DOI: 10.1594/pangaea.947837
title: Biocrust and sediment characteristics of biological soil crusts in coastal sand dunes in northern Germany

description: This dataset comprises environmental parameters for biological soil crusts in coastal sand dunes in northern Germany. Biological soil crusts (biocrusts) are autonomous ecosystems consisting of prokaryotic and eukaryotic microorganisms growing on the topsoil. They colonize global climatic zones, including temperate dunes. This study examined changes in the community structure of biocrust phototrophic organisms along a dune chronosequence at the Baltic Sea compared to an inland dune in Northern Germany. The community composition and their shift between different successional stages of dune development were related to physico-chemical sediment properties. A vegetation survey followed by species determination and sediment analyses were conducted. The sampling took place on the 25th of April and on the 5th of May 2020. The samples were collected at a costal dune area, namely the Schaabe spit on the island Rügen, Mecklenburg Wester-Pomerania, Germany, and in an inland dune area at Verden (Aller), Lower Saxony, Germany. Biocrust samples were taken along one transect per study site. Each transect followed a natural succession gradient in the dune area. Along each transect, the different successional dune stages were visually identified and further named as dune subsites. At each subsite, a sampling plot of 1 m2 was established and used for further vegetation analyses, biocrust and sediment sampling. Along the Schaabe spit transect four subsites with one sampling plot each were established and three subsites were established in the inland dune in Verden. For the vegetation survey seven different functional groups were defined describing the overall surface coverage: Thin (1-3 mm) green algae-dominated biocrusts were defined as early successional stages. Later successional stages, in which the green algae biocrusts became slightly thicker (3-8 mm) and moss-covered, were defined as the intermediate successional biocrust stage. Moss-dominated biocrusts and those who additionally lichenized characterized the mature successional stages of biocrusts. Vascular plants, and litter (dead material, i.e., pine needles, leaves, and branches) were two of the non-cryptogamic but still biotic functional groups. Bare sediment was the only abiotic functional group. The predefined functional groups were recorded within each plot according to the point intercept method by Levy and Madden (1933). Each of the seven sampling plots was divided into 16 equal subplots (0.0625 m2). A 25 cm x 25 cm (0.0625 m2) grid of 25 intersections was placed randomly into 4 of these subplots. Within each sub-plot, the functional groups were recorded by 25 point measurements according to the approach of Williams et al. (2017). That allowed 100 point measurements per sampling plot (1 m2).
description: Sample collection:For biocrust sample collection, Petri dishes (92 mm in diameter) were used. Within each plot, three biocrust samples were collected for chlorophyll a analyses, sequencing, as well as for algae community cultivation, direct microscopy and identification. Redundant numbering. Replicates number 1-3 in taxa table and sequencing data refer to the same Petri dishes. Replicate numbers 1-3 in the environmental data set are additional Petri dishes NOT the same as for species determination.Additional six biocrust samples were taken for analyses of sediment properties (moisture and organic matter content, and nutrient concentration) in the lab. If detectable, one additional sediment sample was collected in unvegetated areas within each of the seven sampling plots and stored in zip lock bags. These samples represented the crust-free area of each plot and were used for sediment pH measurements. All mosses and lichens detected in the sampling plots were collected by hand and stored in paper bags.Sample processing:In the lab, biocrust and sediment samples used for further analyses were removed from the six Petri dish per plot dedicated to analyses of sediment properties. A razor blade was used to separate the visible biocrust from the underlying sediment. Three of them were used for environmental (moisture and organic matter content) and the remaining three for nutrient (Ct, Nt, Pt) analyses. For environmental analyses the biocrust material from the three Petri dishes was separately weighed for fresh mass (FM g) determination. Afterward, the samples were dried at 105 °C for 24 h and weighed again to determine the dry mass (DW g) and calculate the water content. The organic matter (OM) content was calculated based on the weight loss after combustion at 450 °C for 5 h. The moisture content was expressed as a percentage of total fresh mass (% FM) and the organic matter content as a percentage of total dry mass (% DW). Each of the sample for nutrient analyses was further sieved (2 mm mesh size). From the now homogenized samples, three subsamples each were filled into PVC tubes for further analysis. The pH of the each crust-free sediment sample was measured in a calcium chloride (0.01 M) solution after one hour (w/v ratio 1:4) with a pH meter (METTLER TOLEDO SevenMulti). Chlorophyll a (Chl a) content was taken as a measure for the photosynthetic biomass (chlorophyll a m-2). Chlorophyll a was extracted in 3 ml of 96% ethanol (v/v) for 30 min at 78°C. Samples were shaken afterward and cooled on ice for 10 min followed by centrifugation at 5088 g for 5 min at 5°C to decrease turbidity. The supernatant was carefully pipetted into a 1 cm quartz cuvette. A spectrophotometer (Shimadzu UV-2401 PC, Kyoto, Japan) was used for measuring the Chl a absorbance at wavelengths of 632, 649, 665, and 696 nm. The chlorophyll a content was calculated according to (Ritchie 2008) and normalized to a square meter (m2).