MEPS 256:75-85 (2003)  -  doi:10.3354/meps256075

ABSTRACT: The effects of solar radiation, both visible and ultraviolet (UV-R, 290 to 400 nm), on the kelp Laminaria saccharina (L.) Lamour were examined in relation to depth distribution and tidal height in the Gulf of Maine. Despite the high attenuation of visible and UV-R, shallow subtidal L. saccharina exhibited significant decreases in midday measurements of steady-state quantum yields of Photosystem II (PSII) fluorescence compared to deeper conspecifics. Decreases in PSII quantum yields under low-tide conditions at midday were more pronounced and associated with higher coefficients of non-photochemical quenching than yields of the same algae measured at midday under high-tide conditions. Steady-state quantum yields in shallow algal populations had not recovered by early morning of the following day. Laboratory experiments were conducted to partition out the effects of high visible radiation versus UV-R effects on steady-state quantum yields observed in the field. Algae exposed to solar visible PAR (photosynthetically active radiation; 400 to 700 nm), PAR + UV-A (ultraviolet-A radiation; 320 to 400 nm), and PAR+UV-A+UV-B (ultraviolet-B radiation; 290 to 320 nm) treatments exhibited daily midday depressions in steady-state quantum yields, similar to those observed in field samples at low tide. The PAR+UV-A+UV-B treatment showed a significantly greater depression in PSII quantum yields than field samples at low tide, and took longer to recover at night. Algae exposed to the UV-R component of the spectrum also showed a significant decrease in gross primary production and contained less areal chlorophyll a than field samples, while non-photochemical quenching (a measurement of the dissipation of excess excitation energy) was significantly higher during midday exposures to UV-B. Significantly greater concentrations of the UV-B-absorbing compound mycosporine-glycine, a mycosporine-like amino acid (MAA), were present in the shallow field and experimental PAR+UV-A+UV-B-treated algae. Our results suggest that exposure to midday visible irradiances resulted in dynamic photoinhibition, while exposure to visible and UV-R irradiance in the Gulf of Maine results in both dynamic and chronic photoinhibition that causes a decrease in gross primary production of L. saccharina in shallow waters or under low-tide conditions.

KEY WORDS: Macroalgae · Chlorophyll fluorescence · Laminaria saccharina · UV-B radiation · Photoinhibition · Mycosporine-like amino acids · Gulf of Maine