Introduction

Natural lakes cover approximately 2.67 million km2 worldwide or 1.8% of the land surface area across nearly all climate zones (Messager et al. 2016). In contrast to terrestrial forests, where organic carbon (OC) is stored mainly in living biomass and soils, aquatic ecosystems accumulate organic carbon (OC) in their sediments (Mcleod et al. 2011; Mendonça et al. 2017). Lakes generally have high OC burial rates (Mendonça et al. 2017), which account for substantial contributions to regional carbon budgets, and overall, to the global carbon cycle (Anderson et al. 2014). Even though lakes cover < 2% of the entire Earth surface, these freshwater ecosystems accumulate almost half as much OC as the world’s oceans, i.e. 42 and 100 Tg C year−1, respectively (Dean and Gorham 1998). Large lakes >500 km2 account for 93% of the total surface area of the world’s waterbodies (Herdendorf 1998), and small lakes < 500 km2 account for 60–70% of the total OC stored in lake sediments (Alin and Johnson 2007).

In continental aquatic ecosystems, water level fluctuations, catchment land use, and human activities influence aspects of the carbon cycle, including burial and export (Anderson et al. 2014; Mendonça et al. 2017; Almeida et al. 2019). Land conversion from grasslands to agriculture releases nutrients that increase aquatic productivity (Anderson et al. 2013, 2014; Dietz et al. 2015), and indirectly enhances OC burial (Anderson et al. 2014). For example, Mendonça et al. (2017) examined OC burial values in > 400 lakes and reservoirs and concluded that the areal proportion of croplands, temperature and runoff in the basins have a positive influence on OC burial rate, whereas the area of the aquatic ecosystem and the average slope of the terrain have a negative influence, i.e. the greater the lake area, and/or the steeper the terrain, the lower the OC burial rate. Therefore, knowledge of catchment characteristics and an understanding of the quantity and distribution of carbon accumulated in lake sediments can help determine how lakes sequester OC.

The primary objective of this study was to estimate OC burial rates over the last century in sediments of three coastal lakes of the Mirim-Mangueira Lagoon system, which are subject to different levels of human impact. This was done to investigate if agricultural expansion had affected carbon sequestration. The study region is part of the largest coastal lagoon ecosystem on Earth, covering > 500 km of coastline in Brazil and Uruguay where wetlands have been largely converted to agricultural land, mimicking trends observed in many coastal ecosystems worldwide.

Study area

The Mirim-Mangueira Lagoon ecosystem is located in a watershed shared by Uruguay and Brazil that covers an area of 62,250 km2, almost half of which is located in Brazilian territory (Borba 2016). It was formed by Quaternary glacio-eustatic sea level fluctuations, which gave rise to a shallow lagoon complex with four depositional systems (sedimentary barriers), each representing a transgressive–regressive cycle (Tomazelli et al. 2000).

The Mirim Lagoon basin is characterised by the presence of thousands of small mounds. These mounds (cerritos de indios in Spanish, or aterros in Portuguese) are elevations of land built by the indigenous populations that inhabited the region for five thousand years before the seventeenth century, and constitute archaeological remains of the pre-colonial populations. Although the mounds are located on the most fertile soils, pre-colonial populations obtained their sustenance almost exclusively from hunting and gathering (Bracco Boksar 2006). After European colonisation in the seventeenth century, the low-lying humid landscape along the Atlantic coast in this region of South America started being replaced by agricultural fields (Mertz et al. 2007). Currently, irrigated rice cultivation is the main economic activity in the catchment. The first paddy fields date back to 1903 in Brazil and 1928 in Uruguay (MIN 2008; ACA 2017). By 1960 there was an exponential growth in rice production and the area planted in the catchment (FAO 2019) associated with new technologies (including pesticides, fertilisers, and the mechanisation of previously manual processes) (Alegre et al. 2014). In addition, in the 1980s the Brazilian Federal Government provided financial support to encourage the use of the wetlands for agriculture through the Pro-Várzea Progam, which also promoted rice irrigation in the area (Menegheti 2010).

