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 Bolingen 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 r(2) 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 r(2) 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 102-758degreesE. 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

Hydrological and land-use changes in the Cuzco region (Cordillera Oriental, South East Peru) during the last 1200 years: a diatom-based reconstruction

A quantitative diatom analysis was carried out on a sediment core from the small infilled lake basin of Marcacocha (Cuzco-region, SE Peru), in order to reconstruct environmental changes between 800 and 1850 AD. Five stratigraphical zones were distinguished by means of constrained cluster analysis. Very few diatoms were present between 790 and 1070 AD, probably reflecting dry and cool conditions, at a time when anthropogenic impact was limited around the basin. The transition at ca. 1070 AD was the most pronounced, and corresponded with a marked shift towards higher temperatures, as deduced from plant macroremains and the pollen record. This coincided with a sudden climate shift from cold and dry conditions towards warmer and even drier conditions, as recorded elsewhere in tropical South America. Between 1070 and 1650 AD diatoms (mainly the genus Epithemia Ktzing) became more abundant, together with charophyte oospores, suggesting the existence of a stable, shallow lake. The transition in diatom composition and abundances at ca. 1650 AD, with peaks centered on 1700 AD, lag behind the start of the Little Ice Age (around 1490-1530 AD), though match increased cooling at the end of the 17th and early 18th century as recorded in Peruvian ice cores. This could be caused by a threshold that was passed after the lake level had lowered sufficiently due to a cooling and drying climate, as well as infilling processes. Further transitions in the diatom community may be interpreted less in terms of climatic change, but as increasing sensitivity to local environmental changes, such as a lake level decrease and lake infilling. A hiatus in diatom abundance observed between ca. 1750 and 1810 AD, could be associated with increasing colonization of Juncaceae around the lake margin and rapid infilling, possibly linked to the construction of drainage canals clearly visible today. The final stage of infilling occurred after ca. 1845 AD, with complete colonization of the remaining lake surface by Juncaceae, with further accumulation of peats

Historical processes constrain patterns in global diatom diversity

There is a long-standing belief that microbial organisms have unlimited dispersal capabilities, are therefore ubiquitous, and show weak or absent latitudinal diversity gradients. In contrast, using a global freshwater diatom data set, we show that latitudinal gradients in local and regional genus richness are present and highly asymmetric between both hemispheres. Patterns in regional richness are explained by the degree of isolation of lake districts, while the number of locally coexisting diatom genera is highly constrained by the size of the regional diatom pool, habitat availability, and the connectivity between habitats within lake districts. At regional to global scales, historical factors explain significantly more of the observed geographic patterns in genus richness than do contemporary environmental conditions. Together, these results stress the importance of dispersal and migration in structuring diatom communities at regional to global scales. Our results are consistent with predictions from the theory of island biogeography and metacommunity concepts and likely underlie the strong provinciality and endemism observed in the relatively isolated diatom floras in the Southern Hemisphere

Coastal oceanographic conditions in the Prydz Bay region (East Antarctica) during the Holocene recorded in an isolation basin

Information on East Antarctic coastal environments during the Holocene is relatively sparse. This is surprising as sedimentary records from the interface between land and sea can provide chronologies of climatic change, isostatic uplift, relative sea level and the colonization of newly formed biomes. Here we examine a sediment core from Pup Lagoon, a coastal lake in the Larsemann Hills, East Antarctica. Sediment stratigraphy, fossil pigments and diatoms were used to infer the sequence of Holocene environmental and climatic change. Results show that between 5800 and 5500 cal. yr BP the marine coast of Prydz Bay was characterized by stratified, open-water conditions during spring and summer and seasonally warm conditions. From 5500 to 2750 cal. yr BP sea-ice duration in Prydz Bay increased with the coast being ice-free for 2-3 months each year, conditions which are similar to the present day. A return to stratified, open-water conditions and a reduction in winter sea-ice extent between 2750 and 2200 cal. yr BP is signalled by enhanced biogenic production and more open-water diatom taxa. This is consistent with evidence for the mid-Holocene Hypsithermal detected in other records in East Antarctica. Isostatic isolation of the Pup Lagoon basin from the sea between 2200 and 2000 cal. yr BP slightly precedes the emergence of lakes with comparable sill heights from the nearby Vestfold Hills. The colonization of Pup Lagoon after its isolation as a freshwater lake was initiated by a siliceous flora dominated by stomatocysts with microbial mat development being prevented by mechanical or physical stress. A brief period of marine incursion following the mid-Holocene Hypsithermal may be related to local events such as iceberg calving or to minor sea-level change. Weighted averaging regression, used to infer salinity in the lacustrine zone, shows that from 1500 cal. yr BP Pup Lagoon is a freshwater lake, where the flora is dominated by stratified cyanobacterial mats, with green algae and diatoms as co-dominants, comparable to modem Pup Lagoon and other lakes in the Larsemann Hills