Florida’s urban stormwater ponds are net sources of carbon to the atmosphere despite increased carbon burial over time

Carbon burial rates increased with pond age

Areal and annual C burial rates were calculated for four to six sediment cores extracted per pond in June 2019 using total OC (TOC), dry weight, and piston corer dimensions. Mean TOC content in whole cores of ponds ranged from 1.7 to 26.5{6d6906d986cb38e604952ede6d65f3d49470e23f1a526661621333fa74363c48} (Table 1 and Supplementary Fig. 1). The areal OC stock ranged from 306 ± 8.2 g OC m⁻² in Pond 14 (14-year-old pond) to 7380 ± 56.1 g OC m⁻² in Pond 34 (34-year-old pond), increasing linearly with increasing pond age (R² = 0.99, p < 0.001, Fig. 1a and Table 1). Scaling this OC stock by pond age (assuming constant burial rates over time) yields estimated annual burial rates ranging from 21.8 ± 7.5 g OC m⁻² y⁻¹ in Pond 14 to 217 ± 50.2 g OC m⁻² y⁻¹ in Pond 34, with annual rates increasing logarithmically with pond age (R² = 0.93, p < 0.01, Fig. 1b). These OC burial rates, especially in the 23- and 34-year-old ponds (141 and 217 g OC m⁻² y⁻¹, respectively), are comparable to or exceed previously reported rates from a range of aquatic ecosystems (Supplementary Table 1) including critical blue C sinks such as mangrove forests, salt marshes, and seagrasses²¹. The OC burial values reported here are also similar to reported rates in other SWPs and constructed systems^9,16,22,23, but remain below results for some impoundments around the globe (Supplementary Table 1).

Organic carbon stock (g OC m⁻², a) and the estimated rate of burial (g OC m⁻² y⁻¹, b) increase with pond age. Individual core values (n = 4–6 per pond) are plotted as small gray points and the larger black points represent the pond mean, which was used for regressions. Both linear and logarithmic regressions are statistically significant (p < 0.01).

The relationship between C burial and pond age suggests that internal sediment and/or aquatic properties allow ponds to become increasingly stable environments for C deposition and storage. Burial rates were not related to pond morphology (perimeter, area, depth, area:perimeter). SWPs are unique ecosystems that receive a dramatic quantity of sediment from impervious and piped catchments. It is possible that over time and as more particulate material is added to the sediment surface, sediment thickness and anaerobic conditions beneath newly deposited material increase, slowing down the rate of OM decomposition. A higher degree of recalcitrant OM can also be left behind, increasing the pool of stable OM more resistant to degradation. Further, the addition of SWPs to newly developed landscapes is associated with the addition of new landscaping vegetation to the urban watershed, such as trees which can increasingly contribute leaf litter to ponds as they mature over time, suggesting that allochthonous C inputs may increase concurrent with in-pond conditions favorable for C burial. Natural factors such as precipitation, length of the growing season, and littoral vegetation also affect C accumulation in urban SWPs^9,22. In addition to allochthonous inputs, SWPs can support highly productive algal communities and phytoplankton turnover represent an internal OC input²⁴. Littoral vegetation may provide a similar autochthonous OC source, but sites in this study contained no littoral vegetation and also had not been dredged (a common management practice). Despite the apparent relationship between age and C burial, our sample size is small, and this relationship may be spurious. A prior study found no relationship between sediment OC and age in an analysis of 45 aquaculture ponds²⁵, but the aquaculture activities may have offset natural successional dynamics. Furthermore, an experimental study of small ponds found that new ponds (3 years old) exhibit significantly less C burial in sediments than mature ponds¹². Nonetheless, in the process of preventing urban sediment from moving into downstream waters, constructed SWPs have emerged as significant reservoirs of sediment C, as shown here and in other anthropized ecosystems (reservoirs, impoundments, SWPs) or small ponds^3,9,17,22,23.

