Ecological characterization of soil-inhabiting and hypolithic soil crusts within the Knersvlakte, South Africa

TLDR: In this paper, a transect approach was used to determine the frequency of hypolithic growth depending on the size, weight, and embedding depth of the quartz pebbles.

Abstract: Within the Knersvlakte, cyanobacteria occur hypolithically underneath translucent quartz stones in areas with quartz pavement and, outside pavement areas, they are soil-inhabiting within the uppermost millimeters of the soil. Both habitats were characterized in terms of biomass and growth patterns of cyanobacteria. Long-term microclimatic conditions were determined. Biomass of organisms within both habitats was determined by means of chlorophyll analyses. A transect approach was used to determine the frequency of hypolithic growth depending on the size, weight, and embedding depth of the quartz pebbles. Organisms were identified by means of microscopic analyses of the samples. Microclimatic conditions within both habitats, i.e., temperature, light intensity, air humidity, and soil moisture, were recorded bi-hourly from September 23, 2004 through September 7, 2006. The biomass of hypolithic and soil-inhabiting crusts was almost identical, 88 vs. 86 mg Chla/m2 and 136 vs. 134 mg Chla+b/m2. Within the quartz fields, 46.8% of the surface area was covered by quartz stones with 69% of translucent quartz stones colonized by hypolithic cyanobacteria and algae. Colonized quartz stones were significantly thicker, heavier, and more deeply embedded in the soil than uncolonized ones. Whereas the annual mean temperature on top of quartz stones was nearly identical to that underneath thin and thick quartz stones, daily temperature amplitudes were largest on the stone surface (up to 48.1K), compared to the hypolithic habitats (up to 39.4K). Light intensity in the hypolithic habitat was between 15 and 30% of the ambient light intensity during daytime. Water condensation in the absence of rain occurred during 50% of the nights on the quartz stone surface, but only during 34% of the nights on the soil surface during winter months within 1 year. Soil moisture beneath quartz layers was greater and less variable than beneath soil-inhabiting crusts. In spite of the large differences in the microclimatic conditions, both habitats seem to be similarly well suited for cyanobacterial growth, resulting in equal biomass values but some differences in taxonomic composition.