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Gametophytes of three Laminaria species occurring near Helgoland, North Sea, were cultivated 4 wk in a 12:12 LD regime at different temperatures in artificial light fields, and in the sea at different water depths. In the artificial light fields underwater spectral distribution was simulated according to Jerlov water Types 5, 7, 9. Blue light in the simulated light fields amounted to 17, 12 or 4% of total quanta. The rate of vegetative growth did not depend on spectral distribution, was light‐saturated at 4–6 W · m−2, and increased with temperature up to 15 C. L. saccharina (L.) Lamour. exhibited the highest tolerance towards temperature, light and UV. Gametophytes survived 1 wk at 21 C ± 0.1, but not 22 C ± 0.1. Gametophytes of L. hyperborea (Gunn.) Fosl. and L. digitata (Huds.) Lamour. survived 1 wk at 20 C ± 0.1, but not at 21 C ± 0.1. In sunlight, and in the light field of a xenon lamp, 50% of L. saccharina gametophytes were killed by a quantum dose of 50 μEin · cm−2, and 100% of the plants by 90 μEin · cm−2. Approximately half of these quantum doses killed the corresponding percent of the other species gametophytes. Appreciably higher quantum doses were survived in visible light, with red being the most detrimental. Fertility depended on a critical quantum dose of blue light which decreased almost exponentially with decreasing temperature. The quantum dose (400–512 nm) required for induction of fertilization of 50% of the female gametophytes (males react similarly) was 90 μEin · cm−2 at 5 C, 110 μEin · cm−2 at 10 C, 230 (560 in L. digitata)μEin · cm−2 at 15 C, and 560 (L. hyperborea) or about 850 (other 2 species) μEin · cm−2 at 18 C. In the sea the gametophytes survived the dark winter months in the unicellular stage, with almost no vegetative growth of the primary cell, due to lack of light. In early spring the female gametophytes matured in the unicellular, and the males in a few‐celled stage at the depth of 2 m, as did the laboratory cultures under conditions inducing maximal fertility.

Zoospores, gametophytes, young sporophytes and discs cut from mature sporophytes of Laminaria digitata, L. hyperborea and L. saccharina were exposed in the laboratory to UV-radiation, with a spectral composition and irradiance similar to natural sunlight, for periods ranging from 15 min to 8 d, and were then returned to white light. Germination of zoospores and the growth of gametophytes were reduced after exposures to UV longer than 1 h, whereas UV had little effect on the growth of young or mature sporophytes unless exposure continued for more than 48 h. The variable fluorescence (Fv:Fm) of all stages was strongly reduced immediately after short exposures to UV, but recovered almost completely within 24 h. However, exposure of gametophytes to UV for > 4 h resulted in little or no recovery of F~:Fm, whereas > 16 h of UV were required to produce this result in young sporophytes, and > 48 h in mature sporophytes. Thus, sensitivity to UV-radiation decreased from gametophytes to sporophytes, and with increasing age of sporophytes, but, in gametophytes, growth appeared to be a more sensitive indicator of UV-damage than Fv: F,, after 24 h recovery. The responses to UV of the zoospores and gametophytes of all three species were similar, but both growth and fluorescence measurements suggested that the sporophytes ofL. saccharina were more sensitive to UV than those of the other two species.

Three eulittoral algae (Ulva lactuca, Porphyra umbilicalis, Chondrus crispus) and one sublittoral alga (Laminaria saccharina) from Helgoland (North Sea) were cultivated in a flowthrough system at different temperatures, irradiances and daylengths. In regard to temperature there was a broad optimum at 10-15 °C, except in P. umbilicalis, which grew fastest at 10 °C. A growth peak at this temperature was also found in four of 17 other North Sea macroalgae, for which the growth/temperature response was studied, whereas 13 of these species exhibited a growth optimum at 15 °C, or a broad optimum at 10-15 °C. Growth was light-saturated in U. lactuca, L. saccharina and C. crispus at photon flux densities above 70 #E m2s 1, but in P. umbilicalis above 30 pE m2s -1. Growth rate did not decrease notably in the eulittoral species after one week in relatively strong light (250 pE m-2s~), but by about 50 % in the case of the sublittoral L. saccharina, as compared with growth under weak light conditions (30 #E m2sl). In contrast, chlorophyll content decreased in the sublittoral as well as in the eulittoral species, and the greatest change in pigment content occurred in the range 30-70/zE m2s -1. Growth rate increased continuously up to photoperiods of 24 h light per day in L. saccharina and C. crispus, whereas daylength saturation occurred at photoperiods of more than 16 h light per day in U. lactuca and P. umbilicalis.

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