Pieter Ledeganck

Habitat preferences in freshwater diatom communities from sub-Antarctic Îles Kerguelen

A limited sampling campaign on the main island of Îles Kerguelen revealed 170 freshwater diatom taxa, belonging to 29 genera. Four different communities were distinguished in two major groups, being determined principally by habitat type. The Fragilaria vaucheriae–Fragilaria germainii assemblage occurs exclusively in running waters. The three other communities, observed in standing waterbodies, are separated by their geographical position on the island with the Fragilaria exigua–Aulacoseira alpigena and the Fragilaria exigua–Fragilaria vaucheriae assemblages, mainly found in the Lac Studer area and the Fragilaria pinnata var. pinnata assemblage, recorded near the coastal line and the research station. Analysis of different waterbody types revealed the habitat preference of the various diatom taxa and identified a diatom flora for running waters and one preferring stagnant waterbodies.

Plant Functional Group Diversity Promotes Soil Protist Diversity

We tested whether effects of plant diversity can propagate through food webs, down to heterotrophic protists not linked directly to plants. To this end we synthesised grassland ecosystems with varying numbers of plant functional groups (FGN) and assessed corresponding changes in testate amoebae communities. The number of plant species was kept constant. When FGN was increased from 1 to 3, species number and total community density of live testate amoebae were enhanced according to a linear and a saturating function, respectively. From FGN 1 to 2, the appearance of new testate amoebae species did not affect the presence of the resident species, whereas, from FGN 2 to 3 about one quarter of the resident testate amoebae species was replaced, without altering the total species number. Overall, density by species increased, while evenness of the testate amoebae community was not affected by FGN; although Trinema lineare, one of the most common species, became more abundant. The observed relationship between plant functional group diversity and testate amoebae diversity could shed new light on the biogeographical distribution patterns of protists.

Are soil biota buffered against climatic extremes? An experimental test on testate amoebae in arctic tundra (Qeqertarsuaq, West Greenland)

Climate warming is likely to have pronounced impacts on soil biota in arctic ecosystems. In a warmer climate, heatwaves are more frequent and intense, but it is unclear to what extent soil communities are buffered against this. We studied the effects of an artificially induced heatwave on the structure of testate amoebae communities in dry heath tundra in Qeqertarsuaq (Disko Island, West Greenland) during the summer of 2003. While the heatwave was severe enough to induce significant leaf mortality in the aboveground vegetation, overall testate amoebae abundance did not react to the difference in temperature. However, in the heated plots transient shifts in species populations occurred during the exposure, followed by increases in species richness weeks after the heatwave had ended. The most important taxa appearing after the heating period belonged to bacterivorous genera, in agreement with a transient peak in bacterial colony forming units, caused by the heatwave. Lobose testate amoebae resisted the heating and its associated desiccation better than their filose counterparts.

Soil testate amoebae from sub-Antarctic Îles Crozet

An ecological study of the soil testate amoebae (Protozoa, Rhizopoda) communities on the sub-Antarctic island of Île de la Possession (Îles Crozet) revealed 65 taxa, belonging to 20 genera. Trinema lineare and Euglypha laevis were the most dominant species in all types of soils on the island. A Redundancy Analysis revealed three communities that characterize specific habitats: i. fellfield soils, characterized by low moisture values and low organic material content, contained fewer taxa and fewer individuals, ii. peaty valley soils, with lower pH values, showed high abundances and diversity, and iii. soils influenced by sea spray and marine animals had higher nutrient contents, elevated chloride and conductance values, resulting in more tests of Difflugiella oviformis, Nebela dentistoma and Edaphonobiotus campascoides. Special attention was given to the species composition in relation to the soil type and its moisture content. A comparison with aquatic habitats on Île de la Possession was made.

Soil diatom communities from Ile de la Possession (Crozet, sub-Antarctica)

Abstract. An ecological study of soil diatoms on Ile de la Possession (Crozet Archipelago, sub-Antarctica) was carried out during the austral summer of 1998/1999. Both diatom and chemistry data were collected. A highly diverse diatom flora of 230 taxa, belonging to 39 genera were identified from 104 samples. Several of them are considered to be new. The most abundant genera were Diadesmis, Achnanthes and Pinnularia. Principal Component Analysis (PCA) was used to classify the samples based on their chemical characteristics. Moisture and nutrients were the main factors separating three groups of samples: dry fellfield soils, soils influenced by marine animals and mesic valley soils. Species assemblages correspond well with this division. Forward Selection with Monte Carlo permutations was used to reduce the number of significant variables. The analysis selected moisture, phosphate and sulphate concentrations. Based on the analysis, the same three groups of the PCA were found. Weighted averaging and calibration were used to develop a statistical transfer function to infer the moisture content of soils from the diatom assemblages.

LUTICOLA BEYENSII SP. NOV., A NEW AEROPHILOUS DIATOM FROM ILE SAINT PAUL (INDIAN OCEAN, TAAF)

The terrestrial diatom Luticola beyensii sp. nov. is described &om relatively dry soil samples on the subtropical Saint Paul Island (Indian Ocean, TAAF). The detailed morphology of this taxon is examined using both light and scanning electron microscopy (SEM) and compared with similar species. The structure of the raphe terminations combined with the valve outline and the structure of the central area are the main morphological features. L. beyensii is found in bare, dry soil samples dominated by Diadesmis-taxa.

Testacean communities in perturbed soils: the influence of the wandering albatross

On Île de la Possession (Crozet Archipelago, sub-Antarctica), the testate amoebae (Protozoa, Rhizopoda) fauna in soils around abandoned and occupied nests of the wandering albatross (Diomedea exulans) was investigated. A comparison with control samples, a cluster analysis and several ordination techniques indicated that the presence of the breeding albatrosses induced modifications in physico-chemical soil characteristics and in the soil inhabiting testacean fauna. Only 11 testate species occurred frequently in the soils in the albatross’ zone. Soils around occupied nests had significantly higher moisture values, less acid pH values, an increased specific conductance and elevated phosphate and ammonium concentrations. Highly influenced testacean communities were characterised by high abundances of Difflugiella oviformis and extremely high abundances of Trinema lineare, resulting in a very low diversity and evenness within these communities. The intermediary situation of one abandoned nest indicated that soils around abandoned nests evolve gradually back to undisturbed soils. Although an overall negative influence on the testacean diversity, a limited albatross’ influence may increase the living fraction within the testacean soil communities.

The Distribution of Soil Testate Amoebae under Winter Snow Cover at the Plot-scale Level in Arctic Tundra (Qeqertarsuaq/Disko Island, West Greenland)

Summary. Understanding the spatial distribution of soil protozoa under the snow cover is important for estimation of ecosystem responses to climate change and interpretation of results of fi eld experiments. This work explores spatial patterns of soil testate amoebae under the snow cover at the plot scale (the range of metres) in arctic tundra (Qeqertarsuaq/Disko Island, West Greenland). To explain spatial patterns in abundance, species diversity and assemblage composition of testate amoebae, we measured microtopography, snow depth and substrate density. The results indicate that the abundance of active testate amoebae under the snow cover was quite low. The empty shell assemblage was characterised by the presence of linear spatial trends in the species composition across the site, whereas no patterns were detected within the plot. The distribution of the abundance and the species diversity were unstructured. The linear trends in the species composition corresponded to the site microtopography and were controlled by the topography-related soil moisture. Snow depth also affected the linear trends presumably by controlling soil temperatures. Overall, the results suggest that population processes do not generate spatial patterns in protozoan assemblages at the plot scale so that protozoan distribution can be considered random at macroscopically homogeneous plots.