The study results pointed out to a connection between the different dune successional stages and the phototrophic community living on it. The enrichment of organic matter, moisture, and phosphorus content with advanced biocrust development was detected.
The vegetation survey revealed a successional development of biocrust stages from a thin topsoil layer of algae to stable moss and lichen-dominated ‘mature’ communities, along the two investigated dune chronosequences. This confirmed our first hypothesis. In addition, the overall areal cover of biocrusts across the sampling plots increased along the succession gradient. These results are in line with a strong zonation from grey dunes to mature dunes regarding the vascular plant communities at the German Wadden Sea coast described with (Isermann 2011). Along the dune chronosequences and the succession series, respectively, the organic matter and moisture content increased at both our study sites, thus reflecting a shift in the biocrust communities.
Foredunes or equivalent the center of the inland dune are characterized by the influence of strong wind resulting in a highly mobile substrate. Additionally, nearshore higher sediment salinity and air-borne salt spray cause harsh conditions at coastal dunes. Along with a scarcity of nutrients, neither a stable plant nor a biocrust cover could establish at both study sites (García Novo et al. 2004; Hesp 1991; Lane et al. 2008; Martínez et al. 2004). Consequently, the organic matter and chlorophyll a content were lowest in these mobile dune types. In addition, microorganisms such as filamentous green algae or cyanobacteria, which might glue sand particles together by excreted exopolysaccharides (van Ancker et al. 1985; Belnap 2006; Zhang 2005), were missing. Hence, no coherent top-soil layer can be formed. Consequently, precipitation will easily seep away in loose sandy sediment (Gypser, Veste, et al. 2016). The earliest successional stages of biocrusts were found in the chronologically following intermediate dune at the Schaabe spit and on the dune slope in the inland dune (Verden). They were mainly formed by green algae, covering approximately half of the dune’s surface, forming patchy and thin microbial layers. The occasionally growth of mosses and lichens could be observed as well. However, the establishment of mature biocrust types could hardly be observed in intermediate dunes due to the frequent disturbance by erosive wind forces leading to sand mobility (Martínez et al. 2001) or due to water limitation caused by the low-holding capacity of sand (Chamizo et al. 2016; Gypser, Veste, et al. 2016). Soil moisture can positively affect carbon and nitrogen fixation by microorganisms, as it was observed for biocrusts in arid regions (Belnap and Eldridge 2001). Especially the accumulation of organic carbon by microbial biomass formation plays an essential role in the early pedogenesis (Kaviya et al. 2019). Organic carbon accumulation is controlled by the turnover rates of soil organic matter (Šourková et al. 2005). The present organic matter accumulation was assumed to be based on faster biomass formation and litter input than decomposition within the studied two dune areas. It is assumed that the increase in organic matter within the biocrust has the potential of increasing the plant-available P concentration in the sediment. Inorganic bound P can be solubilized from parent material either by the secretion of organic acids by the microorganisms (Fox 1995) or by an increase in the pH (Blume et al. 2010). Phosphatases, produced by the biocrust organisms are known to hydrolyze organic phosphates releasing P (Nannipieri et al. 2011). The increase in total P in the biocrusts along both chronosequences can be explained by sedimentation, but mainly by the general increased biomass formation. Increased phototrophic biomass is revealed by the chlorophyll a content. Depending on sampling site, season, biocrust communities, and successional type the Chl a content varies clearly. Lange (2001) reported a Chl a content up to 100 mg m-2 in biocrusts formed by cyanobacteria and eukaryotic algae, whereas lichen- or bryophyte-dominated biocrusts reached values above 900 mg m-2. Just as it is proposed by Büdel et al. (2009) the presented study could prove a significant increase in the chlorophyll content from biocrusts of the early successional stage to later ones. In a semi-arid desert, Büdel et al. (2009) measured a mean Chl a value of 118.9 ± 35.8 mg m-2 in an area with strong coverage of biocrusts with bryophytes, dominated by mosses.These findings fit in with the Chl a value of the dune slope of the inland dune, which was also dominated by green algae biocrusts associated with mosses (112.07 ± 20 mg m-2). The Chl a value of the intermediate dune (161.34 ± 56.51 mg m-2) and grey dune (150.92 ± 26.49 mg m-2) on the Schaabe spit exceeded this value slightly but lay close together. This finding highlighted these two dune types as an interface of succession concerning their phototrophic biomass accumulation. One reason could be their largely shared moss and lichen community composition. Along the dune chronosequences, the successional stages of biocrusts gain more biomass due to thicker green algae layers and their by-products. Moreover, the gain in biomass (Chl a) and sediment organic matter might be caused by the increased establishment of mosses. The values of moss-dominated biocrusts in the presented study (DF inland dune = 210.80 mg m-2; MD coastal spit = 287.63 mg m-2) were significantly higher than those measured by (Gypser, Herppich, et al. 2016). Gypser, Herppich, et al. (2016) described the mean Chl a content of moss- and lichen-dominated biocrusts in an artificial temperate sand dune are of 68.9 mg m-2.