Currently, the Mirim Wetlands and the area between the Mangueira and Mirim Lagoons are cultivated (Villanueva et al. 2000) (Fig. 1), and irrigated rice production accounts for >99% of regional water usage (Santos et al. 2008a). Rice irrigation disrupted natural ecological processes as a consequence of the transformation and fragmentation of habitats, as well as changes in hydrology and release of contaminants (Menegheti 2010). In addition to land-use change, there has also been intense use of fertilisers, which has caused the continuous export of nutrients to the adjacent aquatic environments (Santos et al. 2004; Andrade et al. 2012). Mirim Lagoon (Table 1) is connected with the Patos Lagoon Estuary through the São Gonçalo channel (Fig. 1). In general, water flows from Mirim Lagoon to Patos Lagoon (Oliveira et al. 2019). During the dry season, however, brackish water from Patos Lagoon can reach Mirim Lagoon and damage the surrounding rice paddies. To prevent this, a lock was built in the São Gonçalo channel in 1977 (Hirata et al. 2010) (Fig. 1), and Mirim Lagoon is currently a freshwater ecosystem. Mangueira Lagoon is separated from the Atlantic Ocean by a sand barrier. Despite its large size, freshwater inputs are only through direct rainfall and groundwater seepage (Santos et al. 2008a). The only surface connection to other aquatic environments is through the Taim Wetland (Villanueva et al. 2000; Borba 2016). The protected Taim Wetland, located north of Mangueira Lagoon (Fig. 1), contains shallow freshwater lakes like Flores, Jacaré and Nicola. The Taim Wetland reserve was created in 1978, with the primary objective of protecting the remaining wetland ecosystems and endangered wildlife. In 2017, the Taim Wetland was designated a wetland of international importance by the Ramsar Convention (Ramsar site 2298), which recognised its important heritage and biological diversity. It is, nevertheless, subject to threats such as human settlements, intensive rice cultivation and cattle ranches (Villanueva et al. 2000; RAMSAR 2017).

Fig. 1
figure 1

Main waterbodies in the Mirim—São Gonçalo catchment and coring sites in Mirim Lagoon (MIR1, MIR2, MIR3), Mangueira Lagoon (MAN2) and Nicola Lake (NIC). Colour code: Green: Agricultural land; Red: Taim Wetland; Red dot: Lock in São Gonçalo channel; Black dots: artificial drainage canals between Mirim and Mangueira lagoons

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Table 1 Basic characteristics of the three study sites
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Materials and methods

Sediment core sampling and analysis

Five sediment cores were collected using a gravity corer with pre-cleaned plastic core tubes (50-mm diameter and 1 m long), three in Mirim Lagoon (MIR1, MIR2 and MIR3), one in Mangueira Lagoon (MAN2) and one in Nicola Lake (NIC) (Table 2, Fig. 1). Cores NIC and MIR1 were collected in August 2006 and cores MAN2, MIR2 and MIR3 in January 2008. The cores were extruded from the bottom up, sectioned into 1-cm intervals in the upper 10 cm and subsequently at 2-cm intervals throughout the rest of the cores, and stored in plastic bags. Once in the laboratory, the sediment core slices were dried in an oven at 60 ºC overnight or until the weight was stable.

