ABSTRACT
A continuous record of photosynthetically active radiation (PAR) was collected for one year at the maximum depth limits of
Thalassia testudinum
meadows in Tampa Bay, Florida. Attenuation by epiphytic material, growth, and condition measurements were performed periodically at both the maximum depths and at shallower (non-light-limited) locations. Water clarity (Secchi depth) during the project year was comparable to the previous 5 years, and grassbeds were presumed to be growing at a steady state with respect to light. The deepest station in the study exhibited morphological adaptations to lowered light levels: increased blade lengths, decreased blade production per year, higher ratios of above-to below-ground biomass, and a decline in shoot density in contrast to expected seasonal patterns. Annual water column PAR totals were at or above 4860 moles m
yr
for sites with no evidence of stress; 3730 moles m
yr
at the light-stressed site. Annual means of epiphytic attenuation were an additional 33.1% of PAR remaining at depth. Annual percentages of immediately subsurface irradiance at canopy height ranged between 20.1 and 23.4%. Incorporating conservatively high estimates of bottom reflectance resulted in estimates of nearly 20.6% as an annual percentage. Total PAR received was a more sensitive predictor of shading stress than annual average attenuation; timing of PAR deficits also appears crucial. Percentages of water column PAR can be used as transferrable light requirements only if epiphytic growth (and attenuation) is comparable. Seasonal patterns of light and growth emphasize that the two parameters are temporally decoupled.
