# Seasonal dynamics of detritus flows and decomposition across ecosystem boundaries

Eva Cereghetti, Tianna Peller, Silvana Kaeser, Isabelle Gounand, Florian Altermatt

► To cite this version:

Eva Cereghetti, Tianna Peller, Silvana Kaeser, Isabelle Gounand, Florian Altermatt. Seasonal dynamics of detritus flows and decomposition across ecosystem boundaries. Current Biology, 2025, 35, pp.2139 - 2145.e3. 10.1016/j.cub.2025.03.019. hal-05315396

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# Current Biology

## Seasonal dynamics of detritus flows and decomposition across ecosystem boundaries

### Highlights

* Leaf litter flows and decomposition rates showed different phenologies

* Recalcitrant and labile litter had inverse flow and decomposition dynamics

* Microorganisms were the main contributors to decomposition in forests and streams

* Recalcitrant litter might buffer ecosystem responses to resource pulses

### Authors

Eva Cereghetti, Tianna Peller, Silvana Kaeser, Isabelle Gounand, Florian Altermatt

### Correspondence

eva.cereghetti@eawag.ch (E.C.), florian.altermatt@ieu.uzh.ch (F.A.)

### In brief

Cereghetti et al. demonstrate with empirical data the strong seasonality of leaf litter fluxes within and across temperate forests and streams. The authors highlight how material flows and decomposition rates of leaf litter operate on different phenologies and how recalcitrant and labile litter present distinct temporal decomposition dynamics.
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Report

Seasonal dynamics of detritus flows
and decomposition across ecosystem boundaries

Eva Cereghetti,<sup>1,2,4,*</sup> Tianna Peller,<sup>1,2</sup> Silvana Kaeser,<sup>2</sup> Isabelle Gounand,<sup>3</sup> and Florian Altermatt<sup>1,2,*</sup>

SUMMARY

Material fluxes are ubiquitous in nature within and across ecosystems, connecting habitats with vastly different characteristics, like forests to rivers and lakes. <fnref n="1" />⁻<fnref n="3" /> Although individual fluxes and their cascading effects are well known, <fnref n="4" />⁻⁶ very few studies address the intra-annual phenology of ecosystem processes, despite the pronounced seasonality of fluxes. Here, we empirically quantified and resolved fluxes of recalcitrant and labile types of leaf litter in temperate riparian forests and streams across a year, representing one of the most emblematic examples of seasonal systems. We quantified intra-annual variation in litter inputs from terrestrial plants to forest floors and streams and estimated aquatic and terrestrial decomposition rates across the year at 6-week intervals. Our data show that the autumn pulse of leaf litter is complemented by smaller magnitude but more constant-through-the-year lateral flows to the stream ecosystems. Decomposition of labile litter fluctuated seasonally, on a different phenology, with generally higher rates in summer, but rates of recalcitrant litter decomposition remained largely constant. Microorganisms were the main contributors to the decomposition process in both forests and streams. Overall, our work highlights the asynchronous and seasonally variable changes in decomposition rates between recalcitrant and labile detritus despite their initial synchronized availability and suggests that the dominating presence of recalcitrant litter buffers ecosystem responses to the concentrated temporal distribution of litter resources. ⁷,⁸ Investigating such ecological processes both across ecosystem borders and at fine intra-annual resolutions is imperative to understand complex system responses in the context of species' shifts in phenologies and resource quality. ⁹⁻¹¹

RESULTS

We measured the intra-annual fluxes of leaf litter, covering both leaf litter fall and decomposition, within and between riparian forests and stream ecosystems in a seasonal temperate region (NE Switzerland) at roughly 6-week intervals. Our system is emblematic of highly seasonal landscapes, where a recalcitrant resource (slow-decomposing litter) is dominant and a more labile resource (fast-decomposing litter) is scarcer. This is typical and representative for old-growth forests in the northern temperate hemisphere, such as beech or oak forests. We assessed two types of fluxes: leaf litter flows and subsequent decomposition rates by the decomposer community. We used leaf litter traps to measure the direct vertical flow of leaf litter (g m⁻<fnref n="2" />) from terrestrial plants to the riparian forest (Figure 1: B_t → D_t) and stream detritus pool (Figure 1: B_t → D_a), as well as the secondary lateral flow (i.e., transport, g m⁻<fnref n="1" />) from the forest floor to the stream (Figure 1: D_t → D_a). We used serial incubations of leaf litter bags with two mesh sizes to measure changes in decomposition rates throughout the year for microbes and invertebrate consumers in both terrestrial and aquatic ecosystems. The assessed decomposition rates reflect a decomposition potential