In Florida, there are at least 76,000 SWPs⁵. Given the role of SWPs in storing sediment C and their ubiquity on the landscape, these anthropized ecosystems represent a large pool of C in urban aquatic sediments. Using the mean OC burial rates in the youngest and oldest pond combined with the estimated surface area covered by SWPs in Florida (627 km²)⁵, we estimate that SWPs in Florida can store between 0.2 and 4.6 Tg C annually. The upper end of this estimate would be enough to offset 1.4{6d6906d986cb38e604952ede6d65f3d49470e23f1a526661621333fa74363c48} of the global CO₂ evasion from inland lakes and reservoirs²⁶.

Carbon gas fluxes are controlled by physical and chemical factors within ponds

The mean floating chamber flux of CO₂ over the study period (biweekly measurements June 2019 to March 2020) was 5940 ± 857 mg CO₂ m⁻² d¹¹ with a range of −13600 to 34300 mg CO₂ m⁻² d ⁻¹ (Supplementary Fig. 2). CH₄ fluxes were consistently positive but more variable and lower than CO₂ fluxes with a mean of 47.8 ± 17.1 mg CH₄ m⁻² d⁻¹ and range of 0.12–1400 mg CH₄ m⁻² d⁻¹ (Supplementary Fig. 3). This maximum CH₄ flux was accompanied by an unexplained water quality change that coincided with cloudy white water and DO levels depressed to 0–5{6d6906d986cb38e604952ede6d65f3d49470e23f1a526661621333fa74363c48} throughout the water column. Using median values, annual fluxes equate to 1290 g CO₂ m⁻² yr⁻¹ and 5.0 g CH₄ m⁻² yr⁻¹. The median estimate for CH₄ is far below the 2019 Intergovernmental Panel on Climate Change estimate for manmade freshwater ponds (18.3 g CH₄ m⁻² yr⁻¹), although the mean CH₄ flux in this study is on par at 17.4 g CH₄ m⁻² yr^{−1 27}. On a C mass basis, the daily mean flux of C gases combined (CO₂-C + CH₄-C) equates to 1636 mg C m⁻² d⁻¹ (CO₂ making up 98{6d6906d986cb38e604952ede6d65f3d49470e23f1a526661621333fa74363c48}), and an annual flux of 324 g C m⁻² yr⁻¹ (using the median). Diffusive CH₄ contributed a small percentage of total pond C fluxes in this study. Converting CH₄ to CO₂ equivalents with a sustained-flux global warming potential of 45²⁸ and summing with CO₂ fluxes results in a mean of 8090 mg CO₂ eq m⁻² d⁻¹, of which CH₄ makes up 27{6d6906d986cb38e604952ede6d65f3d49470e23f1a526661621333fa74363c48}, and annual median of 1660 g CO₂ eq m⁻² yr⁻¹. This is a smaller fraction of CH₄ contribution to overall CO₂ equivalent fluxes compared to other urban SWPs such as 94{6d6906d986cb38e604952ede6d65f3d49470e23f1a526661621333fa74363c48} of 28.2 g CO₂ eq m⁻² d⁻¹ (excluding ebullitive emission)²⁹ and 62{6d6906d986cb38e604952ede6d65f3d49470e23f1a526661621333fa74363c48} of 9.2 g CO₂ eq m⁻² d⁻¹ (including ebullitive emissions)¹⁶. Using the median CO₂-equivalent fluxes in the oldest and youngest pond combined with the estimated surface area covered by SWPs in Florida (627 km²)⁵, we estimate that SWPs in Florida can emit between 52 and 204 Tg CO₂-equivalents annually (or 12–50 Tg C annually).

