CO2-exchange of emersed plants and 0,-exchange of submersed plants were measured in 5 species of brown algae from different tidal heights on shores in Ireland and Helgoland (southern North Sea). The photosynthesis of emersed fucoids and Laminaria digitata increased as up to 25 % of tissue water was lost, but then declined with further desiccation. The relationship between decrease in photosynthesis and loss of tissue water was similar in 3 species of Fucus, and the photosynthetic apparatus of F. spiralisappeared to be no more resistant to desiccation than that of F. serratus. Recovery from severe desiccation took about 2 h in all species, regardless of their typical position on the shore, but the extent of recovery from a given degree of desiccation was greater in upper shore species. Pelvetia canaliculata and F. spiralis showed complete recovery from 80 to 90% water loss, F. vesiculosus from about 70 %, F. serratus from 60 %, and Laminaria digitata from 55 % water loss. The photosynthetic rate of each species after full recovery decreased linearly as water loss increased beyond these values. It is in the extent of recovery of photosynthesis after desiccation that intertidal brown algae show the clearest correlation with their heights in the zonation pattern on European shores.

An underwater light-measuring station, consisting of 3 sensors at each of 2 depths (4.5 and 5.5 m below MLWS), was established off Helgoland (southern North Sea) to provide continuous measurements of underwater irradiance at 3 wavelengths (452,552 and 653 nm, isolated by interference filters). The percentage transmittance of each wavelength through l m of seawater was calculated from the readings at the 2 depths. Irradiance and transmittance were recorded every 75 s during 3 periods of 2 mo between August 1990 and June 1991. During autumn and winter 1990, in spite of variations in weather conditions, there was a strong 2-weeMy cycle of total daily irradiation, with peaks when low tide occurred at around midday (neap tides at Helgoland) and troughs when high tide occurred at this time (spring tides). The amplitude of this cycle was much greater than could be accounted for by the difference in tidal height at midday, but could be explained if light penetration through the water was greater during neap tide series than during spring tides. This hypothesis was supported by the transmittance measurements, and by wind speed data and Secchi disc depths from the same site. However, no association between water clarity and the spring-neap cycle was apparent in Apnl-June 1991. Nevertheless, the variable relationship between underwater and surface irradiance through the spring-neap cycle needs to be taken into account in any attempt to estimate benthic primary productivity from surface light data.

Little is known of the ecological effects of harvesting littoral algae although this is a worldwide commercial activity. In 1976 an attempt to establish harvesting in Strangford Lough, Northern Ireland, was opposed on mainly theoretical conservation grounds. The attempt began and stopped within a single small bay leaving a sharp boundary between cut and uncut areas. A subjective survey apparently confirmed the predicted loss of cryptic fauna, decline through predation and the resorting of interboulder sediment. In April 1979 the cut and uncut areas were examined in detail to determine whether any of these effects had persisted and were demonstrable scientifically. Beach and boulder transects and various other studies showed some increases in the cut area. There was significantly more Fucus, •nteromorpha and Ulva; Cirratulus (inhabiting Rhodochorton-bound sediment on boulder surfaces) had a greater biomass. Some changes in Littorina colour morphs were apparent. Sediment in the cut area was coarser and had significantly more crustacean meiofauna. Ascophyllum internodal length and lateral branching were increased but it still provided 20 % less shore cover than in the uncut area. There were significant decreases in the cover of Cladophora on the sides of boulders and of Halichondria, Hymeniacodon and Balanus on undersurfaces. Indeed on the habitable underside of boulders total animal cover had been reduced by nearly two-thirds and the average number of species per boulder by one-third. It is concluded that Ascophyllum harvesting has a significant and persistent effect on shore ecology.Littoral algae are a valuable commercial asset but it is important that some fairly large intertidal areas should be left unharvested for general conservation purposes.

The transient stimulation of light-saturated photosynthesis In Laminaria digitata (Huds.) Lan~our, and L. saccharina (L.) Lamour., which has been observed following pulses of blue light, was found to persist when low irradiances of continuous blue light were given as a supplement to saturating irradiances of red or yellow light. The degree of stimulation was directly proportional to the logarithm of the irradiance of blue light, with a 50% response at 0.28 1.1mol m-' S-' and saturation above 1 pm01 m-' S-'. These irradiances represented about 0.2 % and 0.5 %, respectively, of the total irradiance incident on the plants. In natural underwater light fields, such low proportions of blue wavelengths would be found only close to, or below, the lower depth limit for Laminaria spp., where photosynthesis, if it occurred at all, would be light-limited and, therefore, not subject to blue light stimulation. Irradiances of blue light measured in the Laminaria zone during periods when the total irradiance was high enough to saturate photosynthesis were always higher than 1 pm01 m-' S-', and photosynthesis by Laminaria spp. in simulated underwater light fields in the laboratory was not affected by additional blue light. Unlike Laminaria, other brown algae (e.g. Asperococcus. Ectocarpus) exhibited stimulation by blue light in irradiances of red light which are limiting for photosynthesis, and their photosynthetic rates can, therefore, be limited when blue light is present as a higher proportion of the total irradiance than for Lam~naria. However, these plants are mostly found in the littoral zone, and will rarely experience low blue light environments. The stimulation of photosynthetic capacity by blue light in brown algae occurs at such low irradiances of blue light that photosynthesis by these plants, in their natural habitats, is unlikely ever to be limited by a shortage of blue light.