Watershed Nutrient Loading Research Articles

Simulating climate change in a coastal watershed with an integrated suite of airshed, watershed, and estuary models

AbstractIn 2020, the Chesapeake Bay Program moved to offset impacts from climate change for the 30‐year period from 1995 through 2025 by having its seven watershed jurisdictions (Delaware, Maryland, New York, Pennsylvania, Virginia, West Virginia, and the District of Columbia) apply additional nutrient pollutant reduction practices. The climate change assessment was performed with integrated models of the Chesapeake watershed, airshed, and estuary. Scenarios run for the years 2025, 2035, 2045, and 2055 estimated effects from the different future climatic conditions. This article presents the results of that assessment and is intended to provide a guide to assist other modeling practitioners in assessing climate change impacts in coastal watersheds. Major influences of climate change that were quantified include increases in precipitation volume, potential evapotranspiration, watershed nutrient loads, tidal water temperature, and sea level. Minor influences quantified in the climate change analysis include changes in nutrient speciation and increases in wet deposition of nitrogen, CO2, rainfall intensity, tidal wetland loss, up‐estuary salt intrusion, and phytoplankton biomass. To offset climate change impacts from 1995 to 2025 on water quality, the scenarios indicate an additional 2.3 million and 0.3 million kg of nitrogen and phosphorus per annum, respectively, will need to be reduced beyond what is called for in the Chesapeake Total Maximum Daily Load.

Linking watershed nutrient loading to estuary water quality with generalized additive models.

Evaluating estuary water quality responses to reductions (or increases) in nutrient loading attributed to on the ground management actions can be challenging due to the strong influence of environmental drivers on nutrient loads and non-linear relationships. This study applied generalized additive models to calculate watershed nutrient loads and assess responses in estuary water quality to seasonally-adjusted freshwater inflow and flow-adjusted nutrient loads in Lavaca Bay, Texas. Lavaca Bay is a secondary embayment on the Texas coast displaying early potential for eutrophication and water quality degradation. Use of flow-adjusted nutrient loads allowed the study to evaluate the response in water quality to changes in nutrient loads driven by anthropogenic sources. Cross-validation indicated that, despite data constraints, semiparametric models performed well at nutrient load prediction. Based on these models, delivered annual nutrient loads varied substantially from year to year. In contrast, minimal changes in flow-normalized loads indicate that nutrient loadings were driven by natural variation in precipitation and runoff as opposed to changes in management of nonpoint sources. Models indicated no evidence of long-term changes in dissolved oxygen or chlorophyll-a within Lavaca Bay. However, site specific long-term increases in both organic and inorganic nitrogen are concerning for their potential to fuel eutrophication. Further analysis found freshwater inflow had strong influences on nutrient and chlorophyll-a concentrations but there was no evidence that changes in watershed nutrient loading explained additional variation in dissolved oxygen and limited evidence that watershed nutrient loadings explained chlorophyll-a concentrations. In addition to providing a baseline assessment of watershed nutrient loading and water quality responses in the Lavaca Bay watershed, this study provides methodological support for the use of semiparametric models in load regression models and estuary assessments.

A tale of two contaminants: stark differences in the response of N and P to urban lake mitigation efforts

Abstract A constructed in-lake water quality mitigation system has proven itself to be effective at reducing Machado Lake phosphorus (P) levels, but ineffective at reducing nitrogen (N) levels. A combination of lake sediment dredging and capping, oxygenation, and a recirculating wetland have reduced lake water column P levels by nearly 50%, as compared to pre-project levels. Key to this result has been the dampening of seasonal P recycling in the sediments. A new lake water quality numerical model is presented, with applications to both pre- and post-project conditions. Model auditing has revealed very good results with respect to predicting mitigation impacts on P but poor results with respect to predicting the performance, or lack thereof, of the N mitigation system. Model sensitivity analyses indicate that the P reductions are primarily attributable to the sediment dredging and capping. Conversely, seasonal data, supported by modeling, suggest that the poor performance of the N mitigation system may be attributable to incomplete removal, or sequestration, of sediment N mass during dredging and/or a lack of impact from the oxygenation system. Future mitigation efforts for the lake should focus on reducing the substantial watershed nutrient loads to the lake and further in-lake P inactivation.

