Three macroalgal species, Fucus vesiculosus, Laminaria saccharina and Ulva lactuca were exposed to natural daylight in UV-transparent md UV-non-transparent Plexiglas vessels at different water depths. Uptake of tSNH4 + and 15NO3-was measured at different light intensities. In general, uptake rates in August surpassed those in May 1987 and Ulva assimilated significantly more nitrogen than Fucus and Laminaria.
negative influence of ambient solar UV radiation on nitrogen uptake was found in Fucus and Ulva. Additionally, the impact of UV of Jifferent wave bands, using cut-off filters (WG 295, WG 305 and WG 320) and special UV-A and UV-B lamps, on pigments and tSN ammonium assimilation by several macroalgae was studied under controlled laboratory conditions. UV-B irradiation of shorter wave bands ed to a reduction in the main pigments of Fucus except antheraxanthin and those of Ulva except lutein and violaxanthin. Uptake of ~Nammonium by Fucus and Ulva was reduced after UV-B radiation whereas an increase was observed after UV-A irradiance in Ulva. Ammonia Jtilization by Halidrys was more damaged by UV-A than UV-B. The impact of UV radiation on the labelling of free amino acids and the pattern of pool sizes varied with the species and the UV wave bands. A marked reduction in 15N incorporation into the amino acids of Plocamium was found after UV-B exposure. The ~SN labelling of the amino acids of Halidrys and Rhodomela was less affected. Results are :ti scussed with reference to photoinhibitory effects on the enzymes of the nitrogen metabolism.