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Warming enhances sedimentation and decomposition of organic carbon in shallow macrophyte‐dominated systems with zero net effect on carbon burial

Temperatures have been rising throughout recent decades and are predicted to rise further in the coming century. Global warming affects carbon cycling in freshwater ecosystems, which both emit and bury substantial amounts of carbon on a global scale. Currently, most studies focus on the effect of warming on overall carbon emissions from freshwater ecosystems, while net effects on carbon budgets may strongly depend on burial in sediments. Here, we tested whether year‐round warming increases the production, sedimentation, or decomposition of particulate organic carbon and eventually alters the carbon burial in a typical shallow freshwater system. We performed an indoor experiment in eight mesocosms dominated by the common submerged aquatic plant Myriophyllum spicatum testing two temperature treatments: a temperate seasonal temperature control and a warmed (+4°C) treatment (n = 4). During a full experimental year, the carbon stock in plant biomass, dissolved organic carbon in the water column, sedimented organic matter, and decomposition of plant detritus were measured. Our results showed that year‐round warming nearly doubled the final carbon stock in plant biomass from 6.9 ± 1.1 g C in the control treatment to 12.8 ± 0.6 g C (mean ± SE), mainly due to a prolonged growing season in autumn. DOC concentrations did not differ between the treatments, but organic carbon sedimentation increased by 60% from 96 ± 9.6 to 152 ± 16 g C m−2 yaer−1 (mean ± SE) from control to warm treatments. Enhanced decomposition of plant detritus in the warm treatment, however, compensated for the increased sedimentation. As a result, net carbon burial was 40 ± 5.7 g C m−2 year−1 in both temperature treatments when fluxes were combined into a carbon budget model. These results indicate that warming can increase the turnover of organic carbon in shallow macrophyte‐dominated systems, while not necessarily affecting net carbon burial on a system scale.

