No abstract

Stratospheric ozone depletion and the concomitant increase in irradiance of ultraviolet-B radiation (UVB) at the earth's surface represent major threats to terrestrial and aquatic ecosystems. In costal rocky shore environments, seaweeds constitute a group of organisms of particular significance to ecosystem function. Thus, impairment of seaweed performance by UVB-exposure may result in severe changes in the functioning of coastal ecosystems. Here we present our view on how UVB radiation affects seaweed physiology and ecology and, thus, shapes the coastal environment by affecting the spatial, species and functional structure of seaweed communities.

Measurements of photosynthesis, germination capacity and assessment of DNA damage were carried out in the laboratory to determine the effect of different conditions of ultraviolet (W) and photosynthetically active radiation (PAR) on zoospores of various large brown algae collected on Spitsbergen (Svalbard, High Arctic) and Tarifa (Cbdiz, southern Spain). Results were correlated to in situ light conditions and indicated that zoospores suffer photoinhibition of photosynthesis, loss of viability and DNA damage in relation to the growth depth of parental sporophytes. At both sites, germination capacity of zoospores in species collected in deep waters was more strongly mpaired after exposure to the same UV doses than in species from shallower waters. In general, zoospores exposed to PAR+UVA+UVB showed higher mortality rates than after exposure to PAR+UVA or PAR alone. For Larmnana digitata from Spitsbergen, it was found that the loss of zoospore viability is the result of DNA damage and photodamage of the photosynthetic apparatus. UVB irradiances occurring in southern Spain at water depths shallower than 7 m prevented the germination of spores of deep water Laminariales from this region.

Macroalgal rafts frequently occur floating in coastal waters of temperate regions of the world's oceans. These rafts are considered important dispersal vehicles for associated organisms with direct development. However, environmental factors may limit the floating potential of kelp and thereby the dispersal of associated organisms. To examine the effect of water temperature and grazing on growth, reproductive output, and survival of floating Macrocystis spp., experiments were conducted in outdoor tanks during austral summer 2006/2007 at three sites along the Chilean Pacific coast (20° S, 30° S, 40° S). At each site, Macrocystis spp. was maintained individually at three different water temperatures (ambient, ambient - 4°C, ambient + 4°C) and in the presence or absence of the amphipod grazer Peramphithoe femorata for 14 d. High water temperatures (>20°C) provoked rapid degradation of Macrocystis spp. rafts. At moderate temperatures (15°C-20°C), algal survival depended on the presence of associated grazers. In the absence of grazers, algal rafts gained in biomass while grazing caused considerable losses of algal biomass. Algal survival was the highest under cooler conditions (<15°C), where raft degradation was slow and grazer-induced biomass losses were compensated by continuing algal growth. Our results indicate that floating kelp rafts can survive for long time periods at the sea surface, but survival depends on the interaction between temperature and grazing. We suggest that these processes limiting the survival of kelp rafts in warmer temperatures may act as a dispersal barrier for kelp and its associated passengers.

Photosynthesis, dark respiration, chlorophyll a contents and daily metabolic C balance were determined in 5 species of brown and red algae from Potter Cove (King George Island) during the Antarctic spring. In sitii irradiance data were used to d e t e r m~n e the light requirements of plants collected at 10, 20 and 30 m depth. Average daily maximum quantum irradiances measured in springsummer reached up to 23 pmol photons m-2 s-' a t 30 m depth indicating that macroalgae can effectively be exposed to non-limiting quantum irradiances for photosynthesis. Net photosynthetic rates (P,,,,,) were high in the brown alga Desmarestia anceps and the red algae Palmaria decipiens with values close to 33 and 36 pmol O2 g-l FW h-', respectively, at 20 m depth. With the exception of the brown alga Himantothallus grandifolius, all the species showed lower P,,,,, in plants collected at 30 m than at 10 and 20 m depth. The photosynthetic efficiency (a) varied strongly among species, but no clear depthdependent relations were found. Saturation (&) and compensation (L) points for photosynthesis were, in general, lower in plants growing at deep locations. In plants from 10 and 20 m, photosynthesis was saturated at significantly lower irradiances than in situ quantum irradiances. Values of I, varied between 58 pm01 photons m-' S ' in D. anceps and 15 pm01 photons m-2 S-' in the red alga Gigartina skottsbergii, while I, ranged between 1 and 10 pmol photons m-2 S-' in most of the species. D. anceps exceptionally had I, values close to 26 pm01 photons m-2 S-' in plants from 10 m depth. Overall, photosynthetic performance in these species was comparable to rates measured in macroalgae from upper littoral zones and did not provide evidence for metabolic acclimation with depth. Apparently, the daily periods for which plants are exposed to saturation and compensation irradiances (H,,, and H,,,,,) and, consequently, the metabolic C balance account for the acclimation of macroalgae to d e e p sublittoral zones. At 10 m, H,,, for many species was between 12 and 14 h, while at 30 m these periods decreased to 7 h in D. anceps or 9 h in the red alga KaUymenia antarctica. The H,,,,,, periods were longer, in the case of the red algae up to 16 h. The daily carbon balance decreased with depth. At 30 m, algae exhibited C gains lower than 1 mg C g-l FW d-' and in D. anceps, due to its high respiration rates, carbon balance was negative at saturation and compensation irradiances. In general, greater C gains relative to losses were found in plants growing at 20 m depth. Although data on P,,.,,, a. I, and I, indicate that Antarctic macroalgae are metabolically able to inhabit greater depths during spring-summer, the shortening oI the daylengths for which algae are exposed to saturating or compensating irradiances Impose a maximum depth limit at depths around 30 m.