Table 2 Core locations and water depths at the collection sites
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Gamma spectrometry was used to quantify 137Cs, 210Pb and 226Ra activities using a Canberra well-type hyperpure Ge detector. Dried sediment samples were packed into small vials to a constant height, covered with a layer of epoxy to prevent the escape of 222Rn, and counted after a 3-week hold time to ensure radioactive equilibrium between 226Ra and 222Rn daughters 214Pb and 214Bi. 226Ra activity was estimated from the 214Pb and 214Bi photopeaks (295, 352, and 609 keV), 210Pb was determined from its direct photopeak at 46.5 keV and 137Cs from the 661.6 keV photopeak. The gamma system was calibrated using IAEA natural matrix sediment and soil standards. The excess 210Pb activity was estimated by subtracting the 226Ra (supported 210Pb) activity from the total 210Pb activity. The excess (unsupported) 210Pb was used to estimate the ages of the sediment intervals using different approaches. In all cores, the ages of all intervals were calculated using the constant flux constant sedimentation (CF:CS) dating model (Appleby and Oldfield 1983; Sanchez-Cabeza et al. 2012). In cores MIR1 and MAN2, the entire inventory of 210Pbxs was obtained and the constant rate of supply (CRS) model was also applied for comparative purposes. The CRS model assumes that the flux of 210Pb to the accumulating sediment has been constant through time (Appleby and Oldfield 1978).

Fine-grained sediment (FGS) (< 63 μm) was quantified using a standard wet-sieving method (Suguio 1973). Briefly, a dried and weighed core interval was mixed with water and then placed in a 63-μm sieve shaker with a continuous supply of water for 5 min. Material retained in the 63-μm sieve (coarse sediment) was then dried and the FGS calculated as the difference between the initial weight and the coarse sediment weight.

Dry bulk density (g dry cm−3 wet) was calculated by dividing the dry sediment weight of the selected intervals by the initial wet sediment volume. Total organic matter was determined by loss on ignition (LOI), by placing weighed dry samples in a muffle furnace at 450 °C (LOI-450) for 3 h, which combusts the organic matter and avoids loss of carbonates (Craft et al. 1991). LOI-450 was converted to OC by dividing LOI-450 by 1.724 (Schumacher 2002). This conversion factor has been used to estimate OC from organic matter content in sediments from similar coastal settings (Brown et al. 2016; Sanders et al. 2017).

Organic carbon accumulation rates in sediments (g OC m−2 year−1) were obtained by multiplying the 210Pb-derived bulk mass sedimentation rate by the proportion OC content. Because sedimentation in shallow ecosystems shows high spatial variability (Whitmore et al. 1996), OC accumulation rates were corrected for sediment focusing, a process whereby water turbulence causes sediments in shallow areas to be resuspended and deposited in deeper zones of the lake (Whitmore et al. 1996). To estimate the sediment focusing factors (SFF), we compared the flux of 210Pbxs for each core (derived from the integrated total 210Pbxs value) to the average 210Pbxs flux from the three cores collected in Mirim Lagoon. The three Mirim Lagoon cores were collected from different water depths, capturing some of the spatial variability of sedimentation rates within the system (Mendonça et al. 2017). To compare the measured OC burial rates with other ecosystems, we compiled published values of OC burial in other inland lakes and coastal ecosystems across the globe, using the search terms organic carbon, carbon accumulation, carbon burial, carbon sequestration, carbon sink, lakes, lagoons (Electronic Supplementary Material [ESM] Table S1).

Results

Core chronologies and sedimentation rates

In core MIR3 (collected at a shallow site near the mouth of the Jaguarão River) and core NIC (the shallowest station), upper layers had similar 210Pb activities, suggesting that the near-surface layers of the cores had been mixed (Fig. 2).137Cs values were below the detection limit for most cores, so the 210Pb model could not be validated with 137Cs profiles.