Temperature-dependency of carbon burial in macrophyte-dominated systems is of particular interest as global mean surface temperature is projected to increase over the coming century by 3-5°C (i.e., RCP 8.5 scenario in IPCC (2014)). Rapid warming of lakes has already been observed around the globe over the last decades (Adrian et al., 2009;Mooij, De Senerpont Domis, & Hülsmann, 2008;O'Reilly et al., 2015;Woolway et al., 2017). Temperature changes can modify the metabolic balance of freshwater lakes (Yvon-Durocher, Jones, Trimmer, Woodward, & Montoya, 2010), as aquatic respiration processes show a stronger response to warming than primary production (O'Connor, Piehler, Leech, Anton, & Bruno, 2009). Modifications to the metabolic balance in combination with increasing inorganic carbon loading from the catchment (Weyhenmeyer et al., 2015) can lead to enhanced dissolved carbon concentrations in already supersaturated lakes and ponds (Atwood et al., 2015;Flanagan & McCauley, 2010;Kosten et al., 2010) and decreased carbon burial efficiency (as suggested by Mendonça et al. (2016)) in warmer climates. Thus, a detailed understanding of how temperature affects carbon cycling on a system scale is important to predict how warming will influence the ratio between carbon emissions and burial in lakes (Mendonça et al., 2012).
Currently, most studies focus on the effect of warming on overall carbon emissions from freshwater ecosystems, while net effects on carbon budgets may strongly depend on carbon burial in sediments.
Autochthonous carbon cycling can be divided into three main processes: the uptake of inorganic carbon by primary producers and its conversion into organic carbon bound in their biomass (net primary production), the subsequent sedimentation of senesced organic matter and the remineralization of organic carbon through decomposition (Yvon-Durocher, Allen, . Once arrived at the bottom of the lake, part of the sedimented organic material can be buried for a longer period of time in lake sediments. Each carbon-cycling process can depend on environmental conditions including lake temperature. Warming can enhance decomposition rates (Fernandes, Seena, Pascoal, & Cássio, 2014;Flanagan & McCauley, 2010;Gudasz et al., 2010;Song, Yan, Cai, & Jiang, 2013;Zhou, Chen, Yan, & Duan, 2016). Sedimentation of organic material can be positively (Kritzberg et al., 2014) or negatively (Flanagan & McCauley, 2008) affected by warming, most probably related to temperature-driven changes in primary production (Kritzberg et al., 2014). However, temperature-driven prolongation of growing seasons (Netten, Van Zuidam, Kosten, & Peeters, 2011) and changes in seasonal timing (Hansson et al., 2013;Velthuis, Domis et al., 2017;Zhang, Bakker, Zhang, & Xu, 2016), as well as shifts in vegetation type (Moss et al., 2011;Peeters et al., 2013) and plant carbon:nutrient stoichiometry (Velthuis, van Deelen, van Donk, Zhang, & Bakker, 2017), can also indirectly affect the impact of warming on aquatic carbon cycling. Understanding how temperature affects the intricate balance of primary production, sedimentation, and decomposition is essential for predicting carbon burial under global warming (Yvon-Durocher, Hulatt, Woodward, & Trimmer, 2017).
To our knowledge, no integrative studies on the effect of elevated temperature on submerged aquatic plant biomass and its fate via sedimentation and decomposition exist to date. Therefore, we tested the effect of warming on the carbon stocks in plant biomass and carbon fluxes in sedimentation and plant litter decomposition in a year-round 1,000 L mesocosm experiment, stocked with Eurasian watermilfoil (Myriophyllum spicatum L.). M. spicatum is a submerged, rooted freshwater plant species native to Europe, Asia, and has spread to North America (Smith & Barko, 1990). The mesocosms were exposed to temperature scenarios representing temperate (Dutch) conditions and a +4°C scenario, representing the RCP 8.5 scenario (IPCC 2014). During the experiment, we measured plant abundance, as well as sedimentation and decomposition of particulate organic matter. We hypothesized that 4°C warming would prolong the growing season of M. spicatum by advancing its spring phenology and delaying its decline in autumn. We further hypothesized that the prolonged growing season would subsequently increase the production of particulate organic carbon and consequently enhance sedimentation. Similarly, temperature-driven increases in decomposition rates of plant litter were expected. To determine the net burial of autochthonously produced organic carbon over the course of the experiment, we integrated the sedimentation and decomposition fluxes into a carbon budget model.

| Experimental set-up
Eight 988 L indoor mesocosms referred to as limnotrons (1.35 m average depth, 0.97 m diameter, see also Verschoor, Takken, Massieux, and Vijverberg (2003)) were filled on March 4, 2015 with 912 L of tap water (12.8 ± 7 and 0.85 ± 0.04 μM DIN and DIP, respectively) and 75 L of pre-sieved sediment (5-mm mesh size) from the top layer (top 5 cm) of a mesotrophic shallow pond in Wageningen, The Netherlands (51°59′16.3″N 5°40′06.0″E). As this work was part of a 3-year mesocosm project, the sediment collected from the field was mixed with sediment from the experiment performed the year before (2-1 parts, v/v, for information on the previous experiment, see Frenken et al., 2016;Velthuis, Domis et al., 2017).
Prior to plant and nutrient additions, water was circulated between all mesocosms for 2 days to promote similar starting conditions. Five shoots of Myriophyllum spicatum (30 to 60-cm length, with attached roots), originating from the same mesotrophic pond as the sediment, were added on March 6, 2015 by attaching a small pebble to the stem and letting them sink to the bottom of the mesocosm. Due to initial limited growth of the plants, additional shoots were planted in the same manner on April 28 and June 30 (Supporting Information Table S1). Nutrients were added to the water column on March 9, 2015 to come to final concentrations of 52, 1.9 and 65 μM of NO À 3 , PO 3À 4 and Si, respectively. The incident light at the water surface was kept constant throughout the experiment at 188 ± 5 (mean ± SE) μmol photons m −2 s −1 , provided by two HPS/ MH lamps (CDM-TP Elite MW 315-400 W, AGRILIGHT B.V., Monster, The Netherlands). The light:dark cycle followed typical Dutch seasonality (Supporting Information Figure S1). Details on experimental conditions (TIC, pH, and nutrient availability) can be found in Supporting Information Table S2. Surface mixing during the experi-

