Major Nutrients and Their Stoichiometry over 45 Years (May 1963-May 2008) at the Northern and Southern Basin of Lake Biwa, Japan

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2.1. Total Phosphorus, TP

A fixed amount of unfiltered sample (200 mL) was taken into a beaker and dried on a hotplate to dryness, followed by both incineration with 2 mL of 60% HClO₄ and quantization of phosphorus content according either to hetropoly blue spectrophotometric method at 700 nm ^[11] from 1963 to 1980 observation and to the indirect spectrophotometry at 545 nm ^[12] from 1981 to 2008 observations.

2.2. Total Nitrogen, TN

A fixed amount (100 mL) of unfiltered sample was subjected to total Kjeldahl digestion and the resulting ammonia-N was analyzed spectrophotometrically at 660 nm according to the indophenol method ^[13]. For NO₃-N 100 mL of unfiltered sample was dried in a beaker and nitrate in the residue was quantitized spectrophotometrically at 410 nm by using phenoldisulphonic acid ^[14]. TN defined as total Kjeldahl nitrogen plus NO₃-N.

3. Results and Discussion

3.1. Vertical Distribution of Total Phosphorus at Ie-1

The long term variation of monitored total phosphorus concentrations are shown in Figure 2. It is indicated from the available data that, in the early stage the total phosphorus concentrations were low at the bottom, but increased after some time due to rapid industrial growth. The loading of total phosphorus into increased from 1960 to 1975 and then decreased, reflecting economic growth and regulation of total phosphorus ^[15]. The phosphorus concentrations reached a maximum in 1976 and then declined until 1982, and fluctuated around a stable value thereafter. A water treatment regulation was enforced in 1982, and nutrient loading was progressively reduced and then stabilized after 1985 in ^[16]. In , there was an unusually cool and rainy summer in 1993 while an unusually hot and dry summer was recorded in 1994. In 1993, abundant rain and weak thermal stratification supported high nutrient concentrations in the epilimnion. In 1994, nutrients were depleted and a high transparency and low concentration of chlorophyll a were observed in the epilimnion of the . This is because the supply of nutrients was limited owing to little rain and a well-developed thermocline ^[17]. Over the whole stage, the TP concentration at Ie-1 has been increasing at a rate of 3.0 × 10⁴μM per month; in other words, the amount of phosphorus contained in the water phase of the entire lake (2.75 × 10¹³ dm³), has increased by 3.0 ton yr^-1. The total phosphorus concentrations were high in 2000, 2003 and 2006 because of high dissolved organic phosphorus and particulate phosphorus concentrations, respectively ^[18]. In the surface waters, phosphorus was essentially present in particulate phosphorus form, while bottom waters contained an important fraction of the element in dissolved organic phosphorus and orthophosphate form. The recent increase of total phosphorus at Ie-1 can better be attributed to supply from coastal regions where phosphorus compounds accumulated, and also from the Southern Basin ^[10].

Figure 2. Mean annual vertical distribution of Total phosphorus (ppb) at Ie-1 (0~70 m depths)

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3.2. TP Concentration at Nb-5

Since the water depth at Nb-5 is around 3.5 m and stratification hardly occurs throughout the year, the concentration profile of TP along the depth is mostly uniform and, in consequence, the concentration at the surface may well represent the average concentration of TP to the full length of Nb-5. The high TP concentration in 1963-1982 can be attributed firstly to higher anthropogenic activity in the surrounding urbanized area, secondly to the geological nature of the Southern Basin having a shallower and more gradually shaped bottom ^{[3, 19]}, and, thirdly to high population level of seston ^[20]. Recently, waters of the Southern Basin showed a greater and more rapid reduction in total phosphorus concentration as compared to the due to improvement in the water quality of tributary streams entering the Southern Basin. The quality was much better at the beginning of the 1990s, compared with the 1980s. In fact, the mean concentration of total phosphorus contained in the river water flowing into the Southern Basin during 1978-1999 was decrease from 455 to 93 mg/L ^[21], whereas, total phosphorus concentration in the river water flowing to the Northern Basin during 1996-2007 was increased or stable ^[4]. According to monitored phosphorus records over the whole stage, the mean value was 19.6 (min. 8; max. 37), whereas, the average total phosphorus concentration from 1963 to 1985 and 1996 to 2008 were 21.3 (min. 8; max. 43) and 20.5 (min. 10; max. 37), respectively (see Figure 3).

Figure 3. Mean annual variations in Total Phosphorus concentrations at Nb-5

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3.3. Vertical Distribution of Total Nitrogen at Ie-1

Since had an apparently oligotrophic nature in 1963 and exhibited a truly eutrophic nature in 1973, the variation depicted in Figure 4 represents the eutrophication processes in terms of total nitrogen. It is indicated from the available data that, at the bottom (60~70 m depths) in the early stage the total nitrogen concentrations were generally lower mainly due to low nitrate nitrogen. The total nitrogen concentrations were high during 1975 to 1980 due to rapid industrial growth, whereas, after 1981 deceased rapidly due to regulation of law. Most of the nitrogen load during 1976 to 1985 decreased from the non-point sources as the Japanese government tried many ways to control the non-point sources pollution ^[22]. The high TN concentrations were observed in 2000 and 2006 due to NO₃-N and particulate organic nitrogen, respectively ^[18]. The Basic Law for Environmental Pollution Control (1967), Water Pollution Control Law (1970), Pollution Prevention Ordinance (1973) and Shiga Prefecture Basic Environment Ordinance (1996) were implemented to control the increased TN concentrations in the . The submerged macrophyte growth also increased by 15% in the in 2002, compared to 1997 ^[23]. However, in spite of these various countermeasures, the TN concentrations in the have not improved due to loads from nonpoint sources are unexpectedly greater than those from point sources such as domestic wastewater. Total nitrogen concentration has increased in the Northern Basin due to increased NO₃-N concentration during the past 40 years ^[24].

