Andrew McMinn

Marine introductions in the Southern Ocean: an unrecognised hazard to biodiversity

This study investigated the potential for transport of organisms between Hobart, Macquarie Island and the Antarctic continent by ships used in support of Antarctic science and tourism. Northward transport of plankton in ballast water is more likely than southward transport because ballast is normally loaded in the Antarctic and unloaded at the home port. Culturing of ballast water samples revealed that high-latitude hitchhikers were able to reach greater diversities when cultured at temperate thermal conditions than at typical Southern Ocean temperatures, suggesting the potential for establishment in the Tasmanian coastal environment. Several known invasive species were identified among fouling communities on the hulls of vessels that travel between Hobart and the Southern Ocean. Southward transport of hull fouling species is more likely than northward transport due to the accumulation of assemblages during the winter period spent in the home port of Hobart. This study does not prove that non-indigenous marine species have, or will be, transported and established as a consequence of Antarctic shipping but illustrates that the potential exists. Awareness of the potential risk and simple changes to operating procedures may reduce the chance of introductions in the future.

The spatial structure of Antarctic biodiversity

Patterns of environmental spatial structure lie at the heart of the most fundamental and familiar patterns of diversity on Earth. Antarctica contains some of the strongest environmental gradients on the planet and therefore provides an ideal study ground to test hypotheses on the relevance of environmental variability for biodiversity. To answer the pivotal question, “How does spatial variation in physical and biological environmental properties across the Antarctic drive biodiversity?” we have synthesized current knowledge on environmental variability across terrestrial, freshwater, and marine Antarctic biomes and related this to the observed biotic patterns. The most important physical driver of Antarctic terrestrial communities is the availability of liquid water, itself driven by solar irradiance intensity. Patterns of biota distribution are further strongly influenced by the historical development of any given location or region, and by geographical barriers. In freshwater ecosystems, free water is also crucial, with further important influences from salinity, nutrient availability, oxygenation, and characteristics of ice cover and extent. In the marine biome there does not appear to be one major driving force, with the exception of the oceanographic boundary of the Polar Front. At smaller spatial scales, ice cover, ice scour, and salinity gradients are clearly important determinants of diversity at habitat and community level. Stochastic and extreme events remain an important driving force in all environments, particularly in the context of local extinction and colonization or recolonization, as well as that of temporal environmental variability. Our synthesis demonstrates that the Antarctic continent and surrounding oceans provide an ideal study ground to develop new biogeographical models, including life history and physiological traits, and to address questions regarding biological responses to environmental variability and change.

Chlorophyll a in Antarctic sea ice from historical ice core data

Sea ice core chlorophyll a data are used to describe the seasonal, regional and vertical distribution of algal biomass in Southern Ocean pack ice. The Antarctic Sea Ice Processes and Climate – Biology (ASPeCt – Bio) circumpolar dataset consists of 1300 ice cores collected during 32 cruises over a period of 25 years. The analyses show that integrated sea ice chlorophyll a peaks in early spring and late austral summer, which is consistent with theories on light and nutrient limitation. The results indicate that on a circum-Antarctic scale, surface, internal and bottom sea ice layers contribute equally to integrated biomass, but vertical distribution shows distinct differences among six regions around the continent. The vertical distribution of sea ice algal biomass depends on sea ice thickness, with surface communities most commonly associatedwith thin ice (<0.4m), and ice ofmoderate thickness (0.4– 1.0 m) having the highest probability of forming bottom communities.