Novel predictors related to hysteresis and baseflow improve predictions of watershed nutrient loads: An example from Ontario's lower Great Lakes basin

Eutrophication has re-emerged in the lower Great Lakes basin resulting in critical water quality issues. Models that accurately predict nutrient loading from streams are needed to inform appropriate nutrient management decisions. Generalized additive models (GAMs) that use surrogate data from sensors to predict nutrient loads offer an alternative to commonly applied linear regression and may better handle relationship non-linearities and skewed water quality data. Five years (2015–2020) of water quantity and quality data from 11 agricultural watersheds in southern Ontario were used to develop GAMs to predict total phosphorus (TP) and nitrate (NO3−) loads. This study aimed to 1) use GAMs to predict nutrient loads using both common and novel predictors and 2) quantify and examine the variability in seasonal and annual nutrient loads. Along with routine surrogate model predictors (i.e., flow, turbidity, and seasonality), the addition of the baseflow proportion and the hydrograph position of flow observations improved model performance. Conversely, including the antecedent precipitation index minimally affected model performance, regardless of constituent. Seasonal and annual patterns in TP and NO3− load predictions mirrored that of the hydrologic regime. This study showed that parsimonious GAMs featuring novel model predictors can be used to predict nutrient loads while accounting for the partitioning of surface and subsurface flow paths and hysteresis between streamflow and water quality parameters that are frequently observed in a wide range of environments.

Carbon Sequestration, Nitrogen, and Phosphorus Accumulation in a Freshwater, Estuarine Marsh: Effects of Microtopography and Watershed's Nutrient Loads

Carbon Sequestration, Nitrogen, and Phosphorus Accumulation in a Freshwater, Estuarine Marsh: Effects of Microtopography and Watershed's Nutrient Loads

Implementation of a watershed modelling framework to support adaptive management in the Canadian side of the Lake Erie basin

The present study aims to provide guidelines with respect to the implementation of a watershed modelling framework that can support adaptive management in the Canadian side of the Lake Erie basin. Our primary objective is to highlight the establishment of a multimodel ensemble as an appealing strategy that can address a wide range of conceptual and operational uncertainties typically underlying any modelling exercise. We identify three ensemble strategies that have different implications for the granularity of the analysis, empirical knowledge and data input demands to constrain the individual models, and required timeframe to operationalize them. The first option partly resembles the watershed modelling work in the Maumee River watershed, where the characterization of the watershed attributes and functioning that modulate nutrient loading will be based on multiple independent SWAT applications with different process characterizations. Before implementing SWAT, we propose the use of SPARROW —a data-driven model— to delineate hot-spots, examine landscape predictors for nutrient mobilization and retention, and use these results to inform the SWAT models. Alongside the SPARROW and SWAT applications, the second option considers two additional models (HYPE, HSPF) that are conceptually sound, have been applied in the area, and can increase the structural and functional diversity of the ensemble of catchment models by offering alternative representations of fundamental processes/functions associated with the water cycle, soil erosion, biogeochemical cycles, and soil-plant interactions. The third option introduces local (farm, urban or natural vegetation) and edge-of-field monitoring and modelling (EPIC or APEX tools) to refine the spatial resolution and design in-field conservation practices that mitigate nonpoint source pollution from agriculture and other sources. Our study also pinpoints data monitoring needs to elucidate critical unknowns of the watershed functioning, such as the role of legacy phosphorus (P), the causes and consequences of the increasing long-term trends in dissolved reactive P loading, the challenges in reproducing spring-freshet or event-flow conditions, and the dynamic characterization of water/nutrient cycles under the non-stationarity of a changing climate. The strong linkages between environmental processes and management decisions that occur along the watershed-lake continuum require the presence of a holistic model ensemble to provide rigorous BMP efficiency assessment and simulation of the human-environment system responses to current and future watershed nutrient loads, land uses, climatic forcing, and socioeconomic conditions. The proposed model ensemble framework can be used as an integrative tool for ecosystem policy analysis and design of proper management action plans in Lake Erie and elsewhere.

