Urban Stormwater Ponds Research Articles

Internal loading in stormwater ponds as a phosphorus source to downstream waters

AbstractWe assessed the prevalence and causes of sediment phosphorus (P) release within urban stormwater ponds, a process that may reduce P removal by sedimentation. Data collected from surface water of 98 urban stormwater ponds in Minnesota showed that nearly 40% had median summer total P concentrations in excess of average stormwater runoff (0.38 mg L−1), implying effects of internal loading. We sampled seven ponds more intensively and found four were strongly stratified with persistent hypolimnetic anoxia, despite mean depths <2 m. Sediment core incubations revealed that, unlike in most lakes, both labile organic P (NaOH minus persulfate extractions) and redox‐sensitive P (NH4Cl and Na2S2O4 extractions) contribute to P release. Together, these analyses suggest P accumulated in stormwater ponds is highly susceptible to internal release and potentially contributes to downstream eutrophication. Understanding how frequently these conditions occur and how they affect different P forms is vital to improving pond design and management.

Planktonic algae abundance and diversity are similar in urban stormwater ponds of different geographic locations and natural shallow lakes

Elevated levels of different contaminants are typical to stormwater management ponds. Despite that, a number of works report stormwater ponds serving as habitats for a variety of biota. In this study we aimed to examine phytoplankton communities of urban ponds, as the basis of the aquatic food web, and compare them to those of natural shallow lakes. Stormwater ponds were selected from two distant geographic locations: three in Denmark and three in Canada. As a reference to natural systems, three Danish shallow lakes were sampled. The sampling was carried out in the spring, summer and fall of 2014. The phytoplankton communities in ponds were found to be at least as rich in taxa as natural shallow lakes. Their abundance and biovolume varied highly among the types of water bodies as well as in each pond or lake individually, depending on the sampling month. We did not find any significant differences among ponds and natural shallow lakes at the investigated taxonomic level, despite some distinction observed by multivariate DCA and CCA analyses. Little difference was found between Canadian and Danish ponds as well, even though they are separated by a large geographic distance. This study shows that stormwater ponds are habitats for diverse planktonic algae communities which have some similarities to those of natural shallow lakes. Also, the similarities observed between Danish and Canadian ponds indicate some consistence with the urban homogenization hypothesis, although this should be further looked into by future works examining a larger number and distinct types of ponds.

It Is Not Easy Being Green: Recognizing Unintended Consequences of Green Stormwater Infrastructure

Green infrastructure designed to address urban drainage and water quality issues is often deployed without full knowledge of potential unintended social, ecological, and human health consequences. Though understood in their respective fields of study, these diverse impacts are seldom discussed together in a format understood by a broader audience. This paper takes a first step in addressing that gap by exploring tradeoffs associated with green infrastructure practices that manage urban stormwater including urban trees, stormwater ponds, filtration, infiltration, rain gardens, and green roofs. Each green infrastructure practice type performs best under specific conditions and when targeting specific goals, but regular inspections, maintenance, and monitoring are necessary for any green stormwater infrastructure (GSI) practice to succeed. We review how each of the above practices is intended to function and how they could malfunction in order to improve how green stormwater infrastructure is designed, constructed, monitored, and maintained. Our proposed decision-making framework, using both biophysical (biological and physical) science and social science, could lead to GSI projects that are effective, cost efficient, and just.

Stormwater ponds as habitat for Odonata in urban areas: the importance of obligate wetland plant species

Urbanization significantly alters hydrological regimes in cities by reducing infiltration rates and increasing runoff. Stormwater ponds have been constructed in North American cities to mitigate the effects of increased urban runoff by dampening floods and filtering out contaminants. However, these ponds may also provide habitat for wetland species in cities. This study aimed at determining the significance of stormwater ponds as attractive habitats for the adult stages of Odonata (dragonflies and damselflies), widely considered bioindicators of aquatic and wetland ecosystem health. A total of 41 urban stormwater ponds and ten rural natural ponds were sampled across the National Capital Region of Canada. On average, stormwater ponds had fewer species and lower abundance of dragonflies but, in contrast, more species of damselflies. Stormwater ponds had a higher total plant species richness because of a higher number of non-native species. However, some stormwater ponds had similar odonate and plant species assemblages to natural ponds. The variation in odonate abundance and species composition was largely explained by plant community composition and significantly linked to the presence of specific obligate wetland plant species. Overall, this study highlights the importance of wetland features in cities and points to design elements of stormwater ponds that could be implemented to enhance biodiversity and ecosystem services.

