Main aim of the study was to search for possible differences in diatom colonization and their communities under the influence of glacier meltwater inflow and when unaffected by glacier meltwater, and also to define the time needed for the development of diatom communities on newly submerged substrates at small depths in Antarctica. We used artificial substrates (Plexiglass© tiles), submerged at a depth of 1 m below the sea surface at two locations at the South Bay of Livingston Island: (1) Johnsons Dock – a cove, known to receive glacier meltwater with sediments, and (2) outside the cove, generally unaffected by glacial meltwater. Samples from the natural epilithon at similar depth were also taken as a reference for diatom community structure. Statistical testing the differences between the two sites was not possible this time, but the samples allowed us to compare the sites in terms of diatom growth, species richness, diversity and evenness changes in diatom communities along the time of the experiment at both sites and with the natural epilithon at similar depths. Diatom colonization followed the three-phases scheme (colonization, logarithmic growth and equilibrium) as in other latitudes. Based on the valve density and community indices e.g. species richness, diversity (1-D) and evenness (J’), we consider that at least three weeks might be necessary to obtain sufficiently representative for the environment diatom communities on new substrates at small depths in Antarctica, in conditions similar to those of South Bay. No particular differences between the sites were noted in the colonization scheme, but the diversity (1-D) and evenness (J’) were higher at glacier influenced site, as well as the number of the valves on the substrates. Sea ice diatoms prevailed at the glacier influenced site. We suggest that species exchange between the sea ice and other hard substrates do exist, at least for some taxa, and such species might be indicative for variations in both salinity and water transparency, related to glacial meltwater inflow.

We used an artificial substratum (plexiglass tiles) to compare diatom communities at three different depths at two sites differing in their hydrological conditions and glacier melt-water influence. Samples at 1 m depth were taken during early summer in 2018, whereas samples at 3 m and 6.5 m were obtained in late summer 2020. The tiles were submerged for a period of up to 45 days in 2018, and up to 34 days in 2020. Water temperature, salinity, conductivity, oxygen saturation and concentrations, and Secchi depth were measured multiple times at both sites. During late summer of 2020 Photosynthetically Active Radiation (PAR) was also measured at depths of 3, 6.5 and 10 m at both sites. The communities constituted of a total of 50 taxa. Colonization and community development followed the same scheme at both sites and at all depths, with an early establishment of the dominant taxa, and a decline in species richness, diversity, and evenness indices over the time towards relatively stable low values. Based on the results of PERMANOVA, ANOSIM and SIMPER analyses, diatom communities were site-specific, with 49% dissimilarity between the sites. Mechanical disturbances, such as wave action and ice scouring, as well as depth (and light availability) seemed to be the main factors driving the differences. The motile Navicula aff. perminuta dominated under mechanical disturbances at various light conditions, Navicula glaciei preferred calm shallow waters, and erect diatom growth forms were present in higher numbers in deeper waters with deteriorated light conditions.