The availability of Dissolved oxygen (DO) sensors and open-source modelling tools has greatly expanded the breadth of knowledge regarding the metabolism of aquatic systems. River metabolism is one of the most integrative indicator of ecological health, it can be applied across diverse water systems, through continuous measurement systems, and is highly sensitive to environmental stressors as it reflects shifts in anthropogenic disturbance or management actions. While former studies showed that the estimation of aquatic ecosystem metabolism can be used as a relevant tool for urban water management, long-term metabolism investigations remain currently scarce, especially in large rivers.

Relying on the MeSeine Observatory, a sensor network of 8 sites measuring  continuously DO and water temperature in the Seine River from upstream to downstream of Paris region, we examined 25 years of spatial and temporal variability of the river metabolism and its underlying drivers. Daily metabolic rates, including gross primary production (GPP), ecosystem respiration (ER), which both contribute to the net ecosystem production (NEP), were estimated using the single-station open-channel method, integrating hourly DO, temperature, and river discharge data.

Preliminary results reveal distinct seasonal and inter-annual patterns in GPP, ER and NEP rates. ER and GPP peaked in late spring and in summer, driven by high light availability, warm temperatures and low discharge, while the lowest rates occurred in winter across all sites. Inter-annual variations were primarily influenced by hydroclimatic conditions and sewage inputs. Annual mean rates of GPP, ER and NEP across all sites ranged from ~0.29 to ~2.3, ~-5.65 to ~-0.53, and ~-4.16 to ~0.51 gO₂.m⁻³.d⁻¹, respectively. ER consistently exceeded GPP, indicating a predominantly heterotrophic status downstream of Paris. Notably, the upstream site of the observatory (the only one before Paris) exhibited several years with positive annual NEP values. Along the river, net heterotrophy increased downstream, likely due to urban organic matter inputs. While GPP and ER displayed similar temporal patterns, NEP followed a distinct trajectory, aligning more closely with water quality, organic matter and nutrient concentrations. Moreover, in recent years, NEP tends to increase, reflecting a decrease of the heterotrophy and nutrient concentrations in the river.

The analysis demonstrates that the Seine River's metabolism has exhibited distinct seasonal and upstream-to-downstream trends over the past 25 years, driven by urban impacts, seasonal dynamics, and hydroclimatic conditions. Future work will focus on refining these observations to uncover long-term trends and establish clear relationships between metabolic rates and key environmental stressors across shorter timescales, with special focus on the structural components, hydrological information and sewage effluents. To assess ecological status and develop effective management tools, it is essential to understand the interactions between river components and the cause-effect mechanisms underlying aquatic ecosystem alterations.

How to cite: Njapou Mawa, P., Mouchel, J.-M., Escoffier, N., and Mougin, J.: Long-term variability in the metabolism of an aquatic ecosystem: results from 25 years of continuous dissolved oxygen monitoring in a large urban river., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17491, https://doi.org/10.5194/egusphere-egu25-17491, 2025.

The river is one of the most important freshwater ecosystems serving as a hub for water, gas, and energy exchange among land, ocean, and atmosphere. River ecosystem metabolism vibrates with external stresses such as solar radiation, flow stability, and temperature. We discovered that the internal factor dissolved inorganic carbon (DIC) plays a hidden role in amplifying riverine metabolic sensitivity, which, however, is rarely concerned. In this study, machine learning is used to reproduce global riverine DIC datasets, and then the global rivers are divided into high-DIC and low-DIC rivers. Apparent oxygen utilization (AOU) is used as an indicator of ecosystem metabolism intensity. High-DIC rivers exhibit intensified ecosystem variability, which is more obvious in colder climate zones. Facilitated gross primary production (GPP) by DIC boosts ecosystem respiration, which increases the risk of hypoxia and biological stress in high-DIC rivers. A modified Michaelis-Menten equation-based model is developed and well simulates the historical variation of global mean annual AOU. The model was further applied to project DIC’s role in long-term river ecosystem variation under different future climate scenarios. The model results demonstrate that high-DIC rivers have a higher risk of oxygen depletion in all scenarios with different applications of fertilizer situations compared with low-DIC rivers.

How to cite: Qi, H. and Liu, Y.: Hidden Threats of Dissolved Inorganic Carbon to River Ecosystem Metabolism, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17639, https://doi.org/10.5194/egusphere-egu25-17639, 2025.

Rapid expansion of Moso bamboo (Phyllostachys edulis) has been noted in East Asian countries, and replacement of riparian vegetation by dense bamboo has been observed in many areas. Changes in riparian vegetation have significant effects on stream ecosystems, but the effects of bamboo remain uncertain. In this study, leaf litter breakdown and leaching of dissolved organic matter (DOM) and nutrients in stream were compared between Moso bamboo and other riparian tree species to assess the effects of bamboo expansion on stream organic matter and nutrient dynamics.