| Macrophyte PVI and biomass
Macrophyte abundance was recorded by weekly PVI (Percent Volume Infested, Canfield et al., 1984) estimates. Accordingly, plant height was measured with a ruler and cover (%) was estimated by eye in each mesocosm. From these measurements, the PVI of M. spicatum was calculated as: After a full year, the experiment was terminated. Above-and belowground biomass was harvested and dried at 60°C until constant dry weight, weighed, and stored dry and dark until further analysis for elemental composition.

| Sedimentation
Sedimentation rates were determined each month by placing custom-made tube-shaped sedimentation traps (9-cm diameter, 18-cm height, and 1.1-L volume) at one-meter depth (measured from the water surface) in the center of the mesocosm for a period of 3 days.
The contents of the sediment trap (i.e., sedimented material and sestonic POC in the water inside the trap) were thereafter filtered over pre-washed GF/F filters (Whatman, Maidstone, UK), dried at 60°C overnight, and stored dry and dark for elemental analysis. To correct for sestonic POC (<220 μm), water samples were taken with a tubesampler in the middle of the mesocosm on the same day as the sedimentation traps were taken out and handled in the same manner.
Sedimentation rates (SRs) were calculated as the amount of sedimented POC (corrected for sestonic POC present inside the traps) divided over the time that the sedimentation traps hung in the mesocosms. The amount of sedimented organic carbon was then calculated as the area under the curve of these sedimentation rates.

| Decomposition
Microbial decomposition of sedimented macrophyte litter on the sediment surface was determined using a litterbag method (Benfield, 2006  The bags were dried at 60°C until constant dry weight. The weight loss of the litterbags with plant material in them was corrected for the weight gain from the control litterbags. Plant material from the litterbags was stored dry and dark until elemental analysis. As we did not observe any sedimented stem material, we focus on the decomposition of leaf material. The results for the decomposition of stem material can be found in Supporting Information Figure S2. To determine whether temperature affected dissolved oxygen concentrations near the sediment (and thereby oxygen availability for decomposition), oxygen was measured at the sediment interface (120-cm depth) every 2 weeks using a multi-parameter meter

| Elemental analysis
The carbon (C) content was analyzed for the harvested plant biomass at the end of the experiment, as well as of sedimentation, seston, and decomposition samples throughout the experiment.

