Exploring the Trends in Sediment and Phosphorous Concentration and Loads: Case Study over Part of the Great Lake Basin (Ontario, Canada)

doi:10.21203/rs.3.rs-3239892/v1

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Exploring the Trends in Sediment and Phosphorous Concentration and Loads: Case Study over Part of the Great Lake Basin (Ontario, Canada)

https://doi.org/10.21203/rs.3.rs-3239892/v1

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In this study, trend analysis of total suspended solids (TSS) and total phosphorus (TP) concentrations and loads are performed, combining bootstrapping with the Weighted Regressions on Time, Discharge, and Season i.e., WRTDS_BT technique. The technique is used at ten selected monitoring stations of Northern Lake Erie, Eastern Lake Huron, and Lake Ontario & Niagara Peninsula in Ontario, Canada. Trend analysis over major tributaries reveals that trends in TSS concentrations and loads were highly variable, while there was a significant decline in TP concentrations and loads. However, in most tributaries, TSS and TP concentration levels are significantly higher than the provincial and national guidelines and objectives. To be precise, TSS concentration levels are significantly higher than Canadian Water Quality Guidelines (CWQG) for the TSS concentration in tributaries of 30 mg/L (following Toronto Region Conservation Authority (TRCA), Ontario) and TP concentration levels are significantly higher than the Ontario’s provincial water quality objectives (PWQO) guidance for the TP concentration in tributaries of 0.03 mg/L. Moreover, our findings suggest that changes in land management practices in agricultural areas, such as tillage, tile drainage and fertilizer/manure application may play an important role for the analysed trend.

WRTDS_BT

Trend analysis

Total Phosphorous

Total Suspended Solids

Water Quality

Total Suspended Solid (TSS) concentration were significantly higher than CWQG limit of 30 mg/L
Total Phosphorous (TP) concentrations/ loads, over most tributaries, got significantly declined
TP concentration were significantly higher than PWQO limit of 0.03 mg/L

Increased nutrient loading/concentration, especially of phosphorus, and sediment in water bodies such as lakes and coastal waters is a complex problem and growing global concern due to its adverse impact on water quality and environmental health (Smith 2003; Chislock et al. 2013; Stammler et al. 2017). The nutrients get exported from the near by basins into the water bodies and affect aquatic ecosystems, human and animal health, drinking water qualities and local amenities accelerating frequent algal blooms due to eutrophication in water bodies (Smith et al. 1999). Consequently, depletion of dissolved oxygen, due to decomposing algae, happens commonly in lakes and stratified estuaries including the Great Lakes which are five large lakes located between Canada and United States border.

In the recent times, high phophsorus and sediment is entering Great Lakes from the contributing basins from Canadian and USA side. In this paper, we focus on Canadian side. To be precise, excess sediment and phosphorus associated primarily with soil erosion and associated agricultural runoff and fertilizers originate primarly from the three dominant basins located in Southern Ontario, Canada: Northern Lake Erie, Eastern Lake Huron, and Lake Ontario & Niagara Peninsula. Over the past few decades, the expansion of croplands, livestock production and new industrial developments in the basins have raised serious concerns about the future of water quality in the downstream waters due to increased manure and fertilizer use as well as increased erosion (R. Halliday & Associates Ltd et al. 2009). Also, the 2003 census report (Statistics Canada 2003) indicated that a 40% human population increase was expected over the following 25 years, and significant increases in water consumption and wastewater discharge to streams were expected. This prediction seems to have similarity with the observed consequences of anthropogenic changes in Lake Ontario since European settlement (Estepp and Reavie, 2015). In the recent past, beneficial management practices have been implemented in various basins in southern ontario to promote the reduction of nutrient losses from agricultural fields (Miller et al. 2010). To assess the effectiveness of ongoing and past efforts, we need to keep analysing the historical monitored data. However, the current monitoring data is grab sampled (i.e one or two samples per month). It is beneficial to have continuous data to effectively perform analysis of historical data. Trend analysis is one of such analysis techniques which helps us in understanding of the spatiotemporal variation in nutrient loads and concentration in large and highly complex basins.

