Seasonal characteristics of vertical profiles of water quality

Figure 4.1 shows the time-series data of daily rainfall and measured values of transpar-ency from April to December in 2015 and 2016. The observations shown in Fig. 4.2 and Fig.

4.3 summarize the seasonal changes in the vertical profile of water quality parameters including light intensity, water temperature, DO, ORP, NH4-N, NO3-N, PO4-P, and sulfide in both years.

Here, the light intensity at each measuring point denotes the value normalized by the result measured at the water surface. The water quality parameters are summarized in Figs. 4.1, 4.2, and 4.3.

The average transparency was generally low (about 2 m) in 2015 and 2016, but in 2015,

Fig. 4.1 Seasonal change in daily rainfall and transparency in 2015 and 2016.

150 100 50 0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 150

100 50 0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 Daily rainfall (mm)TransparencyDs(m)

2015 2016

5/1 6/1 7/1 8/1 9/1 10/1 11/1 12/1 4/1 5/1 6/1 7/1 8/1 9/1 10/1 11/1 12/1 4/1

5/1 6/1 7/1 8/1 9/1 10/1 11/1 12/1 4/1 5/1 6/1 7/1 8/1 9/1 10/1 11/1 12/1 4/1

Date Date

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there was a high frequency of daily rainfall exceeding 30 mm/d from April to June; thus, trans-parency was even lower (about 1 m) due to the increased inflow of turbid substances. As a result of daily rainfall exceeding 50 mm/d during the rainy season starting in June, a scarce underwa-ter light environment with about 1 m transparency persisted until September. The inflow of turbid water due to heavy rains had little impact after October, improving the underwater light environment with transparency values reaching 3 to 5 m. In 2016, transparency reached 3 to 5 m until mid-June; however, muddiness of the reservoir became pronounced due to daily rainfall exceeding 50 mm/d from late June to mid-July and from late August to early October, reducing the transparency to less than 2 m. Due to the continued effects of the inflow of turbid water, scarce underwater light environment conditions lasted longer in 2016 than in 2015. In both years, from spring to summer, the extinction of underwater light in the vertical direction was large and the water depth, at which the light intensity turned to zero, became shallower than 3 m. In 2015, the effect of heavy rains was small starting from October; thus, the attenuation of underwater light intensity in the vertical direction was limited such that the light intensity turned to zero at a water depth of 4 m. In 2016, as a result of the long-term effects of turbid water in late August, the water depth, at which light intensity turned to zero, was shallow at about 2 m and the large attenuation of underwater light intensity lasted until December.

The vertical profile of water temperature in 2015 and 2016 indicated similar seasonal changes. In April, the water temperature difference between the water surface and the bottom bed was large, approximately 10°C, and thermal stratification formed with a structure consist-ing of an epilimnion, thermocline, and hypolimnion. From April to August, the water tempera-ture gradient increased in the thermocline located at 2 to 4 m; thus, the development of a firm thermal stratification could be confirmed. In September, radiative cooling at the water surface was significant and a mixing layer of constant water temperature attributed to the settling of cold water mass was formed. The mixing layer reached a depth of 4.5 m in October and the two-layered stratification with a clear thermocline could not be confirmed in mid-November.

In early December, thermal stratification was disrupted by vertical circulation reaching all water depth zones and the water temperature showed uniform distribution. Considering the observa-tion that the destratificaobserva-tion occurred in mid-November so far, the duraobserva-tion of stratificaobserva-tion in

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Fig. 4.2 Seasonal data for water quality in 2015.

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0 0 200 400 600 800 Normalized underwater light quantum

Water depth (m)

2015/04/15 2015/06/17 2015/08/07 2015/10/07 2015/11/25

Water depth (m)

2015/12/09 Water temperature ( ^oC) DO (mg/L)

NH₄-N (mg/L) NO₃-N (mg/L)

PO₄-P (mg/L) Sulfide (μg/L) ORP (mV)

Water depth (m) Water depth (m)

Water depth (m) Water depth (m)

Water depth (m) Water depth (m)

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Fig. 4.3 Seasonal data for water quality in 2016.

8 7 6 5 4 3 2 1

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8 7 6 5 4 3 2 1

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8 7 6 5 4 3 2 1

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Normalized underwater light quantum

Water depth (m) Water depth (m)

Water temperature ( ^oC) DO (mg/L)

NH₄-N (mg/L) NO₃-N (mg/L)

PO₄-P (mg/L) Sulfide (μg/L) ORP (mV)

Water depth (m) Water depth (m)Water depth (m)

Water depth (m)Water depth (m) Water depth (m)

2016/04/20 2016/06/15 2016/08/22 2016/10/07 2016/11/25 2016/12/09

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2015 and 2016 was long compared to the average years. Therefore, the effects of fall warming might weaken the thermal convection caused by water surface cooling that inhibits the devel-opment of a mixing layer from September to November.