Ponds were a source of CO₂ over the vast majority of the study period excluding the two oldest ponds, which reported negative flux values during 6 of 17 sampling events each (35{6d6906d986cb38e604952ede6d65f3d49470e23f1a526661621333fa74363c48}), reaching as low as −507 mg CO₂-C m⁻² d⁻¹ (Pond 23) and −3730 g CO₂-C m⁻² d⁻¹ (Pond 34, Supplementary Fig. 2). Ponds 15 and 18 only exhibited negative CO₂ gas flux one time each. These results suggest that at some points in the year, older ponds may switch from net heterotrophy to net autotrophy. A gradual decrease in mean CO₂ flux with increasing pond age can be seen in Supplementary Fig. 3, but the same pattern is not evident with CH₄ fluxes. Similarly, a survey on hydroelectric reservoirs around the globe (n = 85) found that CO₂ and CH₄ emissions were inversely related to reservoir age³⁰ a pattern that was also found on a temporal study of a single hydroelectric reservoir (CO₂ only)¹⁸.

The CO₂ fluxes from this study were similar to or higher than other aquatic ecosystems, CO₂ fluxes were similar or higher, whereas CH₄ fluxes were typically lower than other urban studies (Supplementary Table 2). Ebullition is considered an important pathway for methane emission in lentic ecosystems with anoxic sediments³¹. Because we did not measure ebullition our values for overall CH₄ flux are likely underestimated. Small patches of bubbles were occasionally captured underneath our flux chamber, causing the internal concentration of CH₄ to spike to ~130,000 ppb. For reference, the global mean atmospheric CH₄ concentration for 2019 was 1870 ppb³². There was not a similar spike in CO₂ concentrations when bubbles were captured.

Linear mixed-effects models (LMM) were used to estimate the relationship between gas fluxes and environmental variables measured in this study (Supplementary Table 3). For both CO₂ and CH₄ models, site (five levels) and sampling date (seventeen levels) were set as random effects and pond age was a similar fixed effect. Including the variation from random effects, 68{6d6906d986cb38e604952ede6d65f3d49470e23f1a526661621333fa74363c48} of the variation in CO₂ fluxes was explained by inverse relationships with surface {6d6906d986cb38e604952ede6d65f3d49470e23f1a526661621333fa74363c48} DO, pH, pond age, and the interaction between {6d6906d986cb38e604952ede6d65f3d49470e23f1a526661621333fa74363c48}DO and pH. In addition to the negative relationship between CO₂ and pH and the observed decrease in mean CO₂ flux with age (Supplementary Fig. 3), an associated observation was the slight increase in mean pH with pond age, up to 10.4 in Pond 34 (Fig. 2 and Supplementary Fig. 4). Surface {6d6906d986cb38e604952ede6d65f3d49470e23f1a526661621333fa74363c48}DO and pH were strongly related to CO₂ in the LMM, and when combined can be considered indicators of primary production. Photosynthetic fixation of CO₂ from the water column results in an increase in pH and DO. Concurrently, the chemical form of CO₂ in the water column (and its concentration) is controlled by pH, speciating into carbonic acid, bicarbonate, and carbonate at higher pH values. According to the kinetics of this reaction, free CO₂ in freshwater becomes depleted at pH > 8.3. Of the 14 observations of negative fluxes, all but one occurred above pH 8.3 (Fig. 2). So, while primary production contributes to a decrease in CO₂ emissions, the increased pH can exert a compounding effect, rendering high pH ponds and lakes to be weak CO₂ sources, similar to observations in in other urban ponds¹¹, saline lakes³³, and in agricultural reservoirs³⁴. Other abiotic factors may contribute to an increase in pH. Florida is known to have shallow carbonate bedrock which, coupled with the weathering of urban concrete infrastructure (calcium hydroxide, portlandite, calcium silicate hydrates), can contribute carbonates and calcium salts, increasing alkalinity in aquatic ecosystems^35,36. Additionally, in high CaCO₃ systems, inputs of CO₂ can contribute to increased alkalinity over time via calcium release. Thus, ponds have the potential to become CO₂ sinks over time as a result of increasing eutrophication and accumulation of natural- and anthropogenically sourced alkaline ions increasing pH.