Therefore, gap-filling of data based on limited monitored/gauged data, for having continuous sample (on daily basis), is essential for analysis such as trend analysis to support existing modeling and monitoring efforts. To address this, the United States Geological Survey (USGS) has developed a modified weighted regression method–Weighted Regression on Time, Discharge, and Season (WRTDS) (Hirsch et al. 2010). WRTDS can also detect and describe temporal trends that may not conform to linear or quadratic functional forms. On the other hand, combination of bootstrap technique with WRTDS (known as WRTDS_BT) facilitates a straightforward way to derive the estimates of standard errors and confidence intervals only using limited number of samples, without further sampling.

In this junction, the decision to use any given trend analysis procedure depends on two major considerations: one is the extent to which the test outputs can describe the nature and magnitude of the trend that may have occurred. The other is the degree to which it can accurately represent the level of variability in the results. Over the years, a few related studies have been carried out like Taylor and Hamilton (1993), Rock and Mayer (2004) and Burque (2013). Thus, the lack of comprehensive studies (in combination with limited data) and robustness of WRTDS_BT has motivated us to implement the methodology in order to compute sediment and phosphorus trends based on limited nutrient information. The specific objective of this research is to explore the long-term trend of total phosphorus (TP) and total suspended solids (TSS) concentrations and loads in various streams/rivers in the Great Lakes Basin (Ontario, Canada) along with related statistical analysis, including confidence interval, to check the variability. This study is the first of a kind in the southern Ontario using WRTDS_BT, as per best of the knowledges of the authors. This study may help us not only to get the trends (with confidence intervals) of the nutrients, at selected tributaries, also to find out whether WRTDS_BT results have a match with previous trend analysis results with other techniques. In this regard we would like to mention that, though, a few reports describing local trends using different approaches have been been produced by the Conservation Authorities in the study area; but comprehensive work combinedly for Southern Ontario is limited.

The study area included three dominant basins of Southern Ontario, Canada: Northern Lake Erie, Eastern Lake Huron, and Lake Ontario & Niagara Peninsula (Fig. 1). Northern Lake Erie basin is located in Southern Ontario and is mainly dominated by agricultural row crops (60.8% of total landuse) (Table S1) with a drainage area of around 22,647 km². A major portion of the Northern Lake Erie basin drains into Lake Erie, while some portion along Thames River within the basin drains into Lake St. Clair and then to Lake Erie. Eastern Lake Huron basin is located in South-Western Ontario and covers a drainage area of around 15000 km² and drains majorly into Lake Huron and some parts of Georgian Bay. Eastern Lake Huron basin is also an agriculture-dominated watershed with 42.8% of the total landuse for agricultural activities. Lake Ontario and Niagara Peninsula basin is located in south-east Ontario and comprises 40% forests. Lake Ontario & Niagara Peninsula watershed has the largest drainage area among the selected basins covering a drainage area of around 28,500 km² and draining into Lake Ontario. This study mainly focused on analyzing two water quality constituents i.e., total suspended solid (TSS) and total phosphorus (TP). The time periods of considered data from each monitoring station is shown in Table S2 with other details in supplementary section S1.

The details of WRTDS method and its computational techniques are described in Hirsch et al. (2010) and Hirsch and De Cicco (2015). The details of WRTDS_BT implementation are omitted in the main text for brevity, and have been provided in the supplementary section S2.

The brief and theoretical description of the WRTDS_BT for trend analysis has been summarized in Fig. 2.

4.1. Trends and Related Statistics of Total Suspended Sediments (TSS)

In this section, trends of TSS concentrations and loads were analyzed and discussed at selected tributaries across Southern Ontario. The temporal trends of TSS concentration and loads over major tributaries are shown in Fig. 3 (a) and 3 (b), respectively, using mean annual estimates of the flow normalized (FN) concentration and load. Of the 10 stations where TSS concentrations were successfully modeled, seven exhibited decreasing trends, two showed increasing trends and one had no trend. On the other hand, for TSS load modelling, five exhibited decreasing trends, two showed increasing trends and three had no trend. In short, the results indicate that trends of TSS concentration and loads were highly variable over all tributaries.