Habitat heterogeneity determines plant species richness in urban stormwater ponds

Urbanisation is seen as one of the most important drivers for species extinction. The aim of our study was to compare vascular plant species richness of 35 artificial urban stormwater ponds (STOPON) designed to control water flow, with 35 control ponds (CONTROL) in the surrounding landscape. For both plot types, we gathered a variety of environmental parameters to determine the drivers of plant species richness. Based on the results, we discuss the relevance of STOPON for biodiversity conservation in urban areas and give recommendations for biodiversity-friendly construction and pond management.Our study showed that the environmental conditions clearly varied between STOPON and CONTROL. STOPON were especially larger than CONTROL; as a consequence, the aquatic, semi-aquatic and terrestrial zones had a greater extent at STOPON. Accordingly, the overall and mean number of aquatic, salt-tolerating aquatic and threatened plant species was higher at STOPON than at CONTROL. However, species density of all three plant indicator groups did not differ among the two plot types.The greater plant species richness of STOPON was driven by a higher habitat heterogeneity which was especially explained by (i) the genuine effect of habitat area on habitat heterogeneity, (ii) the existence of three similarly large habitat zones (terrestrial, semi-aquatic, aquatic) and (iii) the regular management, which represses shading plants, creates open soil and activates the soil seed bank. Due to the high species richness of threatened plant species STOPON play an important role in biodiversity conservation in urban areas.

Environmental variables drive differences in the beta diversity of dragonfly assemblages among urban stormwater ponds

Stormwater ponds are beneficial to urban landscapes because these man-made systems can reduce the negative effects of flooding in urban areas and restrain the distribution of pollutants. In additio ...

Mobility and Bioavailability of Sediment Phosphorus in Urban Stormwater Ponds

AbstractStormwater ponds can serve as retention hotspots for phosphorus (P) moving out of the urban environment. This retention may be reduced by P speciation that reduces the bioavailability of P to primary producers and alters its mobility in sediments. Here we examined the mobility and fate of dissolved P in urban stormwater ponds with a set of complementary field measurements and short‐term laboratory and field experiments. We measured the types and amount of P in water column and sediments of urban stormwater ponds. We further assessed the mobility of different P types in pond sediments in the field and rates of P release from sediment cores maintained under laboratory conditions. Finally, we assessed P uptake rates by pond algal communities using short‐term bioassay experiments. We found that dissolved organic P was highly prevalent in urban pond water and sediments and that this type of P was mobile within sediments and could be released under high or low O2 conditions. We also found highly variable P demand by algae among stormwater ponds and that algal growth responses to P was correlated to water column N:P ratios. Altogether, our results indicate an important role for organic phosphorus cycling in urban stormwater ponds, which likely constrains the overall retention efficiency in these aquatic ecosystems.

Nutrient processes and modeling in urban stormwater ponds and constructed wetlands

Stormwater ponds, such as wet ponds and constructed wetlands, are widely used to manage stormwater runoff volume and water quality. Controlling nutrient levels in flows through stormwater ponds is of increasing concern, with many regions implementing loading limits for receiving waters. Nitrogen and phosphorus loading to stormwater ponds and wider catch basin sources are established. Dominant stormwater pond internal processes and prevalent nutrient forms are discussed. Relative impact of removal vectors and key factors such as temperature, dissolved oxygen, and seasonal impacts are reviewed. The direct and secondary impacts of design considerations such as macrophytes and floating treatment wetlands are discussed. Established design guideline performance standards and actual pond performance are compared. Modern approaches to modelling water quality changes within stormwater ponds are reviewed, within the context of improved pond design moving forward. This review paper intends to provide a broad overview of the nutrient processes in stormwater ponds and potential avenues for designers to improve future performance.

Distribution, Toxic Potential, and Influence of Land Use on Conventional and Emerging Contaminants in Urban Stormwater Pond Sediments.

