Sensitivity of freshwater macrophytes to UV radiation: relationship to depth zonation in an oligotrophic New Zealand lake

The ultraviolet radiation ( UVR) responses of photosynthesis by two freshwa ter vascular plants, Potamogeton cheesemanii and Isoetes alpinus, and the c haracean algae Chara fibrosa and C. corallina in Lake Coleridge, New Zealan d, were investigated. Experiments comprised 4-5 h of exposure to different UV wavelengths followed by 17 h of recovery in low light. Photosynthetic co mpetence was assessed by pulse-amplitude-modulated fluorometry. The four sp ecies showed different sensitivities to UVR, which were consistent with the ir upper depth limits. The shallowest-growing species, P. cheesemanii, was uninhibited by UVR, whereas after 5 h of exposure to UVR, inhibition of 15% , 38% and 48% was measured for I. alpinus, C. fibrosa and C. corallina coll ected from 4 m, 6.2 m and 16.5 m, respectively. Not all plants recovered fu lly from UVR inhibition. Plants from upper and lower depths of their growth range did not generally differ in inhibition sustained or ability to recov er photosynthesis. The species with greatest tolerance of UVR also containe d the highest concentrations of UVR-absorbing pigments. Freshwater macrophy tes have differing abilities to tolerate UVR exposure through repair and or protection strategies and these may be related to their vertical zonation.