4.1.1. Tributaries draining into lake Ontario

All two tributaries draining into Lake Ontario displayed a substantial decline in the magnitude of FN TSS concentration and loads over the period from 1979 to 2019 (i.e., Humber River (84 mg/ L concentration decrease and 55.96 t/ Year load decrease) and Don River (128 mg/ L concentration decrease and 36.31 t/ year load decrease)). The decline in trend is similar to the TSS trend declination at Don River and Humber River, described in TRCA (2017). Meanwhile, the Canadian Water Quality Guidelines (CWQG) for acceptable TSS concentrations in rivers is 30 mg/L (TRCA, 2017) (Fig. 3 (a)). The levels of TSS concentration were marginally above the suggested levels based on CWQG guidelines in the Humber River and Don River (Credit Valley Source Protection Area, 2015).

4.1.2. Tributaries draining into lake Erie

Trends analyzed over 1976–2019 in the Central Thames tributary (Thames River at Thamesville) showed a decrease in TSS concentration (10 mg/L), but no trend for TSS load (though decrease of 13.82 t/year). Kao et al. (2022) stated seasonality as the probable reason for the trend. On the other hand, increase was observed in TSS concentration and loads at Big Otter Creek (16.9 mg/L concentration increase and 25.45 t/year load increase) over the time period 2002–2019. TSS concentration and loads in Big Otter Creek increased. However, the TSS concentration levels in both these tributaries were well above the CWQG of 30 mg/L (Fig. 3 (a)). Changing agricultural practices, including the implementation of conservation tillage, have been suggested as the remedy to address issue related to increased sediment loads (Baker et al. 2014; Daloğlu et al. 2012; Muenich et al. 2016; Stow et al. 2015). Moreover, the higher TSS concentration level of Thames River at Thamesville matches with findings from Upper Thames River & Lower Thames Valley Source Protection Areas (2008), Nürnberg and LaZerte (2015) and Kao et al. (2022).

However, TSS load increased (2.63 t/ year) with no trend for concentration (little increase of 1.23 mg/ L) in Sydenham River from 2002 to 2019 (Staton et al. 2003; Upper Thames River & Lower Thames Valley Source Protection Areas 2008; St. Clair Conservation authority 2018). Staton et al. (2003) identified the topography and geomorphology as the main reasons for the trend.

Other tributary from Thames river i.e., Thames River at Innerkip shows increase in trend (0.14 mg/L) while trends in Ausable river at Springbank and Big Creek near Walsingham were decreasing (3.56 mg/L and 15.60 mg/L, respectively), for concentration. Thames River at Innerkip and Ausable river at Springbank show no trend for loads. However, load trend in Big Creek near Walsingham were decreasing (8.81 t/year). The results for Thames River at Innerkip show match with Nürnberg and LaZerte (2015). However, Ausable river at Springbank shows the similar trend as Nelson et al. (2003). Big Creek near Walsingham TSS trends express that the suggestions by Lake Erie Source Protection Region Technical Team (2008) have been followed over the years to improve the situation. Besides the point to be noted that only TSS concentration trends at Thames River at Innerkip and Big Creek near Walsingham are below the suggested limit based on CWQG guidelines.

4.1.3. Tributaries draining into lake Huron

Besides, mean annual concentrations and loads over the Saugeen River were decreasing over the time period 2002–2019 (Fig. 3 (a) and 3 (b)) (3.35 mg/ L concentration and 13.26 t/ year load). The TSS levels in the Saugeen River were well below the CWQG of 30 mg/L (Fig. 3 (a)) (Saugeen Valley Source Protection Area 2015; Falk et al. 2021). We can see that for Naottawasaga river at Baxter trend decreased for both concentration and load of TSS (10.8 mg/ L and 11.37 t/ year, respectively). Rutledge and Chow-Fraser (2019) held the variable land management practices responsible for the trends of TSS over acceptable limit.

Figure 3(a) Long term trends in FN TSS concentration over major tributaries.

