Linking Nutrient Dynamics with Urbanization Degree and Flood Control Reservoirs on the Bahlui River

: This work analyzed the nutrient dynamics (2011–2022) and discharge (2005–2022) for the Bahlui River at four distinctive locations: Parcovaci—a dam-protected area that has been untouched by agriculture or urbanization; Belcesti—a primarily agricultural area, also dam-protected; Podu Iloaiei—a region influenced by agriculture and urbanization; and Holboca—placed after a heavily urbanized area. The analysis focused on determining a series of statistical indicators using the Minitab 21.2 software. Two drought intervals and one flood interval were analyzed to highlight daily discharge evolution during the selected period, showing that the constructed reservoirs successfully control the streamflow. For the entire period, the evolution of mean and median values of the streamflow is consistent, considering the locations’ positions from the source to the river’s end. The total nitrogen and total phosphorus were selected as representative quality indicators. The study follows the influence of the analyzed areas’ characteristics and reservoirs’ presence on nutrient dynamics. The results showed that the most influential factor that impacts nutrient dynamics is the reservoirs’ presence, which controls the discharge, creates wetlands and swamps, and implicitly impacts nutrient concentration.


Introduction
Rivers' eutrophication is not a new problem [1], and it is an expected natural phenomenon in climates with prolonged dry seasons [2].It is caused by an uncontrolled increase in nutrient concentration (e.g., nitrogen and phosphorus) followed by excessive growth of various aquatic plants that leads to water poisoning, death of aquatic animals, and sometimes irreversible damage to aqueous ecosystems.In regions without natural dry seasons, "nutritional pollution" can result from climate changes and human activities like urbanization, intensive agriculture, and industrial growth, all of which are interconnected and contribute to climate change [3].It is generally recognized that extensive urbanization affects river courses [4]; industrial expansion is the cradle for countless pollutants, while intensive agriculture is the primary source of inorganic nutrients like nitrogen and phosphorus.Climate change causes intense events that generate heavy rainfalls and floods followed by severe and prolonged droughts.The former washes the lands and transports the nutrients to rivers and water reservoirs.At the same time, the latter ensures the concentrations increase due to water evaporation and the temperatures required for the occurrence of unnatural eutrophication.Unfortunately, this global phenomenon affects the rivers and coastal regions on all continents [5][6][7].There are numerous reports in the Water 2024, 16, 1322 2 of 24 literature regarding the spatiotemporal evolution of nutrients in various rivers across the globe-see Table S1 and Figure S1 in Supplementary Materials.
A bibliometric study performed using the Scopus platform and the VOSviewer software, version 1.6.19,yielded 1210 results for "climate change and eutrophication and nutrients or nitrogen or phosphorus" search terms in title/abstract/keywords, starting from 2000 till the present day.The four significant clusters identified by VOSviewer were eutrophication, nitrogen, water quality, and nutrients, as shown in Figure 1.
Water 2024, 16, x FOR PEER REVIEW 2 of 24 the occurrence of unnatural eutrophication.Unfortunately, this global phenomenon affects the rivers and coastal regions on all continents [5][6][7].There are numerous reports in the literature regarding the spatiotemporal evolution of nutrients in various rivers across the globe-see Table S1 and Figure S1 in Supplementary Materials.
A bibliometric study performed using the Scopus platform and the VOSviewer software, version 1.6.19,yielded 1210 results for "climate change and eutrophication and nutrients or nitrogen or phosphorus" search terms in title/abstract/keywords, starting from 2000 till the present day.The four significant clusters identified by VOSviewer were eutrophication, nitrogen, water quality, and nutrients, as shown in Figure 1.Based on previous experiences, predictions can be made to prevent unwanted phenomena through various actions: reducing the amount of fertilizers during the rainy season and using reservoirs and dams for sustainable river-flow management during drought periods.
Unfortunately, for various reasons, e.g., political-some countries keep the data private; economic-some countries cannot afford such studies, some regions are not yet integrated into the global perspective [8].In some countries, there are records for hundreds of years regarding river floods, discharge variation, and drought periods [9], while in others, such types of analysis and data records are incipient.Nevertheless, the importance of such studies on a global scale is generally acknowledged nowadays, and the impact of climate change on nutrient dynamics reflected in water quality in rivers is undeniable [8].
This paper contributes to the knowledge in the particular field of river nutrient monitoring with information regarding the long-term (2011-2022) evolution of nitrogen and phosphorus in Bahlui River, Iasi County, Romania.The nutrient dynamics were linked with urbanization degree and flood control reservoirs based on the data recorded at four Based on previous experiences, predictions can be made to prevent unwanted phenomena through various actions: reducing the amount of fertilizers during the rainy season and using reservoirs and dams for sustainable river-flow management during drought periods.
Unfortunately, for various reasons, e.g., political-some countries keep the data private; economic-some countries cannot afford such studies, some regions are not yet integrated into the global perspective [8].In some countries, there are records for hundreds of years regarding river floods, discharge variation, and drought periods [9], while in others, such types of analysis and data records are incipient.Nevertheless, the importance of such studies on a global scale is generally acknowledged nowadays, and the impact of climate change on nutrient dynamics reflected in water quality in rivers is undeniable [8].
This paper contributes to the knowledge in the particular field of river nutrient monitoring with information regarding the long-term (2011-2022) evolution of nitrogen and phosphorus in Bahlui River, Iasi County, Romania.The nutrient dynamics were linked with urbanization degree and flood control reservoirs based on the data recorded at four specific locations: (i) Parcovaci-as a pristine location-close to the river's spring, in a forest region, where the first dam on the river course was built; (ii) Belcesti-as a rural location-where the main activity is agriculture/farming, the place of the second reservoir (Tansa); (iii) Podu Iloaiei-a small town with mixed industrial/farming activities; and (iv) Holboca-located close to the river mouth, after Iasi, the third largest city in Romania.The primary hypothesis is that a correlation between the analyzed aspects is present and can be the basis for predicting river discharge and nutrient variation in a specific context.

