Nonpoint source reduction to the nearshore zone via watershed management practices: Nutrient fluxes,fate, transport and biotic responses — Background and objectives

Studies that evaluate the linkages between watershed improvement through Best Management Practices (BMPs) and downstream outcomes are few. Water quality of coastal waters is often impacted by soil and nutrient loss from watersheds in agriculture. Mitigation of these impacts is of concern in the Great Lakes, the Finger Lakes Region of New York State, and generally in water bodies of North America. In this issue, we report on hypothesis-based research at the watershed level evaluating the impact of BMPs on mitigation of nonpoint sources of nutrient and soil loss to streams and the nearshore zone of a lake. Specifically, we hypothesize not only reductions in nutrient and soil losses from watersheds but also a resultant decrease in metaphyton (filamentous algae), coliform bacteria, and macrophyte populations in the nearshore at stream mouths draining sub-watersheds where BMPs were introduced. Small experimental sub-watersheds, predominantly in agriculture (> 70%), were selected to ensure that effects on downstream systems would not be confounded by other land use practices often observed in large watershed approaches. In this introductory paper, we provide background information on Conesus Lake, its watershed, and the Conesus Lake watershed project, a large multi-disciplinary study evaluating agricultural management practices. The series of papers in this volume consider the effect of BMPs designed to control nonpoint sources on water chemistry, metaphyton, macrophytes, and microbial populations in the coastal zone of a lake. Ultimately, this volume expands the basic understanding of the ability of BMPs to control nonpoint source pollution while contributing toward the goal of improving water quality of downstream systems including streams, embayments, and the nearshore of large lakes.     

典型农村小流域磷营养输移模拟研究

磷是湖泊水体富营养化的关键元素之一。农村小流域作为农业生产和生活污水的排放区域,磷的输出过程仍不清晰,难以量化。以直接入巢湖的小流域烔炀河流域为研究对象,结合流域自然汇水特征和行政区划划定了 24 个子流域网格单元,细化建立了以子流域为基本单元、以水量过程为基础驱动的流域磷营养输移数学模型。基于实测流量和自动站总磷浓度的过程对比检验表明,数学模型可以合理地模拟降水驱动下的小流域水量和磷营养变化过程,为量化流域磷营养的输出负荷、明确其关键来源和路径提供了科学工具。     

Influence of Riparian and Watershed Alterations on Sandbars in a Great Plains River

Anthropogenic alterations have caused sandbar habitats in rivers and the biota dependent on them to decline. Restoring large river sandbars may be needed as these habitats are important components of river ecosystems and provide essential habitat to terrestrial and aquatic organisms. We quantified factors within the riparian zone of the Kansas River, USA, and within its tributaries that influenced sandbar size and density using aerial photographs and land use/land cover (LULC) data. We developed, a priori, 16 linear regression models focused on LULC at the local, adjacent upstream river bend, and the segment (18–44 km upstream) scales and used an information theoretic approach to determine what alterations best predicted the size and density of sandbars. Variation in sandbar density was best explained by the LULC within contributing tributaries at the segment scale, which indicated reduced sandbar density with increased forest cover within tributary watersheds. Similarly, LULC within contributing tributary watersheds at the segment scale best explained variation in sandbar size. These models indicated that sandbar size increased with agriculture and forest and decreased with urban cover within tributary watersheds. Our findings suggest that sediment supply and delivery from upstream tributary watersheds may be influential on sandbars within the Kansas River and that preserving natural grassland and reducing woody encroachment within tributary watersheds in Great Plains rivers may help improve sediment delivery to help restore natural river function. Copyright © 2014 John Wiley & Sons, Ltd.     