because we provided standardized leaf litter substrates throughout the year (i.e., senesced leaves collected in autumn from three leaf species, provided in monocultures) independently from the changing availability or quality of the surrounding litter stock. We averaged the observed fluxes, measured at 6-week intervals across four seasons, to ease interpretation and comparability and also to reflect commonly used phenological and meteorological periods: spring, summer, autumn, and winter. Details about the data collection and analysis are given in the **supplemental information**. With this data, we address three major questions on detritus dynamics at the terrestrial-freshwater ecotone. (1) How do magnitudes of fluxes of leaf detritus change across seasons? (2) Are these changes synchronized between resource input flows and processing rates? (3) Do recalcitrant and labile litter types differ in their seasonal dynamics?

Leaf litter budgets

Vertical flows of leaf litter from terrestrial plants to the ecosystem below (i.e., forest floor or stream, Figure 1: B<sub>t</sub> → D<sub>t</sub> and B<sub>t</sub> → D<sub>a</sub>) totaled 611.34 g m<sup>-2</sup> across the whole year. The largest share of inputs (82.8%) was expectedly observed in autumn, and it

<footnote n="1">¹Department of Evolutionary Biology and Environmental Studies, University of Zurich, 8057 Zurich, Switzerland</footnote>

<footnote n="2">²Department of Aquatic Ecology, Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Dübendorf, Switzerland</footnote>

<footnote n="3">³Sorbonne Université, Université Paris Cité, Univ Paris Est Créteil, CNRS, IRD, INRAE, Institut d'écologie et des sciences de l'environnement de Paris, IEES Paris, 75005 Paris, France</footnote>

<footnote n="4">⁴Lead contact</footnote>

<footnote>*Correspondence: eva.cereghetti@eawag.ch (E.C.), florian.altermatt@ieu.uzh.ch (F.A.)</footnote>

<footnote>https://doi.org/10.1016/j.cub.2025.03.019</footnote>
<figure><img src="image_3.png" /><figcaption>**Figure 1. Ecosystem fluxes within and between forest-stream ecosystems across seasons**</figcaption></figure>

Vertical flows of leaf litter detritus (solid arrows) connect the terrestrial leaf biomass (B) compartment to the terrestrial and aquatic detritus compartments (D to the left and right side, respectively). A lateral flow from terrestrial to aquatic systems connects the two detritus compartments. Magnitudes of flows are given in g day⁻¹ m⁻² (vertical flows) or g day⁻¹ m⁻¹ (lateral flows). Decomposition by microbes (M) and consumers (C, i.e., invertebrates) in their respective ecosystem is indicated by dashed arrows. The magnitude of the decomposition is given by the decomposition rate k_D. Yellow arrows represent fluxes of an abundant and recalcitrant litter type (beech litter), whereas red arrows represent fluxes of more labile and scarcer litter types (alder, poplar, and mixed litter). Standard errors of the fluxes are given in Figure 2.

See also Figures S1 and S2.

averaged 7.68 g day⁻¹ m⁻² (Figures 1 and 2A). The catchment systems we studied are located in European beech (*Fagus sylvatica*)-dominated forests, and the abundance of this tree species is well reflected in the results: 77.3% of all vertical flows of leaf litter were made up of beech litter, well known to be recalcitrant, with only 22.7% made up of a mix of other species. This mix included ash (*Fraxinus excelsior*), willow (*Salix* sp.), maple (*Acer* sp.), hazel (*Corylus avellana*), alder (*Alnus glutinosa*), and oak (*Quercus* sp.), constituting all together a more labile litter than beech, despite the presence of some recalcitrant litter (most notably, oak). Lateral flows of leaf litter from the forest floor into the stream were more constant across seasons, with only the summer season showing a clearer and lower departure from the averages observed the rest of the year (Figure 1: D_t → D_a; Figure 2A). In comparison with vertical flows, however, the absolute magnitude was more than four times inferior

(144.00 g year⁻¹ m⁻¹). The relative contribution of the two litter types was once again dominated by beech litter (79.1% of flows across the year), but this proportion tended to fluctuate more across seasons than it did for the vertical flows (Table 1).