Moss-dominated biocrusts took over and covered about half of the investigated plot within the grey dune at the Schaabe spit. These biocrust types formed a denser and coherent layer on the sediment surface. The growth of lichens increased significantly. No bare sediment was detected in this dune area. The increase in moss and lichen biomass along the chronosequence characterized the transition from the intermediate dune to the established grey dune area (Vázquez 2004). However, the percentage coverage by vascular plant was highest in this dune type, mostly dominated by Poaceae. The development of grey dunes was characterized by increased sand stabilization due to less sand accumulation (Martin 1959). Therefore, vegetation cover could expand and support the formation of an organic matter layer on the sediment surface (Isermann 2011). Such a layer was composed of dead plant material along with living phototrophic and heterotrophic biomass, originating from the biocrust. A similar trend was reported by (Gypser, Herppich, et al. 2016), who showed a progressively increase of biomass and the total chlorophyll content from initial to moss-lichen dominated biocrusts at post-mining sites in Lower Lusatia, north-east Germany.The mature dune area landwards at the Schaabe spit and respectively the area close to forest in the inland dune represented the oldest successional stages of dunes along the investigated dune chronosequences. They differed from (younger) dune types by a closed biocrust cover dominated by mosses with the highest occurrence of lichens. These covers grew under a light canopy of a coastal pine forest. Mature dunes, as the latest phase of dune succession, represented a high organic matter content as an indicator of soil formation (Dümig et al. 2014). This assumption is consistent with the observations made in the present study and in line with Isermann (2011). Here, the organic matter accumulation originated from annual vascular plants, scrubs, and smaller trees. The observed increase in the sediment moisture content could be ascribed to the thick biocrust, by absorbing water into the cellular mucilage and reducing evaporation, thereby stimulating infiltration (Belnap et al. 2013; Chamizo et al. 2012) and soil water content (Berdugo et al. 2014). Chamizo et al. (2016) highlighted the later stages of biocrust succession as dominated by lichens and mosses by their higher infiltration capability and water retention finally leading to higher soil moisture. The presented results showed similar patterns and are in line with a study of Gypser, Veste, et al. (2016) in which green algae tend to inhibit water infiltration due to pore-clogging by algal filaments, thereby increasing water runoff. In contrast, later successional stages of biocrusts, dominated by mosses and lichens, can absorb more water and reduce runoff compared to the algae-dominated biocrusts. Particularly moss rhizoids can facilitate water infiltration into deeper soil layers (Dümig et al. 2014).
As proposed in the second hypothesis, the phototrophic biocrust community composition was shaped by the developmental stage of the dunes along each chronosequence. While algae were found in all biocrust-holding plots, their individual species compositions and most frequently detected taxa shifted between the developmental dune stages. The same trend applied for the moss and lichen species. Only a few species were omnipresent along each chronosequence. Comparing both study sites, regional differences concerning the community structures became visible. While the algae community showed many similarities, the moss and lichens community differed significantly more between the two study sites. Based on these findings, the third hypothesis could be partly confirmed. The differing geographical regions had a noticeable impact on the specific biocrust community.