Role of Sponge City Development in China’s battle against urban water pollution: Insights from a transjurisdictional water quality management study

The recent Sponge City Development (SCD) policy enacted in China has attracted tremendous investment, although its role in tackling water pollution remains vague. This study investigated the impact of SCD in Shenzhen, a national pilot city for SCD, on water quality in the transjurisdictional bay between Shenzhen and Hong Kong. A system modeling approach that coupled watershed nutrient load calculations with three-dimension bay water quality simulations was adopted. The main study findings include the following. While the planned SCD approach may result in much higher load reductions in total phosphorus and biochemical oxygen demand (BOD) and a comparable reduction in total nitrogen compared with those obtained by upgrading wastewater treatment plants, the contribution from green infrastructure (23.2%, 21.1% and 17.4% for total nitrogen, total phosphorus and BOD, respectively), which is a definitive component of SCD, is much smaller than that of pipeline construction (i.e., a gray infrastructure component of SCD). The overall cost-effectiveness ranking is green infrastructure > pipeline > upgrading wastewater treatment plants. On average, SCD in Shenzhen could reduce the total nitrogen and total phosphorus concentrations in the bay by 2.4% and 20.2%, respectively, but increase the chlorophyll a concentration by 5.5% due to improved light conditions. This study represents one of the few attempts to quantitatively assess the water quality impact of the SCD policy at a regional scale. The study findings suggest that SCD emphasizing green infrastructure should be preferably adopted by small-to medium-sized cities, and SCD for an individual city may only have a local impact on water quality and needs to be harmonized with other policies within the regional context.

Estimation of phosphorus loads from septic systems to tributaries in the Canadian Lake Erie Basin

The contribution of septic systems to watershed nutrient loads is poorly quantified although they are often cited as potentially important nutrient sources. The study used a geospatial model to estimate P loads from septic systems to the tributaries of the Canadian Lake Erie Basin to inform Lake Erie nutrient management initiatives. There is currently no inventory of septic systems in the Lake Erie Basin (e.g., numbers and locations of septic systems). Therefore a geospatial model was developed to automatically locate individual septic systems and to use these locations to estimate P load contributions to tributaries. The model was first tested on three subwatersheds in the Canadian Lake Erie Basin before being applied across the Basin. Present-day basin-wide P load estimates reveal that: (i) only a fraction of septic effluent is currently reaching the tributaries due to slow transport and other delays, as well as (ii) P attenuation in the subsurface, range from 23 ± 11 to 68 ± 32 MT/yr. Based on these estimates, septic systems may currently contribute 1.7 ± 0.8–5 ± 2.3% of the P loads to Lake Erie from Canada. However, maximum P load estimates and transient model results show that the contribution of septic systems to P loads will increase over time as slow moving septic-derived groundwater P plumes reach tributaries if aging septic systems are not maintained. This study provides widely applicable new knowledge and methodology; as well as specific findings needed to inform nutrient and septic system management in the Lake Erie Basin.

The effects of Hurricane Irma on seagrass meadows in previously eutrophic estuaries in Southwest Florida (USA)

In six contiguous estuaries in Southwest Florida (USA) focused management actions over the past several decades have reduced watershed nutrient loads, resulting in an additional 11,672 ha of seagrass meadows between 1999 and 2016, an improvement of 32%. However, in September of 2017, Hurricane Irma made landfall in the state of Florida, affecting the open water and watersheds of each of these six estuaries. In response, seagrass coverage declined by 1203 ha between 2016 and 2018, a system-wide decrease of 3%. The range of decreases associated with Hurricane Irma varied from less than a 1% loss of seagrass coverage in St. Joseph Sound to declines of 7 and 11% in Clearwater Harbor and Lemon Bay, respectively. Areas with the largest losses between 2016 and 2018 were those systems where seagrass coverage had declined in prior years, indicating the effects of Hurricane Irma might have been intensified by prior impacts.

