REGULATION OF RUBISCO ACTIVITY AND ITS POTENTIAL EFFECT ON PHOTOSYNTHESIS DURING MIXING IN A TURBID ESTUARY

The recent demonstration that Rubisco activity in phytoplankton is reg ulated in response to changes in irradiance allows development of a me chanistic model of the photosynthetic response to fluctuating irradian ce. The model was used to predict photosynthetic responses during mixi ng in estuarine systems, the only environments in which the rate of ch ange of irradiance is likely to occur on time scales comparable to ind uction of photosynthesis. Non-steady-state rates of photosynthesis wer e calculated based on the lags associated with activation and deactiva tion of Rubisco in response to an increase or decrease in irradiance. Non-steady-state rates were compared with steady-state rates, calculat ed assuming an instantaneous change in photosynthetic response to a ch ange in irradiance. Simulations were run for a deep, relatively clear estuary and a shallow, turbid estuary, using input parameters measured in Delaware Bay (DE, USA) and San Antonio Bay (TX, USA), respectively When estimates of production are based on steady-state rates of photo synthesis, the model predicts average overestimates of 14% in the form er and 22% in the latter. Sensitivity analyses show that, within the r ange of reported values, the model is more sensitive to changes in tur bidity than to changes in the depth of the mixed layer, incident irrad iance or diffusivity. When compared with published data, the model ten ded to overestimate the reduction in photosynthesis in the deep bay co ndition, possibly because of a compensating increase in photosynthesis due to alleviation of feedback limitation by mixing. In contrast, the model provided reasonably accurate estimates in the shallow bay condi tion. This suggests that while induction is unlikely to impose a const raint on production in relatively deep and clear estuaries such as Del aware Bay, it may impose a severe constraint in the shallow, turbid es tuaries typified by San Antonio Bay.