Fig. 2
figure 2

Profiles of log 210Pbxs activity versus cumulative mass. Excess 210Pb was fitted using the least squares procedure and the slope of the log-linear curve was used to determine the mass accumulation rates and sedimentation rates using the CF:CS model (Appleby and Oldfield 1983; Sanchez-Cabeza et al. 2012). MAR = Mass accumulation rate, SR = Sedimentation rate, SR CRS = Average sedimentation rate calculated from the CRS model

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Profiles of log 210Pbxs activity versus cumulative mass, excluding the mixed layers, revealed log-linear decay, enabling the use of the constant flux constant sedimentation (CF:CS) dating model (Appleby and Oldfield 1983; Sanchez-Cabeza et al. 2012) (Fig. 2), and linear regression analysis to calculate the mass accumulation rate (MAR, g cm−2). Using the mass accumulation rates, sedimentation rates ranged from 0.4 mm year−1 to 1.2 mm year−1 (Fig. 2). In cores MIR1 and MAN2, sedimentation rates calculated with the CF:CS model were consistent with the average values calculated with the CRS model (Fig. 2). In core MAN2, the CRS model displayed an increase in the sedimentation rate after 1960, with average values of 4.0 ± 0.5 mm year−1 and 8.3 ± 2.2 mm year−1 before and after 1960, respectively. This trend, however, was not observed in core MIR1. All the results for the measured radionuclides in the studied sediment cores are shown in ESM Table S2 following Mustaphi et al. (2019) guidelines for reporting and archiving 210Pb sediment chronologies.

Sediment grain-size and organic carbon content

In Mirim Lagoon, the fine-grained sediment (FGS) content was slightly higher in the core collected near the central area (MIR2, 86.6 ± 12.8%), than in the cores collected near the lagoon margins (core MIR1 79.6 ± 6.2% and MIR3 77.3 ± 7.2%) (Fig. 3). MIR2, which had higher sedimentation rate, and thus better temporal resolution, showed a decline in the FGS fraction at the end of the 1970s. In core MAN2, the grain size was highly variable and coarser than in Mirim Lagoon, with a mean of 62.4 ± 9.9% FGS. After 1960, a sustained decrease in fine sediment content was observed in MAN2 (Fig. 3). In core NIC, sediments had high FGS content, with an average of 83.9 ± 7.9%.

Fig. 3
figure 3

Vertical distribution of fine-grained sediments (FGS) (%), dry bulk density (DBD) (g dry/cm3 wet), estimated total organic carbon (LOI-derived) (OC) (%) and carbon fluxes (g m−2 year−1) in the MAN2, NIC, MIR1, MIR2 and MIR3 sediment cores over the last 120 years. Black stars mark the times of the main historical changes that influenced the land cover in the catchment: 1903: First rice plantations, 1960: Intensification of agriculture, 1970: Main hydrological changes (construction of dams and reservoirs)

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The Mirim and Nicola cores showed similar dry bulk density values (0.6 ± 0.1 and 0.5 ± 0.0 g dry cm−3 wet, respectively). The Mangueira core displayed significantly lower values (< 0.1 g dry cm−3 wet) (Fig. 3). In the past ~ 120 years, Mirim Lagoon sediments displayed low and constant OC concentrations (Fig. 3), with an average of 3.9 ± 0.4%. In contrast, in Mangueira Lagoon and Nicola Lake, the organic carbon contents were higher and more variable, averaging 20.5 ± 5.2% and 26.1 ± 15.0%, respectively (Fig. 3).

Organic carbon accumulation rate

During the twentieth century, carbon flux remained relatively low near the margins of Mirim Lagoon, 9.6 ± 7.0 g OC m−2 year−1 (MIR1) and 10.4 ± 0.8 g OC m−2 year−1 (MIR3) (Fig. 3). The MIR2 core, which is located between cores MIR1 and MIR3 showed the highest bulk sediment and organic carbon accumulation rate (24.7 ± 3.7 g OC m−2 year−1). This core also displayed an increase in organic carbon flux since the mid-1970s (Fig. 3). Overall, Mirim Lagoon showed an average OC accumulation rate of 14.9 ± 8.5 g C m−2 year−1. After correction for potential sediment focusing (SFF = 2.13), Nicola Lake presented the highest modern OC accumulation rate, 69.9 ± 38.5 g C m−2 year−1, and Mangueira Lagoon the lowest, with an average of 6.4 ± 3.7 g C m−2 year−1.

