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.

Thirteen species from the red algal flora of Helgoland (southern North Sea) were exposed to UVA ÷ UVB radiation for various periods in the laboratory, and dark-adapted variable fluorescence (F v : Fro) was measured immediately after the UV treatment and again after various recovery times in white light. With the exception of Porphyra ~mbilicalis, all species showed a decrease in F v : F m on exposure to UV radiation, followed by recovery towards the initial values during the next 24-48 h in white light. The rate of the initial decrease was greater, and the extent of recovery was less, in deep subtidal species (e.g. Delesseri~ s~guinea, Placamium carfflagineum) than in intertidal or shallow subtidal species, although there was no direct correlation between these indicators of sensitivity to UV radiation and the depth range of a species, and only slight differences were detected between populations of a single species collected from different depths. There was also little evidence of a seasonal change in sensitivity to UV radiation in Delesseria or Plocamium. The decrease in variable fluorescence in Delesseria and Plocamium was proportional to the logarithm of the exposure to UV radiation, and reciprocity between irradiance and the length of irradiation appeared to hold for up to ~6h. The exposure to UVA that reduced F~ : F m by 50% ('50% exposure') immediately after the treatment was about I0 kJ m -2 for both Delesseria and Plocamium, but Plocamium appeared to recover more rapidly than Delesseria because the 50% exposure after 24 h recovery was about 37 kJ m -2 for Delesseria compared with 70-120 kJ m -z for different populations of Plocamium. The removal of UVB from the UV radiation treatments had no detectable effect o~ the inhibition of F v : F~ in either Ddesseria or a deep-water population of Plocamium, but reduced the inhibition by 30-50% in a shallow-water population of Plocamium. When only half an intact blade of De]esseria was exposed to UV radiation, variable fluorescence was reduced in the irradiated half but was completely unaffected in the unirradiated half, and steep gradients of Fv : F m values were measured which persisted for over 48 h after the irradiation treatment.

This book provides an introduction to recent analytical and experimental studies of plant growth in the sea. The physiology and ecology of marine plants are, therefore, emphasized at the expense of a more traditional taxonomic or morphological treatment. The physics and chemistry of the marine environment are examined with specific reference to the requirements of marine plants, and much of the book concentrates on those aspects of their physiology which are unique to marine plants, or which help us to understand their ecology. Since over 90% of the species of marine plants are algae, most of the book is devoted to the marine representatives of this group, with examples from all oceans and coasts of the world where detailed work has been done. Phytoplankton and seaweeds are discussed together in chapters on photosynthesis, growth and productivity, and geographical distribution, in order to provide an integrated picture of the biology of marine plants in general. There is, however, a deliberate bias towards the seaweeds in certain chapters (e.g. morphogenesis, rocky shore ecology, economic utilization) since the ecology and physiology of these plants have received less attention in books at this level than has the ecology of phytoplankton. Marine angiosperms are also discussed alongside the autotrophic algae, and the ecological roles of bacteria and fungi in the sea are covered in a separate chapter.

Underwater irradiance was measured at intervals of 20 min for one year at 2 water depths (2.5 and 3.5 m below M.L.W.S.) and in 3 spectral regions in the sublittoral region of the rocky island of Helgoland. Data are presented for spectral and total irradiance at water depths ranging from 2 to 15 m (below M.L.W.S.). 90 % of the total annual light reaching sublittoral habitats is received during the period from April to September, when Jerlov water type 7 (occasionally water type 5) dominates. During the other half of the year, the water is very turbid, and transparency is so low that long dark periods occur even at moderate water depths. The total annual light received at the lower kelp limit (Larninaria hyperborea), at 8 m water depth, is 15 MJ m -2 year -1 or 70 E m -2 year -1, which corresponds to 0.7 % of surface irradiance (visible). At the lower algal limit (15 m water depth) these values are 1 MJ m -2 year -1 or 6 E m -2 year -1, corresponding to 0.05 % of surface irradiance. These data are similar to measurements at the same limits in several different geographical areas, and may determine the depth at which these limits occur.

Fucus vesiculosus from the northern Baltic Sea (5 psu) and from the Irish Sea (35 psu) were cultivated at different temperatures, salinities and dissolved inorganic carbon (DIC) concentrations with the addition of different nutrient concentrations. The influence of these abiotic factors was assessed by measuring photosynthesis as electron transport rate (ETR) and growth as relative growth rate (RGR ). When Baltic F. vesiculosus was cultivated at a DIC concentration similar to that of the Irish Sea, the ETR as well as RGR increased, but never equalled the rates of the marine F. vesiculosus from the Irish Sea. Cultivation at different salinities showed that F. vesiculosus from the Baltic has a higher ETR max and RGR at low salinities (5-10 psu) than F. vesiculosus from the Irish Sea, whose ETR and RGR decreased sharply in salinities below 20 psu. Plants from both sites grown at high nutrient concentrations, however, performed better at low salinities than those grown under low nutrient conditions. Salinity had the greatest impact on differences in ETR and RGR between the two populations, followed by differences in DIC and nutrient concentrations. There was a highly significant correlation between ETR max and RGR in plants from both sites and across the full range of culture conditions, indicating that the same amount of energy from photosynthesis is used for growth in both varieties of the species at different salinities. The photosynthesis of F. vesiculosus in the northern Baltic is close to the minimum demand for growth, which may account for their small size. The temperature optimum for F. vesiculosus from the Baltic was 4-10 C, while that for F. vesiculosus from the Irish Sea was 15-20 C. The photosynthesis of Irish Sea plants was less strongly affected by exposure to high irradiances than that of plants from the Baltic.