This study examined the distribution and toxic potential of conventional and emerging contaminants in composite sediment samples from 15 stormwater ponds in the Minneapolis-St. Paul, MN metropolitan area. Previously, coal tar-based sealants were shown to be a major source of polycyclic aromatic hydrocarbons to these ponds, and concentrations of carcinogenic benzo[a]pyrene (B[a]P) equivalents were influencing management options about pond maintenance. For the second component of this study, a complex mixture of 13 metal(loid)s, 4-nonylphenols, 8 brominated diphenyl ethers (BDEs), and total polybrominated diphenyl ethers (PBDEs) were detected in all surficial samples. Contaminants with detection frequencies ≥ 20% included: silver (46.7%), beryllium (20.0%), chloride (60.0%), bis(2-ethylhexyl)phthalate (60.0%), 10 per- and polyfluoroalkyl substances (PFASs; 26.7-80.0%), 4-nonylphenol monoethoxylate (66.7%), 4-nonylphenol diethoxylate (40.0%), bifenthrin (20.0%), total permethrins (33.3%), and 24 other BDE congener groups (20.0-93.3%). Five stormwater ponds had contaminants exceeding benchmarks likely to be associated with harmful effects to benthic organisms. Ponds with watersheds dominated by either commercial and/or industrial land uses had significantly higher (p < 0.05) concentrations of zinc, 4-nonylphenol, six BDEs (28 + 33, 47, 99, 100, 154, and 209), and total PBDEs than those dominated by residential land uses. Multivariate statistical analyses verified that updated B[a]P equivalents were an effective chemical proxy for making management decisions about excavated pond sediment. Jurisdictions that do not test their stormwater pond sediments prior to maintenance dredging should consider the environmental ramifications of applying this potentially contaminated material to land.

Odonate assemblages of urban stormwater ponds: the conservation value depends on pond type

Urbanisation is among the most severe drivers of the recent biodiversity crisis. It has been shown that stormwater ponds have a high value for the conservation of dragon- and damselflies in urban areas. However, information on the relevance of different types of stormwater ponds is lacking so far. The aim of this study was to compare the Odonata assemblages of three types of urban stormwater ponds (n per type = 10): (i) ponds only containing temporary water bodies (TEMP), (ii) ponds with temporary and small perennial water bodies (TEMP/PERE) and (iii) ponds with one large perennial water body (PERE). We observed distinct differences in environmental conditions and Odonata assemblages among the three types of stormwater ponds. In particular, vegetation structure and the partly interrelated microclimate differed considerably between TEMP on the one hand, and TEMP/PERE and PERE on the other hand. Odonate species richness and exuviae density of threatened species differed, too. Due to their early successional stages with low cover of riparian woodland and shallow water bodies, TEMP were characterised by a warmer microclimate than TEMP/PERE and PERE. Odonate species richness and exuviae density of threatened species were highest in TEMP and lowest in PERE. Moreover, indicator species were only identified for TEMP. Our study showed that stormwater ponds with a temporary hydroperiod play an important role for the conservation of odonates in urban areas. This is especially the case for specialised threatened species, such as Libellula depressa and Ischnura pumilio.

Low cost media can filter particulate phosphorus from turbid stream water under short retention times

Wood particle biofilters are being used for remediation of nitrate in watercourses but so far, have received little attention for the possibility of phosphorus (P) treatment. Phosphorus export in many agriculture and urban stressed watercourses however, is dominated by particulate rather than dissolved P and particulate P is potentially treatable by physical filtration. Five low cost, highly permeable media types, including woodchips, sawdust, bark mulch, corn cob chips and peastone, were tested in eight pilot scale biofilters over a six month period for their ability to remove suspended solids (TSS) and particulate P from turbid stream water. The biofilters were operated at high loading rates, with retention times averaging 1.9–4.2 h and achieved TSS removal averaging 85–97%. Excluding a 30 day startup period, seven of the eight biofilters removed an average of 53–64% of stream total P (TP) averaging 65.6 µg/L. The corn cob media provided lower TP removal (1%) because P leached from the media. These filters provided TP removal at hydraulic loading rates that were an order of magnitude higher than when similar media is used for nitrate remediation. These low cost filter media, with capacity for high loading rates, could offer a practical solution for P removal in urban storm water ponds, sewage lagoons and agriculturally impacted water courses.

