Mieke Sterken

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.

Revision of the genus Navicula s.s. (Bacillariophyceae) in inland waters of the Sub-Antarctic and Antarctic with the description of five new species

Van de Vijver B., Zidarova R., Sterken M., Verleyen E., de Haan M., Vyverman W., Hinz F. and Sabbe K. 2011. Revision of the genus Navicula s.s. (Bacillariophyceae) in inland waters of the Sub-Antarctic and Antarctic with the description of five new species. Phycologia 50: 281–297. DOI: 10.2216/10-49.1 A revision of the diatom genus Navicula s.s. in inland water samples from the Sub-Antarctic and Antarctic resulted in the description of five new species, viz. Navicula australoshetlandica Van de Vijver sp. nov., N. dobrinatemniskovae Zidarova & Van de Vijver sp. nov., N. cremeri Van de Vijver & Zidarova sp. nov., N. conveyi Van de Vijver sp. nov. and Navicula bicephaloides Van de Vijver & Zidarova sp. nov. The new species are extensively compared with other similar species. The morphology and taxonomy of three other Navicula taxa present in the same materials are briefly discussed. A critical revision of literature reports of Navicula taxa in the Antarctic and Sub-Antarctic region indicates that despite the large number of records the genus is rather species-poor in the Sub-Antarctic and Antarctic. From a biogeographical point of view, there is a clear separation between the Sub-Antarctic localities in the southern Indian Ocean and the islands in the southern Atlantic Ocean, with several species showing a restricted biogeography.

FOUR NEW NON-MARINE DIATOM TAXA FROM THE SUBANTARCTIC AND ANTARCTIC REGIONS

A recent survey of the non-marine diatoms from islands in the Atlantic sector of the Southern Ocean (King George Island, Deception Island, Livingston Island, James Ross Island, Signy Island, South Georgia and Beak Island) resulted in the description of four new diatom species: Chamaepinnularia gerlachei Van de Vijver & Sterken sp. nov., Craticula antarctica Van de Vijver & Sabbe sp. nov., Craticula subpampeana Van de Vijver & Sterken sp. nov. and Diadesmis australis Van de Vijver & Sabbe sp. nov. The morphology of each species is described on the basis of light and electron microscopy. The new species are compared to similar taxa, especially to those present in the Subantarctic and Antarctic Region. Preliminary observations on the ecology and biogeography of each species are provided.

Congruence and disparity in benthic diatom community structure of small lakes in New Zealand and Tasmania

The ecological characteristics of benthic diatom genera from lakes and tarns in mountainous areas of Tasmania (76 lakes) and the South Island (65 lakes) and Stewart Island (6 lakes) of New Zealand were investigated. Community composition and diversity were mainly governed by gradients in calcium, pH and the monovalent/divalent ionic (M/D) ratio, with typical acidophilous and calciphilous communities present in both regions. Highest genus diversity occurred in the pH range between 5.5 and 7.5. Marked interregional differences were present in both calciphilous and acidophilous diatom community assemblages, which were at least partially related to variations in the concentration of the chloride, sodium, potassium and humic substances. Acidophilous communities in New Zealand were typically dominated by Frustulia, Brachysira and Kobayasiella, whereas Eunotia and Actinella dominated in Tasmania. Calciphilous communities in New Zealand were characterised by higher relative abundances of the genera Hantzschia, Diploneis, Nupela, Stauroneis and Synedra, whereas their Tasmanian counterparts were typified by the genera Amphora (subgenus Psammamphora), Biremis, Navicula and Psammothidium. The provinciality of the floras underscores the need for continued protection and conservation of high latitude aquatic ecosystems worldwide and in the Australasian region in particular.

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.