Responses of lake macrophyte beds dominated by Eurasian watermilfoil (Myriophyllum spicatum) to best management practices in agricultural sub-watersheds: Declines in biomass but not species dominance

Long-term studies of macrophyte beds growing near streams in Conesus Lake, New York, have revealed a high biomass and continuing dominance of the invasive rooted species Eurasian watermilfoil (Myriophyllum spicatum). We tested whether agricultural best management practices (BMPs) designed to reduce tributary nutrient and soil loss from the watershed could reduce populations of Eurasian watermilfoil downstream in the lake littoral. Six macrophyte beds were monitored during a 3-year baseline period (2001–2003) prior to the implementation of BMPs and for a 4-year experimental period after a variety of agricultural BMPs were implemented in three sub-watersheds. For three macrophyte beds downstream from sub-watersheds managed as part of our project, quadrat biomass decreased by 30–50% and was statistically lower than Pre-BMP baseline values in 7 of 11 experimental sample years. Biomass loss primarily in the form of the dominant Eurasian watermilfoil ranged from 6.2 to 10 t wet weight for each bed. The declines in biomass coincided with significant annual and January–August decreases in the concentrations and fluxes of dissolved nutrients, total phosphorus, and total suspended solids in nearby streams. For three macrophyte beds downstream from watersheds in which landowners applied less extensive or no new agricultural management, biomass was statistically indistinguishable from Pre-BMP baseline values in all 12 experimental sample years. Milfoil remained the overwhelmingly dominant species at all sites during the entire study period. These results provide impetus for the use of watershed nutrient management to control the nuisance growth of Eurasian watermilfoil on a local scale in the lake littoral.     

Evaluation of stormwater and snowmelt inputs,land use and seasonality on nutrient dynamics in the watersheds of Hamilton Harbour,Ontario, Canada

Between July 2010 and May 2012, 87 24-hour level-weighted composite samples were collected from a variety of catchment states (rain, snowmelt, baseflow) from all four major tributaries to Hamilton Harbour, Ontario, Canada. Samples were analyzed for phosphorus- and nitrogen-based nutrients, and concentrations were examined for trends with catchment state, land use, and seasonality. Total phosphorus (TP) and phosphate concentrations were consistently higher during rain/melt events relative to baseflow. Nitrogen parameters, however, exhibited either concentrating behavior or little change in concentration across a range in flows (chemostasis) depending on the parameter and catchment. Despite differences in land use among the four watersheds, TP concentrations during rain/melt events did not vary among stations; however, spatial variability was observed for other parameters, especially nitrate which was elevated in watersheds on the north shore of the Harbour. Seasonal variability was generally not observed for TP concentrations, mirroring the lack of temporal trends for TSS. In contrast, elevated concentrations of nitrate and phosphate were observed during the fall and/or winter period, except in the primarily agricultural watershed where concentrations were elevated during the summer growing season. Highly elevated concentrations of ammonia and nitrate were observed in some watersheds during the unseasonably cold winter of 2010–2011 but not in the comparatively warm winter of 2011–2012. Implications of the study are discussed including the inferred potential impacts of climate change on nutrient dynamics given the strong contrasts in weather patterns observed between years, and exploration of the feasibility of mitigation measures given the data trends.     

Landscape-scale modeling of water quality in Lake Superior and Lake Michigan watersheds: How useful are forest-based indicators?

The Great Lakes watersheds have an important influence on the water quality of the nearshore environment, therefore, watershed characteristics can be used to predict what will be observed in the streams. We used novel landscape information describing the forest cover change, along with forest census data and established land cover data to predict total phosphorus and turbidity in Great Lakes streams. In Lake Superior, we modeled increased phosphorus as a function of the increase in the proportion of persisting forest, forest disturbed during 2000–2009, and agricultural land, and we modeled increased turbidity as a function of the increase in the proportion of persisting forest, forest disturbed during 2000–2009, agricultural land, and urban land. In Lake Michigan, we modeled increased phosphorus as a function of ecoregion, decrease in the proportion of forest disturbed during 1984–1999 and watershed storage, and increase in the proportion of urban land, and we modeled increased turbidity as a function of ecoregion, increase in the proportion of forest disturbed during 2000–2009, and decrease in the proportion softwood forest. We used these relationships to identify priority areas for restoration in the Lake Superior basin in the southwestern watersheds, and in west central and southwest watersheds of the Lake Michigan basin. We then used the models to estimate water quality in watersheds without observed instream data to prioritize those areas for management. Prioritizing watersheds will aid effective management of the Great Lakes watershed and result in efficient use of restoration funds, which will lead to improved nearshore water quality.     