Assessment of the treatment efficiency of an urban stormwater pond and its impact on the natural downstream watercourse

During the last few decades, stormwater ponds have become an alternative management practice in order to avoid flooding and to contain rainwater and runoff in urban areas where impervious land cover has increased. A second purpose of stormwater ponds is to improve the quality of runoff water that is usually contaminated with nitrogen, phosphorus, metals and organic micropollutants. Processes used are based on natural methods such as settlement and contribute to minimize the impact of these inputs to the natural aquatic system. This study aims to better understand the behavior of a wet stormwater pond, Heron Lake (33 ha) located in the city of Villeneuve d’Ascq in northern France through various indicators [trace metals, PAHs, PCBs, caffeine (CAF), carbamazepine (CBZ), nutrients and pathogens]. For that purpose, water quality was monitored for 1 year, mainly at the entrance and at the outlet of the lake. Sampling have also been done in the downstream aquatic environment, the Marque River. Sediments were sampled in the lake to evaluate the pollution trapped during sedimentation. Our results of both water and sediment sampling highlight: (i) the wastewater input into the Heron Lake is estimated to be equivalent to that of roughly 3800 inhabitants; (ii) the removal rates observed at the outlet, relative to concentrations at the entrance channel, vary as follows for these dissolved species: 24% for NO3− and PO43−, 28% for CBZ, 35% for Cu, 63% for Pb, 78% for CAF, 84% for Zn and up to 93% for NH4+; (iii) there are high levels of sediment contamination with metals, PAHs and PCBs at the entrance channel; (iv) the eutrophication of this pond is attributed to persistent high nutrient concentrations in both water and sediment, and has contributed to the development of an invasive macrophyte, the Elodea nuttallii; and (v) there appears to be only a negligible impact of the discharge from the lake to the natural watercourse, contributing annual loads of <2 up to 6% of the total amount of Cu, Pb, Zn, CAF, CBZ and nutrients measured in the Marque River, and having a slight diluting effect on concentrations in the Marque River.

Sediment chemistry of urban stormwater ponds and controls on denitrification

AbstractStormwater ponds and retention basins are ubiquitous features throughout urban landscapes. These ponds are potentially important control points for nitrogen (N) removal from surface water bodies via denitrification. However, there are possible trade‐offs to this water quality benefit if high N and contaminant concentrations in stormwater pond sediments decrease the complete reduction of nitrous oxide (N2O), a potent greenhouse gas, to dinitrogen (N2) during denitrification. This may occur through decreasing the abundance or efficiency of denitrifiers capable of producing the N2O reductase enzyme. We predicted that ponds draining increasingly urbanized landscapes would have higher N and metal concentrations in their sediments, and thereby greater N2O yields. We measured potential denitrification rates, N2O reductase (nosZ) gene frequencies, as well as sediment and porewater chemistry in 64 ponds distributed across eight U.S. cities. We found almost no correlation between the proportion of urban land cover surrounding ponds and the nutrient and contaminant concentrations in the stormwater pond sediments within or across all cities. Regression analysis revealed that the proportion of potential N2 and N2O production that could be explained was under different environmental controls. Our survey raises many new questions about why N fluxes and transformations vary so widely both within and across urban environments, but also allays the concern that elevated metal concentrations in urban stormwater ponds will increase N2O emissions. Urban stormwater ponds are unlikely to be a problematic source of N2O to the atmosphere, no matter their denitrification potential.

A Review of the Factors That Determine Whether Stormwater Ponds Are Ecological Traps And/or High-Quality Breeding Sites for Amphibians

Stormwater ponds were originally constructed to control the quantity and quality of runoff on urban roads and highways before it was released to the environment. Often, stormwater ponds were designed in a technical feat of civil engineering, with no particular ecological or landscape objective in mind. Nevertheless, they are colonized spontaneously by diverse species, including amphibians. Through an initial review of the scientific literature, the objective of this study was to understand which factors determine that a pond can be considered as an ecological trap or a valuable breeding site fort amphibian. The first step was to question the role of the pond environment as a major factor in its colonization by amphibians, demonstrating that not all ponds are colonized by the same variety of species. The internal factors in the ponds that define them as ecological traps or sustainable breeding sites for amphibians was also considered. After confirming the functional and structural similarity between highway and urban stormwater ponds, 25 publications were compared, with study sites mostly located in Europe and North America, and concerning the colonization of stormwater ponds by amphibians in urban or highway areas. Several factors were identified that may affect the ecological viability of these basins: 1) the factors related to the shape of the ponds (inclination of the banks, materials used, etc.); 2) the biotic factors (aquatic vegetation, presence of predators, etc.); 3) the abiotic factors (luminosity, water level in the ponds, etc.); and 4) pollutants. The low number of publications on this subject, as well as the law variety in the location of study sites, only allows for cautious conclusions be drawn. In particular, stormwater ponds located in highly anthropogenic landscapes can be both ecological traps and suitable habitats for amphibian breeding. This depends on the species that colonize each pond, many internal factors, and the environmental context in which it is embedded. Additional research is therefore needed in other parts of the world - particularly in amphibian biodiversity hotspots - as well as other impact factors such as the effects of different maintenance practices.

