Kjersti Sjøtun

Bacterial diversity in relation to secondary production and succession on surfaces of the kelp Laminaria hyperborea

Kelp forests worldwide are known as hotspots for macroscopic biodiversity and primary production, yet very little is known about the biodiversity and roles of microorganisms in these ecosystems. Secondary production by heterotrophic bacteria associated to kelp is important in the food web as a link between kelp primary production and kelp forest consumers. The aim of this study was to investigate the relationship between bacterial diversity and two important processes in this ecosystem; bacterial secondary production and primary succession on kelp surfaces. To address this, kelp, Laminaria hyperborea, from southwestern Norway was sampled at different geographical locations and during an annual cycle. Pyrosequencing (454-sequencing) of amplicons of the 16S rRNA gene of bacteria was used to study bacterial diversity. Incorporation of tritiated thymidine was used as a measure of bacterial production. Our data show that bacterial diversity (richness and evenness) increases with the age of the kelp surface, which corresponds to the primary succession of its bacterial communities. Higher evenness of bacterial operational taxonomical units (OTUs) is linked to higher bacterial production. Owing to the dominance of a few abundant OTUs, kelp surface biofilm communities may be characterized as low-diversity habitats. This is the first detailed study of kelp-associated bacterial communities using high-throughput sequencing and it extends current knowledge on microbial community assembly and dynamics on living surfaces.

Spore dispersal in Laminaria hyperborea (Laminariales, Phaeophyceae)

Abstract Large areas of barren sea floor are found in northern Norway. These are limited to the inner coast: the outer coast is undisturbed with an intact kelp forest where the dominant species is Laminaria hyperborea. The kelp forest is an important spore source for the reforestation of barren areas, and its dispersal range is of vital importance. Experiment using sampling devices at varying distances from the kelp forest have shown the dispersal range of a populaiion of L. hyperborea plants to be at least 200 m. Large numbers of zoospores were found in samples taken from the water column at a depth of 5 m both within and 50 and 200 m from the kelp forest. Laboratory experiments have shown that spores probably swim as long as they can and thereafter sink passively out of the water column to settle.

Population studies of Laminaria hyperborea from its northern range of distribution in Norway

Eight populations of Laminaria hyperborea from wave-exposed localities in Finnmark (70–71°N) were compared with two populations in Vega (65 °N). Standing stock in shallow water (3–5 m) was within the same range in the two areas (6–16 kg fr. wt. m-2). Both the highest and lowest value of standing stock were registered in Finnmark. Degree of wave-exposure was determined by the numbers of sectors exposed to open sea at each locality, with each sector given a relative wind force value. The most wave-exposed locality in Vega had a slightly higher annual biomass production as a function of plant age compared with the populations from Finnmark within a similar range of wave exposure. Of the two localities in Vega the most wave-exposed one had a higher annual biomass production per plant than the more sheltered one. The most wave-exposed locality examined was situated in Finnmark and exhibited the lowest annual biomass production per plant. Recruits (one-year-old plants) were found at all the localities except at one locality in Finnmark. The plants reached a higher age in Finnmark (13–18 years) than in Vega (8–9 years).

The newly introduced species Heterosiphonia japonica Yendo (Dasyaceae, Rhodophyta): geographical distribution and abundance at the Norwegian southwest coast

The newly introduced red alga Heterosiphonia japonica, previously denoted as Dasysiphonia sp., has, since its first record in Europe in 1994, established populations in the Netherlands, Spain, France and Norway. In order to investigate the geographical distribution of the species along the Norwegian southwest coast, 83 localities were dredged along a total distance of 830 km, and the abundance of H. japonica recorded on a semi‐quantitative scale in 2000–2001. H. japonica was present in the entire range investigated. The local abundance of H. japonica at 40 localities in an area south of Bergen was recorded by dredging. H. japonica was not present at wave‐exposed localities, but was abundant at the more sheltered localities where it could constitute up to 65% of the total algal content of the dredge. The size distribution in a population was also studied at one locality during 1 year. Plants in good condition were found throughout the year and a large number of plantlets were present during all seasons. The results of this study suggest that the species has a high dispersal and recruitment capacity. The effect of H. japonica on indigenous ecosystems has yet to be determined.

Vegetative reproduction in “Heterosiphonia japonica” (Dasyaceae, Ceramiales, Rhodophyta), an introduced red alga on European coasts

Abstract Vegetative reproduction by means of abscised fragments was studied in the introduced red alga “Heterosiphonia japonica”. Small side branches (pseudolaterals) are actively shed in all seasons, but mostly during late summer/autumn. The development of excised pseudolaterals was studied in the laboratory under different culture conditions. At 0°C, few pseudolaterals survived and no development was observed. At 4°C, survival was high but development was slow, while at 12 and 20°C the majority of pseudolaterals developed rhizoids and attached to the substratum. Aquarium studies of fragmentation rate of pseudolaterals and estimates of tetraspore production showed that the instantaneous fecundity by fragments could be almost as high as the estimated fecundity by tetraspores. Field observations showed recruitment of “H. japonica” by fragments during all seasons. There was a weak trend of higher densities of recruits originating from fragments during late summer, consistent with the high degree of pseudolateral shedding and optimal temperature for pseudolateral development at this time of the year. The results from this study show that the newly introduced species has a high capacity for regenerating from small branchlets, and a continuous release of these vegetative propagules throughout the year.