4.2. Trends and Related Statistics of Total Phosphorus (TP)

Trends in TP concentration and load were analyzed over major tributaries across Southern Ontario. Trends are shown using mean annual estimates of the FN concentration and load. The temporal trends in TP concentration and loads are shown in Fig. 4 (a) and Fig. 4 (b), respectively, over major tributaries. Of the 10 stations where TP concentrations were successfully modeled, eight exhibited decreasing trends, two showed no trend. On the other hand, for TP load modelling, eight exhibited decreasing trends, one showed increasing trends and another one had no trend. In short, the results indicate that trends of TP concentration and loads were highly variable over all tributaries.

4.2.1. Tributaries draining into lake Ontario

For TP decreasing trend is visible for both concentration and load in both the tributaries of two highly urbanized watersheds Humber and Don (Concentration decrease of 0.08 and 0.39 mg/ L for Humber and Don River, respectively; Load decrease of 0.05 and 0.07 t/ year for Humber and Don River, respectively). No stations have a phosphorus concentration trend less than the PWQO level. However, the results match with the results in TRCA (2017; 2021).

4.2.2. Tributaries draining into lake Erie

Three tributaries along the Thames River were analyzed for trends in TP concentration and load. Trends over upstream and downstream Thames tributaries were analyzed for the period between 1978 to 2018, while trends at Thamesville were only analyzed for the period between 2012 to 2019. TP concentration levels were considerably high (ranging between 0.20 to 0.25 mg/L) over the upstream Thames tributary for the period between 1978 to 2000. However, there was a significant decline in TP levels after the year 2000 and further showed a consistent decrease in TP concentrations until 2018 (Stammler et al. 2017). Trends in TP concentrations and loads were highly variable at the downstream Thames tributary for the overall time period. TP concentrations showed no sign over the period from 1980 to 1990, while concentration levels increased dramatically after the 1990s and further showed a consistent surge in TP levels until 2001, with a concentration level reaching up to 0.20 mg/L, which was observed to be the highest TP concentration over the time period. On the contrary, TP concentration levels started to decline after 2001 and showed a consistent decline until 2018 with a TP concentration value of around 0.12 mg/L. Overall, there was a decline in TP concentration by -0.057 mg/L and in TP load by -168.2 t/ year over the period from 1980 to 2018. Upper Thames River & Lower Thames Valley Source Protection Areas (2008); Nürnberg and LaZerte (2015) and Kao et al. (2022) supported the above-mentioned analysis.

Descriptive statements and statistical values indicated that a decline in concentration and load at this over Big Otter Creek tributary was likely for concentration and load decline (Loomer, 2011; Lake Erie Source Protection Region, 2020). The large watershed size and distributed nature of agricultural nutrient sources are a challenge for reducing nutrient loads. These represent that the areas are minimally affected by human activities. Phosphorous loading from these undeveloped sites may not be immediate threat to eutrophication. Still, they emphasize the need for continued and increased monitoring of undeveloped areas to preserve watersheds that are minimally affected by human activities.

TP concentration and load declined at Sydenham River over the period from 2002–2019 (Stammler et al. 2017). To be precise, TP concentration and load declined by 0.02 mg/L and 0.01 t/ year, respectively. Descriptive statements and statistical results displayed that a decline in concentration and load was likely with a likelihood of 0.76 (p-value = 0.49) for both the cases.

This was mainly due to an increase in TP concentration levels from 2012 to 2019, which would have influenced the overall likelihood estimate (Station et al. 2003; Upper Thames River & Lower Thames Valley Source Protection Areas 2008; St. Clair Conservation Authority 2018). The increasing trends may be due to the changes in land managment practices and shift in tillage practices (Stammler et al. 2017). A further concern is that nutrient increases were common in estuaries with relatively little information on eutrophication status, suggesting that a lack of monitoring information could hamper our ability to determine high-risk estuaries and track related changes in nutrient loading to those estuaries.

