Control of Lake Biwa eutrophication

The first serious sign of a deterioration in Lake Biwa water quality came suddenly, in the form of a large-scale red tide along the eastern coastline of the northern basin in early 1977. The sighting of this phytoplankton, Uroglena Americana, was quite a shock as the northern basin had been, up till then, believed to be nearly pristine. The red tide has since been sighted almost every year. Prompted by the red-tide incident, the Shiga Prefecture enacted the Eutrophication Control Ordinance of 1980 to ban the use and sale of phosphorus-containing synthetic detergents. A wide array of control measures has been introduced over the decades for improving Lake Biwa water quality. Already by the mid-1970s it was becoming difficult for large-scale industries to get away with discharging polluting wastewater into watercourses leading to the lake, thanks to the Water Pollution Control Law of 1970, which had stringent punitive provisions. With the enactment of the Eutrophication Control Act of 1980, the regulatory provisions for industries became even more precise and stringent, with additional controls on nutrient discharges. Wastewater from smaller-scale industries has been progressively brought under control, although very small industries (whose discharges are insignificant in comparison with total industrial discharges) are yet to be regulated fully. Despite all the environmental programmes, typical water quality indices, such as COD, total phosphorus (TP), and total nitrogen (TN), reveal that the improvement in lake water quality has not been very impressive; indeed, figure 3.10 shows a worsening trend in COD in recent years.

What about trends in water quality over the much longer term? Water transparency at specified sites and bottom dissolved oxygen in the northern lake are two indicators of longer-term trends that happen to be available. They show that Lake Biwa water quality had slowly been deteriorating even before the red-tide incident of 1977. In addition, there are other more subtle indicators of what is taking place in the lake ecosystem. For example, figure 3.11 shows that there is a worrying change in the dominant species of nuisance-causing phytoplankton in the lake. Putting all the indicators together, it can be noted that the deteriorating trend was accelerating up until the mid-1980s and, although the rate has since slowed down, the trend itself has not been reversed yet.

Fig. 3.10 Trends in Lake Biwa water quality: TP and COD, 1979-1996 (Source: Shiga-ken, 1997)

Are there good prospects of reducing polluting inputs into the lake? The regulation of polluting industries and the provision of sewerage to households and commercial establishments are the two main point source management strategies. Time (how soon?) and the mobilization of financial resources (how much?) are the basic considerations affecting progress now that the special budgetary provisions for environmental projects in the LBCDP have been terminated.⁶

Concern about the control of waste load discharge has been gradually shifting in the past decade or so from point sources of pollution to non-point sources of pollution. The four major sources of non-point pollution are: (1) rainwater, (2) forest and field runoffs, (3) paddy runoff, and (4) urban runoff. Not only will the development of a comprehensive non-point source control system be expensive, but it will also require legal and institutional measures yet to be elaborated. This is particularly so with wet weather non-point sources of pollution, or stormwater runoff into the lake. Moreover, financial resources are likely to be tied up in point source control for some time.

Elaboration of control programmes for non-point pollution from both urban and agricultural sources has just begun. Paddy field runoff under dry weather conditions may be managed very effectively by a combination of structural and non-structural means. The current trend in policy is to regard individual paddy fields as point sources rather than non-point sources under dry weather conditions. Proper management of irrigation water and a reduction in the wasteful use of fertilizers and pesticides are the keys to successful control of dry weather runoffs. The promotion of agricultural best management practice and technological developments for the more efficient use of resources will also be necessary.

Fig. 3.11 Trends in algal bloom incidents in Lake Biwa, 1977-1996 (Source: Shiga-ken, 1997)