Abteilung Aquatische Ökologie

Spatial environmental heterogeneity is an important driver of aquatic biodiversity. Ecological and evolutionary theory often consider spatial heterogeneity as being driven by exogenous factors, yet heterogeneity can also be generated and modified by organisms. Here we used a mesocosm experiment to investigate if consumers influence the build-up of spatial heterogeneity. We expected that consumer effects on heterogeneity would depend on consumer composition and differ among response variables. We constructed metacommunities consisting of three mesocosms and manipulated the presence and composition of consumers, using four treatment levels: (1) no consumers, (2) two genotypes of Daphnia galeata, (3) D. galeata and Daphnia longispina, and (4) D. galeata and a hybrid of D. galeata × D. longispina. We then continuously increased heterogeneity among the three patches of each metacommunity by adding nutrients and dissolved organic carbon (DOC), respectively, to two of the three mesocosms. We found that consumers affected the build-up of heterogeneity, but the direction and magnitude of this effect differed among consumer compositions. Metacommunities with only D. galeata had increased heterogeneity in phytoplankton biomass, whereas metacommunities with D. longispina or the hybrid had low phytoplankton heterogeneity. The differential effects of Daphnia taxa on phytoplankton heterogeneity cascaded down to the abiotic environment and resulted in taxon-specific effects on heterogeneity in light extinction, dissolved inorganic nitrogen, and total inorganic carbon. Our results imply that changes in consumer species (e.g., due to environmental change or invasion) might affect not only the local environment but could also impact heterogeneity among environments, with important consequences for aquatic biodiversity.

Biodiversity underpins the functional integrity of ecosystems. At present, our understanding of the relationship between biodiversity and ecosystem functioning (BEF) is essentially based on manipulative experiments. Compelling data at large spatial scales are scarce, especially for river networks. BEF patterns across landscapes are complex because they unfold in the context of environmental gradients and compositional turnover of natural communities. Leaf litter decomposition, a pivotal ecosystem process in streams, is no exception to this context dependency. The dendritic structure of river networks plus the unidirectional water flow shape both environmental conditions and the distribution of leaf resources and consumers. However, it is difficult to predict how spatial gradients of resource and consumer composition can overlap across a river network, and thus govern spatial patterns of decomposition. Here, we investigated the capacity of macroinvertebrate biodiversity to control decomposition rates of heterogeneous leaf resources at the river-network scale. We deployed five litterbags containing either one of four single leaf species or a mixture of all species at 51 sites across the Thur River network (Switzerland). We measured litter decomposition rates, variation of decomposition among leaf resources, and the effect of leaf litter diversity on decomposition. We found that decomposition rates decreased from headwaters to downstream reaches mainly due to the parallel decrease in the abundance of key shredder taxa (namely, Amphinemura, Nemoura, Leuctra, Habroleptoides, and Stenophylacini). Macroinvertebrate diversity had a minor, negative effect on decomposition rates. However, high functional macroinvertebrate diversity at the reach scale reduced the variation of decomposition among leaf resources, thus alleviating nutritional constraints exerted by nutrient-poor leaf resources. Furthermore, litter mixtures were preferably decomposed by communities with low evenness and dominated by a few taxa. These findings point to a critical role of macroinvertebrates in controlling litter decomposition at the network scale beyond environmental effects. While shredder abundance and community composition are key to determining decomposition rates across the river network, functional diversity is important in decreasing the variation of decomposition rates among leaf resources. Our results stress the importance of biodiversity controlling ecosystem functioning not only at the local but also at the river network scale.

Leaf litter decomposition (LLD) is a key ecosystem function where invertebrate shredders produce large amounts of fine particulate organic matter (FPOM) that serves as a substrate for microbial assemblages. Here, we explore the shredder-produced FPOM composition and activity of FPOM-associated microbial communities in response to different leaf species and their conditioning. In a laboratory experiment, we fed leaves of different elemental compositions (alder, beech and maple), conditioned under oxic or anoxic conditions, to caddisfly larvae (Sericostoma sp.). We hypothesized differences in FPOM elemental and fatty acid composition and FPOM-associated microbial activity among the leaf species, conditioning, and two types of shredder-produced FPOM, i.e. shredded leaves and faecal pellets. Our results suggest that leaf conditioning and shredder activity play pivotal roles in shaping FPOM composition and FPOM-associated microbial activity. We observed lower C/N ratios with high-C/N litter (beech and maple leaves) after conditioning and no change in the elemental composition of the faecal pellets compared to the leaves. However, we observed differences in microbial fatty acid proportions and composition on leaves and faecal pellets with significantly higher fractions of bacterial fatty acids on faecal pellets than on leaves. We also noted a significant impact of leaf conditioning on the microbial activity of shredded leaves and faecal pellets, with a higher microbial growth efficiency observed on faecal pellets compared to ingested leaves. These findings highlight the crucial influence of leaf species and conditioning on the activity of shredder-produced FPOM, emphasizing the complex interplay between leaf properties and fate and microbial processes in streams.