| Calculations and statistics
All calculations and statistics were carried out in R (R Core Team 2015), using the packages nlme, minpack.lm, stats, xlsx, and ggplot2 (Dragulescu, 2014;Elzhov et al., 2016;Pinheiro, Bates, DebRoy, & Sarkar, 2015;Wickham, 2010). All R-code is archived in the Dryad repository belonging to this paper. To determine whether warming advanced the spring phenology of M. spicatum, we determined the date at which half of the maximum PVI was recorded for each individual mesocosm (Rolinski, Horn, Petzoldt, & Paul, 2007;Zhang et al., 2016). When this cardinal date occurred between two sampling dates, we approximated it by linear extrapolation between these dates.
The carbon stock in the form of aboveground plant biomass at the end of the experiment was determined by multiplying dry weight with its carbon content for each individual mesocosm. To determine the decomposition rate and the fraction recalcitrant organic carbon (i.e., remaining carbon fraction) in the decomposition experiment, we used a two-phase decomposition model (as described by Harmon et al., 2009): where C 0 is the initial fraction of organic carbon in the litter bags (= 1) and C t the remaining fraction of organic carbon at time (t) since the start of the experiment (in days), k is the decomposition rate (in day −1 ), and s is the fraction recalcitrant organic carbon. This decomposition model was fitted through the experimental data of leaf and stem litter for each individual mesocosm with the function nlsLM, from which the parameters k and s were derived through nonlinear algorithms. The parameter decomposition rate (k) and recalcitrant fraction (s) were tested for differences between treatments using Student's t tests (function t test).
We used a carbon budget model to estimate the amount of autochthonously produced organic carbon during the experimental year that was buried in the sediment of the mesocosm. In this model, we incorporated the dynamics of labile and buried particulate organic carbon throughout the season and assumed that the recalcitrant fraction s and the decomposition rate k determined from the decomposition dynamics are representative for the burial and decomposition rates of the sedimented carbon. With these parameters, we modeled the dynamics of labile organic carbon (LOC in g C/ m 2 ) at the bottom of the system over the season for each individual mesocosm in the following manner: here, LOC is defined as the remainder of build-up of labile particulate organic carbon at the earlier time point, (LOC t−1 ) plus the labile fraction (1-s) of the sedimented particulate organic carbon (at rate SR in day −1 ) over the respective time interval (Δt in day −1 ), minus the amount of organic carbon that was decomposed during that period (at rate k, in day −1 ). For the dynamics in organic carbon burial (OCB in g C/m 2 ), the following equation was used for each individual mesocosm:

| Macrophyte development, carbon stocks and carbon:nutrient stoichiometry
The abundance of M. spicatum gradually increased over the first 3 months of the experiment, until a maximum abundance of approximately 75% PVI was reached at the end of September in both treatments. After this peak, abundance declined to 35% in the control treatment, while it remained around 65% PVI in the warm treatment. Over the entire experimental year, the abundance of M. spicatum was significantly higher in the warm treatment (Table 1).
Furthermore, warming significantly affected spring phenology of plant growth as the date on which half of the maximum PVI was attained advanced by 10 days in the warm treatment (Student's t test; t(3) = 3.9, p = 0.02; Figure 1).
At the end of the experiment, the carbon stock in aboveground M. spicatum was 1.9-fold higher in the warm treatment compared to the control (Table 2). This carbon stock was significantly positively related to plant abundance (Linear regression, R 2 = 0.88, p < 0.001, n = 8, Supporting Information Figure S3). Warming did not affect macrophyte carbon content, aboveground C:P ratios, belowground carbon stocks, or belowground carbon:nutrient ratios. The above-  Figure S4, Table S3).

| Sedimentation
Sedimentation rates varied seasonally, with maximum rates at the end of August of 0.89 ± 0.20 and 1.70 ± 0.38 g C m −2 day −1 (mean ± SE) in the control and warm treatment, respectively (Supporting Information Figure S5, Table S3). This sedimented material consisted mostly of detritus (unidentified particulate organic matter and senesced leaves of M. spicatum). The cumulative amount of sedimented organic carbon over the season was significantly higher in the warm treatment (Table 1; Figure 2). At the end of the experiment, 95.6 ± 9.5 and 152 ± 16 g C m −2 had sedimented in the control and warm treatment, respectively ( Table 2). Quality of the sedimented material was not significantly affected by warming, as carbon:nutrient ratios did not differ between treatments (Supporting Information Figure S6, Table 1). However, a gradual increase in these ratios was observed over the course of the experiment as indicated by a significant time effect. Oxygen concentrations at the sediment did not differ between temperature treatments (Supporting Information Figure S7, Table S3).