Polycyclic aromatic hydrocarbon contamination and source profiling in watersheds serving three small Wisconsin, USA cities

Polycyclic aromatic hydrocarbons (PAHs) continue to be common environmental contaminants. The anthropogenic sources of these compounds are broadly classed as petrogenic and pyrogenic, but more importantly specific sources including activities such as coal burning, oil spills, and application of coal tar sealants can be identified based on several types of data analysis. Several studies have focused on PAHs in sediments of lakes, streams, and stormwater ponds in larger urban areas, finding contamination arising from a number of different sources and correlating well to land use in the nearby watershed. We report here a study of PAH concentrations and source identification for river and lakebed sediments in and upstream of three smaller Wisconsin municipalities: Eau Claire (Eau Claire River), Stevens Point (Plover River), and Racine (Root River). PAH concentrations increased with increasing developed land cover and impervious surface. Concentrations within the cities and upstream agricultural or residential areas do not rise to the level found in larger urban areas or stormwater ponds servicing industrial or commercial land use, but can rise to a level that exceeds the Threshold Effects Concentration (TEC). Concentrations in areas with natural landcovers were very low, with the exception of one sample in a wetland with unusually high organic content. Multiple lines of evidence indicate that coal tar-based pavement sealants are a primary source of the contamination in all three cities. PAH concentrations reported here are likely conservative, and these results indicate that even smaller cities using detention ponds as a stormwater management practice should be prepared for costs of contaminated sediment disposal.

Bioturbating invertebrates enhance decomposition and nitrogen cycling in urban stormwater ponds

Stormwater retention ponds manage runoff in developed areas, reducing flooding and erosion, and improving downstream water quality. Urban ponds also serve as habitat for aquatic organisms in an otherwise terrestrial habitat. However, the relationship between common pond species and biogeochemical function of ponds is poorly understood. Using a microcosm experiment, we evaluated the influence of two bioturbating invertebrates, Chironomus sp. and Lumbriculus variegatus, on sediment–water column nutrient exchange and benthic metabolism in stormwater ponds with divergent management histories and sediment organic matter (OM) content. Both organisms facilitated net release of reactive inorganic nitrogen to the water column, flipping the net reactive nitrogen balance of sediments relative to controls. Release of ammonium to the water column and sediment oxygen consumption were enhanced by both species, especially during colonization, but sediment uptake of nitrate was strongly enhanced by Chironomus and inhibited by L. variegatus. Comparison with direct animal respiration and excretion rates allows attribution of direct relative to indirect effects of bioturbation and bioirrigation. The influence of L. variegatus is predominantly direct, whereas Chironomus has strong direct and indirect biogeochemical effects. After 30 days, microalgal photosynthetic efficiency was reduced in all L. variegatus treatments, and context specific interactions appeared, with a reduction in OM in sediment with initially higher values, and reduced nitrate uptake in sediments with lower OM. Animal impacts on key processes ultimately influence downstream water quality and this study illustrates the important role of tolerant benthic invertebrates in mediating ecosystem functions in even the most disturbed aquatic ecosystems.

Developing an ecosystem model of a floating wetland for water quality improvement on a stormwater pond

An ecosystem model was developed to assist with designing and implementing a floating wetland (FW) for water quality management of urban stormwater ponds, focusing on nitrogen (N) removal. The model is comprised of three linked submodels: hydrology, plant growth, and nitrogen. The model was calibrated with the data that resulted from a FW constructed and implemented as part of an interdisciplinary pedagogical project on a university campus, titled “The Rain Project”, which raised awareness of stormwater issues while investigating the potential application of green infrastructure for sustainable stormwater management. The FW had been deployed during the summer of 2015 (i.e., May through mid-September) on a major stormwater pond located at the center of the Fairfax Campus of George Mason University near Washington, D.C. We used the model to explore the impact of three design elements of FW (i.e., hydraulic residence time (HRT), surface area coverage, and primary productivity) on the function of FW. Model simulations showed enhanced N removal performance as HRT and surface area coverage increased. The relatively low macrophyte productivity observed indicates that, in the case of our pond and FW, N removal was very limited. The model results suggest that even full pond surface coverage would result in meager N removal (∼6%) at a HRT of one week. A FW with higher plant productivity, more representative of that reported in the literature, would require only 10% coverage to achieve similar N removal efficiency (∼7%). Therefore, macrophyte productivity appears to have a greater impact on FW performance on N removal than surface area coverage or pond HRT. The outcome of the study shows that this model, though limited in scope, may be useful in aiding the design of FW to augment the performance of degraded stormwater ponds in an effort to meet local water quality goals.