Research Area and Sampling Locations
The length of the hydrographical network of the Bahlui River is more than 3.100 km, of which only 119 km are included in the main river from spring to mouth [16].The surface of the Bahlui River catchment covers 2025 km 2 and is the most important tributary of the Jijia River [17].The river basin has a hilly terrain with large successive valleys and terraces.The valleys are deep and narrow, flanked by high hills with a slope of around 10%.The river basin has an average altitude of approximately 155 m.The mean slope of the river system varies, being 3‰ in the most upstream part, 1.6‰ in the middle part, and 0.5‰ in the downstream part.Precipitation runoff is the main water source for the river, accounting for 85-95% of the inflow [18].The Bahlui River basin is located in an area with an annual precipitation of approximately 500 mm.Several factors contribute to the high complexity of the main river, its tributaries, and reservoirs.(i) Intense soil erosion during flood waves and moderate soil erosion in agricultural areas, leading to high levels of suspended solids.(ii) Sedimentation, gradually altering riverbeds and reservoirs, reducing water storage capacity and flood protection.High precipitation in the Bahlui River basin causes significant soil erosion [19], occasionally resulting in landslides [20].
The Bahlui River is a rain-fed river system with many ungauged and temporary tributaries for which no data were available [13,21].The average discharge ranges between 2.8 m 3 /s [10,14] and 4 m 3 /s [19,22].Only approximately 30% of the length of the network has permanent flow [22].The length and surface area of the main tributaries of the Bahlui River are presented in Table 1 [16,22].Four sampling sites were selected from the Bahlui River catchment, as shown in Figure 2: Pârcovaci, Belces , ti, Podu Iloaiei, and Holboca.Each site has specific particularities: (i) Pârcovaci is placed relatively close to Bahlui's spring, in a forest area, and is actually Water 2024, 16, 1322 4 of 24 the first village crossed by the river-there is no significant influence of agriculture or urbanization on this site; the first important dam on the Bahlui River is placed in Pârcovaci; (ii) Belces , ti is a commune, in an agricultural region; in the vicinity of Belces , ti, the important Tansa-Belces , ti dam was built on the Bahlui River in order to prevent flooding; (iii) Podu Iloaiei is a small town with less than 10,000 inhabitants; a dam lake performing as a water reservoir is located on the Bahluet , River from the Bahlui River basin; and (iv) prior to reaching the commune of Holboca that is a part of the Iaşi metropolitan area, the Bahlui River crosses Iasi city, the third largest city in Romania by number of inhabitants.To summarize, the data come from four sites, representing (i) a region with no influence from agriculture or urbanization that is dam protected; (ii) a predominantly agricultural region that is dam protected; (iii) a mixed region influenced by both agriculture and urbanization; and (iv) a highly urbanized region.The GPS coordinates where the measurements were performed are 47 urbanization on this site; the first important dam on the Bahlui River is placed in Pârcovaci; (ii) Belcești is a commune, in an agricultural region; in the vicinity of Belcești, the important Tansa-Belcești dam was built on the Bahlui River in order to prevent flooding; (iii) Podu Iloaiei is a small town with less than 10,000 inhabitants; a dam lake performing as a water reservoir is located on the Bahlueț River from the Bahlui River basin; and (iv) prior to reaching the commune of Holboca that is a part of the Iaşi metropolitan area, the Bahlui River crosses Iasi city, the third largest city in Romania by number of inhabitants.To summarize, the data come from four sites, representing (i) a region with no influence from agriculture or urbanization that is dam protected; (ii) a predominantly agricultural region that is dam protected; (iii) a mixed region influenced by both agriculture and urbanization; and (iv) a highly urbanized region.The GPS coordinates where the measurements were performed are 47°27′03.7″N, 26°49′08.