Sedimentation of 13C-rich organic matter from Antarctic sea-ice algae: A potential indicator of past sea-ice extent

Organic matter collected in sediment traps at a nearshore marine site in Prydz Bay, East Antarctica, displayed a pronounced seasonal variation in carbon isotopes (expressed as δ 13 C OM ). Winter values were fairly constant near −20‰ before increasing to above −15‰ in response to inputs from sea-ice algae in early spring. Particulate organic matter obtained directly from sea-ice cores exhibited very high δ 13 C OM values, typically −15‰ but as much as −8‰, much above the values of organic matter suspended in the water column below the sea ice. The sea-ice algae contributed ∼20% of the annual flux of organic matter, and the δ 13 C OM measurements from a sediment core at the site suggest that this value is representative of the recent past. All of these carbon isotopic compositions are considerably above the extremely δ 13 C-depleted compositions (generally −25‰ to −30‰) that have been found in many other Antarctic and Southern Ocean studies. The elevated δ 13 C OM values derive from considerable depletion of dissolved carbon dioxide and nutrients within the sea ice and to a lesser extent within stratified surface waters induced by melting sea ice. Thus, elevated δ 13 C OM values in Southern Ocean sediment cores may indicate the presence of sea-ice-hosted algae, rather than temperature or surface CO 2 pressure ( p CO 2 ) variations.

Paleolimnological studies from the Antarctic and subantarctic islands

To compile reference data for palaeolimnological studies using fossil pigment, we examined the extent to which environmental variables, gross morphology and species composition influence the modern pigment content of in situ microbial communities in 62 east Antarctic lakes. Pigment contents, measured using HPLC, were compared with 32 environmental variables, gross microbial mat morphology and cyanobacterial species composition in each lake. Results showed low concentrations or an absence of pigments in the water columns of most lakes. For benthic microbial communities, multivariate statistical analyses identified lake depth as the most important factor explaining pigment composition. In deeper lakes the pigment composition was dominated by chlorophylls, in intermediate depth lakes by chlorophylls and carotenoids, and in shallow lakes by scytonemins, ultraviolet-screening pigments found in cyanobacteria. In addition to lake depth, conductivity, turbidity, dissolved oxygen, sulphate and geographical location were all significant (p<0.05) in explaining variance in the pigment content. Significant differences in microbial mat gross morphologies ocurred at different lake depths (p<0.01), and were characterised by significant differences in their pigment content(p<0.004). Despite the high abundance of scytonemin in shallow lakes, there were only limited changes in the absolute concentrations of chlorophylls and carotenoids. We conclude that lake depth is the most significant factor influencing both gross mat morphology and pigment content presumably as a result of its influence on the light climate. In general, the ability of the cyanobacteria to regulate their pigment content, morphology, community composition and motility to best exploit thelight environment at different lake depths may explain their dominance in these systems.

Nutrient stress gradient in the bottom 5 cm of fast ice, McMurdo Sound, Antarctica

Abstract One-centimeter-scale vertical sampling of fast ice from McMurdo Sound, Antarctica reveals evidence of progressive nutrient limitation with distance above the ice/water interface. Over the bottom 6 cm photosynthetically active radiation increases by between 1.8 and 3 times, C:N increases from 6.8 to 19.8 and δ13C increases from −18 to −12. Fatty acid composition also changes with a consistent decline in polyunsaturated fatty acids and a rise in saturated fatty acids. These factors all suggest severe and progressive nutrient limitation with distance from the ice/water interface.

A diatom-based palaeosalinity history of Ace Lake, Vestfold Hills, Antarctica

A comprehensive diatom stratigraphy is used to calculate a palaeosalinity history for an Antarctic lake via an established diatom-salinity transfer function for the Vestfold Hills, Antarctica. A sediment core taken from Ace Lake in 1995 shows three distinct changes in diatom assemblage constituents: initial benthic hyposaline – freshwater taxa are replaced by marine planktonic and sea-ice taxa with these taxa in turn replaced by the benthic hypersaline taxa dominant in the lake today. These changes in assemblage composition enable the lakewater salinity of each stage to be determined, and the Holocene evolution of the lake to be refined. Deglaciation of the Vestfold Hills at the beginning of the Holocene exposed Ace Lake basin; following this, fresh lacustrine diatoms were deposited from| 11 380 to| 8110 corrected 14C yr BP. Relative sea-level rise after this time led to the progressive marine inundation of the lake and the deposition of marine diatom taxa. Marine taxa were dominant in the sediment for more than 6000 years. Isostatic rebound and stabilization of the sea-level isolated Ace Lake and at| 1480 corrected 14C yr BP saline lacustrine diatoms became the domi nant taxa, indicative of the concentration of dissolved salts through evaporation after isolation.

