The importance of submerged aquatlc macrophytes to coastal ecosystems has generated a need for knowledge of minlmum l~g h t levels that wlll support the m a~n t e n a n c e and restoration of healthy populations Our goals were (1) to evaluate the s e n s~t~v~t y to natural, non-sinusoidal fluctuatlons in irradiance I of analytical integration techniques for calculating daily carbon gain (2) to evaluate the H,,, (the daily period of I-saturated photosynthesis) model of daily production relat~ve to models based on Instantaneous photosynthesis vs Irradiance ( P v s I ) and (3) to provlde some guidance for the temporal d e n s~t y of Irradiance data r e q u~r e d for accurate estimation of dally carbon gain Monthly measures of the P v s I response of an eelgrass Zostera m a n n a L population were used to p~e d l c t rates of dally carbon galn from continuous in sjtu recordings of I Dally ~n t e g r a t e d I was not a reliable predictor of daily product~on Numencal (iterative) integration of H,,, was much more r e l~a b l e but r e q u~r e d repeated measures of I within a day as did numencal integrat~on of P vs I Analytical (non-iterative) models based only on observations of l,, (noon) could not p r e d~c t daily production accurately Analyt~cal models of P v s I and H,,, agreed with each other, however, indicating that the analyt~cal models may be useful where the daily pattern of I is sinusoidal Given the h~g h degree of temporal vanability in coastal 11ght environments continuous monltonng of hght a v a~l a b~l i t y may be required for calculation of daily production and rehable management of aquatlc macrophyte populations

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