Modeling Photosynthetically Active Radiation in Water of Tampa Bay, Florida, with Emphasis on the Geometry of Incident Irradiance

Abstract

Field studies that compare the spatial and temporal variation in light attenuation often neglect effects of solar elevation angle, yet these effects can be significant. To approximately correct for these angular effects, we developed a model that uses a simplified geometric description of incident direct solar beam and diffuse skylight. The model incorporates effects of solar elevation angle and cloudiness on the amount of in-air photosynthetically active radiation (PAR) that passes through the air-water interface and onK0in waters of relatively low turbidity. The model was calibrated with 3266 5-min averages of scalar PAR measured in air and at two depths in water and permits the value ofK0to be adjusted approximately for the effects of time of day, season and cloudiness. The model was then used with 255 days of in-air PAR data (15-min averages) to evaluate irradiance that entered the water and attenuation in the water. On an annual basis, 49% of the incident scalar irradiance, or 380μmol m−2s−1, was estimated to enter the water of Tampa Bay. The value ofK0was estimated to vary as much as 41% on a clear summer day due to changes in solar elevation angle. The model was used to make estimates of the depth to which sea-grasses might receive adequate light for survival for a range of values ofK0. This approach should be useful for projecting the effect of changes in water clarity on the depth of sea-grass survival and for comparing values ofK0collected at different times of day and in different seasons.