Leaf litter of Moso bamboo and two evergreen species in riparian area, Camphor laurel (Cinnamomum camphora) and Japanese cedar (Cryptomeria japonica), that provide leaves to the stream in the same season as bamboo were compared. Litter bags with 5 g of leaf litter were incubated on the streambed in four riffles in July and August. Litter bags were collected 1, 8, 15, 28 and 42 days after the start of incubation to measure breakdown rates and leaching of DOM and nutrients. Macroinvertebrates in the litter bags at day 8 were also examined.  

Breakdown rates of Moso bamboo was significantly lower than those of Camphor laurel and Japanese cedar. Despite the low macroinvertebrate breakdown rates, the number and species richness of macroinvertebrate present were highest in the bamboo litter bags. These results suggest that bamboo leaves were not palatable compared to Camphor and cedar litter, but they functioned as habitat for macroinvertebrate. Leaching of DOM was highest from Moso bamboo leaves at day 0, and it rapidly declined as breakdown progressed. Japanese cedar, on the other hand, released smaller amount of DOM, but maintained the similar rates despite the progress of breakdown. Leaching of bioavailable DOM (BDOM) was lowest from bamboo, 0.64 (± 0.84) mg/g of litter, and highest from Camphor leaves, 5.48 (± 0.99) mg/g, at day 0, and by day 8, leaching of BDOM became similar among species, 3.30 (± 0.67) mg/g, 3.67 (± 0.16) mg/g, 2.37 (± 0.86) mg/g respectively for Moso bamboo, Camphor laurel, and Japanese cedar. Thus, bamboo leaves leached larger amount of DOM, but BDOM was low, and the effects on in-stream nutrient processes may be smaller than other two species.

How to cite: Kasahara, T., Gourlaouen, A., and Tanaka, A.: Effects of bamboo expansion on organic matter and nutrient dynamics in mountain streams, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3873, https://doi.org/10.5194/egusphere-egu25-3873, 2025.

Greening is the optimal way to mitigate climate change and water quality degradation caused by agricultural expansion and rapid urbanization. However, the ideal sites to plant trees or grass to achieve a win-win solution between the environment and the economy remain unknown. Here, we performed a 12-year comprehensive observation in the Jiulong River watershed (southeastern China) and a nationwide survey on groundwater in China (n = 90), combining them with statistical and AI models to explore the linkages between land use within hydrologically sensitive areas (HSAs) and nitrogen concentrations/fluxes from the perspective of hydrological connectivity. We found that HSAs occupy approximately 20% of the total land area and are hotspots for transferring nitrogen from the land surface to rivers and groundwater. Increasing the proportion of natural lands within HSAs improves river and groundwater quality and reduces the exports of riverine nitrogen to coastal zones. These new findings suggest that prioritizing ecological restoration in HSAs is conducive to achieving harmony between the environment (improving watershed water quality and reducing river nitrogen export flux) and the economy (reducing investment in area management).

How to cite: Wang, Y., Chen, N., and Luo, X.: Ecological restoration guided by hydrological connectivity reduces nitrogen fluxes from river to coast, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10937, https://doi.org/10.5194/egusphere-egu25-10937, 2025.

Watershed ecosystems rely on energy and nutrient exchanges between terrestrial and aquatic systems, which influence carbon and nutrient cycles, biodiversity, and overall ecosystem dynamics. The synchronization of terrestrial and riverine productivity, referred to as coupling strength (CS), provides a useful metric for assessing ecosystem integration and responses to environmental change. Despite its importance, the spatial variability of CS and its environmental drivers remain poorly understood, particularly across regions with diverse natural conditions and human impacts. This study quantifies CS across over one hundred river sites and their upstream watersheds in the continental United States. Using explainable machine learning, we identified key environmental factors, including water temperature, river width, leaf area index, and watershed area, that exhibit distinct nonlinear relationships with CS. Clustering analyses revealed spatially diverse coupling regimes, influenced by a combination of ecohydrological processes and anthropogenic activities. These findings advance the understanding of how environmental conditions mediate synchronization between terrestrial and aquatic productivity. The results provide a foundation for future research into the mechanisms of land-water interactions and their responses to environmental stressors. By integrating these insights into broader ecological and hydrological frameworks, this work can support the development of predictive tools for watershed management under changing environmental conditions.

How to cite: Chen, S., Blougouras, G., Calamita, E., Lee, S.-C., Huang, J., and Jiang, S.: Patterns and potential drivers of land-water productivity coupling across U.S. river systems, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6815, https://doi.org/10.5194/egusphere-egu25-6815, 2025.

