| An apparatus was designed, fabricated and field-tested which measures and records the photosynthetic response of a contained, axenic, single-species phytoplankton culture exposed to natural irradiance fluctuations in the water column. Sampling of irradiance exposure, culture temperature, carbon uptake (as pH change) and oxygen evolution is on a one-minute interval. The culture media pH and dissolved oxygen concentration are controlled by the apparatus as necessary to maintain these parameters within the normal seawater range over day-long deployments. This apparatus is self-contained and operates unattended with data acquisition, data storage and media chemistry control handled by a battery-powered, onboard microcomputer.;Photosynthetic carbon uptake is calculated from the seawater media pH change. The calculation method is based on chemistry first principles using the alkalinity, pH, salinity and temperature of the culture media. Correction for the alkalinity change generated by the cellular uptake of nitrate is included. Calculated values of carbon uptake are compared with laboratory-derived empirical relationships and with the photosynthetic oxygen evolution signal from a field trial.;The operation during and results from a field trial are discussed with emphasis on the ability of the apparatus to resolve short time scale variations of the photosynthetic performance of the culture under investigation. Light saturation of oxygen evolution is apparent in the morning at moderate irradiance levels above the growth incubation levels, but it is not apparent in carbon uptake. Adaptation to the higher mid-day irradiance levels is inferred from the disappearance of the light-saturated characteristic of the oxygen evolution response in the afternoon. Since the photosynthetic quotient decreases with increasing irradiance in the morning, a shift from normal, light-sufficient biosynthesis utilizing nitrate as a nitrogen source to protective, light-saturated shunting of excess energy is implied. |