## Decomposition rates

Rates of decomposition of leaf litter differed across ecosystems, decomposer groups, and litter types (Figures 1 and 2B). Forests had lower average decomposition rates than streams, with smaller seasonal fluctuations. Decomposition by terrestrial microbes (Figure 1: D_t → M_t) was constantly higher than that by invertebrates only (Figure 1: D_t → C_t), in particular that of the labile litter (Figure 2B). Among the various treatments (ecosystems, decomposer groups, and litter type), the highest decomposition rates were always observed for the microbial decomposition of labile litter in streams (Figure 1: D_a → M_a red arrow). The rates
<figure><img src="image_4.png" /><figcaption>B Decomposition rates</figcaption></figure>

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<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

of decomposition in this treatment peaked in summer and were overall lowest in winter, although the individual data point with the lowest decomposition was in early spring. The contribution of aquatic invertebrates to the decomposition of labile litter (Figure 1: D_a → C_a red arrow) was lower than the microbial contribution (D_a → M_a red arrow) but analogously peaked in summer (Figure 2B). Aquatic decomposition rates of the more recalcitrant beech litter slightly increased in summer and autumn for microbes (D_a → M_a yellow arrow) but remained very low across the seasons for macroinvertebrates (D_a → C_a yellow arrow).

## DISCUSSION

Plant detritus is a fundamental resource for both the form and function of ecosystems.<sup>12,13</sup> Its role as a food and nutrient source has been shown to not only impact the detritivores depending on it but also to feedback on overall communities, food webs, and ecosystem functions within and across biomes, including terrestrial forests,<sup>4,14</sup> marine systems and their nearby shores,<sup>15,16</sup> and freshwater ecosystems such as lakes and streams.<sup>1,5</sup> The sheer amount of plant detritus alone renders its movement and cycling through ecosystems a major ecological process,<sup>2,17-19</sup> and, thus, most past work focused on the single most prominent time point in the year when fluxes peak. Yet, despite the strong seasonality, specific quantifications addressing the intra-annual phenology of such fluxes are largely lacking (but see Wallace et al.<sup>20</sup>). This is surprising, as climate change is not only expected to change magnitude but also, especially, timing of events, including resource fluxes coupling ecosystems.

<caption>**Figure 2.** Fluxes of leaf litter across seasons</caption>

(A) Vertical (upper) and lateral (lower) flows of leaf litter detritus (mean ± SE, weighted by the number of measurements within the season). The dashed line serves to visualize the difference in the y axis scale.

(B) Decomposition rates of leaf litter split by ecosystem type and decomposer group (mean ± SE).

See also Figure S3.

Thus, it is important to quantify and describe the phenology and temporal distribution of this resource.<sup>9,10,21</sup> In fact, alongside their magnitude, the timing and duration of detritus fluxes can control the availability, quality, and overall viability of the detritus over time, thus determining how strongly and through which mechanisms the food webs and ecosystem functioning of donor and recipient ecosystems will be affected.<sup>4,6,22,23</sup>

Our field data show that the magnitude of leaf litter fluxes within and across terrestrial and aquatic ecosystems are strongly seasonal and that the seasonal changes among these fluxes are often not synchronous. Peaks of leaf litter flows did not coincide with peaks in decomposition rates, indicating a temporal mismatch between when detritus resources are mostly available and when they could most efficiently be processed. Further, we show how inverse dynamics in the magnitude of fluxes can arise within the same broad category of resources (in our case, leaf litter) when their palatability is considered: recalcitrant litter entered the system in large flows, but its processing rates were low, whereas the opposite was true for more labile litter. Overall, we highlight how meta-ecosystem fluxes have inherent and distinct phenologies, and our findings advocate for more explicit integrations of temporal dynamics to better understand the functioning of ecosystems in seasonal landscapes.