Seasonal changes in the vertical profile of DO in 2015 and 2016 revealed differences in the spring season. In 2015, there was no development of anoxia or hypoxia in April, even at the bottom bed, but in 2016, hypoxia was observed in mid-April at depth ≥ 7 m. Starting from June in both years, the hypolimnion was in a hypoxic state at depth ≥ 5 m and the DO showed a distribution similar to the vertical profile of water temperature. That is, the DO profile had a three-layer structure with a top layer in a largely saturated state, a rapidly decreasing quick change layer, and a hypoxic bottom layer. In particular, in August, hypoxia developed at depth ≥ 2.5 m when strong stratification formed with the large water temperature difference of approximately 20°C between the water surface and the bottom bed. This vertical profile of DO may be con-sidered an important feature of the water environment in the targeted area. Owing to the devel-opment of a mixing layer in the fall season, water depth zones with a sufficient DO concentra-tion enlarged to depth ≤ 4 m in October and depth ≤ 6 m in November. Thus, the DO environ-ment improved across all zones in the vertical direction, starting with destratification, which occurred by the development of vertical circulation in December. In both 2015 and 2016, the disappearance of hypoxia due to destratification occurred in early December, approximately one month later compared to the years (from 2010 to 2014(unpublished data)), but the anoxia of hypolimnion could be observed in April 2016 and May 2015. Therefore, the anaerobic state near the bottom sediment in 2016 lasted approximately one month longer than in 2015. This difference is important when considering the water quality dynamics under anoxic states.

Seasonal fluctuations in the vertical profile of ORP both in 2015 and 2016 could be found in the hypolimnion at depths of 5 m or more, where anoxia occurred. First, a reductive state of ORP<0 with the same timing as DO=0 was confirmed at the bottom. In addition, ORP rapidly dropped to approximately 150 mV and this strong reductive state was maintained until anoxia disappeared. In the hypolimnion, except for the neighborhood of the bottom, there was a time lag of one month or longer between the timings of DO=0 and ORP<0. For example, at a water depth of 7 m in 2015, anoxia was confirmed in mid-June, but ORP was negative in August.

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In 2016, an anoxic state was reached in August at depth ≥ 3 m, but the depths ranging from 3 to 5 m remained in an oxidative state of ORP>0 through the observation period. Moreover, ORP was negative across the widest range, which was deeper than 5 m in October 2016. The point, at which a strong reductive state of ORP= 200 to 100 mV was maintained at depth ≥ 6 m until late November in both 2015 and 2016, may be considered a feature of the water environment in the tar-geted area.

NH4-N, PO4-P, and sulfide showed uniform concentration distributions and no seasonal changes at ≤ 6 m. Furthermore, these inorganic ions showed seasonal increases at a water depth of 7 m and near the bottom. In particular, the vertical profile was determined with a maximum concentration at the bottom, which may be an important feature of the water environment, sug-gesting that NH4-N, PO4-P, and sulfide increased as a result of the interaction between the water quality and the sediment with long-term anoxia and strong reduction caused by anaerobic bio-chemical reactions. Therefore, NH4-N and PO4-P would be eluted from the sediment as a result of organic matter decomposition by anaerobic microorganisms at the bottom sediment and sul-fide was generated by the reduction of sulfate ions in the water above the bottom. Furthermore, the increase in the concentrations of NH4-N, PO4-P, and sulfide right above the bottom could result in an increase in their concentrations at a water depth of 7 m through substance transpor-tation upward by molecular diffusion. NO3-N remained at a very low concentration during the summer regardless of water depth after gradually decreasing over time from the spring to the summer. Considering this change from the perspective of the nitrogen cycle in water bodies, nutrient intake due to algal activity (photosynthesis) and reduction of nitrate ions by denitrifiers (denitrification) can be regarded as biochemical processes related to decreased NO3-N. The former depends on sufficient underwater light, while the latter occurs under anaerobic condi-tions. Consequently, reduced NO3-N from spring to summer in all water depth zones is at-tributed to different biochemical processes depending upon the water depth and NO3-N de-creased with photosynthesis in the epilimnion and with denitrification in the hypolimnion. The temporal changes in inorganic nitrogen in the epilimnion show that NO3-N levels had decreased since spring, while the variations in NH4-N were not significant during the observation period.

The phytoplankton would prefer NO3-N over NH4-N in the intake of inorganic nitrogen for

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Above the bottom layer in the targeted area, long-term anoxia occurred owing to thermal strat-ification from spring to fall; thus, the sediment was in a strong reductive state, represented by a gray color. However, in the radiant heat period following the fall season, anoxia generally disappears because of the development of vertical mixing of the water mass across all water depth zones resulting in a recovery from the anaerobic state to the aerobic state even near the bottom. As a result, a dark reddish-brown oxidation layer formed at the surface of the sediment in the spring of 2015; such sediment was confirmed in the period prior to late April (Fig. 4.4a).

However, in 2016, the sediment was in a reductive state presenting a gray color at the start of April and it was not possible to confirm the evidence of an oxidation layer in the sediment (Fig.