Other tributary from Thames river i.e., Thames River at Innerkip shows decrease in trend along with Big Creek near Walsingham for both TP concentration and load. For Thames River at Innerkip, concentration decrease is 0.05 mg/ L, while load decrease is 0.03 t/ year. For Big Creek near Walsingham, concentration decrease is 0.03 mg/ L, while load decrease is 0.01 t/ year. However, trends in Ausable river at Springbank show no trend and increase for TP concentration and load (0.0002 t/ year), respectively. The results for Thames River at Innerkip show match with Nürnberg and LaZerte (2015). However, Ausable river at Springbank shows the same risk with similar trend mentioned by Nelson et al. (2003). Big Creek near Walsingham TP trends express that the suggestions by Lake Erie Source Protection Region Technical Team (2008) have been followed over the years to improve the situation.

4.2.3. Tributaries draining into lake Huron

The Saugeen River reported a decrease in TP loads from 1975 to 2018. The Saugeen River also reported the lowest TP concentration magnitude among all the tributaries, with concentration levels consistently below PWQO levels from 2008 through 2018 (Fig. 4 (a)) (Saugeen Valley Source Protection Area 2015; Falk et al. 2021). Falk et al. (2021) also specified that well management of non-point sources is responsible for lower the TP concentration. We can see that for Naottawasaga river at Baxter trend decreased for both concentration and load of TP (0.03 mg/ L and 0.03 t/ year, respectively). Rutledge and Chow-Fraser (2019) held the variable land management practices responsible for the trends of TP over acceptable limit.

Figure 4(a) Long term trends in FN TP concentration over major tributaries.

4.3. Variability of the confidence intervals

We can see from Figs. 3 and 4 that the widths of the confidence bands were relatively wide at the beginning and the end of the analysis period. This appears to be closely related to more extreme variability in the data during that periods. It is to be noted that a few of the yearly estimates (black dots in Figs. 3 and 4) lie outside the confidence band. In this regard, we would like to argue that we are not astonished with these results, at all. The probable reason, may be, the temporal variability of the yearly estimates, particularly for load, due to the variability in discharge. However, the FN values are computed after removing the year-to-year variability due to discharge. Therefore, these are less variable. This is again analogous to confidence regions in regression (Hirsch and De Cicco, 2015).

Another point to be considered that the variability of the end points of the confidence intervals diminishes substantially as the number of replicates increases (As seen for Don and Big creek River monitoring stations). It is also clear that FN concentration confidence intervals are less variable than those for FN load. The probable rationale behind the instance may be FN load trend results are strongly influenced by the few observations that were taken at the highest discharges (Hirsch and De Cicco, 2015). Because, the flow conditions can heavily influence estimates of concentration and load. For instance, in the case of a constituent for which the concentration rises with streamflow, an extreme streamflow period of one or two years close to the end of the time-period may indicate a decrease in water-quality. Thus, streamflow-driven variations produce a significant amount of obvious "noise", making it problematic to recognize the trend (Hirsch et al., 2010).

4.4. Spatial assessment

Spatial presentation of TP and TSS trends describing increase/decrease/no-trend in tributaries across Southern Ontario revealed clear patterns (Fig. 5). In particular, all four modelled tributaries of Lake Ontario and Lake Huron, showed decreasing trends of concentration and load for both TSS and TP. We have also found the decreasing trends for Big Creek near Walsingham tributary of Lake Erie, For both the concentration and load for both TSS and TP. Moreover, at Thames River at Thamesville, we can see similar results except the no trend for TSS load (Fig. 5). Besides, we also can see decreasing trend for TP concentration and load at Sydenham River at Florence and Thames River at Innerkip. In contrast, for other cases we identify high variability in trend direction for concentration and load for both TSS and TP at tributaries of Lake Erie, with some stations showing increasing trends in all metrics of all parameters and some stations exhibiting a mix of increasing, decreasing and no trend depending upon the parameter examined (Fig. 5). Therefore, it is difficult to find a particular pattern for those tributaries. However, we have identified that TSS concentration is under CWQG suggested level of 30 mg/ L at Saugeen River near Port Elgin, Thames River at Innerkip and Big Creek near Walsingham. While TP concentration is under PWQO suggested level of 0.03 mg/ L at only Saugeen River near Port Elgin. Variability in precipitation, temperature, increased urbanization, agricultural applications of fertilizer, manure i.e., different land management practices and shift in tillage practices may play a big role in the spatial pattern of TSS and TP trends at the ten selected monitoring stations. This deduction gets support from the state of the Great Lakes report (Wittnebel 2022). According to the report, evaluation of the water quality data shows negative influence of increased urbanization and modernised agricultural applications in the tributaries of the Great Lakes. In this circumstance, we would like to suggest future measures, specially for the tributaries of Lake Erie where high variability in trend direction is identified. To be precise, if TSS is increasing, but TP is decreasing or showing no trend, specially in urban dominating watershed, focus should be on erosion control measures. For opposite scenario, specially in agriculture dominated watersheds, focus should be on better management practices in agriculture.