The combined effect of canopy shading and sea urchin grazing on recruitment in kelp forest (Laminaria hyperborea)

Abstract The aim of this study was to determine the impact of sea urchin grazing (Echinus esculentus) and canopy shading on the recruitment of the kelp Laminaria hyperborea in mid-Norway. A spatially variable distribution of sea urchins was observed, and recruitment processes were studied both after disturbance, caused by kelp harvesting removal of the canopy kelps, and in pristine kelp forests. The combination of sea urchin density and the density of canopy-forming kelps had the strongest influence on the density of small kelps in pristine kelp forest, suggesting that both grazing from sea urchins and shading from the canopy contributed to the mortality of small kelps. High densities of small kelps (>20 m−2) were only found in pristine kelp forest together with ≤6 canopy-forming kelps m−2 or <3 sea urchins m−2 on average. However, within the observed range of sea urchin densities these had no effect on the density of large, canopy-forming kelps. Large L. hyperborea were apparently not subject to grazing. In addition, only a small number of surviving kelp recruits was needed to maintain the density of canopy-forming kelps, as L. hyperborea specimens may survive many years. These conditions result in high stability of the kelp forest. A different picture was seen after kelp harvesting, when high recruitment and survival of recruits are the conditions for rapid restoration of the kelp vegetation. After removal of the canopy-forming plants, the kelp recruits were temporarily released from high density-dependent mortality due to shading. Some influence of sea urchin grazing on the density of recruits was observed, but this was small compared with the strong canopy effect. However, the accumulated impact of grazing during a period of time had a strong overall effect on the regrowth of kelp. After 2.5 years the accumulated biomasses at the harvested stations were strongly related to average sea urchin density, and a density of between 4 and 5 sea urchins m−2 resulted in very little biomass accumulation. This suggests that the L. hyperborea kelp forest vegetation has a high degree of stability, but shows less resilience after disturbance, when exposed to moderate sea urchin grazing.

Changes of macroalgal biodiversity in sublittoral sites in southwest Norway: impact of an introduced species or higher temperature?

Abstract The Pacific red alga Heterosiphonia japonica has dispersed rapidly along European shores. Due to the species’ high abundance in many habitats, an impact on species richness and composition of local macroalgal communities might be expected. Higher sea temperatures may also influence local macroalgal composition, by providing more favourable conditions for species requiring higher temperatures. Macroalgal composition at 22 sublittoral sites along the south-western coast of Norway investigated prior to the establishment of H. japonica (1994–1995) were reinvestigated in 2003–2004, using similar methods. The total number of species collected in the area was approximately the same in the present investigation as in the previous study. With regard to number and composition of species at each site, there were no significant differences between sites with high abundance of H. japonica and sites with low or no abundance. Similarity percentage analysis (SIMPER) showed that there were temporal changes in composition of the macroalgal communities, mainly caused by higher frequency of ‘southern species’ (species with a northern limit on our coast). There was a significant increase in the percentage share of such species in the reinvestigations. The temporal differences observed are most likely caused by several warm summers/autumns and mild winters since the first investigation, which may favour a higher abundance of ‘southern species’. Heterosiphonia japonica was the most important species contributing to temporal dissimilarity in the area. This paper concludes that H. japonica has caused no negative impact on regional algal species richness in the relatively short time span since its introduction.

Seasonal growth pattern of an Icelandic Laminaria population (section Simplices, Laminariaceae, Phaeophyta) containing solid and hollow-stiped plants).

Lamina elongation and content of mannitol, laminaran and nitrate were measured during one year in Laminaria saccharina sensu lato from Iceland. The population contained both solid- and hollow-stiped plants. Growth rate was at its minimum from October to December, and started to increase in mid-winter, slightly earlier at 3 m than at 5 m. The increase in growth rate coincided with a strong reduction in stored carbohydrates and an increase in nitrate content of the laminae, indicating that stored mannitol and laminaran provided extra energy for increased lamina growth and/or for nitrate uptake. The results showed that stored mannitol was utilised before laminaran. The growth rate was at its maximum from April to June, and was reduced from June to July. The ambient nitrate concentration at the locality was low from May to August. The nitrate content of the lamina tissue in relation to dry weight was high during spring but was reduced to low values by July, indicating that nitrate levels limited growth during...

Mass-occurrence of a brown filamentous endophyte in the lamina of the kelp Laminaria hyperborea (Gunnerus) foslie along the southwestern coast of Norway

Abstract The mass-occurrence of spot disease on the dominant kelp species (Laminaria hyperborea (Gunnerus) Foslie) in Norway is described. The spots were associated with an endophyte provisionally assigned to the genus Streblonema and growing mainly in the mucilage canals in the outer cortex of the lamina. Spot disease had a mean cover on old plants (< 4 years) of 2.3–16.7 % of the lamina on two stations and was found on more than 20 % of the plants on all stations examined. After a period of strong wind, heavily infested plants had major lamina loss, and many kelp plants were observed without lamina. Sori were not normally found on infested parts of the lamina. There was no correlation between infested areas of the old lamina and the formation of the new lamina in early winter.

Being abundant is not enough: a decrease in effective population size over eight generations in a Norwegian population of the seaweed, Fucus serratus

The brown alga Fucus serratus is a key foundation species on rocky intertidal shores of northern Europe. We sampled the same population off the coast of southern Norway in 2000 and 2008, and using 26 microsatellite loci, we estimated the changes in genetic diversity and effective population size (Ne). The unexpectedly low Ne (73–386) and Ne/N ratio (10−3–10−4), in combination with a significant decrease (14%) in allelic richness over the 8-year period, suggests an increased local extinction risk. If small Ne proves to be a common feature of F. serratus, then being abundant may not be enough for the species to weather future environmental changes.