Eutrophication, usually considered an overproduction of biomass in water bodies due to an enrichment of nutrients such as phosphorus (P) and nitrogen (N), is a common thread to riverine ecosystems. In Europe, chlorophyll a (Chl-a) concentrations, an indicator for algae biomass, have been successfully reduced from the 1980s by limiting phosphorus input into rivers. However, elevated Chl-a levels in European rivers are still found until today. As recent algae blooms were rather linked with drought periods than with particularly high phosphorus concentrations, the question is raised whether other parameters than phosphorus concentrations might be more crucial for eutrophication management in the future.

To understand the conditions under which rivers are particularly prone to an efficient conversion of phosphorus into algae biomass, we analyzed a Germany-wide dataset of Chl-a and total phosphorus (TP) concentrations with 31661 measurement pairs at 330 stations between 2000 to 2019. To quantify this conversion efficiency, we used the measure of the degree of realized eutrophication, α_realized, which is the ratio between the realized (i.e. the Chl-a measurement) and the potential eutrophication (i.e. a theoretical upper Chl-a concentration at a given TP level if all TP is converted to biomass). In a preceding study, we found that station-wise medians of α_realized are mainly controlled by water residence time with high median α_realized being either related to large rivers with a long distance to source or small rivers with close upstream lakes. As management not only asks where but also when Chl-a concentrations are at critical levels, we here analyze the temporal variability of α_realized at different stations. To that end, we calculate the coefficients of variability of α_realized, TP, and Chl-a, and statistically relate these characteristics to other instream parameters and catchment attributes at each station.

We find both stations with low α_realized variability and a positive TP - Chl-a correlation and stations with high α_realized variability and no or even a negative TP and Chl-a correlation. The former case suggests stable controls of α_realized and good predictability of Chl-a based on TP concentrations at the respective stations. The latter case implies either variable water residence times or additional controls by parameters with strong seasonality, such as light availability, water temperature, or ecological community shifts.

In this contribution, we will present whether high temporal variability of α_realized is predictable from instream parameters or catchment attributes and discuss the underlying processes. We will further conclude on the implications of the results for river management, particularly in terms of algae control and in light of climate change.

How to cite: Hubig, A., Scharfenberger, U., and Musolff, A.: Temporal variability of the realized eutrophication in rivers across Germany, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20204, https://doi.org/10.5194/egusphere-egu25-20204, 2025.

Recent studies highlight the critical role of changes in connectivity, erosion processes and sediment sources in shaping biological communities in palaeo-environmental reconstructions (e.g., Giguet-Covex et al., 2023; Morlock et al., 2023). Such processes can lead to misinterpretations attributing shifts in biodiversity around lakes to environmental changes, when in fact they may be due to the introduction of previously unconnected sediment sources driven by human activities (e.g. land management) or extreme climate events.

To assess the impact of changes in connectivity and accelerated erosion on biological communities, we analysed sediment archives from the Dzoumogné reservoir (Mayotte Island, France). This reservoir drains a small catchment (1038 ha) that underwent significant land-use changes (e.g. deforestation, agricultural intensification) during a well-documented period (2011-2021). We reconstructed land-use changes, erosion rates and sediment sources, as well as biological communities using sediment DNA (sedDNA) based on seven genetic markers targeting plants (trnL, rbcL), fungi (ITS), metazoa (16S, 18S) and vertebrates (12S).

Our results show a 450% increase in operational taxonomic units (OTUs) and shifts in OTUs detected in 23 taxonomic groups following the first deforestation phase (2012-2013) and subsequent agricultural intensification resulting in landscape fragmentation. During this period, sedDNA identified an increase in forest-derived species (e.g. Streptophyta and terrestrial fungi) and agricultural species (e.g. banana, cassava, cattle). These changes coincide with accelerated sediment delivery and erosion rates (+310% between 2011 and 2015). Between 2015 and 2021, declining water levels (driven by climate and human activities) combined with high sediment and nutrient inputs continued to drive major shifts in aquatic communities. These included increases in OTUs belonging to the taxonomic groups Chlorophyta and Ciliophora - key indicators of eutrophication and water quality degradation. The high primary production associated with algae and microorganisms likely explains the observed increase in invertebrate and fish communities higher up the trophic chain within just two years of changes in sediment connectivity and sources.

This study highlights how rapid shifts in biodiversity in both terrestrial and aquatic systems are driven by increased erosion and connectivity of previously isolated land use areas (e.g. forests, croplands). Understanding these connectivity dynamics is crucial to avoid misinterpretation of biodiversity change in lake sediment records.

References:

Giguet-Covex, C., Jelavić, S., Foucher, A., Morlock, M. A., Wood, S. A., Augustijns, F., Domaizon, I., Gielly, L., & Capo, E. (2023). The Sources and Fates of Lake Sedimentary DNA (pp. 9–52).