| Decomposition
The fraction of carbon in plant leaf litter that was lost due to decomposition was significantly larger with warming, with the remaining fraction recalcitrant organic carbon (s) being 39% and 23% in the control and warm treatment ( Figure 3, Table 2). No effect of warming could be detected on decomposition rates of leaf litter (k), which were 0.024 ± 0.0031 and 0.019 ± 0.0017 day −1 in the control and warm treatment, respectively (Table 2). thereafter decreased in both treatments (Figure 4a). The amount of LOC throughout the experiment was higher in the warm treatment than in the control, while organic carbon burial (OCB) did not differ between treatments (Table 1). However, there was a significant time effect on organic carbon burial, indicating its accumulation over the experiment (Figure 4b). The amount of remaining organic carbon in the sediment (LOC + OCB) after a full experimental year was 41.2 ± 5.9 and 39.5 ± 6.4 g C m −2 in the control and warm treatment, respectively. No effect of warming on this organic carbon burial was found (Table 2).

| DISCUSSION
Global warming is predicted to have strong impacts on the carbon balance in aquatic ecosystems (Yvon-Durocher, Allen et al., 2010).

| Warming effects on submerged plant abundance and DOC concentrations
As warming can accelerate growth rates of organisms and prolong their growing seasons (Brown, Gillooly, Allen, Savage, & West, 2004;Netten et al., 2011;Peeters et al., 2013), a positive response of warming on standing stock biomass of Myriophyllum spicatum was anticipated in our experiment. Indeed, the abundance of M. spicatum was significantly higher in the warm treatment throughout the experiment even though more initial biomass was added to the control treatment (Supporting Information Table S1). Higher M. spicatum abundance in the warm treatment is most probably attributed to higher production rates, as indicated by enhanced sedimentation rates in the warm treatment. Warming has been shown to enhance the growth of M. spicatum (Li et al., 2017), as well as of other aquatic (Barko & Smart, 1981;Kaldy, 2014;Peeters et al., 2013;Velthuis, van Deelen et al., 2017) and terrestrial plants (Rustad et al., 2001), indicating that this is a common response across ecosystems.
Additionally, warming can advance the spring phenology of aquatic (Hansson et al., 2013;Velthuis, Domis et al., 2017;Winder & Schindler, 2004;Zhang et al., 2016) and terrestrial organisms (Parmesan, 2007  F I G U R E 5 Summary of autochthonously produced organic carbon pools and fluxes (mean ± SE) in macrophyte-dominated systems in the present-day situation (control) and under global warming (warm), with standing stock (macrophyte biomass) at the end of the experiment in g C, DOC in g C and sedimentation, decomposition, and burial in g C/m 2 /y VELTHUIS ET AL.

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Although not the main focus of this study, dissolved organic carbon (DOC) concentrations can be an outcome of leaching from aquatic primary production (Barrón, Apostolaki, & Duarte, 2012) and consequently a temperature-induced increase related to enhanced macrophyte biomass was anticipated. However, no effect of warming on DOC concentrations was observed ( Figure 5), which may be due to simultaneous temperature-driven increases in DOC mineralization rates (Gudasz et al., 2010). Nonetheless, a climate change-related increase in DOC concentrations is to be expected in lakes, often linked to enhanced DOC inflow from the catchment area (Larsen, Andersen, & Hessen, 2011).