Temporal changes in the hydrology and nutrient concentrations of a large tropical river: Anthropogenic influence in the Lower Grijalva River,Mexico

Dam construction and nutrient loading are among the greatest threats to freshwater ecosystems, altering ecological processes and the provisioning of ecosystem services. Temporal change in hydrology and ambient nitrogen and phosphorus concentrations was studied on the Grijalva, a large tropical river in southern Mexico, where four hydroelectric dams operate and where land conversion has impacted the freshwater environment. Temporal changes in discharge and in river chemistry were examined by analysing long‐term discharge and nutrient data using the software Indicators of Hydrologic Alteration and Mann–Kendall tests. Furthermore, additional water chemistry samples were collected to examine seasonal nutrient dynamics in the lower Grijalva. Long‐term discharge data indicated dam construction has severely altered temporal patterns in discharge and other hydrological characteristics. The lower Grijalva has also experienced increase in nitrate concentrations through time, which may be attributed to the expansion of agricultural and urban areas in the watershed. In contrast, total phosphorus appeared to decline at the sites influenced by dam construction. Lower nutrient concentrations were recorded upstream from the city of Villahermosa, suggesting that inputs from urban areas may have contributed to nutrient loading. Additionally, higher nitrate and total phosphorus concentrations were detected in tributaries draining intensive agricultural and suburban areas. Collectively, the results from the study suggest that dam construction and land conversion in large, tropical watersheds can produce chemical and hydrological changes, which may negatively impact important ecosystem services—such as fisheries and the provisioning of sources of drinking water—and may compromise the integrity of coastal zones.     

Widespread microplastic pollution in Indiana,USA, rivers

Microplastics (i.e., plastic particles <5 mm in size) are aquatic contaminants of emerging concern but are poorly quantified in flowing waters of the midwestern USA. Microplastics enter streams and rivers through a variety of pathways (e.g., wastewater effluent, breakdown of larger plastic debris, atmospheric deposition) and can potentially harm aquatic organisms through both direct consumption and indirect contamination from sorbed toxins. In this study, we quantified microplastic concentrations and types (i.e., beads, fibers, films, foams, fragments) in nine Indiana watersheds representing a gradient of dominant land use (i.e., agricultural, urban, and forested). We predicted that microplastic concentration would be higher in watersheds with higher percentages of urban and agricultural land use than in forested watersheds. Our results revealed measurable quantities of microplastics in samples from all watersheds, but microplastic concentration did not vary significantly with land use or longitudinally within watersheds. Fibers were the dominant form of microplastic at all sites, suggesting that fibers may be transported primarily through atmospheric deposition rather than via direct runoff from the surrounding landscape. We conclude that rivers have a different microplastic “signature” than large lakes, likely due to retention characteristics of flowing water ecosystems, unique microplastic sources, and a shorter legacy of microplastic pollution.     

Stream nutrient and agricultural land-use trends from 1971 to 2010 in Lake Ontario tributaries

Urbanization is generally recognized as the most widespread form of landuse/landcover change (LULC) within populated regions, including southern Ontario, and is often at the expense of surrounding agricultural land. However, changes in agricultural LULC within these peripheral regions should be considered when interpreting water quality changes in watersheds containing mixed LULC. The objectives of this study were to first, quantify changes in LULC within twelve Lake Ontario tributaries between 1971 and 2010, and secondly, to determine whether these changes co-occurred with changes in total phosphorus (TP) and nitrate‑nitrogen (NO₃-N) concentrations in streams. Water quality data were obtained from the Ontario Provincial Water Quality Monitoring Network while historical land use was reconstructed from agricultural census reports, historical land cover maps, and modern remotely sensed datasets. Urban cover increased, although percent increases in urban cover were small in the most agriculturally dominated watersheds (3–8%). The area of agriculture declined across all watersheds, yet the proportion of agricultural land dedicated to crop (corn, soybean, and wheat) production increased, including in the most urbanized watersheds (e.g. Mimico 89% urban; 2009–11). Total P concentrations in streams were highest at the urbanized watersheds, particularly in the 1970s, before declining in recent decades. In contrast, NO₃-N concentrations were highest (>1.5 mg/L; 2000–10) within the most agricultural watersheds (e.g. Gages 71% agriculture) and have increased over the same period of row crop expansion. Further research is needed to determine the mechanisms behind the potential relationship between expanding row crop cover and stream NO₃-N concentrations in southern Ontario.     

