Historical changes in primary production in the Seto Inland Sea, Japan, after implementing regulations to control the pollutant loads

Abstract

A total pollutant load control system (TPLCS) was implemented in the Seto Inland Sea in 1979 to reduce the water pollution and the frequency of red tides. We estimated primary production from 1981 to 2010 to determine the effects of reducing the nutrient loadings from the surrounding land. While primary production has decreased overall in the Seto Inland Sea in response to the TPLCS and the associated reductions in the total nitrogen (T-N) and phosphorus (T-P) loads from land since 1981, the reductions were limited to 4 of its 11 subareas. Primary production has increased in the Harima Nada but has been stable in the Bingo Nada subarea, reflecting the fact that the T-N and T-P stocks have not decreased in these subareas over the study years. The inconsistent responses of the 11 subareas suggest that the characteristics of each subarea should be considered when environmental management measures are established and implemented in the Seto Inland Sea. The controls on the nutrient loadings according to the TPLCS should be modified to permit better management of this semi-enclosed sea.

Long-term trends in pH in Japanese coastal waters

“…Previous studies have reported that nutrient loadings in Japan have decreased over recent decades (e.g., Yamamoto-Kawai et al 2015;Kamohara et al 2018;Nakai et al 2018), with variable effects on summer pHinsitu in coastal waters. TN was monitored for a shorter period than pHinsitu (1995 to 2009).…”

“…8). The pH trends in coastal areas of western Kyushu, where the anthropogenic nutrient loadings are relatively low, therefore reflect the decreases in nutrient discharges, resulting in variations between regions (e.g., Nakai et al 2018;Yamamoto and Hanazato 2015;Tsuchiya et al, 2018). Several cities in this area have introduced advanced sewage treatment to prevent eutrophication in coastal waters (Nakai et al 2018;Yamamoto and Hanazato 2015).…”

Management of the west-central Seto Inland Sea, Japan: factors controlling the spatiotemporal distributions of chlorophyll a concentration and Secchi depth

“…type, magnitude, frequency and timing), connectivity with adjacent systems and differing water quality parameters (Carstensen et al, 2011;Duarte et al, 2015). In successful cases, the improvement appeared in nutrients, Chl.a, dissolved oxygen concentrations and seagrass cover after implementation of the programme (Greening et al, 2014;Riemann et al, 2016;Nakai et al, 2018;Nishijima et al, 2018), whereas little improvement or even worsening were unfortunately reported too (Williams et al, 2010;Riemann et al, 2016).…”

“…On the other hand, stable and appropriate primary production by phytoplankton, as well as by seagrasses and benthic microalgae, is essential to sustain the healthy functioning of ecosystems and the sustainable supply of fishery resources (Takai et al, 2002;Hoshika et al, 2006;Nakai et al, 2018). Phytoplankton growth will directly respond to nutrient supply, whereas the growth and distribution of benthic macro-and microalgae will be determined by both nutrient supply and light availability; the latter will also be affected by nutrient supply through phytoplankton growth.…”

“…One effect of TPLCS implementation on the Seto Inland Sea has appeared in certain ecosystem components (Yamamoto, 2003;Nakai et al, 2018;Nishijima et al, 2018), although it varies in the subareas. The west-central Seto Inland Sea, including Hiroshima Bay and Aki Nada (Figure 1), receives substantial anthropogenic nutrient loading from its watersheds.…”