In this study, the WRTDS_BT was used to analyze the trends in TSS and TP concentration and loads from major tributaries in Southern Ontario, Canada. Trends in TSS concentration and loads were found to be highly varying among the tributaries. The varying trends may be due to the changes in land management practices in agricultural areas, such as tillage, tile drainage and fertilizer/ manure application. However, the results may contribute to understand the effects of natural and anthropogenic factors on sediment and nutrient losses. Besides, it is also hoped that the trend analysis results would be useful in informing future policies and legislation related to water quality.

The precise conclusions of this study are listed below:

Five tributaries displayed a decreasing trend in TSS concentration and loads, while the other five tributaries displayed varying trends in TSS concentration and load.
Whereas most tributaries reported the TSS concentration levels that were significantly higher than the CWQG for the suggested total suspended solids concentration in tributaries of 30 mg/L.
Trend analysis over major tributaries for TP concentrations and loads further revealed that there was a significant decline in TP concentrations and loads (decreasing trend likelihood > 95 and p-value < 0.10) over most tributaries.
However, most tributaries reported TP concentration levels that were significantly higher than the PWQO for total phosphorus concentration in tributaries of 0.03 mg/L.
If TSS is increasing, but TP is decreasing or showing no trend, specially in urban dominating watershed, focus should be on erosion control measures. For opposite scenario, specially in agriculture dominated watersheds, focus should be on better management practices in agriculture.

Acknowledgements, Statements & Declarations.

-Ethical Approval: Not Applicable

-Consent to Participate: We agree among ourselves to outline the roles and responsibilities towards one another throughout the whole research and publication process.

-Consent to Publish: We all give consent to publish.

-Authors Contributions: Pranesh Kumar Paul: Conceptualization, data management, analysis, First draft, review of draft; Anant Goswami: analysis; Ramesh Rudra: review of draft; Pradeep Kumar Goel: review of draft; Prasad Daggupati: project management and review of draft.

-Funding: This research was funded by the Natural Science and Engineering Research Council of Canada Discovery Grant (#401257).

-Competing Interests: There is no competing interest between us.

-Availability of data and materials: Due to privacy, ethical concerns, and confidentiality agreements the supporting data cannot be made available.

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Tables are available in Supplementary Files section.

No competing interests reported.

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Exploring the Trends in Sediment and Phosphorous Concentration and Loads: Case Study over Part of the Great Lake Basin (Ontario, Canada)

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Abstract

Figures

Highlights

1. Introduction

2. Study Area and Data

3. Methodology

4. Results and Discussion

4.1. Trends and Related Statistics of Total Suspended Sediments (TSS)

4.1.1. Tributaries draining into lake Ontario

4.1.2. Tributaries draining into lake Erie

4.1.3. Tributaries draining into lake Huron

4.2. Trends and Related Statistics of Total Phosphorus (TP)

4.2.1. Tributaries draining into lake Ontario

4.2.2. Tributaries draining into lake Erie

4.2.3. Tributaries draining into lake Huron

4.3. Variability of the confidence intervals

4.4. Spatial assessment

5. Conclusion

Declarations

References

Tables

Additional Declarations

Supplementary Files

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