photochemical responses to bright light operate on time scales that, in conjunction with vertical mixing, should limit the occurrence of photoinhibition to extreme irradiance environments. Our results support the hypothesis that the photoinhibition of phytoplankton productivity occurs less commonly than is indicated by fixed‐depth incubation measurements.