Discussion

Shallow-water sediments presented clear evidence of mixing in the upper layers (Table ESM S2). Mirim Lagoon and Nicola Lake have low densities of benthic organisms (Santos et al. 2004; Würdig et al. 2007), so the sediment mixing observed at stations MIR3 and NIC was likely a consequence of wind-driven resuspension in these shallow ecosystems, rather than bioturbation. As a consequence, estimated very recent bulk sediment accumulation rates and organic carbon burial estimates should be considered maximum values. Sedimentation rates and carbon burial in the sandy margins of the lagoons are likely lower because of organic sediment focusing into deeper areas. Nonetheless, linear sedimentation values obtained here are lower than those measured in other Brazilian coastal environments, which range from 1.2 to 22 mm year−1 (Santos et al. 2008b). This includes the nearby Patos Lagoon, where sedimentation rates during the last century were estimated to be 4.7 ± 0.7 mm year−1 in the freshwater region (Bueno et al. 2019) and 3 mm year−1in the estuarine area (Niencheski et al. 2014). Since the water column of these shallow lakes is constantly mixed by winds (Fragoso et al. 2011; da Silva et al. 2019; Vieira et al. 2020), the low measured sedimentation rates may be a consequence of high sediment resuspension and export to nearby wetlands, sediment deposition in deeper areas of the lagoon, and/or high rates of carbon degradation under high oxygen and light conditions. For example, da Silva et al. (2019) determined that Mirim Lagoon has a short water residence time and exhibits water-column mixing, both of which are related to slow sediment accumulation. Furthermore, Vieira et al. (2020) showed that the water depths where cores were obtained in our study correspond to a nearshore zone above the 6 m isobath, depths at which resuspension of fine sediment is prevalent. Recent increases in sediment accumulation rate at the MAN2 site were likely a consequence of higher erosion in the catchment that resulted from rice irrigation activities, which intensified after 1960 (IRGA 2019). This increasing trend was not found at the MIR1 site, which is not located in a depositional zone.

Coarser sediments were encountered in surface deposits collected near the margins of Mirim Lagoon (cores MIR1 and MIR3), whereas muddier sediments were more concentrated in deeper areas (MIR2), matching earlier grain size analysis by Santos et al. (2003) and Vieira et al. (2020). These grain size distributions are likely the result of sediment focusing. Water turbulence and resuspension preferentially move finer particles from shallower to deeper zones of lakes, resulting in greater accumulation, i.e. focusing, in those areas (Blais and Kalff 1995; Mendonça et al. 2017). Although the low sedimentation rates preclude detailed temporal assessments, the MIR2 core showed a decline in the mud fraction during the 1960s and 1970s. During those decades, new technologies for rice production were introduced with an intensification of agriculture, a larger area of rice paddies, and substantial hydrological changes such as the construction of dams and reservoirs (Alegre et al. 2014; Borba 2016; IRGA 2019; FAO 2019). The major hydrological change was the construction of the lock in the São Gonçalo channel between 1972 and 1977 to prevent the entry of brackish estuarine water into the lagoon (Borba 2016), so the observed coarser deposits may be a consequence of intense soil remobilisation for agriculture and the construction of water reservoirs for rice irrigation.

Because of its proximity to the sandy barrier, Mangueira Lagoon (Grimler et al. 2018) has coarser sediments than Mirim Lagoon. Mangueira Lagoon’s water is pumped onto rice paddies through artificial drainage canals (Fig. 1), and eventually drains back into the lagoon. We speculate that fine-grained suspended sediments are retained on rice paddies during this process. Therefore, the consistent decrease of fine-grained sediment deposition after 1950 along the MAN2 sediment profile represents the effect of decades of altered hydrology. Since Nicola Lake is located farther inland, sediments in the lake are less coarse than those in marine-influenced Mangueira Lagoon.

