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 s upport the maintenance and restoration of healthy populations. Our goa ls were (1) to evaluate the sensitivity to natural, non-sinusoidal flu ctuations in irradiance I of analytical integration techniques for cal culating daily carbon gain, (2) to evaluate the H-sat (the daily perio d of I-saturated photosynthesis) model of daily production relative to models based on instantaneous photosynthesis vs irradiance (P vs I) a nd (3) to provide some guidance for the temporal density of irradiance data required for accurate estimation of daily carbon gain. Monthly m easures of the P vs I response of an eelgrass Zostera marina L. popula tion were used to predict rates of daily carbon gain from continuous i n situ recordings of I. Daily integrated I was not a reliable predicto r of daily production. Numerical (iterative) integration of H-sat was much more reliable but required repeated measures of I within a day, a s did numerical integration of P vs I. Analytical (non-iterative) mode ls based only on observations of I-m (noon) could not predict daily pr oduction accurately. Analytical models of P vs I and H-sat agreed with each other, however, indicating that the analytical models may be use ful where the daily pattern of I is sinusoidal. Given the high degree oi temporal variability in coastal light environments, continuous moni toring of light availability may be required for calculation of daily production and reliable management of aquatic macrophyte populations.