Nutrient and Phytoplankton Dynamics in Harima-Nada, Eastern Seto Inland Sea, Japan During a 35-Year Period from 1973 to 2007

Abstract

Long-term monitoring of water quality and phytoplankton was conducted at 19 sampling stations in Harima-Nada, eastern Seto Inland Sea, Japan for 35 years from 1973 to 2007. There were two significant long-term changes, an increase in winter water temperatures of 0.042°C year −1 , and a decrease in dissolved inorganic nitrogen (DIN) from about 10 μM in the 1970s to~5 μM in the late 1990s due to the reduction in nutrient inputs. DIN concentrations and total phytoplankton cell density were both higher during the 1970s to the early 1980s and then exhibited a significant decrease in the mid 1980s and remained relatively constant thereafter. Diatoms were the dominant phytoplankton group (>90%) over the 35-year period, and there was a dramatic shift from Skeletonema dominance (~70%) to Chaetoceros in the mid 1980s. This shift in diatom species may be attributed to differences in the life cycle of Skeletonema and Chaetoceros and the response to the decrease in DIN concentration.

Long time-series observations in population dynamics of the harmful diatom Eucampia zodiacus and environmental factors in Harima-Nada, eastern Seto Inland Sea, Japan during 1974^|^ndash;2008

“…Nishikawa et al (2010) reported that there were two significant long-term changes, i.e. an increase in water temperature of 0.042°C year…”

“…In Harima-Nada, the Fisheries Technology Institute, Hyogo Prefectural Technology Center for Agriculture, Forestry and Fisheries, initiated measuring of water quality parameters in April 1973, along with the identification and cell counts of phytoplankton to monitor the fishing grounds (Manabe et al 1994, Nishikawa et al 2010. The monitoring has been carried out using similar methods until now, therefore, the data sets allow evaluation of eutrophication and recovery processes in Harima-Nada (Nishikawa et al 2010).…”

“…The monitoring has been carried out using similar methods until now, therefore, the data sets allow evaluation of eutrophication and recovery processes in Harima-Nada (Nishikawa et al 2010). Using the data, Nakamura et al (1989) identified the dominant phytoplankton species in Harima-Nada, and elucidated the pattern of the annual and seasonal changes from 1973 to 1988.…”

“…Using the data, Nakamura et al (1989) identified the dominant phytoplankton species in Harima-Nada, and elucidated the pattern of the annual and seasonal changes from 1973 to 1988. In addition, Manabe et al (1994) and Nishikawa et al (2010) examined the relationship between water quality parameters and phytoplankton using the 20-year (from 1973 to 1993) and 35-year (from 1973 to 2007) environmental monitoring data from Harima-Nada, respectively.…”

“…On the basis of the relationship between growth characteristics and seasonal fluctuation of the environmental factors in Harima-Nada, optimal conditions for the growth of E. zodiacus appear in the autumn season (Nishikawa & Yamaguchi 2006, Nishikawa et al 2010), but its blooms have never occurred in autumn (Fig. 2, ).…”