Effects of habitat and landscape quality on amphibian assemblages of urban stormwater ponds

Urbanisation is one of the most severe drivers of current global biodiversity loss and has contributed to severe declines in many amphibian species. The aim of this study was to determine whether artificial stormwater ponds, designed to control water flow, can act as refuges for amphibians in urban areas. Moreover, we analysed the influence of habitat and landscape quality on amphibian species richness of 46 stormwater ponds (STOPON) in comparison to 46 control ponds (CONTROL).

Monitoring water quality in Toronto's urban stormwater ponds: Assessing participation rates and data quality of water sampling by citizen scientists in the FreshWater Watch

From 2013 to 2015, citizen scientist volunteers in Toronto, Canada were trained to collect and analyze water quality in urban stormwater ponds. This volunteer sampling was part of the research program, FreshWater Watch (FWW), which aimed to standardize urban water sampling efforts from around the globe. We held training sessions for new volunteers twice yearly and trained a total of 111 volunteers. Over the course of project, ~30% of volunteers participated by collecting water quality data after the training session with 124 individual sampling events at 29 unique locations in Toronto, Canada. A few highly engaged volunteers were most active, with 50% of the samples collected by 5% of trainees. Stormwater ponds generally have poor water quality demonstrated by elevated phosphate concentrations (~30μg/L), nitrate (~427μg/L), and turbidity relative to Canadian water quality standards. Compared to other urban waterbodies in the global program, nutrient concentrations in Toronto's urban stormwater ponds were lower, while turbidity was not markedly different. Toronto FWW (FWW-TO) data was comparable to that measured by standard lab analyses and matched results from previous studies of stormwater ponds in Toronto. Combining observational and chemical data acquired by citizen scientists, macrophyte dominated ponds had lower phosphate concentrations while phytoplankton dominated ponds had lower nitrate concentrations, which indicates a potentially important and unstudied role of internal biogeochemical processes on pond nutrient dynamics. This experience in the FWW demonstrates the capabilities and constraints of citizen science when applied to water quality sampling. While analytical limits on in-field analyses produce higher uncertainty in water quality measurements of individual sites, rapid data collection is possible but depends on the motivation and engagement of the group of volunteers. Ongoing efforts in citizen science will thus need to address sampling effort and analytical limits to fully realize the potential value of engaging citizen scientists in water quality sampling.

Phosphorus cycling in urban aquatic ecosystems: connecting biological processes and water chemistry to sediment P fractions in urban stormwater management ponds

Phosphorus (P) dynamics in urban landscapes may differ from that in natural landscapes due to different P sources and unique environmental conditions. However, many aspects of P cycles in urban areas, especially within engineered aquatic ecosystems, remain largely unknown. Through this work, we aim to contribute to better understanding of P cycling in urban aquatic ecosystems by investigating P fractions in sediment and their relationship with ambient chemistry in surface water from six urban stormwater management ponds located in Ontario, Canada. We found that organic P contributed up to 75% of total P in pond sediment, but this percentage decreased significantly between our two sampling events in June and September 2012. This decrease coincided with increased rates of extracellular enzyme (especially phosphatase) activities, which is indicative of fast mineralization processes in these ecosystems. Moreover, the decreased sediment organic P was matched by increased water column P concentration. This inverse relationship suggests that the large organic P pool in pond sediment, and its fast decomposition, contributed to internal release of P from sediment and increased water column P concentrations. The dominance of organic P in sediment and the putative role of relevant biological processes (i.e., decomposition and productivity) in urban ponds found in this study strongly contrast with classic water management expectations of physicochemically controlled P dynamics and long-term P storage in sediment of aquatic ecosystems. This difference suggests that urban stormwater ponds may perform poorly in terms of P retention and thereby contribute to poor water quality in terms of P pollution to downstream urban watersheds. Thus, stormwater pond design and future management strategies should consider these biogeochemical features of urban ponds, including internal P release, to help prevent eutrophication of downstream ecosystems.