Investigating the use of the artificial sweetener acesulfame to evaluate septic system inputs and their nutrient loads to streams at the watershed scale

Septic systems are often cited as an important contributor to rural watershed nutrient loads despite limited quantitative evidence regarding their actual contribution. The measurement of the artificial sweetener acesulfame in streams to quantify upstream inputs of septic system effluent has recently been proposed. However, there is a lack of detailed study on this application, including temporal variability, influence of dry and wet weather conditions, and disentangling the contributions from functioning versus failing septic systems. Further, this approach has not been applied to assess the relative contribution of septic systems to nutrient watershed loads. Here we conducted artificial sweetener and nutrient stream measurements in three watersheds with no other identified sources of acesulfame (e.g., no municipal wastewater treatment plants, landfills) over a 10-month period. Measured acesulfame stream concentrations varied from below detection (<2 ng/L) to 87 ng/L with acesulfame stream loads revealing that septic system inputs to streams vary considerably between low and high stream discharge conditions. Positive acesulfame concentration-stream discharge relationships for all three watersheds reveal the potentially important contributions from failing septic systems and surface flow pathways (i.e., effluent breakout from septic drain field) under high discharge conditions. Further they indicate that, contrary to conventional thinking, sampling under low discharge conditions may underestimate septic effluent delivery to streams. The estimated contribution of septic systems to measured total phosphorus stream loads on a given sampling day varied widely (<1–36%, considering attenuation between the septic tank and stream), but typically septic system contributions to nitrate stream loads were low (<2%). The findings provide guidance on the application of this new tool for managing septic effluent contributions of nutrients and other wastewater contaminants to rural watershed streams.

The Development of Denitrification and of the Denitrifying Community in a Newly-Created Freshwater Wetland

Agricultural growth and urban development have increased nutrient loading into streams to unnatural levels, negatively affecting estuaries downstream. Wetland creation along streams has the potential to mitigate watershed nutrient loading, providing the necessary conditions to encourage bacterial denitrification. We compared denitrification in two wetland systems, one recently created and one well established, along a branch of the St. Martin River, the largest and most degraded tributary flowing into the Maryland Coastal Bays. Ambient and potential denitrification rates were measured using membrane inlet mass spectrometry, and the denitrifying bacterial communities were compared using qPCR with genes nirK, nirS, nosZ-I, and nosZ-II. For all sites, denitrification rates were highly variable, but similar between the two wetlands, while potential rates increased almost 10-fold after a NO3− amendment, indicating that the denitrifying communities can respond to pulses of NO3− delivered via episodic flow events. Overall, the denitrifying community was more abundant at the created wetland, but the reference site had a higher abundance of the nosZ-I gene and, therefore, a lower genetic potential for N2O production. The created wetland has developed denitrification rates and a denitrifying community similar to those of the established wetland and could successfully remove excess nutrients entering the system.

Long‐term assessment of nutrient flow pathway dynamics and in‐stream fate in a temperate karst agroecosystem watershed

AbstractNutrient dynamics in karst agroecosystems remain poorly understood, in part due to limited long‐term nested datasets that can discriminate upland and in‐stream processes. We present a 10‐year dataset from a karst watershed in the Inner‐Bluegrass Region of central Kentucky, consisting of nitrate (nitrate‐N [NO3−]), dissolved reactive phosphorus (DRP), total organic carbon (TOC), and total ammoniacal‐N (TAN) measurements at nested spring and stream sites as well as flowrate at the watershed outlet. Hydrograph separation techniques were coupled with multiple linear regression and Empirical Mode Decomposition time‐series analysis to determine significance of seasonal processes and to generate continuous estimates of nutrient pathway loadings. Further, we used model results of benthic algae growth and decomposition dynamics from a nearby watershed to assess if transient storage in algal biomass could explain differences in spring and downstream watershed nutrient loading. Results highlight statistically significant seasonality for all nutrients at stream sites, but only for NO3− at springs with longitudinal variability showing significant decreases occurring from spring to stream sites for NO3− and DRP, and significant increases for TOC and TAN. Pathway loading analysis highlighted the importance of slow flow pathways to source approximately 70% of DRP and 80% of NO3−. Results for in‐stream dynamics suggest that benthic autotroph dynamics can explain summer deviations for TOC, TAN, and DRP but not NO3−. Regarding upland dynamics, our findings agree well with existing perceptions in karst for N pathways and upland source seasonality but deviate from perceptions that karst conduits are retentive of P, reflecting the limited buffering capacity of the soil profile and conduit sediments in the Inner‐Bluegrass. Regarding in‐stream fate, our findings highlighted the significance of seasonally driven nutrient processing in the bedrock‐controlled streambed to influence nutrient fluxes at the watershed outlet. Contrary to existing perceptions, we found high N attenuation and an unexplained NO3− sink in the bedrock stream, leading us to postulate that floating macrophytes facilitate high rates of denitrification.