| Warming effects on sedimentation
Warming-induced increases in primary production can promote the sedimentation of organic carbon (Kritzberg et al., 2014). Indeed, the amount of sedimented organic carbon was higher in the warm treatment, indicating enhanced production of particulate organic carbon with warming. The annual sedimentation rate in our mesocosms (96-152 g C m −2 year −1 ; Figure 5) falls within the range of sedimentation rates measured in lakes worldwide (4-400 g C m −2 year −1 ; Tartari and Biasci, 1997). To our knowledge, these are the first empirical results to address the effects of warming on sedimentation rates in a macrophyte-dominated freshwater system and our findings demonstrate a way in which warming may increase the carbon flux to lake sediments. Conflicting findings are reported for plankton dominated systems, where 3-4 degrees of warming can lead to an increase (Kritzberg et al., 2014) or decrease (Flanagan & McCauley, 2008) in sedimentation of organic carbon. This discrepancy in warming effects on sedimentation rates may be attributed to the presence of grazers and the top-down control they impose on the carbon stocks in primary producers (Flanagan & McCauley, 2008). As roughly 90% of the consumed organic material by grazers can be respired as CO 2 , their presence introduces a loss of organic carbon from the system (Yvon-Durocher, Allen et al., 2010). In our experiment, no grazing on M. spicatum was observed. On the contrary, warming positively affected macrophyte biomass. As dense macrophyte stands can reduce flow velocity (Madsen, Chambers, James, Koch, & Westlake, 2001), this temperature-driven increase in macrophyte biomass could in theory indirectly enhance sedimentation rates by reducing resuspension of the sediment (Jeppesen et al., 1998). However, as flow velocity did not play a strong role in our mesocosm experiment, we prefer to attribute the positive effect of warming on carbon sedimentation to enhanced production of organic material (Kritzberg et al., 2014).

| Warming effects on decomposition
Decomposition rates of leaf litter in our experiment were around 0.02 day −1 , which is in the same range as field surveys with M. spicatum (0.01-0.06 day −1 ; Carpenter and Adams, 1979) and other aquatic macrophytes (0.0008-0.06 day −1 ; Chimney and Pietro, 2006). In our experiment, the water at the sediment surface where litterbags were exposed remained oxic in both treatments (Supporting Information Figure S7). Anaerobic conditions as often observed below dense macrophyte stands and in the sediment (Frodge, Thomas, & Pauley, 1990;Sukhodolova, Weber, Zhang, & Wolter, 2017) would likely have led to lower decomposition rates than we observed. This could in turn also have altered the balance between sedimentation and decomposition found in our study, resulting in a possible overestimation of decomposition rates under varying oxygen conditions.
Warming significantly reduced the remaining carbon in leaf litter decomposition. This enhanced decomposition is in line with temperature-induced increases in the degree of decomposition of other aquatic plants (Carvalho, Hepp, Palma-Silva, & Albertoni, 2015;Passerini, Cunha-Santino, & Bianchini, 2016;Song et al., 2013), as well as allochthonous leaf litter in aquatic systems (Fernandes et al., 2014) and soil respiration in terrestrial ecosystems (Rustad et al., 2001). Surprisingly, no effect of warming on the decomposition rate k could be detected in our experiment, as the observed results were driven by a lower remaining fraction recalcitrant carbon (s), which was 14% lower in the warm treatment. The unaffected decomposition rates are possibly a result of the relative short time span of the decomposition experiment, leading to overestimations of the recalcitrant fraction (Koehler, Wachenfeldt, Kothawala, & Tranvik, 2012).
Alternatively, our findings could stem from a higher temperature sensitivity of this recalcitrant material to decomposition (Davidson & Janssens, 2006), which in turn can be caused by temperature-driven community shifts toward decomposers that can degrade more recalcitrant organic matter (Dang, Schindler, Chauvet, & Gessner, 2009) and a higher activity of enzymes needed for this recalcitrant matter degradation (Ylla, Romaní, & Sabater, 2012).
As the experiment presented here was conducted over a 1-year period, possible long-term effects of warming on litter decomposition were not taken into account. For instance, sedimentation of the enhanced macrophyte biomass in the warm treatment may lead to rapid changes in sediment redox conditions and related oxygen concentrations, thereby potentially reducing decomposition rates (Passerini et al., 2016). Furthermore, recalcitrant carbon is not necessarily static and can decompose further over a timespan of years, although at a much slower rate than its labile counterparts (Koehler et al., 2012). Relatedly, we may have overestimated the fraction of recalcitrant organic material in the warm treatment as at the end of the experiment the fraction still seemed to decrease, while the decomposition seems to have stabilized in the control treatment (see Figure 3). This would imply an underestimation of the effect of warming on decomposition.