## Leaf litter budgets: Large annual pulses and small constant flows

As expected, our findings show that in temperate systems the major input of leaf litter to both forests and their associated streams occurs once annually, as a pulsed resource (sensu<sup>24</sup>) during the autumn season. This massive annual input can modify the physicochemical proprieties of forest soils<sup>25</sup> and streams,<sup>26</sup> and it largely determines the bulk of the leaf litter resources that will be available to the brown food web for the rest of the year until the next pulse—especially in small streams with high retention characteristics.<sup>20</sup> Microbial communities and invertebrate detritivores can respond strongly to the availability of litter or lack thereof,<sup>27,28</sup> and many of these detritivores can have their own phenological shifts in abundance or life history stages (e.g., aquatic insects). Therefore, there might be complex ecological
<table><caption>Table 1. Relative contribution of litter types to the total litter flows</caption><thead><tr><th rowspan="2"></th><th colspan="2">Spring</th><th colspan="2">Summer</th><th colspan="2">Fall</th><th colspan="2">Winter</th><th colspan="2">Year total</th></tr><tr><th>Beech</th><th>Mixed</th><th>Beech</th><th>Mixed</th><th>Beech</th><th>Mixed</th><th>Beech</th><th>Mixed</th><th>Beech</th><th>Mixed</th></tr></thead><tbody><tr><td>Vertical</td><td>74.7</td><td>25.3</td><td>71.8</td><td>28.2</td><td>77.3</td><td>22.7</td><td>83.2</td><td>16.8</td><td>77.6</td><td>22.4</td></tr><tr><td>Lateral</td><td>90.2</td><td>9.8</td><td>84.4</td><td>15.6</td><td>62.9</td><td>37.1</td><td>80.3</td><td>19.7</td><td>78.2</td><td>20.9</td></tr></tbody></table>

Shown are the relative amounts (%) of beech (recalcitrant) and mixed (labile) leaf litter per season and per input direction. The yearly contribution is weighted according to the different lengths of the seasons.

links associated with the temporal distribution of the detrital pulse that could have cascading effects through the food web.⁹ For instance, climatic changes could shift tree phenologies and consequently the timing or duration—and even the quality—of the litterfall pulse.²⁹ Although some studies have shown that temporal patterns of subsidies can indeed affect consumer dynamics,²²,³⁰ it is currently unclear whether observed and predicted shifts in tree litterfall are significant enough to alter trophic interactions and ecosystem functions.¹⁹,³¹,³²

Smaller inputs (e.g., fall of green, non-senesced leaves) do nonetheless take place between autumn pulses and can provide occasional bouts of nutrient to the systems.³³ These can be critically important for stream ecosystems, whose higher processing rates (discussed more in detail below) and downstream transport result in a shorter-lived leaf litter stock from the autumn pulse. Yet, due to these inequalities in the resource processing speed and magnitude of primary production between forests and streams, aquatic ecosystems receive from the adjacent forest floor an additional input, namely the lateral flow of leaf litter. The amount of leaf litter subsidizing streams via lateral transport can vary depending on the forest slope surrounding the stream, on the forest location, on wind dynamics and also on the vegetation type.³⁴⁻³⁶ Nevertheless, our results are in line with those observed in other deciduous forests.³⁵ Specifically, we found that lateral inputs, though small, were more consistent across the seasons. This characteristic might be central to helping stabilize the stream functioning throughout the year in the face of fluctuating seasonal resource availability (e.g., allochthonous resources⁷,³⁷). Harvey et al.³⁸ further suggested that such cross-ecosystem flows, in light of the inequalities in production and processing, actually result in an overall increased trophic efficiency, thus ultimately benefiting both forests and streams. Whether this efficiency is also tied to specific timings and durations of the cross-system flows remains, however, untested.