Detection of the effects of stormwater control measure in streams using a Bayesian BACI power analysis

The unpredictable timing and magnitude of precipitation events and the spatiotemporal variability of constituent concentrations are major complications to effective monitoring of watershed nutrient and sediment loads. Furthermore, detecting small changes in constituent loads in response to implementation of Stormwater control measures (SCMs) against natural variability is a challenge. Nevertheless, regulatory frameworks that direct reductions of pollutants to streams frequently depend on the ability to quantify changes in loads after management interventions. The before-after-control impact (BACI) sampling design is often used to assess the effects of an environmental change made at a known point in time. However, this approach may be complicated to apply to nutrient and sediment loads in streams as the relative impact of SCMs on nutrient concentration conditional on the long term variability of discharges has not been evaluated. Multi-scale monitoring studies that provide estimates of the natural temporal and spatial variability of discharge and concentrations could provide useful information in designing a BACI study. Here we use data from the Baltimore Long Term Ecological Research (LTER) sites and urban restoration sites to develop multiple statistical measures of the effectiveness of a given monitoring scheme in revealing the hypothesized restoration effects in terms of hydrology and nutrient loads. Stratified sampling over baseflow and stormflow and the use of multiple control streams were useful tools to detect long term cumulative reductions in concentrations due to SCMs. Moderate reductions in concentration (20%), however, were not detectable with the design options considered. We emphasize that appropriate pre-planning of monitoring schemes and sampling frequency is essential to determine if the effects on constituent loads resulting from a given watershed restoration activity are measurable.

The importance of nutrient supply by fish excretion and watershed streams to a eutrophic lake varies with temporal scale over 19 years

Animals transform and translocate nutrients at ecologically relevant rates, contributing to eutrophication in aquatic ecosystems by mobilizing otherwise unavailable nutrients. Yet we know little about how animal-mediated nutrient cycling compares with external abiotic nutrient sources over long periods (years–decades) and at multiple timescales. To address this, we conducted a 19-year study in a eutrophic reservoir examining nitrogen (N) and phosphorus (P) inputs from watershed streams versus excretion by an abundant fish (gizzard shad, Dorosoma cepedianum) at weekly, monthly and seasonal timescales. Over the entire time period, watershed N and P loading was 33- and 3-fold greater than fish N and P excretion, respectively. However, fish N excretion exceeded watershed nutrient loading in 36% of weeks and 43% of months, and fish P excretion in 68% of weeks and 58% of months during the growing season. Fish excretion had lower temporal variability in both supply rate and N:P ratio than watershed loading. Fish excretion also supplied nutrients at a much lower molar N:P ratio than the watershed (mean of daily N:P supply ratios were 15 and 723, respectively). In eutrophic lakes with high fish biomass, fish excretion can strongly influence algal biomass and community composition. Eutrophication management efforts should consider removal of benthivorous fish, like gizzard shad, in addition to other watershed management practices to improve water quality. Future climate change will modulate the interplay between fish- and watershed-mediated nutrient dynamics by altering the geographic distribution of detritivorous fish and the frequency and severity of storm and drought events.