Watershed influences on the transport, fate and bioavailability of mercury in Lake Superior: Field measurements and modelling approaches

The issuance of fish consumption advisories in US states bordering Lake Superior has heightened the need for understanding the biogeochemical cycling and transformations of mercury in this great lake. Major routes of mercury (Hg) transport to lakes include atmospheric deposition (wet and dry), direct discharges and riverine (watershed) inputs. The specific objectives of this ongoing study are to: (i) determine the speciation and bioavailability of Hg transported to Lake Superior (ii) determine the importance of watershed‐specific characteristics that control physical and chemical forms of Hg; (iii) identify key mechanisms controlling Hg bioavailability and speciation in near‐shore zones relative to open lake regions; and (iv) provide process‐level information to compliment concurrent development of Hg fate and transport models of the Lake Superior ecosystem. Three tributaries of Lake Superior were chosen for detailed field study based on previous results and particular watershed characteristics. Mixing zones represent a potentially important zone of Hg entry into the food chain, and were sampled for biota, water and sediment in transects between the tributary mouth and the lake end member. Sampling of open‐water sites was conducted during Environmental Protection Agency‐sponsored cruises on the research vessel Lake Guardian. Results from our work on Lake Superior tributaries demonstrate that dissolved organic carbon and methylmercury (MeHg) export was greatest from watersheds containing wetlands. In Lake Superior, concentrations of Hg species were small at pelagic stations (total mercury (HgT) averaged 0.49 ng/L, MeHg averaged 6.4 pg/L). In contrast, MeHg concentrations in tributaries ranged from 100 to 250 pg/L. Watershed sites dominated by wetlands exhibited the greatest MeHg concentrations, occasionally in excess of 12 ng/L. Samples of bulk zooplankton ranged from 20 to 130 ng/g dry weight HgT and from 15 to 45 ng/g dry weight MeHg, and demonstrated typical bioacculmulation of mercury as well as distinct seasonality in concentrations.     

Influences of land use/cover on water quality in the upper and middle reaches of River Njoro, Kenya

Data from 10 sampling sites along the River Njoro are used to examine the contribution of nutrients from upstream land uses draining each of the sampling sites. The data also are used to assess whether both the proportion of land uses and the size of the subwatersheds account for the variability in water quality in the River Njoro watershed. Geographical Information System analysis was used to determine the spatial distribution of land‐cover types and subwatersheds contributing run‐off to the sampling sites in the River Njoro. Standard Digital Elevation Model‐based routines were used to establish the watershed area contributing run‐off to each sampling site. Water and sediment samples were collected for chemical analysis, and the nutrient levels were related to the upstream land‐use types and the size of the subwatersheds. The mid‐stream portion of the River Njoro (near Egerton University) accounts for the highest nutrient contributions. The percentage contribution is magnified by additions from industrial, human settlements and agricultural land uses around the University. There is a significant decrease in nutrient levels downstream, however, indicating natural purification as the river flows through an area of large‐scale farming with intense, well‐preserved riparian and in‐stream vegetation. Steep slopes of the land upstream of Egerton University enhance erosion and nutrient losses from those subwatersheds. Mixed small‐scale agricultural and bare lands contribute over 55% of the phosphorus load to the upper and mid‐reaches of the River Njoro. The size of the subwatershed accounts for about 53% of the variability in the soluble phosphorus in the river. The land‐use subwatershed proportions are important for characterizing and modelling water quality in the River Njoro watershed. Upland land uses are as important as near‐stream land uses. We suggest that conservation of intact riparian corridor along the river and its tributaries contributes significantly to natural purification processes and recovery of the ecological integrity of the River Njoro ecosystem.     