Historical Milestones: Flood and Drought Periods
Iasi, the historic city that served as the capital of Moldavia and later Romania, has a complex history with the Bahlui River, which cuts through the middle of the city.The evolution of the river's color and smell through the seasons was the subject of many local debates.A long series of floods were recorded during the 19th century.Between 1950 and 2006, 62 floods were reported for the Bahlui River basin [23], of which 15 were with flows

Historical Milestones: Flood and Drought Periods
Iasi, the historic city that served as the capital of Moldavia and later Romania, has a complex history with the Bahlui River, which cuts through the middle of the city.The evolution of the river's color and smell through the seasons was the subject of many local debates.A long series of floods were recorded during the 19th century.Between 1950 and 2006, 62 floods were reported for the Bahlui River basin [23], of which 15 were with flows between 50 and 90 m 3 /s [24].However, there are 4 floods with impressive flows (Table 2) that left a mark in Iasi history [24], the most representative being the one registered in 1932 (Figure S2, Supplementary Materials) [25].Since the Bahlui's discharge is rain-dependent, two distinct periods affect the flow: (i) the winter, when water is stored as snow and ice; and (ii) the drought periods.During the second half of the 19th century, minimum discharges were lower than 0.001 m 3 /s (Table 3), and even water depletion was reported [26].

Hydrotechnical Infrastructure
Nowadays, approximately 70% of the Bahlui basin is hydrotechnically managed [26].No less than 17 reservoirs with different functions, such as flood protection, irrigation, water supply, and fishery, have been built in the river's basin (Table 3) [21,22].
The critical roles of hydrotechnical structures and engineering infrastructure are to act as protection instruments, reduce hydrological vulnerability, and contribute to a sense of security [27].In addition to that, a series of socioeconomic aspects are directly influenced by the presence of hydrotechnical structures such as the formation of characteristic ecosystems (lakes, wetlands, and swamps), diversification of land use given by the water supply, irrigation facilities, fish farming, watersports, and tourism.There are also several drawbacks related to forest loss, sediment and nutrient gathering, and landscape modification, but Water 2024, 16, 1322 6 of 24 the mentioned benefits compensate to some extent [28].The impact of these storage lakes on river discharge, water quality, and nutrient dynamics is fundamental.However, while floods can be prevented and, to some extent, controlled, the prolonged drought periods are difficult to manage for a rainfall-dependent river, particularly for a region where agriculture and farming are the main economic activities.

Statistical Analysis
In order to analyze the available data supplied by the Administratia Nationala Apele Romane (National Administration Romanian Waters, https://rowater.ro(accessed on 5 February 2024)), a set of preprocessing steps was applied.First, the data were verified to ensure their validity; no digitized data were used during the preparation of this manuscript.After that, the data were centralized and analyzed using the Minitab ® 21.2 (64-bit) statistical software.