A Multi-Objective Chance-Constrained Programming Approach for Uncertainty-Based Optimal Nutrients Load Reduction at the Watershed Scale

A multi-objective chance-constrained programming integrated with Genetic Algorithm and robustness evaluation methods was proposed to weigh the conflict between system investment against risk for watershed load reduction, which was firstly applied to nutrient load reduction in the Lake Qilu watershed of the Yunnan Plateau, China. Eight sets of Pareto solutions were acceptable for both system investment and probability of constraint satisfaction, which were selected from 23 sets of Pareto solutions out of 120 solution sets. Decision-makers can select optimal decisions from the solutions above in accordance with the actual conditions of different sub-watersheds under various engineering measures. The relationship between system investment and risk demonstrated that system investment increased rapidly when the probability level of constraint satisfaction was higher than 0.9, but it reduced significantly if appropriate risk was permitted. Evaluation of robustness of the optimal scheme indicated that the Pareto solution obtained from the model provided the ideal option, since the solutions were always on the Pareto frontier under various distributions and mean values of the random parameters. The application of the multi-objective chance-constrained programming to optimize the reduction of watershed nutrient loads in Lake Qilu indicated that it is also applicable to other environmental problems or study areas that contain uncertainties.

Development of an Integrated Water Quality and Macroalgae Simulation Model for Tidal Marsh Eutrophication Control Decision Support

Numerical modeling is an efficient and useful method for understanding the hydrodynamics and water quality responses to nutrient loading changes and other management in estuarine and coastal systems. In this study, the Environmental Fluid Dynamic Code (EFDC) was applied in the Famosa Slough, a small tidal marsh system in urban San Diego County, California, to analyze multiple management scenarios focusing on different aspects of controlling processes: watershed load reduction, macroalgae harvesting, dredging, and the combination of different options. In order to evaluate these management scenarios, a previous EFDC model was enhanced through modifying the model code to allow simulations of both benthic and floating macroalgae as separate state variables, and configuring a sediment diagenesis model to predictively represent the dynamic interactions between the watershed load and benthic nutrient flux. The model was calibrated and verified by comparing model predictions with the observed data of hydrodynamic and water quality parameters throughout 2008. The calibrated model was then applied to simulate the water quality response to various management scenarios. The simulated results showed that combining watershed nutrient load reductions and harvesting floating macroalgae can produce significant water quality benefits. The results provide useful information for hydrological ecosystem protection and can be used for determining cost-effective implementation actions in the future.

A decline in benthic algal production may explain recent hypoxic events in Lake Erie's central basin

Abstract Since the late 1990s, the central basin of Lake Erie has reputedly experienced an increase in the frequency and severity of hypoxic events. However, total phosphorus (TP) loading, in-lake TP concentrations, chlorophyll a (Chl a ), and sediment oxygen demand (SOD) have all declined in the central basin since the 1970s. Water clarity in this basin has declined from the 1970s to 2000s despite the invasion of dreissenid mussels around 1990. In shallow lakes, declines in benthic primary production (PP) can generate positive feedback loops between the internal loading of nutrients/dissolved organic carbon and hypoxic/anoxic conditions in the water column. Such a hypoxia-inducing mechanism driven by declines in benthic PP has not been explored in Lake Erie. To test if a decline in benthic PP might explain hypoxic events in the central basin of Lake Erie, we calculated the inter-decadal changes in benthic and planktonic algal production in this basin from the 1970s to the 2000s. Primary production models using water column Chl a concentrations and light attenuation indicated that benthic PP represents roughly 10% of the basin's total areal PP. However, our calculations show that benthic PP declined from approximately 540 to 200 g C/m 2 y since the 1970s. We propose that a decline in benthic PP may have played a key mechanistic role in the transition from externally-induced hypoxia (i.e. watershed nutrient loading fueling phytoplankton production) in the 1970s and 80s to internally-induced hypoxia (sediment resuspension and internal loading) since the late 1990s.