Low OC content in Mirim Lagoon is likely a consequence of nutrient dilution and high sediment resuspension, which drives organic matter oxidation in the water column (Santos et al. 2004), and stands in contrast to the higher OC content in Mangueira Lagoon and Nicola Lake. The northern region of Mangueira Lagoon has more organic-richer sediments with macroalgae cover near the organic-rich waters of the Taim Wetland (Andrade et al. 2012). Nicola Lake is located north of Mangueira Lagoon and within the protected Taim Wetland, which is not influenced directly by rice cultivation (Villanueva et al. 2000; Motta Marques et al. 2013). We therefore assume that the organic matter in Nicola Lake is autochthonous and sourced from the surrounding wetland vegetation.

Mirim Lagoon displayed higher OC accumulation at the deeper site (MIR2) than at the nearshore sites (MIR1 and MIR3) (Fig. 3) likely as a consequence of sediment focusing. In addition, OC fluxes at the MIR2 site displayed an increasing trend after 1970. Although less evident than in MIR2, higher OC accumulation values were also observed at the MAN2 site after 1960. The increasing OC accumulation observed in MIR2 and MAN2 in the second half of the twentieth century was likely related to the replacement of wetlands by agriculture in the region. Anderson et al. (2013) found that land-use change is the main driver of OC burial in lakes, and suggested that intensification of agriculture and associated nutrient loading, including atmospheric N-deposition, enhance OC sequestration in lakes globally. In Rio Grande do Sul, the area covered by rice paddies increased by 70% between 1961 and 1976 (IRGA 2019). In Uruguay, rice paddies increased from 17,190 ha in 1961 to 177,300 ha in 2006 (FAO 2019), of which ~ 70% are within the Mirim Lagoon basin (Achkar et al. 2012). Hence, irrigation and soil erosion may have contributed to the increase in the OC accumulation through the input of allochthonous material (Anderson et al. 2013; Sanders et al. 2014). Increased OC accumulation associated with land-use change, however, is only implied from one core in Mirim Lagoon and from the muddy location in Mangueira Lagoon. Sediments of Mangueira are mainly fine sands, and the particle size distribution in the different ecosystems also affects their potential as carbon sinks. Upscaling the relationship between grain size and OC accumulation would require assumptions that cannot be tested with the available data. Thus, to determine whether anthropogenic activities influenced sedimentation in these lakes, additional cores that consider the grain size distribution throughout the lakes should be collected, and a refined 210Pb analysis should be considered. In Nicola Lake, the only ecosystem not affected by rice paddies, the substantial shift observed in the OC content and flux could have been driven by drainage canals built before 1950, which modified its natural hydrology (Sobrinho 1951; Grehs 2008).

Mangueira Lagoon is oligotrophic to mesotrophic (Fragoso et al. 2011) and Mirim Lagoon is oligotrophic and nitrogen-limited (Santos et al. 2004). Both ecosystems seem to have retained their trophic state in spite of increasing nutrient inputs. This is likely a consequence of their large size, as well as sediment distribution and resuspension processes (Santos et al. 2004; Fragoso et al. 2011). Land use and climate change had only a minor effect on the trophic status of Mangueira Lagoon, suggesting that hydrodynamics (i.e. advection and diffusion) and dilution processes are related to productivity (Fragoso et al. 2011). Andrade et al. (2012) indicated that advection of nutrient-rich groundwater occurs at the man-made irrigation canals that connect aquifers to the lagoon. Nutrients from groundwater are likely recycled several times in the lagoon, sustaining gross primary production on a continuous basis. Furthermore, the rate of carbon burial is negatively related to dissolved oxygen concentration in the water column (Sobek et al. 2009). Hydrodynamics within these two shallow ecosystems is mainly driven by winds (Fragoso et al. 2011; da Silva et al. 2019; Vieira et al. 2020), which maintain well mixed water columns. The well oxygenated water column and large size thus slow down changes in trophic state, even with enhanced input of nutrients. Hence, despite land-use changes, OC burial is only slightly enhanced in these aquatic ecosystems.