Bahlui River Discharge Monitoring during 2005-2022 Period
There are huge oscillations between the hydro-climatic extremes (droughts and floods) typically reflected by minimum and maximum discharge values.Maximum flows generate concerns about engineering flood risks, water systems infrastructure, and flood response strategies.In contrast, minimum flows generate concerns for agriculture-irrigation engineering, ecosystems, and natural resources management [29].
The classic statistical streamflow indicators for the selected locations on the Bahlui River during the 2005-2022 period are presented in Table 4.The daily discharges from 2005 to 2022 for selected locations are presented in Figure 3. Two insets associated with drought periods (2006-2007, Figure 4; and 2011-2013, Figure 5) are marked in Figure 4 and selected for further discussion in Section 3.1.1.A period of heavy rainfalls (May-August 2010, not marked in Figure 3) is also selected to be individually discussed in Section 3.1.2.Q1 is also known as the lower quartile and shows that 25% of the data fall below this threshold.The second quartile (Q2, or median) shows that 50% of the data are below it.The third quartile (Q3, or upper quartile) shows that 75% of the data are below this value.Thus, the quartiles divide the data into four equal portions.The distance between Q1 and Q3 denotes the interquartile range and reflects the variability.It shows the distribution of the middle 50% of the data.
In Table 4, the highest variability is obtained for Holboca's discharge.This can be explained by the fact that the measurements for this location are taken after the wastewater treatment plant, and it incorporates the runoff gathered from the entire area of Iasi city.Also, in case of heavy floods, this location registered high values, as shown by the peaks in Figure 2, which reach (or go over) 50 m 3 /s.
The evolution of the mean and median values of the streamflow is consistent, considering the locations' positions from the source to the river's end (Figure 2).The considerably higher values computed for Holboca could be related to the tributaries feeding the river after Podu Iloaiei (Figure 2).When it comes to minimum and maximum values, some inconsistencies can be noted, such as that the minimum from Belcesti is 6-fold lower than the minimum in Parcovaci or the maximum in Parcovaci is 1.6 times higher than Belcesti and Podu Iloaiei.These inconsistencies are related to the Parcovaci and Tansa (Belcesti) dams on the main river course.The Tansa is a multi-purpose reservoir, being used as protection against floods, as a drinking water supply for the Belceşti village, the biggest commune in Iasi county, with more than 10,000 inhabitants [30], as an irrigation source, and for aquaculture [31].Therefore, the Bahlui's level is extremely low during drought periods, sometimes close to complete depletion [32], particularly after the Belcesti reservoir.Since the mean and the median are significantly closer to the minimum than the maximum values, it can be concluded that drought periods are predominant compared to flooding intervals (see Table 2).

