| dc.description.abstracteng | The Wadden Sea is one of the largest tidal flats in the world, stretching across the coasts of the Netherlands, Germany and western Denmark. Along sheltered coastlines lie salt marshes which are essential habitats for a variety of organisms. Salt marshes are considered Blue Carbon ecosystems due to their high carbon storage capacity. This is mainly due to the influx of carbon from the ocean and burial of local plant materials in anoxic sediments. Salt marshes show a gradient in shore height, producing distinct zones, each with separate abiotic conditions. In the lowest reaches of the marsh, called the pioneer zone (PZ), inundation occurs daily for several hours. Above the PZ lies the lower salt marsh (LSM), which is inundated during spring tides. Above lies the upper salt marsh (USM) which is only inundated during storm surges. Shore height not only alters inundation frequencies, but it also creates a gradient of allochthonous marine input from the Wadden Sea. Furthermore, due to seasonal changes in tidal range, inundation frequencies across these zones may change.
Despite these spatial and temporal changes in abiotic conditions and allochthonous influx, little is known about how this influences soil microorganisms and soil meso- and macrofauna. Soil microorganisms as well as soil fauna are essential in the cycling of nutrients and thus impact soil health and productivity. Soil microbial communities decompose litter material, making nutrients available for plants or other microbes. In contrast, meso- and macrofauna break down litter, making it accessible to microbes or consume microbes as secondary decomposers. Within salt marshes research of microbial soil communities has largely focused on their composition in relation to abiotic conditions, litter decomposition rates and bacterial or fungal contributions to vascular plant decomposition. However, these studies have predominantly focused on microbial communities associated with the PZ cordgrass (Spartina sp.). Only recently the focus has shifted towards microbial decomposer communities across all salt marsh zones within the Wadden Sea. With indications of spatial changes in abiotic conditions influencing the presence of fungi as well as the exoenzyme activity of bacteria.
Within the soil decomposer system two distinct energy channels exist, the bacterial and the fungal channel. Generally, the bacterial channel is considered to have fast nutrient turnover rates and to depend on easily available resources, whereas the fungal channel has slow turnover rates, processes more recalcitrant resources and may be characterized by top-down control. However, the relative importance of these channels and how the spatiotemporal dynamics of the salt marsh impact the channelling of resources has not been investigated in depth.
The aims of this thesis were to investigate the spatiotemporal dynamics of microbial basal resources, their channelling through the food web as well as changes in trophic position of consumers. In addition, I investigated the use of allochthonous marine resources and their channelling through the soil decomposer food web of the Wadden Sea salt marsh. For this purpose, I sampled soil microorganisms and soil meso- and macrofauna of a Spiekeroog salt marsh in April (spring), July (summer) and October (autumn) 2019. Basal resources and microbial communities were investigated using phospholipid fatty acid patterns in Chapter 2. Channelling of these basal resources to higher consumers was investigated using neutral lipid fatty acids in Chapter 3. Lastly, Chapter 4 investigated spatiotemporal changes in trophic position and allochthonous resource consumption using the stable isotopes ¹⁵N and ¹³C of the soil macrofauna.
The study presented in Chapter 2 showed that the dominant microbial group within the salt marsh are Gram-positive bacteria, with peak fungi concentrations in the LSM and peak algal concentrations in PZ. The microbial communities showed remarkable temporal stability, despite marked seasonal changes in abiotic conditions. Spatial dynamics, i.e. shore height and sediment depth, were the dominant factors shaping the soil microbial community, with bacteria increasing with sediment depth.
The results shown in Chapter 3 demonstrated that the bacterial channel and associated plant resources form the dominant basal resources of the soil fauna, regardless of shore height and season. The contribution of allochthonous marine resources to consumer nutrition was generally low, but depended on season, presumably linked to inundation frequencies and algal productivity.
Lastly, Chapter 4 showed that the salt marsh soil macrofauna food web is simpler than expected, with three trophic levels, regardless of shore height. However, trophic positions of taxa/species varied across shore height and season. Furthermore, Chapter 4 indicated a wider range of resources in the LSM, presumably due to marine influx. However, Bayesian mixing models indicate that autochthonous vascular plant resources are the dominant basal resource for macrofauna consumers of the salt marsh.
Overall, this thesis is the first to investigate spatiotemporal dynamics of basal resources and their channelling through the food web of the Wadden Sea salt marsh. Chapter 2 demonstrated that the dominant microbial decomposer are bacteria. Chapter 3 showed that the dominant basal channel is autochthonous plants and associated bacteria. Lastly, Chapter 4 showed that while spatial dynamics did not impact the number of trophic levels, taxa/species changed their trophic levels across temporal scales. In addition, resource use became more variable with reduced shore height, but overall allochthonous marine resources were a minor addition to the diet of macrofauna consumers. | de |