Few studies have reported OC accumulation in South American lake ecosystems (Mendonça et al. 2017). The closest such study in this region investigated stocks and sources of carbon in the saltmarshes of the estuarine region of Patos Lagoon (Patterson 2016), and found OC burial rates ranging from 5 to 34 g C m−2 year−1. Carbon accumulation in Amazonian floodplain lakes averaged 266 ± 57 g C m−2 year−1, which is on the high end of carbon burial rates reported for lakes and wetlands worldwide (Sanders et al. 2017, Fig. 4). We also compared the OC accumulation rates found in this work to published values from other inland lakes and coastal ecosystems across the globe (Fig. 4, ESM Table S1). Nicola Lake had the highest OC accumulation rate with values similar to those in other small macrophyte-dominated lakes (Gui et al. 2013). Even though Mirim and Mangueira Lagoons were found to have lower OC accumulation rates than Nicola, the results are within the range of lakes in other climate regions (Sanders et al. 2017 and references therein), and very similar to those of other large lakes (1–20 g C m−2 year−1) (Alin and Johnson 2007). Lake area is typically negatively correlated to OC burial rate (Mendonça et al. 2017). Mirim Lagoon, however, had higher OC burial than Mangueira, even though Mirim’s area is five times larger than Mangueira’s. Waters et al. (2019) also found that OC accumulation in Florida lakes did not decrease with increasing lake size, and suggested that the lack of such a relationship was a consequence of the low input of allochthonous OC to lakes in Florida’s flat, well-drained landscape. Even though the Mirim-Mangueira catchment is flat (Tomazelli et al. 2000; Villanueva et al. 2000), Mirim Lagoon has several tributaries that contribute allochthonous organic and inorganic material. Another possible influence on the potential of these ecosystems as carbon sinks is the spatially heterogeneous distribution of fine-grained sediments, which likely limits the area over which OC burial occurs (Waters et al. 2019). For instance, Mirim Lagoon sediments are composed mainly of clayey silt (Vieira et al. 2020), whereas sediment in Mangueira Lagoon is composed of fine sands (Grimler et al. 2018). This may explain the greater OC burial in Mirim than in Mangueira, despite the latter being smaller.

Fig. 4
figure 4

Organic carbon burial rates (g OC m−2 year−1) from lakes analysed in this study (white bars) compared to coastal lagoons and other freshwater system types (grey bars). Carbon burial rates (g OC m−2 year−1) vs. area for different aquatic systems, sources: See ESM Table S1

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The lack of information about OC burial in aquatic ecosystems of the region is notable, even though coastal lakes and lagoons extend over > 1000 km of coastline in southern Brazil and Uruguay alone. Indeed, this is the first investigation to report OC accumulation rates in these aquatic ecosystems in Brazil. Given their large area, our values provide new insights into the role of these ecosystems as carbon sinks and represent a first step toward understanding carbon burial in these shallow lake ecosystems.

Conclusions

This study explored linkages between human modification and carbon burial in large, coastal freshwater lagoons of southern Brazil. The typical negative correlation between lake area and OC burial was not observed here, possibly because of differences in grain-size distributions. Mangueira Lagoon is smaller than Mirim Lagoon, however it possesses sandier sediments that limit its potential as a carbon sink. Despite the well documented catchment land-use change, OC burial was only slightly enhanced in the ecosystems influenced by agriculture. This resistance to change their trophic state is likely a consequence of their large size, as well as sediment distribution patterns and resuspension oxidising sediment carbon. Organic carbon burial rates in the three South American aquatic ecosystems were compared to values determined in lakes around the globe. The present results are within the range of other inland and large lakes in other climate regions, demonstrating the importance of these previously unstudied systems as carbon sinks.