CO₂ fluxes decrease with increasing surface water pH. In agreement with CO₂-carbonic acid equilibrium, a majority of CO₂ fluxes result in negative values at pH > 8.3, the point at which water column CO₂ is nearly all converted to carbonate or bicarbonate. The dotted line denotes zero CO₂ flux.

Compared to CO₂, a lower amount of the variation in diffusive CH₄ flux was explained by variables in this study. With accounted variance from random effects, 51{6d6906d986cb38e604952ede6d65f3d49470e23f1a526661621333fa74363c48} of CH₄ flux variation was explained by an inverse relationship to benthic {6d6906d986cb38e604952ede6d65f3d49470e23f1a526661621333fa74363c48}DO and positive relationships to benthic temperature and log-transformed CO₂ fluxes (Supplementary Table 3). There was no relationship between CH₄ flux and site age. The bottom of most pond water columns remained hypoxic throughout the study period with 65{6d6906d986cb38e604952ede6d65f3d49470e23f1a526661621333fa74363c48} of the data below 3.0 mg/L DO, creating conditions beneficial for anaerobic methanogenesis, which can produce CH₄ alone (by hydrogenotrophic methanogens) or CH₄ and CO₂ (by acetoclastic and methylotrophic methanogens). The positive relationship between CH₄ and CO₂ could be due to this simultaneous production, or due to increased production of CH₄ using CO₂ and H₂ as substrates as a result of its increased availability (by hydrogenotrophic methanogens). CO₂ may also be attributed to aerobic or anaerobic CH₄-oxidation. The negative relationship with water column DO was also indicated during the extreme methane emission during DO depletion (0−5{6d6906d986cb38e604952ede6d65f3d49470e23f1a526661621333fa74363c48} DO through the water column) in Pond 14. Finally, sedimentary methanogens typically become more active at higher temperatures as a result of increasing use of higher redox potential electron acceptors as well as a higher input of primary production-derived substrates that fuel their metabolic activity^37,38.

Younger ponds are net sources of C to the atmosphere

We estimated a C balance for each pond by comparing median organic C burial to median C gas flux (CO₂-C + CH₄-C flux) (Fig. 3). The combination of C gas flux (C export) and burial (C storage) reveals that three of the five ponds were, on average, net sources of C to the atmosphere. However, the median burial values fall within the 5th−95th quantile range for C efflux in Pond 23 and 34, the net burial ponds (Fig. 3 and Table 1). It should be noted that these estimates compare rate measurements determined at separate time scales (seconds vs multi-decadal). As pond age increased, the difference between C gas efflux and C storage in sediments decreased, implying that ponds may become more C-neutral as they age. The degree of this net benefit in older ponds may change if methane ebullition estimates had been included, as others have found that urban pond ebullition accounted for up to 50{6d6906d986cb38e604952ede6d65f3d49470e23f1a526661621333fa74363c48} of total CH₄ emission¹⁶. Previous studies have identified increasing CH₄ emission with ecosystem age¹⁸ and increasing trophic level³⁹, which can be associated with pond succession, converting ponds into net sources in terms of CO₂-equivalents due to increasing CH₄ contributions. Regardless of pond age, this study provides evidence that SWPs, which are constructed to provide ecosystem services, may be providing a disservice by acting as net contributors to GHG emissions, despite storing substantial quantities of C in sediments. Other studies of small ponds support this assertion. For example, both Peacock et al. (2019) and Ollivier et al. (2019) observed release of GHGs from urban and small agricultural ponds at rates high enough to call for their inclusion in global carbon budgets.

A comparison of median C flux (blue circles, CO₂–C + CH₄–C) as positive values and median OC burial rate (brown circles) as negative values. The net C flux (g C m⁻² y¹) in stormwater ponds is shown by the red circles and line and represents the difference between gas flux and burial. Bars on points indicate the 5th and 95th quantiles.