Mitigating lake eutrophication through stakeholder-driven hydrologic modeling of agricultural conservation practices: A case study of Lake Macatawa,Michigan

Lake Macatawa is a hypereutrophic water body that connects with Lake Michigan via a navigation channel. Excess phosphorus (P) concentrations have resulted in a Total Maximum Daily Load (TMDL) for total phosphorus (TP) in the lake, which has not been met. To guide land management and water pollution control in the Macatawa watershed, a Soil and Water Assessment Tool (SWAT) model and scenarios of agricultural best management practices (BMPs) were developed in consultation with stakeholders. Modelling emphasized incorporating practices representative of local agricultural conditions. Approaches to initializing high legacy soil P levels in SWAT were tested. The validated model was used to evaluate the influence of BMPs on lake water quality and identify which practices are necessary for meeting the TMDL. The model showed that eliminating manure applications would have small effect on curbing TP loading, but continuous no-till and high residue combined with already used subsurface manure application would yield notable TP reductions. Achieving TMDL-mandated TP reduction of 72% is possible through a widespread adoption of multiple BMPs (continuous no-till with high residue, cover crops, filter strips, and conversion of some marginal croplands to perennial grasses) across all the watershed’s row croplands. The study highlights how guidance from a local community interested in watershed improvement was integrated with modeling towards addressing eutrophication with informed watershed management. The Lake Macatawa case study presents a tractable system from which management solutions could be transferred to similar small agricultural tile-drained watersheds with high legacy soil P levels in the Great Lakes basin.     

An Integrated Approach for Targeting Critical Source Areas to Control Nonpoint Source Pollution in Watersheds

This study presents an integrated approach for targeting critical source areas (CSAs) to control nonpoint source pollution in watersheds. CSAs are the intersections between hydrologically sensitive areas (HSAs) and high pollution producing areas of watersheds. HSAs are the areas with high hydrological sensitivity and potential for generating runoff. They were based on a soil topographic index in consistence of a saturation excess runoff process. High pollution producing areas are the areas that have a high potential for generating pollutants. Such areas were based on simulated pollution loads to streams by the Soil and Water Assessment Tool. The integrated approach is applied to the Neshanic River watershed, a suburban watershed with mixed land uses in New Jersey in the U.S. Results show that several land uses result in water pollution: agricultural land causes sediment, nitrogen and phosphorus pollution; wetlands cause sediment and phosphorus pollution; and urban lands cause nitrogen and phosphorus pollution. The primary CSAs are agricultural lands for all three pollutants, urban lands for nitrogen and phosphorus, and wetlands for sediment and phosphorus. Some pollution producing areas were not classified into CSAs because they are not located in HSAs and the pollutants generated in those areas are less likely to be transported by runoff into streams. The integrated approach identifies CSAs at a very fine scale, which is useful for targeting the implementation of best management practices for water quality improvement, and can be applied broadly in different watersheds to improve the economic efficiency of controlling nonpoint source pollution.     

Using a weight-of-evidence approach for management of watersheds.

This research used a weight-of-evidence approach to evaluate sources of contaminants in a drinking water watershed that serves as part of the City of Boston's water supply. The approach incorporated land use analysis using GIS, sanitary surveys, traditional water quality monitoring and microbial source tracking (MST) tools. Case-study tributaries were selected based on elevated faecal coliform counts. Land use analysis and sanitary surveys were used to identify suspected microbial sources, including residential septic systems, agricultural animal operations, commercial/industrial operations and wildlife activity. Sampling sites were selected to hydrologically isolate potential contamination sources. Samples were collected seasonally over 1 year and analysed for traditional and MST parameters. Results demonstrated that both septic systems and a horse stable were contributing microbial loads in the first tributary. In the second tributary, septic systems from the townhouses were contributing microbial loads while a plant nursery was contributing organic matter. This evidence was used to evaluate best management practices to mitigate the contamination.     