Drought Periods Examination
During the first drought interval (Figure 4), the discharge in Parcovaci and Belcesti is controlled by the Parcovaci and Tansa (Belcesti) reservoirs.The values are remarkably constant throughout the drought period, even during the few rainy days.The oscillations registered for Podu Iloaiei and Holboca are related to the increased number of permanent and temporary tributaries after Belcesti and the rare rainfalls.
The Parcovaci and Tansa dams considerably impacted the drought period 2011-2013 by maintaining a constant flow rate.Table 5 displays the streamflow statistical indicators for the chosen places on the Bahlui River during the drought years of 2006-2007 and 2011-2013.As observed, although the entire period analyzed is considered as drought, the most severe drought manifestation was in 2007, where, for all sites except Holboca, the lowest mean values and variations in Q1-Q3 are obtained.The somewhat peculiar data obtained for Holboca are related to the location of the measuring site.It is placed after Iasi and its wastewater treatment plant, that discharges into the Bahlui River, sometimes has a higher flow rate than the river itself [33].
During the first drought period, the annual evolution of mean and median values of the streamflow is consistent, considering the locations' positions from the source to the river's end (Table 5, 2006;2007).The evolution of the median and maximum values is inconsistent with the locations' positions, highlighting the influence of the Parcovaci and Tansa (Belcesti) reservoirs.The mean and maximum values analysis during the 2011-2013 drought shows that 2012 was the driest year of the period.The second drought period (Figure 5) was also analyzed by other authors, who highlighted that 2012 was a dry year from the hydrological point of view [34].From a hydrological point of view, the year 2010 is remembered as a year of devastating floods across Central and Eastern Europe, including Romania, Czech Republic, Slovakia, Bosnia-Herzegovina, Hungary, Croatia, southern Poland, and southern and eastern Germany [35,36].Romanescu and Stoleriu analyzed the" exceptional" floods in the Prut basin in the summer of 2010, where river discharge reached 1600 m 3 /s [37].This event was caused by heavy rainfall, snowmelt, and saturated soil conditions.
The highest peaks for discharge values were registered on 28-29 July 2010 (Figure 6).These high discharges were preceded by heavy rainfalls on the 24th and 25th of July [36].Unlike the previous flood intervals at the turn of the century, the urban area of the city of Iasi was protected by Bahlui's waters.
The analysis of the daily discharge statistical indicators during the May-August heavy rainfall interval from 2010 (Table 6) shows that during June 2010 the streamflow achieved the highest peaks.Only the mean values are consistent with the locations' positions on the map; the other indicators were influenced by reservoir activity.The primary anthropogenic sources of N and P in rural areas are fertilizers from agriculture, animal manure from livestock production, and septic systems such as household wastewater [38].Both N and P can be produced via natural soil processes through mineralization, weathering, and/or dissolution [39].However, in some urban areas it was found that household activities such as lawn fertilization and pet ownership contribute more to nutrient watersheds than commercial, municipal, or industrial actions [40].Urban runoff, including stormwater and snowmelt, drainage networks, and yard waste, is the primary carrier of pollutants in the urban environment [41].
As can be observed from Figure 7, the distribution of individual measurements indicates that the lowest values for both TN and TP are for Parcovaci.On the other hand, the largest domains are obtained for Belcesti and Holboca, where outliers are identified.
As expected, the average TN for the Parcovaci location during the analyzed period (0.6303 mg/L, Table 7) is way below the limit value of 10 mg/L [42], and the TN value is relatively constant during the entire period (Figure 8).The same behavior is observed for the TP evolution at Parcovaci (Figure 9).more to nutrient watersheds than commercial, municipal, or industrial actions [40].Urban runoff, including stormwater and snowmelt, drainage networks, and yard waste, is the primary carrier of pollutants in the urban environment [41].
As can be observed from Figure 7, the distribution of individual measurements indicates that the lowest values for both TN and TP are for Parcovaci.On the other hand, the largest domains are obtained for Belcesti and Holboca, where outliers are identified.As expected, the average TN for the Parcovaci location during the analyzed period (0.6303 mg/L, Table 7) is way below the limit value of 10 mg/L [42], and the TN value is relatively constant during the entire period (Figure 8).The same behavior is observed for the TP evolution at Parcovaci (Figure 9).The unexpected high level of TN in the Belcesti area (Figure 8) can be correlated with the uncontrolled use of nitrogen-based fertilizers necessary for agricultural and farming activities.Other authors also observed this aspect [19].The highest level of TN (except 2018) was reported for the Holboca location during the entire period under study, and it is obviously correlated with the urban area of Iasi.The unexpected high level of TN in the Belcesti area (Figure 8) can be correlated with the uncontrolled use of nitrogen-based fertilizers necessary for agricultural and farming activities.Other authors also observed this aspect [19].The highest level of TN (except 2018) was reported for the Holboca location during the entire period under study, and it is obviously correlated with the urban area of Iasi.The highest variability is observed for TN in Belcesti, while the lowest is in Parcovaci.On the other hand, for TP, the highest variability is observed in Holboca and the lowest in Parcovaci.Excluding the 2022 reports where the TP average value is atypically high for Belcesti (Figure 9), there is a "normal" trend for this specific indicator: the pristine location exhibits the lowest values, the agricultural and mixed urban/agricultural locations show comparable values (except for in 2018, where the value is higher for the agricultural area), while after the major urban area, the value almost doubles.For the monitored period, the average values of TP (Table 7) are 0.0819 mg/L for Parcovaci and almost 13-fold higher for Holboca (1.0586 mg/L), yet below the NTPA 001/2002 [42] limit of 2 mg/L.The unexpected high level of TN in the Belcesti area (Figure 8) can be correlated with the uncontrolled use of nitrogen-based fertilizers necessary for agricultural and farming activities.Other authors also observed this aspect [19].The highest level of TN (except 2018) was reported for the Holboca location during the entire period under study, and it is obviously correlated with the urban area of Iasi.
The highest variability is observed for TN in Belcesti, while the lowest is in Parcovaci.On the other hand, for TP, the highest variability is observed in Holboca and the lowest in Parcovaci.Excluding the 2022 reports where the TP average value is atypically high for Belcesti (Figure 9), there is a "normal" trend for this specific indicator: the pristine location exhibits the lowest values, the agricultural and mixed urban/agricultural locations show comparable values (except for in 2018, where the value is higher for the agricultural area), while after the major urban area, the value almost doubles.For the monitored period, the average values of TP (Table 7) are 0.0819 mg/L for Parcovaci and almost 13-fold higher for Holboca (1.0586 mg/L), yet below the NTPA 001/2002 [42] limit of 2 mg/L.