Figure 4. Mean annual vertical distribution of Total Nitrogen (ppb) at Ie-1 (0~70 m depths)

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3.4. TN Concentration at Nb-5

The total nitrogen concentrations were high from 1963 to 1982, likely due to increased anthropogenic activities in the surrounding area. In other words, the Southern Basin was strongly influenced by the input of nitrogen compound over the period. Whereas, in the recent stage low TN concentration at Nb-5 suggests that the TN concentration decreases much faster due to development of macrophyte and improvement of the water quality of tributary streams entering the Southern Basin. The water quality was much more improved in the Southern Basin after 1994 due to development of macrophytes ^[25]. The area covered by submerged macrophytes reached 80% of the entire area of the south basin in 2006 ^[26]. Moss (2001) argued that macrophytes are particularly sensitive to TN loading and will show low species diversity at high TN and high diversity at low TN ^[27]. The dominance of submerged macrophytes usually influences cyanobacteria negatively, and release of allelochemicals by macrophytes is regarded as a major inhibitor of their growth ^[28]. Faafeng and Mjelde (1998) postulated the following mechanism by which macrophytes may maintain a clear-water state; competition with phytoplankton for nutrient, competition with epiphyte algae for light, and the release of allelopathic substances ^[29]. The aquatic plants that form dense vegetation, e.g. reed colonies in , make a great contribution to nutrient binding in the shore zone. In view of these relationships, the recovery of macrophytes presumably played an important role in recent water quality improvement ^[30].

The TN concentration from nonpoint sources from forest, paddy and farm land is decreasing from 1971 to 2006 due to increasing urbanization of Lake Biwa basin from 12.5% to 21.9%, while forest and agricultural land area decreased from 51.2% to 45.2% and 27.6% to 21.8%, respectively ^[31]. Another important factor was the improvement in the water quality of the inflow river water at the beginning of the 1990s compared with the 1980s. At Southern Basin, TN concentration from inflow rivers decreased from 2.80 mg/L in 1980 to 1.17 mg/L in 2007 ^[4]. According to monitored nitrogen records over the whole stage mean value is 364 (min.66; max.784), whereas, the average total nitrogen concentration from 1963 to 1985 and 1996 to 2008 were 402 (min.66; max. 684) and 359 (min.172; max. 784), respectively (Figure 5). Recently, the decline in total nitrogen concentration was principally the result of a marked decline in total organic nitrogen, but not of NO₃-N at the Southern Basin ^[8].

Figure 5. Mean annual variations in Total Nitrogen concentrations at Nb-5

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In the first stage, TN concentration at Nb-5 is 2.6 times higher than that of Ie-1, in other words, the Southern Basin was strongly influenced by the input of nitrogen compounds over the period. In the second stage, however, decrease in the nitrogen concentration at Nb-5 than at Ie-1 suggests that the water quality regarding TN is improving faster than in the Northern Basin ^[18].

3.5. Variations in TN/TP Ratio at Ie-1 (0 m) and Nb-5

Although the absolute value of TN may not be a strong predictor of N limitation, the TN/TP ratio may be useful in determining the potentially limiting nutrient. Changes of TN/TP stoichiometry indicate altered patterns of phytoplankton nutrient limitation ^[32]. The most widely accepted hypothesis about cyanobacterial dominance in low TN/TP ratio was first proposed by Pearsall ^[33], and later elaborated by Schindler ^[34] and Smith ^[35]. This hypothesis maintains that cyanobacteria are masters of survival in environments poor in N and that their dominance in eutrophic systems is related to the low TN/TP ratios of major anthropogenic pollutants.

Tilman (1982) suggested that cyanobacteria should typically be dominant in the lake with a low TN/TP ratio and strong competitors under condition of nitrogen limitation because they can fix new nitrogen from N2, a gaseous source of inorganic nitrogen that is not available for other phytoplankton ^[36]. Smith compiled extensive data sets for several lakes in which cyanobacteria dominance and TN/TP ratios varied, and showed that cyanobacteria dominance became low when the TN/TP ratios were greater than 65 (by atoms). However, when the ratio was less than 65, the potential for cyanobacteria dominance increased dramatically, and in the special case where lakes showed smaller ratios than 25 the dominance of cyanobacteria became mostly 100% ^[35]. The phytoplankton growth is limited by phosphorus but not inorganic nitrogen in the south basin of ^[37]. Freshwater red tide caused by Uroglena americana propagated suddenly in 1977 and changed the surface water to dark brown color and it has occurred every year from 1977 to 1986 (especially frequently and widespread from 1977 to 1979) in ^[38].

Figure 6. (A) Variations of annual mean TN/TP ratio at Ie-1(0 m) and (B) that of Nb-5

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