流域生态环境需水与缺水快速评估（Ⅱ）：应用

采用作者提出的流域生态环境需水与缺水快速评估方法,对延河流域1990年与2000年两期土地利用资料进行了生态环境缺水评估。结果表明安塞县南部、延安市周边及宝塔区生态缺水最为严重。为此,进一步利用位于安塞县的延河二级支流纸坊沟流域1938~1999个时段期间的15个时段土地利用资料,进行了生态环境需水与缺水深入评估。结果表明:纸坊沟流域生态环境用水经历了"自然平衡-严重亏缺-逐渐改善-趋向平衡"的变化过程,这一过程与纸坊沟流域开发治理过程具有很高的一致性。应用研究结果说明,流域生态环境需水与生态缺水快速评估方法是可行的。     

In situ experimental studies of Eurasian Watermilfoil (Myriophyllum spicatum) downstream from agricultural watersheds: Nutrient loading,foliar uptake,and growth

Recent studies in Conesus Lake, New York, documented significant decreases in the biomass of Eurasian watermilfoil (Myriophyllum spicatum) near the mouths of streams draining sub-watersheds where reductions in nutrient loading occurred as a result of the implementation of agricultural Best Management Practices (BMPs). In situ experiments were conducted to further investigate the relationship between stream loading, foliar uptake, and growth of Eurasian watermilfoil. In two of three experiments, plants cropped to a height of approximately 50 cm had the lowest growth (g/m²) downstream from a sub-watershed where major BMPs had been implemented (80% and 0%). In sub-watersheds where minimal or no BMPs were introduced, plants showed significantly higher growth as biomass increased (216% and 22%). In a second set of experiments, shoots of Eurasian watermilfoil plants were incubated for 24 h in ambient lake water and in lake water with enriched concentrations of nitrate and soluble reactive phosphorus comparable to rain event stream effluent concentrations and then allowed to grow in situ for a 2-week experimental period. For all experiments combined, the shoot biomass increased significantly in the enhanced nutrient treatments when compared to the ambient treatment at the Sand Point macrophyte bed (reduced loading) but not at the Eagle Point macrophyte bed (high loading). Overall, the results indicate that foliar uptake of nutrients in stream effluent can contribute to the growth of Eurasian watermilfoil and reinforce the hypothesis that reductions in stream loading through agricultural BMPs can help reduce macrophyte growth in the lake littoral.     

Macroinvertebrate assemblages along a land-use gradient in the upper River Njoro watershed of Lake Nakuru drainage basin, Kenya

A study was conducted in the upper reaches of the River Njoro watershed to test the impacts of changing land‐use patterns, from predominantly forest to pasture and agriculture, on benthic macroinvertebrate communities. Stream sampling sites were chosen to correspond to the main offstream land uses, including forests, grazing, small‐scale agriculture, and intensive agriculture. Physicochemical variables were measured at each sampling site, and from collected water samples. Sampled macroinvertebrates were identified, and taxon diversity, richness, evenness and dominance were estimated for each site. Higher mean temperatures were recorded at the grazed and cultivated sites, compared to the forested sites. Higher ammonia concentrations were characteristic of the grazed parts of the watershed, while higher mean total phosphorus and total nitrogen concentrations were observed at the intensively cultivated sites. Baetis and Simulidae composed 65% by number of all the invertebrates collected. They dominated the benthos of the cultivated sections of the watershed, where they formed up to 75% of the observed invertebrate numbers. However, higher mean diversities, richness and evenness were recorded at forested sites, with a few taxa (notably Lepidostoma hirtum, Potamon sp., Leptophlebia sp. and Helodidae) being restricted to these unpolluted reaches, although many other taxa were common to all sites. Our findings suggest that a change from forestry to agriculture and grazing land uses have affected the physicochemical environment of the River Njoro, leading to a reduction in the diversity and evenness of benthic macroinvertebrates.     

