Substrate specificity and fine-scale distribution of epiphytic diatoms in a shallow tarn in the Brenta Dolomites (South-eastern Alps)

Background and aims – The host-specificity of epiphytic diatom species has long been debated. Scuba divers sampled epiphytic diatoms in the shallow Alpine Lake Valagola (average depth c. 2 m) along seven transects (length: 30–144 m) in West-East direction. The bottom of the tarn was covered by macrophytes dominated by Chara aspera and Potamogeton gramineus. Factors affecting epiphytic-diatom spatial distribution at a fine scale were tested. Methods – Dataset was tested using Redundancy Analysis (CANOCO package) and one-way ANOVA (NCSS package). Key results – The analysis separated sampling sites into two groups: the tarn shore dominated by Potamogeton gramineus, and the central area dominated by Chara aspera. Diatom species richness, diversity, and composition differed significantly between the two main host plants. Potamogeton gramineus assemblages were characterized by higher species richness and diversity, and by the large-celled, adnate diatom species Epithemia adnata, Rhopalodia gibba, Eunotia arcus and E. arcubus. Chara aspera was preferred by the small-celled, motile diatom species Brachysira neoexilis and Encyonopsis cesatii. Conclusions – The spatial distribution of epiphytic diatoms in the shallow, oligo-mesotrophic Lake Valagola is influenced by host plant composition and distribution. Epiphyton size structure suggests that Chara represents a less appropriate substrate for long diatoms

Heterothallic auxosporulation, incunabula and perizonium in Pinnularia (Bacillariophyceae)

The cytology and life cycle of Pinnularia cf. gibba was examined in nine clones from three Scottish localities. This freshwater epipelic diatom is heterothallic and produces two isogametes per gametangium in type IC auxosporulation (Geitler's classification [1973]). The zygote undergoes a highly unusual metamorphosis before beginning expansion, becoming shortly linear-lanceolate; this is accompanied by formation of a complete covering of thin, oxidation-resistant strips and scale-like structures (at the poles), which are quite separate from the perizonium formed during auxospore expansion. Observations of similar incunabular structures in P. acidojaponica show that these elements are siliceous. The P. cf. gibba perizonium also has unusual features, including a remarkably wide primary band. Trikaryotic and haploid auxospores are sometimes formed and haploid 'zygotes' mature and expand like diploids, but do not develop into mature initial cells. Several phases of mucilage secretion take place, from the gametangia, zygotes and auxospores. Triplets of gametangia and polyspermy occurred with high frequency; this and the systematic significance of variation in auxospore, incunabula and perizonium structure, are discussed. Aspects of the taxonomy of the P. gibba group are treated in supplementary material provided on the European Journal of Phycology website

The contribution of epipelon to total sediment microalgae in a shallow temperate eutrophic loch (Loch Leven, Scotland)

Benthic microalgae are known to perform important ecosystem functions in shallow lakes. As such it is important to understand the environmental variables responsible for regulating community structure, positioning and biomass. We tested the hypothesis that the positioning (across a depth gradient of 2 – 22 m overlying water depth) and relative biomass (determined using bulk and lens tissue harvested chlorophyll (Chl) a concentrations) of the epipelon community would vary independently with season (12 monthly samples) and across natural gradients of light and habitat disturbance relative to the total benthic algal community (i.e. all viable microalgae in the surface sediments) in a shallow eutrophic lake. Total sediment microalgal Chl a concentrations (TS-Chl; range: 5 to 874 µg Chl a g-1dw) were highest in winter and in the deepest site (20 m overlying water depth), apparently as a result of phytoplanktonic settling and sediment focussing processes. Epipelic Chl a concentrations (Epi-Chl; range: < 0.10 to 6.0 µg Chl a g-1dw) were highest in winter/spring, a period when water clarity was highest and TS-Chl lowest. Principal components analysis highlighted strong associations between Epi-Chl and sites of intermediate depths (2.5 m to 5.5 m) in all seasons except autumn/winter. Autumn/winter represented the season with the highest average wind speeds preceding sampling, during which the highest Epi-Chl concentrations were associated with the deepest sites. Epi-Chl was associated with intermediate light and habitat disturbance during spring/summer and summer/autumn and varied positively with habitat disturbance, only, in autumn/winter and winter/spring. The epipelon community structure also varied with depth; diatoms dominated shallow water sediments, cyanobacteria dominated deep water sediments, and sediments at sites of intermediate depth returned the highest biovolume estimates and the most diverse communities. This study has strengthened the hypothesis that the structure and biomass of benthic microalgal communities in lakes are regulated by habitat disturbance and water clarity, both of which are expected to respond to climate change and eutrophication. The degree to which these structural responses reflect functional performance requires clarification