Internal cycling, not external loading, decides the nutrient limitation in eutrophic lake: A dynamic model with temporal Bayesian hierarchical inference

Lake eutrophication is associated with excessive anthropogenic nutrients (mainly nitrogen (N) and phosphorus (P)) and unobserved internal nutrient cycling. Despite the advances in understanding the role of external loadings, the contribution of internal nutrient cycling is still an open question. A dynamic mass-balance model was developed to simulate and measure the contributions of internal cycling and external loading. It was based on the temporal Bayesian Hierarchical Framework (BHM), where we explored the seasonal patterns in the dynamics of nutrient cycling processes and the limitation of N and P on phytoplankton growth in hyper-eutrophic Lake Dianchi, China. The dynamic patterns of the five state variables (Chla, TP, ammonia, nitrate and organic N) were simulated based on the model. Five parameters (algae growth rate, sediment exchange rate of N and P, nitrification rate and denitrification rate) were estimated based on BHM. The model provided a good fit to observations. Our model results highlighted the role of internal cycling of N and P in Lake Dianchi. The internal cycling processes contributed more than external loading to the N and P changes in the water column. Further insights into the nutrient limitation analysis indicated that the sediment exchange of P determined the P limitation. Allowing for the contribution of denitrification to N removal, N was the more limiting nutrient in most of the time, however, P was the more important nutrient for eutrophication management. For Lake Dianchi, it would not be possible to recover solely by reducing the external watershed nutrient load; the mechanisms of internal cycling should also be considered as an approach to inhibit the release of sediments and to enhance denitrification.

Sediment and nutrient distribution and resuspension in Lake Winnipeg

Severe algal blooms in Lake Winnipeg since the late 1990s have been attributed to increased watershed nutrient loading, much of which is associated with suspended particles. Within-lake transport and fate of this nutrient fraction and the importance of internal loading via resuspension, however, are unknown. We measured radioisotopes (7Be, 210Pb, 137Cs), metal and nutrient contents of suspended solids in major tributaries and lake-water, in sediment traps and in bottom sediments to estimate sediment resuspension and mass accumulation rates using two models. Sedimentation rates calculated from 137Cs and 210Pb dated cores and sediment traps indicated that most (95–99%) suspended material is derived from bottom sediment; mixing models using7Be/210Pb and 137Cs yielded similarly high estimates (82 and 84%, respectively). 137Cs profiles in cores indicated that up to ~7cm remains actively resuspended for times up to 23years before incorporation into deeper sediments. Total and bioavailable phosphorus (TP, BAP) in this top sediment layer were generally lower in the North than the South Basin, likely reflecting inputs from the Assiniboine and Red Rivers at the southern end of Lake Winnipeg, with an average of ~30% TP as BAP. Estimates of average sediment-associated internal TP loading for the South Basin (0.264g/m2/y) were ~2× those for the North Basin (0.146g/m2/y). Together, this internal loading is comparable to the magnitude of the external loading. Our results indicate that surficial sediments in Lake Winnipeg will remain a significant and active source of internal nutrient loading for several decades, a process which may delay the response of the lake to external nutrient management.

In Brief

The recent Sponge City Development (SCD) policy enacted in China has attracted tremendous investment, although its role in tackling water pollution remains vague. This study investigated the impact of SCD in Shenzhen, a national pilot city for SCD, on water quality in the transjurisdictional bay between Shenzhen and Hong Kong. A system modeling approach that coupled watershed nutrient load calculations with three-dimension bay water quality simulations was adopted. The main study findings include the following. While the planned SCD approach may result in much higher load reductions in total phosphorus and biochemical oxygen demand (BOD) and a comparable reduction in total nitrogen compared with those obtained by upgrading wastewater treatment plants, the contribution from green infrastructure (23.2%, 21.1% and 17.4% for total nitrogen, total phosphorus and BOD, respectively), which is a definitive component of SCD, is much smaller than that of pipeline construction (i.e., a gray infrastructure component of SCD). The overall cost-effectiveness ranking is green infrastructure > pipeline > upgrading wastewater treatment plants. On average, SCD in Shenzhen could reduce the total nitrogen and total phosphorus concentrations in the bay by 2.4% and 20.2%, respectively, but increase the chlorophyll a concentration by 5.5% due to improved light conditions. This study represents one of the few attempts to quantitatively assess the water quality impact of the SCD policy at a regional scale. The study findings suggest that SCD emphasizing green infrastructure should be preferably adopted by small-to medium-sized cities, and SCD for an individual city may only have a local impact on water quality and needs to be harmonized with other policies within the regional context.