Morlock, M. A., Rodriguez‐Martinez, S., Huang, D. Y., & Klaminder, J. (2023). Erosion regime controls sediment environmental‐based community reconstruction. Environmental DNA, 5(6), 1393–1404.

How to cite: Foucher, A., Evrard, O., Maresceaux, J., Debortoli, N., Ambroise, V., Cerdan, O., Landemaine, V., and Desprats, J.-F.: Erosion processes and connectivity shape biological communities recorded by environmental DNA in sedimentary archive: implication for paleo-environmental reconstruction, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11775, https://doi.org/10.5194/egusphere-egu25-11775, 2025.

Northern freshwater ecosystems face a wide range of environmental changes, including climate change, eutrophication, and browning. In Arctic regions, climate warming is occurring nearly four times faster than the global average, leading to higher water temperatures, shorter ice-cover periods, and extended growing seasons for aquatic biota. These changes are expected to have both direct and indirect impacts on these ecosystems, including the microbial communities that underpin their biodiversity and functioning. Building on prior research by the authors, we tested the hypothesis that microbial communities, particularly prokaryotes (bacteria and archaea), exhibit similarities across circumpolar freshwater systems. To investigate this, we surveyed 46 lakes and 30 streams between 2019 and 2022 in Arctic (>70º N) and sub-Arctic (55–70º N) regions spanning Alaska, Canada, Greenland, Norway (including Svalbard), and Sweden. For each waterbody, we collected environmental DNA (eDNA) for 16S rRNA gene metabarcoding and water samples to analyze physical and chemical parameters (temperature, pH, electrical conductivity, and dissolved O₂), major ions, and nutrients (organic C, P, and N).

Bacteria predominantly represented the main prokaryotic group in this study, with archaeal contributions limited to a few lakes in Svalbard and the Canadian Northwest Territories. Our results revealed that latitude (i.e., Arctic vs. sub-Arctic locations) strongly determined community composition (p = 0.001; pseudo-F = 2.634), whereas the type of waterbody (i.e., lakes vs. streams) had a weaker influence on beta diversity (p = 0.012; pseudo-F = 1.813). Latitude, along with water temperature and dissolved O₂, were the main explanatory variables shaping prokaryotic community composition in our study. The core microbiome differed significantly in abundance between Arctic and sub-Arctic locations (p = 0.005). Arctic freshwaters showed the highest alpha diversity (Shannon and Chao1 indices) and were dominated by the genera Rhodoferax, Arcicella, and Polaromonas, all of which positively correlated with increasing dissolved O₂ concentrations. In contrast, sub-Arctic freshwaters were primarily dominated by Limnohabitans, a genus widely distributed in inland freshwater habitats. Regarding waterbody types, lakes were predominantly characterized by Flavobacterium, which positively correlated with increasing nutrient concentrations, and exhibited higher alpha diversity compared to streams. Streams, in turn, were largely dominated by Rhodococcus species, which showed significant positive correlations with water temperature. This study enhances our understanding of prokaryotic diversity across the circumpolar region and aims to further provide valuable insights into the assembly mechanisms of freshwater microbial communities.

How to cite: Valiente Parra, N., Eiler, A., Bertilsson, S., Chaguaceda, F., Christoffersen, K. S., Culp, J., Lavoie, I., Musetta-Lambert, J., Shaftel, R., and Hessen, D. O.: Patterns of prokaryotic diversity in freshwaters across the circumpolar region, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17219, https://doi.org/10.5194/egusphere-egu25-17219, 2025.

Global warming and anthropogenic activities have profoundly altered biodiversity and aquatic ecosystem stability, yet the underlying driving mechanisms remain inadequately understood. Here, we analyzed temporal patterns of biodiversity and community stability over the past century by constructing 29 temporal planktonic network models. These models were based on the sedimentary DNA (sedDNA) extracted from downcore sediments in Lake Chagan, a seasonally frozen lake in Northeastern China, using high-throughput sequencing techniques. Our findings identify the mid-1990s as a critical tipping point, marked by substantial shifts in nutrient levels and annual average temperatures. We demonstrate that the temporal network stability of plankton communities has been predominately compromised by climate warming, followed by nutrient enrichment. Our study highlights the intricate interplay between biotic and abiotic factors in determining the stability of aquatic ecosystems, which have significant implications for the management and conservation of freshwater ecosystems in the face of ongoing climate warming.

How to cite: Yu, S., Cao, X., and Qu, J.: Climate warming and nutrient enrichment destabilize plankton network stability over the past century, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-180, https://doi.org/10.5194/egusphere-egu25-180, 2025.