Discharge-Nutrient Correlation
It is generally recognized that discharge measurement is essential in nutrient load estimation.However, certain degrees of uncertainty should be considered, especially for small streams [43].It was shown that even for the high frequency of data prelevation, paired data levels of uncertainties of up to 25% can occur in correlations between discharge and nutrients [43].In order to correlate the nutrient dynamics with the Bahlui River's discharge, the plots presented in Figures 10-13 were drawn for the selected sites.It is relatively challenging to find rational correlations between nutrient dynamics and river discharge for the first two locations (Figures 10 and 11).The presence of the two dams (Parcovaci and Tansa) on the main river course can disturb the correspondence.The dams usually trap the nutrients and sediments transported by runoff from heavy rains.The dilution phenomenon that should typically occur with the increase in the water flow is rarely present, and at some points the TN level seems unaffected by the discharge values (e.g., January 2013 vs. August 2013, Figure 10         A correlation between the river flow rate, TN, and TP can be observed for Parcovaci between the two discharge peaks in March 2016 and June 2021 (Figure 10).As for the Belcesti site, neither TN nor TP seems to evolve in a visible connection with the river flow oscillations.The nitrogen level is relatively high in this location and exhibits peaks even at low and constant discharge, e.g., April 2019-February 2021, revealing a predisposition to eutrophication that was also reported by other authors [44].A relatively "normal" correspondence can be seen for Podu Iloaiei in Figure 12, where the TN and TP levels typically increase with the discharge growth, with three exceptions: (i) the period June 2011-July 2013, that overlaps with the drought period (analyzed in Section 3.1.1);(ii) the period September 2018-September 2020, when another prolonged drought period was reported [45], when the TN values are high despite the low river level; and (iii) February 2018, when the TP level is atypically high.The best (visual) correlation can be observed for the last location, Holboca (Figure 13), which is close to the river's end.There, the discharge levels are higher than in all other locations, being relatively unaltered by the dam's presence and influenced by the wastewater treatment plant discharge.
A Pearson analysis was performed on the data to determine whether any statistically significant correlations could be obtained.After the data were placed in a scatter plot (Figure S11), the outliers were removed (maximum 5%).Next, the correlation indicators were determined (Table S1).As can be observed, for most cases p > 0.05, indicating that no significant data can support the affirmation that a correlation between parameters exists.The exceptions are for (i) Parcovaci, in the case of TP vs. TN, where a correlation of 0.35 (which is considered moderate positive) was obtained; (ii) for Belcesti, in the case of TN vs. discharge, where a correlation of −0.29 (which is considered low negative) was obtained; and (iii) for Holboca in the cases of TN vs. discharge and TP vs. TN, where correlations of −0.342 (moderate negative) and 0.417 (moderate positive), respectively, were obtained.
In order to determine if a seasonal variation can be observed for each location, analyzed parameter, and season, the means of the values were computed and are shown in Figure 14.The seasons were identified as follows: winter-December, January, and February; spring-March, April, and May; summer-June, July, and August; autumn-September, October, and November.As observed, TN's highest concentrations are obtained during the coldest season of the year for all analyzed locations, while the lowest are in the summer.Most probably due to the dam's presence, no significant seasonal correlations were observed for the other analyzed parameters.A correlation between the river flow rate, TN, and TP can be observed for Parcovaci between the two discharge peaks in March 2016 and June 2021 (Figure 10).As for the Belcesti site, neither TN nor TP seems to evolve in a visible connection with the river flow oscillations.The nitrogen level is relatively high in this location and exhibits peaks even at low and constant discharge, e.g., April 2019-February 2021, revealing a predisposition to eutrophication that was also reported by other authors [44].A relatively "normal" correspondence can be seen for Podu Iloaiei in Figure 12, where the TN and TP levels typically increase with the discharge growth, with three exceptions: (i) the period June 2011-July 2013, that overlaps with the drought period (analyzed in Section 3.1.1);(ii) the period September 2018-September 2020, when another prolonged drought period was reported [45], when the TN values are high despite the low river level; and (iii) February 2018, when the TP level is atypically high.The best (visual) correlation can be observed for the last location, Holboca (Figure 13), which is close to the river's end.There, the discharge levels are higher than in all other locations, being relatively unaltered by the dam's presence and influenced by the wastewater treatment plant discharge.
A Pearson analysis was performed on the data to determine whether any statistically significant correlations could be obtained.After the data were placed in a scatter plot (Figure S11), the outliers were removed (maximum 5%).Next, the correlation indicators were determined (Table S1).As can be observed, for most cases p > 0.05, indicating that no significant data can support the affirmation that a correlation between parameters exists.The exceptions are for (i) Parcovaci, in the case of TP vs. TN, where a correlation of 0.35 (which is considered moderate positive) was obtained; (ii) for Belcesti, in the case of TN vs. discharge, where a correlation of −0.29 (which is considered low negative) was obtained; and (iii) for Holboca in the cases of TN vs. discharge and TP vs. TN, where correlations of −0.342 (moderate negative) and 0.417 (moderate positive), respectively, were obtained.
In order to determine if a seasonal variation can be observed for each location, analyzed parameter, and season, the means of the values were computed and are shown in Figure 14.The seasons were identified as follows: winter-December, January, and February; spring-March, April, and May; summer-June, July, and August; autumn-September, October, and November.As observed, TN's highest concentrations are obtained during the coldest season of the year for all analyzed locations, while the lowest are in the summer.Most probably due to the dam's presence, no significant seasonal correlations were observed for the other analyzed parameters.

