Modeling daily production of aquatic macrophytes from irradiance measurements: a comparative analysis

The importance of submerged aquatic macrophytes to coastal ecosystems has generated a need for knowledge of minimum light levels that will support the maintenance and restoration of healthy populations. Our goals were (1) to evaluate the sensitivity to natural, non-sinusoidal fluctuations in irradiance I of analytical integration techniques for calculating daily carbon gain, (2) to evaluate the Hsat (the daily period of I-saturated photosynthesis) model of daily production relative to models based on instantaneous photosynthesis vs irradiance (P vs I) and (3) to provide some guidance for the temporal density of irradiance data required for accurate estimation of daily carbon gain. Monthly measures of the P vs I response of an eelgrass Zostera marina L. population were used to predict rates of daily carbon gain from continuous in situ recordings of I. Daily integrated I was not a reliable predictor of daily production. Numerical (iterative) integration of Hsat was much more reliable but required repeated measures of I within a day, as did numerical integration of P vs I. Analytical (non-iterative) models based only on observations of Im (noon) could not predict daily production accurately. Analytical models of P vs I and Hsat agreed with each other, however, indicating that the analytical models may be useful where the daily pattern of I is sinusoidal. Given the high degree of temporal variability in coastal light environments, continuous monitoring of light availability may be required for calculation of daily production and reliable management of aquatic macrophyte populations.