Modelling diatom responses to climate induced fluctuations in the moisture balance in continental Antarctic lakes

The water chemistry of lake systems on the edge of the Antarctic continent responds quickly to changes in the moisture balance. This is expressed as increasing salinity and decreasing lake water level during dry periods, and the opposite during wet periods. The diatom composition of the lakes also changes with these fluctuations in salinity and lake water depth. This is important, as their siliceous remains become incorporated into lake sediments and can provide long-term records of past salinity using transfer functions. In order to develop transfer functions, diatoms and water chemistry data were inter-calibrated from five different East Antarctic oases, namely the Larsemann Hills, the Bølingen Islands, the Vestfold Hills, the Rauer Islands and the Windmill Islands. Results indicate that salinity is the most important environmental variable explaining the variance in the diatom flora in East Antarctic lakes. In oligo- saline lakes the variance is mainly explained by lake water depth. This dataset was used to construct a weighted averaging transfer function for salinity in order to infer historical changes in the moisture balance. This model has a jack-knifed r2 of 0.83 and a RMSEP of 0.31. The disadvantage of this transfer function is that salinity changes in oligo-saline lakes are reconstructed inaccurately due to the ‘edge effect’ and due to the low species turnover along the salinity gradient at its lower end. In order to infer changes in the moisture balance in these lakes, a second transfer function using weighted averaging partial least squares (with two components) for depth was constructed. This model has a jack-knifed r2 of 0.76 and a RMSEP of 0.22. Both transfer functions can be used to infer climate driven changes in the moisture balance in lake sediment cores from oligo-, hypo-, meso- and hyper-saline lakes in East Antarctic oases between 102–75°E. The transfer function for lake water depth is promising to track trends in the moisture balance of small freshwater lakes, where changes in shallow and deep-water sediments are readily reflected in changing diatom composition.

A weighted-averaging regression and calibration model for inferring lakewater salinity from fossil diatom assemblages in saline lakes of the Vestfold Hills: a new tool for interpreting Holocene lake histories in Antarctica

The relationship between surface sediment diatom assemblages and measured limnological variables in thirty-three coastal Antarctic lakes from the Vestfold Hills was examined by constructing a diatom-water chemistry dataset. Previous analysis of this dataset by canonical correspondence analysis revealed that salinity accounted for a significant amount of the variation in the distribution of the diatom assemblages. Weighted-averaging regression and calibration of this diatom-salinity relationship was used to establish a transfer function for the reconstruction of past lakewater salinity from fossil diatom assemblages. Weighted-averaging regression and calibration with classical deshrinking provided the best model for salinity reconstructions and this was applied to the fossil diatom assemblages from one of the saline lakes in the Vestfold Hills in order to assess its potential for palaeosalinity and palaeoclimate reconstruction.

Short-term effect of temperature on the photokinetics of microalgae from the surface layers of Antarctic pack ice

Microalgae growing within brine channels (85 psu salinity) of the surface ice layers of Antarctic pack ice showed considerable photosynthetic tolerance to the extreme environmental condition. Brine microalgae exposed to temperatures above −5°C and at irradiances up to 350 μmol photons·m−2·s−1 showed no photosynthetic damage or limitations. Photosynthesis was limited (but not photoinhibited) when brine microalgae were exposed to −10°C, provided the irradiance remained under 50 μmol photons·m−2·s−1. The highest level of photosynthetic activity (maximum relative electron transport rate [rETRmax]) in brine microalgae growing within the surface layer of sea ice was at approximately 18 μmol electrons·m−2·s−1, which occurred at −1.8°C. Effective quantum yield of PSII and rETRmax of the halotolerant brine microalgae exhibited a temperature‐dependent pattern, where both parameters were higher at −1.8°C and lower at −10°C. Relative ETRmax at temperatures above −5°C were stable across a wide range of irradiance.