## Decomposition potential varies across the seasons

Although the consumption of resources inevitably relies on their availability, we observed that the decomposition potential of leaf litter was decoupled from leaf litter inputs. The highest decomposition potential of labile litter was in fact observed in summer, when leaf inputs were low and leaf standing stock was heading toward the yearly minimum (we assume the standing stock to be decreasing until the next input pulse takes place in autumn). This result is in line with the widespread control of temperature on metabolic processes,³⁹ including the decomposition activity of microorganisms and invertebrates in both terrestrial (assuming no moisture limitation) and aquatic habitats.⁴⁰⁻⁴³ On the contrary, recalcitrant litter and its associated lower

decomposition potential could largely be described by constant rates, likely indicating that this litter type was mostly used as a secondary resource.⁴⁴

In our study, microorganisms contributed more than invertebrate consumers to the overall decomposition, which is not uncommon for similar systems.⁴⁵ Though we never assessed the terrestrial community, the dominating aquatic shredder in these streams was the largely aseasonal amphipod *Gammarus fossarum*.⁴⁶,⁴⁷ It could nevertheless be expected that different decomposition phenologies could arise in systems where invertebrate consumers with highly seasonal population dynamics are larger contributors to the decomposition of detritus (e.g., Benstead and Huryn⁴⁸). Aquatic insects, for instance, can have more size-/age-structured populations, such that at some times—e.g., close to their emergence⁴⁹—they can be larger and at other times consistently smaller, yet the possible effect of this phenology could still be masked by temperature effects, as in our study.⁴⁵ Overall, the apparent lack of a spike we observe in processing rates following the autumn pulse of leaf litter could be a stabilizing mechanism for the system⁵⁰ and/or be related to the dominance of a recalcitrant resource and its associated slow metabolism.³⁷,⁵¹

## The roles of recalcitrant and labile resources in time

Beside quantity, the quality of resources can be fundamental to understand the dynamics of ecosystem couplings.⁵²,⁵³ In systems like the old-growth forests where we conducted our study, recalcitrant litter is the dominating resource.³⁶ Despite being traditionally considered a low-quality litter due to its slow decomposition, the importance of recalcitrant litter resources is being increasingly acknowledged.⁵⁴ Past work has shown how recalcitrant resources are critical not for supplying nutrients that are quickly accessible but for supplying nutrients that remain available in the system longer term and could ultimately provide a higher nutrient transfer to upper trophic levels or back to the producers themselves (e.g., Cereghetti et al.,⁸ Trap et al.,⁵⁵ and Robbins et al.⁵⁶). A high abundance of recalcitrant litter could further act as a stabilizer of ecosystem functions because it represents a more constant resource that could potentially fulfill the baseline needs for the metabolism of its consumer, contrary to labile litter that might contain more nutrients but also be more volatile.³⁷,⁵¹ Interestingly, human activities and climate warming are modifying the distribution of recalcitrant litter⁵⁷,⁵⁸ and unequally accelerating the decomposition of some recalcitrant litter types,⁵⁹ possibly threatening the presence or efficiency of such stabilizing effects.

Nevertheless, the quality of litter is not a static variable throughout the year: spring or summer green fall has different traits than senesced litterfall,³³ the dominance in species-specific litterfall can vary through time,²⁹ and once the leaves enter
the detritus pool, the decomposition process modifies their quality too.⁶⁰,⁶¹ Consequently, we expect that the realized decomposition rates might diverge from the observed ones, especially in summer due to a decrease in the natural availability of labile litter. Contrastingly, we do not expect a marked difference in the realized decomposition rates of recalcitrant material, given the generally low and constant decomposition rates observed. Longer-term dynamics, such as the temporal hierarchy of decomposition drivers⁶² or slower palatability changes in recalcitrant litter,⁸,⁶³ could ultimately influence the realized decomposition dynamics in these temperate systems.

## Conclusions

Detrital resources are ubiquitous in nature and their availability is often associated with strong phenological cues. Well-known examples are spring phytoplankton blooms resulting in seasonally large deposits of detritus to the deep sea,⁶⁴ wildebeest drowning in rivers during temporally highly concentrated migration events and amassing carcasses and bones that are subsequently consumed by aquatic decomposers over decades,⁶⁵ and the subsidization of shores by marine kelp production strongly varying throughout the year.⁶⁶ Under the current scenarios of climate warming, the relevancy of temporal events is expected to increase as phenologies and seasonal dynamics are shifting and changing⁶⁷,⁶⁸ (but see also Loughnan et al.³²). Our data suggest that investigating ecological processes not only across ecosystem borders but also at finer temporal resolutions might be imperative to understanding complex system responses to change.¹¹,³¹,⁶⁹

## RESOURCE AVAILABILITY

### Lead contact

Requests for further information and resources should be directed to, and will be fulfilled by, the lead contact, Eva Cereghetti (eva.cereghetti@eawag.ch).

