ALBERT

Description: Kelps (brown algae of the order Laminariales) build highly complex and productive underwater forests and possess microscopic and macroscopic life stages. The microscopic stages (spores, gametophytes, juvenile sporophytes) are usually more sensitive to environmental stressors and may form a bottle-neck for the survival of the population. Future Arctic kelp forests will be especially affected by elevated temperatures and increased sedimentation. Knowledge on grazer impact is still rudimentary. In order to investigate how global change in interaction with grazing may shape future Arctic kelp systems we performed laboratory experiments (2 x 3 x 2 factorial design) on early life stages of the kelps Alaria esculenta, Laminaria digitata and Saccharina latissima from Arctic Kongsfjorden (Svalbard). Spores were exposed to ambient and elevated summer temperatures in combination with 3 levels of sediment and 2 levels of grazing by the limpet Margarites helicinus. The germination and formation of juvenile sporophytes was strongly inhibited in all species with increasing sediment cover, clearly showing the strongest negative effect on sporophyte development of all tested variables. Grazers interacted with temperature and sedimentation affecting kelps in a species-specific way. They had a strong impact on the number of developing sporophytes partially counteracting the negative impact of sedimentation. We conclude that the structure of kelp communities can be shaped by abiotic and biotic variables acting on early developmental stages and that global warming has the potential to alter the strengths and direction of these effects, which may lead to future shifts in community structure.

Description: There is a general lack of information on the succession of marine benthic algae in Antarctica. We performed two colonization experiments in the upper subtidal (3 and 5 metres depth) using artificial substrates in Potter Cove (South Shetland Islands): in the outer cove, an area mainly unaffected by sedimentation, and in the inner cove, in close proximity to a retreating glacier, with high sediment inflow particularly during the melting season. Seasonal and interannual changes in total, diatom and macroalgal cover, species composition and ecological indexes were assessed over four years. Tiles were analysed in spring and summer in the laboratory and by year-round photographic monitoring. Irradiance (photosynthetically active radiation and ultraviolet radiation), salinity and temperature were monitored on a monthly basis. Benthic algae dominated the assemblages, with macroalgae reaching ~70% cover after two years. There were site and temporal differences in the contribution of diatom mats (mainly pennate forms) and macroalgal cover. Diatom cover was higher at the glacier-influenced site, particularly at the early stages, and decreased significantly with time. Between years, macroalgal assemblages changed significantly in a site-specific manner. Assemblages mainly comprised annual and pseudoperennial species at both sites, with absence of adult large Desmarestiales. Although a year separated the establishment of the two experiments, there were convergence patterns in the changes of cover over time - that seemed to be controlled by competitive interactions - and in the patterns of species replacement. However, the inner cove site exhibited lower number of macroalgal taxa and a tendency to decreased diversity over time that could be related to higher the level of stress and disturbance caused by glacial influence.

Description: In Potter Cove, Antarctica, newly ice-free areas appeared due to glacial retreat. Simultaneously, the inflow of sediment increased, reducing underwater photosynthetically active radiation (PAR, 400–700 nm). The aim of this study was to determine the photosynthetic characteristics of two macroalgal species colonizing three newly icefree areas, A1, A2 and A3, with increasing degree of glacial influence from A1 to A3. Turbidity, salinity and temperature were measured, and light attenuation coefficients (Kd) calculated and considered as a proxy for glacial sediment input. The lower depth distribution of the red alga Palmaria decipiens and the brown alga Himantothallus grandifolius was 10 m in A3, 20 m in A2 and 30 m in A1. Both species were then collected, at 5 and 10 m at all areas. Photosynthetic parameters and the daily metabolic carbon balance (CB) were determined. Kd was significantly higher in A3 compared with A1 and A2. The CB of P. decipiens was significantly higher in A1 followed by A2 and A3, and significantly higher at shallower than at greater depth. For H. grandifolius CB was significantly lower in A3 and in A2 at deeper depths compared with the rest of areas and depths. The lower distribution limit of the algae was positively correlated to the light penetration. An increase in the sediment run-off due to global warming might lead to an elevation of the lower depth distribution limit but retreating glaciers can open new space for macroalgal colonization. These changes will probably affect macroalgal primary productivity in Potter Cove with consequences for the coastal ecosystem.

Description: Ensemble habitat modeling is a tool in the multivariate analysis of arbitrary species or community distribution which combines models of best fit to an optimized model (ensemble model, EM). To simulate spatial variation of communities and predict the impact of climate change, it is essential to identify the distribution-controlling factors. Macroalgae biomass production in polar regions is determined by environmental factors such as irradiance, which are modified under climate change impact. In coastal fjords of King George Island/Isla 25 de Mayo, Antarctica, suspended particulate matter (SPM) from glacial melting causes shading of algal communities during summer. Ten different species distribution models (SDMs) were applied to predict macroalgae distribution based on their statistical relationships with environmental variables. The suitability of the SDMs was assessed by two different evaluation methods. Those SDMs based on a multitude of decision trees such as Random Forest and Classification Tree Analysis reached the highest predictive ability followed by generalized boosted models and maximum-entropy approaches. We achieved excellent results for the current status EM (true scale statistics 0.833 and relative operating characteristics 0.975). The environmental variables hard substrate and SPM were identified as the best predictors explaining more than 60 % of the modelled distribution. Additional variables distance to glacier, total organic carbon, bathymetry and slope increased the explanatory power proved by cross-validation. Presumably, the SPM load of the meltwater streams on the Potter Peninsula will continue to increase at least linearly. We therefore coupled the EM with changing SPM conditions representing enhanced or reduced melt water input. Increasing SPM by 25% decreased predicted macroalgal coverage by approximately 38%. The ensemble species distribution modelling helps to identify the important factors controlling spatial distribution and can be used to link causes to effects in (Antarctic) coastal community change.

Description: Macroalgae is a central part of marine shelf ecosystems in the Arctic, both as primary producers and as habitat builders and may contribute substantially to the carbon export into the deep sea. In Kongsfjorden we quantified the zonation of visually dominant macroalgal taxa and of detached macroalgae from underwater videos taken in summer 2009 at six transects between 2 to 138 m water depth. Four transects were located at the south shore along the length axis of the fjord (Kongsfjordneset, Brandal, Prince Heinrich Island, Tyskahytta). Two further transects investigated the steep bedrock of Hansneset with a west-east orientation 50 m apart from each other: Hansneset 1 (north) and Hansneset 2 (south). The georeferenced data (date, depth, coordinates) of all transects were linked to the timecode of the video and imported into a geographic coordinate system (GIS). Presence/absence and cover data of macroalgae along the transects was collated into the GIS. The resulting shape files provide useful information for further investigations of macroalgae in the fjord and the geographical information may enhance the repeatability of the investigation in the future.