C burial and C gas fluxes have been compared in other anthropized ecosystems. In a 20-year old urban pond (the Netherlands) the annual flux of CO₂-C and CH₄-C combined, 391 g C m⁻² yr⁻¹, was substantially higher than the pond’s rate of burial at 29 g C m⁻² yr⁻¹. The aforementioned pond exhibited a lower burial rate and annual emission than Pond 18 (and younger sites) in our study (burial = 126 g C m⁻² yr⁻¹, emission = 429 g C m⁻² yr⁻¹). In a 4-year-old (time since flooding) hydroelectric reservoir, daily estimates of flux and burial were 2.3 g C m⁻² d⁻¹ emission vs. 0.1 g C m⁻² d⁻¹ burial¹⁸.

Our results suggest that after urban ponds are constructed, they emit large proportions of C inputs from the landscape and potentially increase storage efficiency over time. We suspect that when C enters urban SWPs, less C is buried in younger ponds compared to older ponds, as seen in burial studies on aging artificial ponds¹². Younger ponds may exhibit higher C mineralization rates which simultaneously increases emissions while reducing burial. However, identified trends can be drastically altered depending on how ponds are managed, suggesting management actions can be used to modulate pond C storage and C emission. For example, increasing urban pond depth is associated to reduced CH₄ release²⁹, likely due to increased water column contributions to organic matter oxidation prior to sediment settling and CH₄ oxidation, and ponds or lakes larger in area exhibit lower C emissions per unit area compared to smaller ponds². Other suggested properties for anthropized ecosystem C management include water column stratification, water retention time, water source inputs, and landscape position³⁴. It is important to note that drivers of C emission can differ between natural and anthropized ecosystems such that CH₄ emissions from reservoirs globally were correlated to productivity and fluxes increased with increasing area⁴⁰. In contrast, natural lakes were better explained by morphometry and fluxes decreased with increasing area⁴⁰.

Methods for pond maintenance include sediment dredging and muck removal, algaecide application, aeration systems (fountains, bubblers), fish stocking, and vegetation management⁴¹. Although ponds included in this study have not been dredged, dredging could reset the ‘effective pond age’. Dredging may not effectively reset a pond, however, as sediment resuspension that occurs in the process has caused lakes to relapse into a eutrophic state because of the reintroduced availability of sediment nutrients to the water column⁴². The application of aquatic pesticides (i.e., chemical treatments for algal blooms and invasive macrophytes) in SWPs is likely to impact C cycling such that substantial amounts of labile OC are rapidly contributed to the sediment surface, influencing sediment oxygen demand and the respiration of CO₂ and CH₄. Fish are added to ponds for recreation as well as mosquito and algal consumption, and can have varying effects on CO₂-equivalent fluxes. For example, by grazing on zooplankton that consume both algae and CH₄-oxidizing bacteria, fish can elicit a trophic cascade that causes an increase in algal productivity and CH₄ flux and reduced CO₂ flux³⁹. However, fish can also graze on predators to CH₄-oxidizing bacteria, decreasing CH₄ flux⁴³. The presence of vegetation may also be beneficial for reducing GHG emission, as seen in previous studies⁴⁴ and is responsible for increasing C burial rates²². Finally, aeration systems are commonly used to prevent anoxia and could therefore reduce CH₄ emissions, as shown in experimental studies on hypolimnetic DO manipulations⁴⁵.

The number of small constructed ponds was found to increase 18-fold from 1937 to 2005 in the Brandywine watershed of central Pennsylvania and northern Delaware, and land use change associated with urbanization will likely continue to increase the numbers of small constructed ponds regionally and globally⁴⁶. As ubiquitous anthropized aquatic ecosystems such as SWPs continue to be constructed in urban landscapes and have emerged as significant contributors to GHGs and C storage, more work is required to assess their net C footprint, patterns with ecosystem age over a larger number of sites, and biogeochemical responses to management strategies. An improved knowledge of how management actions interact with natural ecological processes is critical for understanding the role of SWPs and other small, anthropogenic aquatic ecosystems in the global C cycle.