### Materials availability

This study did not generate new materials.

### Data and code availability

* Data used in the study have been deposited at Zenodo at https://doi.org/10.5281/zenodo.14989496 and are publicly available as of the date of publication.

* All original code is available at Zenodo at https://doi.org/10.5281/zenodo.14989496.

* Any additional information required to reanalyze the data reported in this paper is available from the lead contact upon request.

## ACKNOWLEDGMENTS

We thank Sarah Bollina, Francesca Cerroti, Dominik Kirschner, Andrin Krähenbühl, Samuel Hürlemann, Nadine Locher, Rebecca Oester, and Paula Rodriguez for helping to carry out the experiments. We thank two anonymous reviewers for the constructive feedback. Funding is from the Swiss Federal Office for Environment (FOEN/BAFU) and the Swiss National Science Foundation (grant nos. PP00P3_179089 and 310030_197410) to F.A.

## AUTHOR CONTRIBUTIONS

E.C. and F.A. conceived the study and designed the methodology. F.A. acquired the resources and funding. E.C. led the field investigation and the analysis of the data. T.P. and F.A. provided input on the methodology and data analysis and S.K. helped with data collection. E.C. led the writing of the

first draft and all authors contributed with comments and editing of the final manuscript.

## DECLARATION OF INTERESTS

The authors declare no competing interests.

## STAR★METHODS

Detailed methods are provided in the online version of this paper and include the following:

* KEY RESOURCES TABLE

* METHOD DETAILS

* Study streams and sites

* Flows of leaf litter material

* Rates of leaf litter decomposition

* QUANTIFICATION AND STATISTICAL ANALYSIS

* Statistical analysis: Rates of leaf litter decomposition

* Statistical analysis: Seasonal summaries

## SUPPLEMENTAL INFORMATION

Supplemental information can be found online at https://doi.org/10.1016/j.cub.2025.03.019.

Received: January 17, 2025

Revised: March 5, 2025

Accepted: March 12, 2025

Published: April 8, 2025

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## STAR★METHODS

### KEY RESOURCES TABLE

<table><thead><tr><th>REAGENT or RESOURCE</th><th>SOURCE</th><th>IDENTIFIER</th></tr></thead><tbody><tr><td colspan="3">Deposited data</td></tr><tr><td>Leaf litter flows and decomposition rates empirical data.</td><td>This study. Data was deposited at Zenodo repository.</td><td>https://doi.org/10.5281/zenodo.14989495</td></tr><tr><td>R code for the analyses</td><td>This study. Data was deposited at Zenodo repository.</td><td>https://doi.org/10.5281/zenodo.14989495</td></tr><tr><td colspan="3">Software and algorithms</td></tr><tr><td>R, version 4.3.3</td><td>R Core Team.<sup>70</sup></td><td>R Core Team<sup>70</sup></td></tr></tbody></table>

### METHOD DETAILS

#### Study streams and sites

We collected data at two small tributary streams of Lake Constance in the NE of Switzerland, the Eschlibach (47° 39.66' N, 9° 2.73' E) and the Manebach (47° 39.77' N, 9° 3.52' E), between September 2020 and December 2021. The Eschlibach measured 3.2 km in total length and had a catchment area of 1.35 km². The Manebach measured 3.3 km and 2.2 km² total length and catchment area, respectively. The total length of the streams may exhibit seasonal fluctuations due to variations in precipitation and flow dynamics, but the overall streams and, in particular, the study reaches presented continuous flows throughout the year. While the Eschlibach was fully forested upstream of the study reach, the upstream Manebach was mostly surrounded by agricultural land. The Eschlibach and Manebach study reaches—within which we conducted the decomposition assays, leaf litter collection and kicknet samples—were 100 and 70 m long, respectively, and were both located in forested areas dominated by *Fagus sylvatica* trees.

