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

Periodic Reporting for period 3 - FLUFLUX (Fluvial Meta-Ecosystem Functioning: Unravelling Regional Ecological Controls Behind Fluvial Carbon Fluxes)

Reporting period: 2019-12-01 to 2021-05-31

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 and its metabolism 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 are (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 investigates 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. This observational work in real river networks will generate hitherto unexisting databases on the spatial distribution of organic matter, biodiversity and ecosystem functioning in river networks. Further, the project uses laboratory-based experimental setups, in which the topology of connected chemostats is manipulated, and modeling approaches studying river meta-ecosystem functioning in-silico.
The FLUFLUX team has so far 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 twice, once in spring at high flow and once in fall at baseflow conditions. Sampling in the Thur was finished in October 2018, technically already in the next project period. 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 traits of this important functional group. The campaign in the Ybbs focused on primary producers exclusively. The decision to include primary producers within the context of FLUFLUX was prompted by its importance for riverine carbon cycling. With the exception of the Ybbs, for which information about dissolved organic matter and microbial biodiversity already existed prior to FLUFLUX, the team collected data on dissolved 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 concert with these measurements, we also progressed on GIS-based descriptions of the various river networks, including analyses of geology and landcover. The team has now entered a phase of data processing after several months of intense field work.

In parallel to this observational work, we also started to test various designs of chemostats considered for building laboratory meta-ecosystems. For the sake of efficient field work, work on this experimental approach has been slower than anticipated and intensification is planned for the third project period.

A strong modeler entered the group in the second year of the project and is currently developing a meta-ecosystem model for river network metabolism using a Bayesian framework. Data from the Mara, Vjosa and Thur will be the first case studies delivering data to use for model testing and validation.
With regard to empirical field work, the team plans to sample at least two more river networks. These additional campaigns will be guided by so far collected data, which still needs to be processed in the coming months. We also expect optimization of field work protocols. The upcoming project periods will otherwise be dedicated to the installation of laboratory-based meta-ecosystems, with which we aim to test explicit hypotheses about the effects of river network topology and fragmentation. Alongside this experimental work, we will progress on developing modeling approaches for “fluvial meta-ecosystems”. By the various approaches, we expect to identify effects of river topology and fragmentation on river network-wide biodiversity, organic matter composition and diversity, as well as functioning as the interaction product of biodiversity and organic matter. We also expect imprints of primary producer diversity on primary production in the empirical dataset and will develop experimental approaches for a follow-up project. Our research activity in the Vjosa River in Albania has further led to fruitful collaboration with Albanian partners from Tirana University and national as well as international NGOs working on river conservation. We will maintain these collaborations and will further invest into arising opportunities where our research results can produce impact through capacity building and informed opinion building of the public.
Confluences landmark emergence in river networks