Nutrient Dynamics: Influence of Urbanization Degree and Flood Control Reservoirs on the Course of Bahlui River
It is evident that the nutrient dynamics are affected by a combination of natural and anthropogenic factors, each having relative influences that change with temporal and spatial scale.Like many other rivers, the Bahlui provides water for the inhabitants of its basin for irrigation, industrial, and drinking purposes.It also assimilates and/or carries industrial and municipal wastewater and manure discharges and runoff from agricultural fields, roadways, and streets [46].From its spring to its mouth, the Bahlui River crosses lands with different degrees of development, including forests, small towns, villages, agricultural regions, and one major urban area.Two major flood-controlling reservoirs were built on the main river course, Parcovaci and Tansa (Belcesti), and another 15 with different functions, such as flood protection, water storage, and fishery, on the tributaries in its hydrographic basin.The existence of these reservoirs attenuates the impact of significant climate change effects such as extreme floods and prolonged drought periods but also affects many physical and ecological aspects and processes, including sediment transport and nutrient exchange [47].The hydrological alterations produced by river-placed reservoirs are changes in flood frequency and magnitude, reduction in overall flow, increased or decreased summer baseflows, and altered timing of releases [48].In addition, the grassy vegetation, especially common reed (Phragmites australis) and bulrush (Typha latifolia), plays an essential role in the retention of nutrients in the wetlands and swamps that usually form at the base of reservoirs or nearby, with small, temporary river tributaries [49,50].

Urbanization Degree
Halecki and coworkers discussed the lower nutrient retention capacity in urban areas compared to rural and suburban areas [51].The high surface runoff, that is correlated with lower retention capacity, could explain the growth in both TN and TP in urban areas [46,[51][52][53].
Holboca, the fourth location from spring to mouth considered for this study, is placed after Iasi, the third city in Romania, close to the Bahlui confluence with the Jijia River.Many authors have studied the effect of anthropogenic activities on the water quality in this part of the river [10,14,15,33,54].As pointed out by the current report, in Holboca the nutrient levels reach the highest values of all the selected locations.Moreover, compared with the nutrient level in the first location, Parcovaci, which is a forest area with minimal human activities, there are considerable differences in magnitude.The mean value for Holboca during 2011-2022 is nearly 14-fold higher for TN and 13-fold higher for TP (Table 7).As for the rural and mixed regions, represented by the Belcesti and Podu Iloaiei locations, respectively, the nutrient level is influenced by the presence of the Tansa reservoir that controls the flow regime and by the increased number of permanent and temporary tributaries after Belcesti, which contributes to the flow development between the two locations.For most of the monitored period, except 2017, when the values were comparable, the TN values were higher in Belcesti than in Podu Iloaiei (Figure 8).As for the TP, except for the peak in 2022 in Belcesti, the values were reasonably low [42], following a similar trend (Figure 9).

Flood Control Reservoirs
During the past century, the Bahlui basin was highly vulnerable to floods [24] and droughts [32,55]; the inundation frequencies and the color and odor of the Bahlui waters during eutrophication periods were notorious.Nowadays, owing to massive investments in hydrotechnical structures, the vulnerability of the studied area, particularly of the selected locations, is highly diminished.Between 1960 and 1990, as many as 17 reservoirs were constructed in the Bahlui basin, as shown in Table 3.As for the selected locations, it is evident that the Parcovaci and Tansa reservoirs control the daily discharge even during drought periods, preventing river depletion (Figure 4).Hydrotechnical developments placed on the main course and/or in a river basin fundamentally impact the environment [56], producing geographical, ecological, and social changes [28], and the Bahlui River is no exception [21].
The reservoir and/or dam-induced hydrological control of the flow regime may affect the nutrient dynamics similarly (yet less aggressively) to the succession of flood and drought periods [48].The formation of wetlands and swamps as tails of the reservoirs is also essential for nutrient dynamics.The balance in nutrient retention and release from swamps and wetlands from the river's hydrographic basin is directly related to the water level, which can be controlled naturally (flood-drought cycles) or artificially (dam discharge).The succession of seasons also plays a vital role in supplying rivers and reservoirs with organic matter that can be further (gradually) decomposed in nutrients (e.g., leaves fallen from the forest canopy in early autumn; leaves on the ground and agricultural waste brought by spring floods) [57].
The TN and TP levels are within natural limits in the Parcovaci location, presenting small fluctuations (seasonal) through the studied interval.On the contrary, the values of nutrients, particularly TN, are relatively high for the Belcesti location, which is protected by the Tansa reservoir (Figure 8).Dughilă et al. [31] analyzed the most significant water quality indicators for the Tansa Lake, including TN and TP, in 2010.It was found that the lake level of TN was much lower than that of the Bahlui River (mainly due to the nitrate component of TN).This resulted from using nitrogen fertilizers in agriculture and wastewater discharge from commercial companies and wastewater treatment plants.The TP values in the reservoir were lower than in the river, though of a different order of magnitude compared to TN (similar to the current study; Figures 8 and 9).The orthophosphate components (that are produced in sewage effluents) were considered responsible for the elevated TP values in the Bahlui River in the Belcesti location [31].The water quality for the main reservoirs (lakes) in the Bahlui drainage basin was studied by Minea, where it was found that it is worsening, most probably due to the contribution of the tributaries that bring pollutants from uncontrolled discharges from industrial sources and malfunctioning water treatment plants [26].