#### Flows of leaf litter material

To measure the input of abscised leaf litter within the riparian forest and stream ecosystems we built two types of leaf litter trap and placed five traps of each type at each of the two studied catchments. The first type of trap measured vertical inputs (i.e., direct fall) of litter and it collected an area of 0.25 m². Since the studied streams were fully covered by a closed canopy, we assumed that the rate of vertical litter input per unit area was the same for both the forest floor and the streams. The second type of trap measured lateral inputs (i.e., lateral transport) of leaf litter to the stream. In this case, the traps were placed adjacent to the stream and collected inputs taking place within 0.5 m of stream length. The exact trap location was selected with a stratified random technique in order to more equally cover the whole study reach length (though for accessibility reasons, the upper part of the Eschlibach was not covered). For the vertical traps, we randomized bank side (left or right) and the distance from the bank (up to 5 m). For the lateral traps, we randomized bank side (left or right) only, as these were positioned directly at the bank (for a schematic of the setup, see Figure S4). Traps were emptied every 6 weeks or at shorter time intervals during the autumn season. Collected material was oven-dried at 30 °C, sorted and weighed to the nearest 0.1 g. We categorized the leaf litter material into either recalcitrant litter (i.e., European beech *F. sylvatica*, the local dominant tree species) or labile litter (i.e., leaves of other tree species which included black alder *Alnus glutinosa*, ash *Fraxinus excelsior*, willow *Salix* sp., oak *Quercus* sp., maple *Acer* sp. and hazel *Corylus avellana*).

The total amount collected from each trap on the collection day was divided over the number of days passed since the previous collection to obtain an estimated daily input for that period. For vertical traps we calculated inputs per area (g m⁻² day⁻¹), whereas for lateral transport into the streams we calculated inputs per length of the terrestrial-aquatic interface (g m⁻¹ day⁻¹). To assess cumulative yearly inputs, we further averaged daily inputs between 2020 and 2021 for days that shared the same day number, i.e., taking an average value across the two years.

#### Rates of leaf litter decomposition

We collected naturally senesced leaf litter of black alder (*Alnus glutinosa*), European beech (*Fagus sylvatica*) and black poplar (*Populus nigra*) and conditioned them for five days in stream water (Chriesbach, 47° 24.26' N, 8° 36.62' E). We oven dried the litter at 30 °C and then filled litterbags (10 x 15 cm) with 3 g of monocultures of the conditioned leaf litter. We randomly assigned each of the three monocultures of litter species to a litterbag mesh size treatment (fine 0.5 mm mesh or coarse 10 mm mesh), a stream treatment (Eschlibach or Manebach), ecosystem treatment (aquatic or terrestrial) and site replicate (1–5). We prepared ten rounds of these treatment combinations, resulting in a total of 1200 litterbags (2 streams × 2 ecosystems × 3 leaf species × 2 mesh sizes × 5 replicates × 10 time points). At each round, we placed litterbags assigned to the aquatic treatment in the stream water column
and the ones assigned to the terrestrial treatment on the ground and then lightly covered them with some leaves and branches from the adjacent environment.

We deployed a new round of decomposition every six weeks, and we collected each round of aquatic decomposition after six weeks and each round of terrestrial decomposition after twelve weeks. In total we deployed ten rounds. At each round we assessed the abundance of the local dominant shredder, the amphipod *Gammarus fossarum* type A.⁴⁷ To do that, we took three kick net samples per sites, counted the amphipods and then released them back into the streams. The collected litterbags were then washed under water, oven-dried at 30 °C and then weighed to measure the remaining dry mass. To obtain more precise mass loss measurements, we burned the remaining leaf material in a muffle oven at 500 °C for 1 h and used ash-free dry mass (AFDM) to calculate mass loss.

