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Fluvial Meta-Ecosystem Functioning: Unravelling Regional Ecological Controls Behind Fluvial Carbon Fluxes

Periodic Reporting for period 4 - FLUFLUX (Fluvial Meta-Ecosystem Functioning: Unravelling Regional Ecological Controls Behind Fluvial Carbon Fluxes)

Reporting period: 2021-06-01 to 2023-03-31

Streams and rivers play a major role in the Earth´s carbon cycle as they metabolize large amounts of terrestrially derived organic material (OM) during hydrological transport. The project FLUFLUX attempts to unravel ecological mechanisms behind the observed metabolism of OM in streams and rivers by connecting the diversity of organic matter (as, for instance, chemical diversity of dissolved OM, or size heterogeneity of particulate OM) with the biodiversity of its consumers (bacteria or invertebrates) at the regional scale of river networks. Importantly, at this spatial scale, both organic matter as well as biodiversity underlie only partially understood spatial constraints that are specific to the dendritic topology of river networks. The different rules governing the movement of organic matter and the spatial distribution of organisms lets us consider river networks as “fluvial meta-ecosystems”. Humans interfere with organic matter and biodiversity in the spatial domain, for instance by changing connectivity through damming-induced fragmentation, channelization and water abstraction, and thus influence fluvial meta-ecosystem functioning and carbon cycling.
Understanding mechanisms behind spatial features of biodiversity, organic matter (or in general resources) and ecosystem functioning is vital for predicting the future role of rivers in the Earth´s carbon cycle. It is further important for maintaining biodiversity and functioning in river networks, which belong to the most biodiverse and most strongly human-affected ecosystems on Earth.
The overall objectives of FLUFLUX were (i) to first principally test whether the interaction of organic matter diversity with consumer biodiversity can help to explain organic matter metabolism to a reasonable degree, and (ii) to test whether gradients of topological variation and fragmentation can lead to sizeable variation of spatial patterns of organic matter metabolism. The project empirically investigated two organic matter-consumer pairs: dissolved organic matter consumed by microbes, and particulate organic matter consumed by invertebrates – different patterns are expected due to differences in transport traits of OM and dispersal traits of consumers. In addition, the project studied the analogue interactions between resources, environmental factors and biodiversity – again in the spatial domain of river networks – controlling the buildup of organic matter (and concomitant removal of CO2) by primary producers.
The FLUFLUX team has achieved empirical data collections in 5 different river networks: The Ybbs and Kamp in Austria, the Mara in Kenya, the Vjosa in Albania and the Thur in Switzerland. The Vjosa was sampled three times, once in spring at high flow and twice in fall at baseflow conditions. Work in the Ybbs also included two field campaigns. In all cases we either established new collaborations with local researchers or strengthened existing ones. The focus of data collection shifted slightly between river networks. Notably, we included biodiversity of primary producers and measures of functional diversity of this important functional group. One campaign in the Ybbs focused on primary producers exclusively. The decision to include primary producers within the context of FLUFLUX was prompted by (i) its importance for riverine carbon cycling, and (ii) the perceived better chance for testing a core hypothesis of FLUFLUX: the translation of network-scale patterns of biodiversity and resources to ecosystem functioning. The team collected data on dissolved organic matter (OM) diversity and microbial biodiversity on at least 50 sites in all rivers. Further data collected include: OM-related extracellular enzyme activities, ecosystem-scale metabolism, carbon dioxide and methane concentrations and emissions. In the rivers Mara and Vjosa, we further collected data on invertebrate communities and food webs with the help of collaboration partners. In the rivers Thur, Ybbs and Kamp we ran river network-scale leaf litter degradation experiments, including investigation of invertebrate biodiversity colonizing the leaf litter bags. In concert with these measurements, we also progressed on GIS-based descriptions of the various river networks, including analyses of geology and landcover.
In parallel to this observational work, we developed a meta-ecosystem model for river networks. This model has meanwhile been made available as a R-package and is capable of modelling network-scale biodiversity, resource distribution and ecosystem functioning as the result of the interaction between biodiversity and available resources. The model is in fact blending a metacommunity model with a biogeochemical transport-reaction model. As such it allows simulation of river meta-ecosystem functioning based on mechanistic principles involving species and resources with particular traits. Besides this mechanistic model, we also advanced on the network-scale statistical analysis of empirical data. Mostly using a Bayesian framework we build models explaining the distribution of microbes, invertebrates and fish, and further develop those to analyze ecosystem functions. In addition to this modelling work, we developed a prototype for a laboratory meta-ecosystem. Work on this experimental approach has been slower than anticipated and could not be brought to the intended level that could support the planned series of hypothesis tests about meta-ecosystem functioning.
By the various approaches, we identified mechanisms behind river network-wide patterns of biodiversity, resources (in particular organic matter composition and diversity but also nutrients), environmental factors, as well as functioning as the interaction product of biodiversity with resources and environmental factors. For periphytic algae we could demonstrate neutral, dispersal-related and deterministic, species sorting-based mechanisms behind river network-wide biodiversity and metacommunity structure. We identified sorting-based dimensions of metacommunity structure to be the only ones relevant for spatial patterns of functioning (primary production). We also showed how such species sorting is strongest in mid-sized rivers, i.e. it follows a hump-shaped pattern with river size in the river network. Also, for leaf litter decomposition we could demonstrate influences of network-wide biodiversity patterns. While analysis of microbial heterotroph biodiversity and organic matter is still underway at the end of the project´s lifetime, FLUFLUX has already moved beyond the state of the art with empirically showing (i) the internal structure of metacommunities in rivers and (ii) how river network-wide biodiversity and metacommunity organization translates to functioning. FLUFLUX also developed novel mechanistic models for fluvial meta-ecosystem functioning by blending metacommunity modelling approaches with transport-reaction models originating in biogeochemical disciplines. Contributions to opinion papers and conferences as well as various outreach activities have spread and defended the idea that – based on the mechanistic principles studied in FLUFLUX – river conservation must be conceptualized and realized at larger, river network scale.
Confluences landmark emergence in river networks