Phosphorus Inputs to Lake Simcoe from 1990 to 2003: Declines in Tributary Loads and Observations on Lake Water Quality

Total phosphorus (TP) inputs to Lake Simcoe have led to hypolimnetic dissolved oxygen (DO) depletion and loss of cold water fish habitat. Since 1990, efforts have been made to reduce the total TP input to the lake below a defined target of 75 t/year, which was predicted to lead to reductions in spring TP concentration and improvements in end-of-summer hypolimnetic DO concentrations. The total TP load to the lake during the most recent period of record (1998/99-2003/04) ranged from 53 to 76 t/yr and averaged 67 t/yr, compared to an average of 114 t/yr estimated between 1990/91 and 1997/98 (range 85-157 t/yr). Reductions in TP loads from the catchment via tributary discharge (∼26 t) accounted for the majority of the decrease in total load between the two time periods. Total P concentrations decreased significantly in four out of six long-term monitored tributaries; however, concentrations in all six tributaries remain above the level recommended to avoid nuisance plant growth (30 μg/L). Although TP loads to the lake are currently below the target 75 t/yr, excessive growths of filamentous algae and macrophytes continue to be a problem in the nearshore zone. End-of-summer minimum hypolimnetic DO concentrations (average 4.3 mg/L, 1998/99-2003/04) remain substantially below the level (7 mg/L) that is considered protective of lake trout. Efforts to reduce TP loads to the lake therefore need to continue.     

A Bayesian hierarchical spatially explicit modelling framework to examine phosphorus export between contrasting flow regimes

We examine the ability of a SPARROW-based model (SPAtially Referenced Regression On Watershed attributes) to assess regional P export coefficients that can assist with evaluation of nutrient mitigation projects and support adaptive watershed management. Limitations in number of tributary monitoring stations were overcome by assembling multi-agency water quality data from provincial, municipal, citizen science, and academic programs. We introduced a Bayesian hierarchical framework designed to guide parameter estimation from tributary nutrient loading in southern Georgian Bay drainage basin during contrasting flow regimes, such as dry and wet years. Agriculture was identified as a major non-point P source representing between 30 and 48% of delivered P loading. Our source apportionment predicted TP loss rates from croplands that exceeded those from forested areas by 320% during dry years and by 360% during wet years, while low intensity agricultural areas (hay and pasture) exceeded P export from forests by a mere 20% and 30%, respectively. Our study identified urban runoff as another significant non-point nutrient source displaying the highest variability between dry and wet years. In particular, owing to the extensive urbanization in the Lake Simcoe watershed, urban runoff contributed nearly half of delivered P loading from tributaries into the lake. The nutrient loading management plan for Lake Simcoe calls for a reduction in P loading by ～40% from a long-term average of 72 t P y^?1 in 2002–2007 to 44 t P y^?1 by 2045. Our analysis emphasizes the importance of mitigating urban non-point sources together with efforts to control agricultural runoff.     

The aquatic environment of Parry Sound,a deep-water embayment complex of Georgian Bay protected from Dreissena

The coastline of eastern Georgian Bay in the Parry Sound region is a distinctive nearshore environment consisting of a large deep-water embayment partially isolated from the open lake and connected with other embayments of diverse morphometric features. The underlying geology of the embayments and their watersheds contrasts with the larger lake basin, resulting in heterogenous water quality stemming from mixing of the high-alkalinity, low phosphorus waters of Georgian Bay with the low-alkalinity and higher phosphorus waters draining watersheds of the Canadian Shield. In 2016, water quality was examined in the deep basin of Parry Sound known as the Big Sound, and six connected embayments to examine the influences of watershed loading and anthropogenic development to enable better detection of changes related to human activity. Inter-basin mixing and river loading accounted for much of the variability in major ions, water clarity and nutrients except for Deep Bay, an embayment exhibiting signs of nutrient enrichment. The oligotrophic conditions in Parry Sound have changed little since 1990 unlike Lake Huron where the dreissenid invasion has correlated with falling phosphorus levels. Low calcium levels may limit dreissenid mussel numbers in Parry Sound and are hypothesized to have pre-empted mussel-related changes in water quality. Apparent productivity, light regime and water column stratification of the Big Sound and connected embayments contrast with the adjacent nearshore of Georgian Bay indicating an aquatic environment distinct from the exposed nearshore of Georgian Bay.