## QUANTIFICATION AND STATISTICAL ANALYSIS

### Statistical analysis: Rates of leaf litter decomposition

All analyses were performed with R, version 4.3.3.⁷⁰

We calculated the decomposition rate using a first-order negative exponential model

$$k = \ln\left(\frac{m_t}{m_0}\right) \times \frac{1}{t}$$

where $m_0$ is the leaf litter biomass at deployment, $m_t$ is the remaining AFDM at time $t$, $t$ is the time elapsed in days and $k$ is the decomposition rate (henceforth $k_D$). To calculate the contribution of invertebrates to the decomposition of coarse mesh bags we used the fragmentation rate $\lambda_F$ developed by Lecerf⁷¹:

$$\lambda_F = k_c - \frac{k_f - k_c}{\ln(k_f) - \ln(k_c)}$$

where $k_f$ and $k_c$ are the decomposition rates of fine and coarse mesh bags, respectively. Since $k_f$ can be greater than $k_c$ due to natural variance and stochasticity, and result in a biologically unfeasible negative $\lambda_F$, we used the average $k_f$ and $k_c$ at the catchment level (averaging the five site replicates) to compute $\lambda_F$ and removed any remaining negative $\lambda_F$ from further analyses. For the rest of our analyses, we used microbial decomposition ($k_f$) and macroinvertebrate fragmentation ($\lambda_F$) as a common response variable for decomposition rate (as they represent the same metric).

To investigate the phenology of leaf litter decomposition (i.e., decomposition rate), we fitted generalized additive models (GAMs) using the R package 'mgcv'⁷². We fitted separated models for all combinations of ecosystem (aquatic and terrestrial) and decomposer group (microorganisms and invertebrates). So, for example, one model was fit for microbial decomposition ($k_f$) of leaf litter in terrestrial systems, and separated model for macroinvertebrate fragmentation ($\lambda_F$) in aquatic systems. All the models followed the formula:

$$\text{decomposition rate} \sim \text{species} + s(\text{day, by} = \text{species, bs} = \text{cc}) + t(\text{species, stream, bs} = \text{cc, re}) + s(\text{amphipods, by} = \text{species, bs} = \text{tp})$$

where $s(\text{day, by} = \text{species})$ is the smooth term that allows each litter species to have their own smoothing parameter across the day of the year [1–366], and $t(\text{species, stream})$ is a tensor product that uses stream catchment identity as a random effect (re), which allows variation between random levels but penalizes functions that strongly depart from the average. The smooth term $s(\text{amphipods, by} = \text{species})$ was only included for modelling aquatic fragmentation and was used to integrate the effect of amphipod abundance on the fragmentation rates of individual litter species. Specifically, this variable was computed using the total count of amphipods per stream assessed at both the deployment and the collection of the litterbags (i.e., at the beginning and at the end of each decomposition assay), and then averaged as the mean number of amphipods per kicknet per stream. The day of the year was modelled using cyclic cubic regression splines to force continuity between the last day and the first day of the year, whereas amphipod abundance was fitted with a thin plate spline.

The GAMs outputs were used to predict the rates of leaf litter decomposition for each day of the year for the given ecosystem type, decomposer group, leaf litter species and stream. Before calculating seasonal rates of decomposition, we first averaged the rates obtained across the two streams. Then, we averaged the rates obtained for the two litter species alder and poplar to obtain the rates of decomposition for labile litter, whereas the values obtained for beech litter represented the rates of decomposition of recalcitrant litter. These rates of decomposition were finally averaged within each season, which are defined in the section below.

### Statistical analysis: Seasonal summaries

To summarize the flows of leaf litter material and the rates of leaf litter decomposition into four seasons, we had to define the starting and ending date of each season. We tested our data using three different cutoffs. The first (“standardized”) used a fixed duration approach, so that spring started on March 1st, summer on June 1st, autumn on September 1st and winter on December 1st. The second (“phenology”) relied on meteorological data, which defines the starting of seasons based on the phenology of tree species.
Specifically, we used the phenological data of the Baden-Württemberg region⁷³ and averaged the tree phenology of 2020 and 2021. We obtained the following seasonal cutoffs: spring started on March 14 (flowering of forsythia trees), summer on June 15 (flowering of lime trees), autumn on September 18 (fruit production of oak trees) and winter on November 10 (leaf fall of oak trees). The third cutoff (“balanced”) is an average of the “standardized” and the “phenology” methods. Finally, to summarize our findings into seasonal data, we averaged the flows of leaf litter material and the rates of leaf litter decomposition within each season according to the three cutoffs. In the main text, we show results obtained with the “balanced” cutoff.