Ultraviolet radiation sensitivity of photosynthesis in phytoplankton from an estuarine environment

We have studied temporal variation in the sensitivity of phytoplankton phot osynthesis to inhibition by ultraviolet radiation (UV; 280-400 nm) using bi ological weighting functions (BWFs) that quantify the biological effect of different wavelengths of UV. Variations in irradiance-dependent BWFs were e valuated for natural phytoplankton assemblages from the Rhode River, a shal low subestuary of the Chesapeake Bay, Maryland, from October 1994 to July 1 996. Phytoplankton assemblages were sensitive to UV throughout the year. Rh ode River assemblages are inhibited more strongly in the UV-B (280-320 nm), particularly below 300 nm, but there is a significant influence well into the W-A (320-400 nm). There was no inhibition of phytoplankton photosynthes is by photosynthetically available radiation (400-700 nm), but there was si gnificant seasonal variation in the saturated rate of photosynthesis (P-s(B )) and in the light saturation parameter (E-s). There was little variation in seasonal average BWFs through the year, but then was considerable variat ion in BWFs during each season. Individual BWFs varied both in absolute val ues of the weightings (reciprocal [mW m(-2)]) and in the spectral shape or relative effect of UV-B versus UV-A, which may be due to changes in species composition, light, temperature, and nutrient availability. Comparison of the most sensitive assemblage (spring) with the least sensitive assemblage (winter) indicates that these BWFs are close to the upper and lower bounds in sensitivity for irradiance-dependent BWFs from all natural and cultured phytoplankton populations. The average, absolute spectral weightings for in hibition of photosynthesis in assemblages from the Rhode River an similar t o an average BWF for Antarctic assemblages.