Algae and cyanobacteria in biocrusts
Comparing the overall green algae and cyanobacteria species richness at the Schaabe spit with the latest studies on biocrust microbial community diversity in this area conducted by Schulz et al. (2015) similarities but also conspicuous differences were obvious. Even though biocrusts are consistently formed by algae rather than cyanobacteria species richness can differ. While Schulz et al. (2015) detected 70 cyanobacterial and non-diatom algal taxa in association with biocrusts in coastal dunes at the Baltic Sea the recent study could reveal only twelve. Since species composition in biocrusts very likely varies with season (Bu et al. 2018) this might be one reason for dominance of green algae in spring biocrusts and cyanobacteria in autumn biocrusts (October Schulz et al. 2015, April/May in this study). Much less species number was detected in biocrusts of the present study mostly concerned with identification of the most frequent and dominating species. While complete species composition of algae and cyanobacteria of biocrusts was showed in Schulz et al. 2015 and related papers (Mikhailyuk et al. 2019, 2021). It was determined using different methods and so-called integrative approach: direct microscopy, enrichment and pure cultures, molecular identification of some species etc.). Biocrusts consisted mainly of few green algae and cyanobacteria in the early successional stages of dune development at both temperate study sites. This is contrasting to studies carried out in arid or semiarid deserts where cyanobacteria where always dominant in early successional biocrusts (Ashley et al. 1985; Büdel et al. 2009; Lange et al. 1992; Veste et al. 2011; Zaady et al. 2000). However, the presented findings are comparable to other studies conducted in temperate areas. In temperate forests Glaser et al. (2017) recorded 52 algal species and only very few cyanobacteria in biocrusts. Additionally, 17 eukaryotic algae and 15 cyanobacteria species were found in initial biocrusts in a sand ecosystem in the northern upper Rhine valley (Germany) (Langhans et al. 2009).
In the center of the inland dune only a few algae taxa could be observed in the bare sand, even if they did not form a thin biocrust layer. Contrary to the foredune close to the Baltic Sea microorganisms at inland dunes do not have to cope with salt and wind stress. This might facilitate their survival in the mobile sediment of the inland dune center. But it can not be excluded that the detected algal species were dormant stages. These algae can act as pioneer colonizers of the mobile sand facilitating further stabilization and colonization. Light green algae dominated biocrusts were the only one containing cyanobacteria along the whole dune chronosequence. One reason for the rare occurrence of cyanobacteria could be the sediment conditions, in particular the pH. For optimum growth of cyanobacteria-containing biocrusts sediment pH should be neutral to slightly alkaline (Singh et al. 2014). Under acidic sediment conditions (pH less than 4 or 5) Brock (1973) could confirm that cyanobacteria were completely absent. The sediment pH along the two dune chronosequences showed a landward strong decrease from 6.4 to 4.0 (coastal spit, Schaabe) and from 4.5 to 3.4 (inland dune, Verden). These acidic conditions in the inland dune might limit the growth of cyanobacteria. The biocrusts in the intermediate dune at the Schaabe spit showed only slightly acidic conditions (6.1) and thus few cyanobacteria could grow. Besides the geological impact on pH dynamics the coniferous vegetation (e.g., Pinus sylvestris L.) typically leads to a decrease in the soil pH due to the microbial decomposition of pine tree litter (Maun 2009). Considering the shifts in vegetation cover along both dune chronosequences, the oldest successional dune areas were located under or close by the dune forest. These areas, dominated at both study sites by pine trees, could be one potential reason for the decrease in sediment pH and causing unfavorable growth conditions for cyanobacteria in the mature dune area. Nevertheless, it could be possible that not all cyanobacteria were detected due to chosen cultural approach. Previous studies showed the necessity to use different determination approaches e.g., morphological and molecular-based revealing a more complete community structure of biocrusts (Mikhailyuk et al. 2019; Rippin et al. 2018).