Conclusions
The current analysis focused on a long period and aimed to cover the gaps in the reported discharge data on the Bahlui River.The results revealed that the reservoirs constructed to regulate the streamflow successfully managed the water discharge.The mean and median streamflow values followed a similar trend for the entire period, consistent with the locations from the river's source to its endpoint.This indicates that the reservoirs effectively controlled the streamflow and mitigated the effects of drought and flood periods.
The analysis of nutrients considered the impact of reservoir presence and the features of the investigated locations on their dynamics.The results indicate that reservoirs have a significant impact because they control water discharge during floods and droughts.Also, reservoirs favor the occurrence of wetlands and swamps, important aquatic habitats, which, in turn, affect the nutrient concentration.Moreover, areas with high levels of urbanization and agricultural activities showed higher nutrient concentrations, which can be explained by the accidental release of pollutants into the water bodies.
A statistical evaluation of discharge and nutrient links in different locations revealed weak correlations only for some particular cases: TN-TP in Parcovaci, TN-discharge in Belcesti, and TN-discharge and TP-TN in Holboca.Therefore, the analyzed data cannot support the initial hypothesis that the analyzed characteristics are correlated.This suggests a highly complex interaction between many factors, and a more in-depth study with a higher frequency of data acquisition, more locations, and a higher number of parameters such as chemical and biochemical oxygen demand, water and air temperature, and precipitation level is required.

Figure 7 .
Figure 7. Variations in TN (a) and TP (b) in the selected locations for 2011-2022.Unfortunately, the TN and TP data are unavailable for all considered locations during the analyzed period.The 2011-2014 interval is missing for Belcesti.

Figure 7 . 24 Figure 8 .
Figure 7. Variations in TN (a) and TP (b) in the selected locations for 2011-2022.Unfortunately, the TN and TP data are unavailable for all considered locations during the analyzed period.The 2011-2014 interval is missing for Belcesti.Water 2024, 16, x FOR PEER REVIEW 14 of 24

Figure 8 .
Figure 8.Average annual variation in total nitrogen.

Figure 8 .
Figure 8.Average annual variation in total nitrogen.

Figure 9 .
Figure 9. Average annual variation in total phosphorus.

Figure 11 .
Figure 11.The variations in discharge, total nitrogen, and total phosphorus at Belcesti for 2011-2022.Figure 11.The variations in discharge, total nitrogen, and total phosphorus at Belcesti for 2011-2022.

Figure 13 .
Figure 13.The variations in discharge, total nitrogen, and total phosphorus at Holboca for 2011-2022.

Figure 13 .
Figure 13.The variations in discharge, total nitrogen, and total phosphorus at Holboca for 2011-2022.Figure 13.The variations in discharge, total nitrogen, and total phosphorus at Holboca for 2011-2022.

Figure 13 .
Figure 13.The variations in discharge, total nitrogen, and total phosphorus at Holboca for 2011-2022.Figure 13.The variations in discharge, total nitrogen, and total phosphorus at Holboca for 2011-2022.

Table 1 .
The main tributaries of the Bahlui River.

Table 2 .
The most important documented floods and drought periods on the Bahlui River.

Table 3 .
The reservoirs in the Bahlui River system.

Table 4 .
Statistical indicators for the river discharge (m 3 /s) for the 2005-2022 period.

Table 5 .
Statistical indicators for the river discharge during selected drought periods (m 3 /s).

Table 7 .
Statistical indicators for some water quality indicators analyzed in the 2011-2022 period.

Table 7 .
Statistical indicators for some water quality indicators analyzed in the 2011-2022 period.