| Warming effects on organic carbon burial
Net organic carbon burial was determined by combining particulate organic carbon fluxes by sedimentation and decomposition into a carbon budget model. This model showed that the individual carbon fluxes counteracted each other in such a way that organic carbon burial was unaffected by 4°C warming. In other words, warming stimulated the carbon turnover rate by enhancing both sedimentation and litter decomposition fluxes. Temperature-driven increases in carbon turnover can also be observed in terrestrial ecosystems (Knorr, Prentice, House, & Holland, 2005) and thus seem to be a common response to warming across ecosystems. We calculated an average organic carbon burial rate of 40 g C m −2 year −1 which falls within the spectrum of burial rates determined for European lakes (0.1-58 g C m −2 year −1 ; Kastowski, Hinderer, and Vecsei, 2011). Furthermore, our model indicates clear seasonal dynamics in the balance between sedimentation and litter decomposition. Until September, labile organic carbon (LOC) accumulated in the sediment in both treatments, as sedimentation rates were larger than decomposition rates (see Figure 2 and  (Cebrian & Lartigue, 2004;Handa et al., 2014;Zhang, Hui, Luo, & Zhou, 2008). Warming can lead to shifts in carbon:nutrient ratios of aquatic plants (Ventura et al., 2008;Zhang et al., 2016), but the direction of change is not uniform across studies (Velthuis, van Deelen et al., 2017). In our experiment, aboveground plant C:N ratios were higher in the warm treatment compared to the control (Table 2), indicating a decreased quality of these plants. As our carbon budget model assumes a fixed litter quality, we may in theory overestimate the effect of warming on litter decomposition.
Nonetheless, decomposition rates of leaf and stem litter of M. spicatum were similar, even though those litter types differed in carbon: nutrient ratios (Supporting Information Figure S2). Furthermore, the quality of the sedimented material did not differ between temperature treatments (Supporting Information Figure S6). Thus, even though litter quality is an important factor to consider in decomposition dynamics, it seems not to be a major driver of temperature effects on decomposition in this particular experimental set-up.
The fate of organic carbon in the sediment is determined by the carbon burial efficiency, which is the balance between buried and sedimented carbon. Fast accumulation of sedimented carbon can lead to higher carbon burial efficiency, as this can reduce oxygen exposure time at the sediment surface (Mendonça et al., 2016;Sobek et al., 2009). Overall, the carbon burial efficiency was 43% in our control treatment and-corresponding with the high turnoveronly 26% in the warm treatment. These percentages are lower than for lakes that receive important allochthonous carbon inputs, but they fall within the range found for lake sediments that receive mainly autochthonous produced carbon (Sobek et al., 2009

| Conclusions and implications
With the predicted climatic warming over the coming century and its potential feedbacks on the global carbon cycle, temperature-driven changes in freshwater carbon cycling are an important component of climate change research (Yvon-Durocher, Allen et al., 2010).
Highly productive macrophyte-dominated freshwater systems (Kazanjian et al., 2018) play an important role in long-term carbon storage at the landscape (Premke et al., 2016) and even at the global scale (Mulholland & Elwood, 1982). Our experimental results suggest that 1 year of 4°C warming will increase production and sedimentation of organic carbon in these systems. Assuming that autochthonous carbon production is the main form of carbon influx, enhanced decomposition with warming can compensate for the increased sedimentation of this organic carbon. Climatic warming may thus increase the turnover rate of aquatic carbon cycling, while not necessarily affecting net carbon burial on a system scale.

ACKNOWLEDG EMENTS
The authors thank Nico Helmsing, Erik Reichman, Dennis de Raaij, and Arjan Wiersma for their technical assistance during the experiment, Ciska Veen, Laura Seelen, and Raquel Ariasfont for their valuable input on the decomposition methods, Christopher Monk for linguistic improvements and Sven Teurlincx for fruitful discussions.
Attribution for the illustration of Myriophyllum spicatum in Figure  | 5239