The biocrusts on the dune slope of the inland dune mostly consisted of Chlorophyta and representatives of the Charophyta, class Klebsormidiophyceae. Moreover, this dune successional stage was the species richest along the whole chronosequence. These findings were identical to those of the intermediate dune on the Schaabe spit. Here, most algal species could be detected. Representatives of the genus Klebsormidium were only found in this dune successional stage. Many studies point to Klebsormidium as a genus, holding species with a wide tolerance range in temperature, water availability, or sun radiation (Holzinger and Karsten 2013; Karsten and Rindi 2010; Kitzing et al. 2014). To cope with such extreme changing environmental conditions Donner et al. (2017) assume certain plasticity of the genus morphology as a benefit to adapting to these varying abiotic stress. These filamentous algae are known as major biocrust forming taxa (Holzinger and Karsten 2013). Glaser et al. (2017) could prove Klebsormidium was the most important biocrust-initiating algae in a temperate forest ecosystem. In an inland dune area in the Netherlands Pluis (1994) found Klebsormidium to be the initial green algae genus in the successional development of the biocrust community. Likewise, representatives of the genus Klebsormidium were found in dynamic dune successional stages along both transects, highlighting the initialization of biocrust development.
With further dune succession, the algal species richness declined. Four species of the family Trebouxiophyceae were found in the area close to forest in the inland dune and the mature dune on the Schaabe spit, having two of those species in common. Such clear differences between the algal community structure of initial and later successional biocrust stages were also shown in a sand ecosystem in the northern Rhine valley (Germany). Here, Langhans et al. (2009) could show a decrease from 17 eukaryotic algae in early successional biocrust stages to 13 species in later successional biocrust stages, which is in common with this study.
Mosses and lichens
Moss- and lichen-dominated biocrusts were distinctive of older stages of development. In phytosociology (Ellenberg 1996), the development of a cryptogam layer marks the beginning of grey dune development, as drifting sand will not cover the small cryptogams. On the other hand, the cryptogam layer helps to fix small amount of still drifting sand and bring organic litter into the sediment.
An incipient moss und lichen growth could be described on the dune slope of the inland dune and the intermediate dune on the Schaabe spit, respectively. Ceratodon purpureus was found in both transitional stages of dune succession. This moss species is typical for mobile dune types due to its high tolerance to sand deposition. A study on response of mosses to experimental burial by Martínez and Maun (1999) could show a high tolerance of C. purpureus of up to 7 cm. This species is the only one the dune slope in the inland dune and the intermediate dune on the Schaabe spit had in common. Along both chronosequences, C. purpureus was observed in transitional dune types as well as in later dune stages, like the area close to forest in the inland dune and on the fixed grey dune on the Schaabe spit. These findings are in line with Gypser, Herppich, et al. (2016) who described the growth of C. purpureus as an initial moss as well as dominant in mature moss-lichen biocrusts. Another species only found on the dune slope of the inland dune is Syntrichia ruraliformis. This species is highly desiccation-tolerant (Mishler and Oliver 1991). This trait is favourable for a moss growing on dune types which are under constant abiotic stress, like higher irradiation and low water holding capacity. Surprisingly, by chance this species was not found along the Baltic Sea chronosequence. Nevertheless, it is quite common in other sand dunes along the Baltic Coast (Meinunger and Schröder 2007). The following later stages of dune succession revealed a shift in community composition along each chronosequence. Pioneer moss species, like C. purpureus, decreased and were supplemented by mosses of later successional stages like Dicranum scoparium and Hypnum cupressiforme var. lacunosum. Gypser, Herppich, et al. (2016) described Polytrichum piliferum as dominant species of mature biocrusts which is in line with this study where P. piliferum was only found in the later successional stage close to forest of the inland dune chronosequence. The mature dune on the Schaabe spit chronosequence was low in moss species richness and was dominated by mosses of the genus Hypnum. The dune area close to forest on the inland dune showed the first growth of lichens along this chronosequence. Exclusively lichens of the genus Cladonia were found. Along the dune chronosequence on the Schaabe spit the intermediate and grey dune were the lichen species richest dune types. Whereas, in the intermediate dune lichen species primarly growing on litter and debris where found. Which includes Bacidina etayana, Lecanora persimilis, and Caloplaca cerinella. Moreover, Placynthiella uliginosa is a species also growing on bare sandy soil and was described to inhabit a sandy inland dune are of Sorgwohld (Schleswig-Holstein, Germany) (Dolnik and Neumann 2020). Later successional dune stages along both investigated chronosequences were dominated by representatives of the genus Cladonia. Cladonia primarily inhabits nutrient-poor soils and forests. This corresponds to the environmental conditions und which they were found.