10:45 AM - 12:15 PM
[ACG31-P08] Secular variation of carbon-nitrogen stable isotope ratios of zooplankton in Toyama Bay, central Sea of Japan.
Keywords:zooplankton, stable isotope, climate change, Toyama Bay, Japan Sea
The current marine environment is changing due to climate change, and concern about the impact on ecosystems. However, little is known about the relationship is limited, and clarifying is an urgent task. Zooplankton are primary consumers of food webs and indicator species for climate and environmental changes, and handling past time series data can provide insight into future changes.
The Japan Sea is reported to have a faster ocean circulation and higher sea surface temperatures than other regions (JMA), and is considered to reflect the effects of climate change more quickly. In addition, a decrease in land-derived nutrients has been reported due to a decrease in snowfall and other factors in Toyama Prefecture, located in the central Sea of Japan(Katazakai and Zhang, 2021; JMA). Therefore, Toyama Bay is model area for investigating the effects of recent climate change on marine ecosystems. In this study, we attempted to evaluate them using carbon and nitrogen stable isotope ratios (δ13C and δ15N) of zooplankton in the deep waters.
Samples were collected at a deep-sea water utilization facility (334 m depth, 100 µm) in Nyuzen Town, located near the Kurobe River in eastern Toyama Prefecture, and were classified by taxonomic group (small copepods, large copepods, chaetognaths, and amphipods). Surveys were conducted in 2010, 2014-2016, and 2019-2020. We evaluated long-term changes in δ13C and δ15N using a generalized additive model and investigated their relationship to environmental changes using open data.
The δ13C of zooplankton is consistent with that of phytoplankton (-24~-19‰, Peterson and Fry, 1987), suggesting that phytoplankton is the major nutrient source. The trend in stable isotope ratios over the 10-year period, there was no change in δ15N, moreover δ13C showed a significant decreasing. Phytoplankton is the major nutrient source for zooplankton, this change may be due to changes in primary producers and inorganic carbon sources utilized by them. The sources of inorganic carbon in Toyama Bay include the atmosphere and terrestrial water. First, we focused on the addition of 12C-rich carbon derived from fossil fuels (Suess effect). Assuming similar level of a decreasing trend of δ13C-DIC in the North Pacific (Quay et al., 2016) , 14~25% of the variability identified in this study was attributed to the Suess effect. Second, the long-term changes in δ13C in each season (spring, summer, fall, and winter), it is clear that the changes are larger in summer and smaller in winter. It is known that land-derived nutrients are an important source of nutrients from summer to autumn in the shallow waters of Toyama Bay below 100 m depth and contribute to phytoplankton and zooplankton growth (Guo et al., 2019). In addition, the river flow of the Kurobe River is large from spring to summer, suggesting that is strongly influenced by snow melt water. The large decreasing trend of 13C during the summer season is considered changes in terrestrial water and snow melt water. The average precipitation in winter did not change from year to year, the total snowfall showed a decreasing, suggesting a change from snowfall to rainfall. In groundwater and submarine ground water in the Katakai River fan in eastern Toyama Prefecture, it caused to decrease dissolved constituents, lower pH, and increase DIC flux (Katazakai and Zhang, 2021). If DIC is supplied in sufficient amounts compared to phytoplankton growth, δ13C will decrease due to preferential utilization of 12C, suggesting that the increase in terrestrial DIC supply may be one of the reasons for the decrease in δ13C observed in this study. These results suggest that the δ13C of zooplankton reflects changes in carbon cycle, and using high frequency zooplankton samples will be an important indicator for understanding the effects of climate change on marine ecosystems.
The Japan Sea is reported to have a faster ocean circulation and higher sea surface temperatures than other regions (JMA), and is considered to reflect the effects of climate change more quickly. In addition, a decrease in land-derived nutrients has been reported due to a decrease in snowfall and other factors in Toyama Prefecture, located in the central Sea of Japan(Katazakai and Zhang, 2021; JMA). Therefore, Toyama Bay is model area for investigating the effects of recent climate change on marine ecosystems. In this study, we attempted to evaluate them using carbon and nitrogen stable isotope ratios (δ13C and δ15N) of zooplankton in the deep waters.
Samples were collected at a deep-sea water utilization facility (334 m depth, 100 µm) in Nyuzen Town, located near the Kurobe River in eastern Toyama Prefecture, and were classified by taxonomic group (small copepods, large copepods, chaetognaths, and amphipods). Surveys were conducted in 2010, 2014-2016, and 2019-2020. We evaluated long-term changes in δ13C and δ15N using a generalized additive model and investigated their relationship to environmental changes using open data.
The δ13C of zooplankton is consistent with that of phytoplankton (-24~-19‰, Peterson and Fry, 1987), suggesting that phytoplankton is the major nutrient source. The trend in stable isotope ratios over the 10-year period, there was no change in δ15N, moreover δ13C showed a significant decreasing. Phytoplankton is the major nutrient source for zooplankton, this change may be due to changes in primary producers and inorganic carbon sources utilized by them. The sources of inorganic carbon in Toyama Bay include the atmosphere and terrestrial water. First, we focused on the addition of 12C-rich carbon derived from fossil fuels (Suess effect). Assuming similar level of a decreasing trend of δ13C-DIC in the North Pacific (Quay et al., 2016) , 14~25% of the variability identified in this study was attributed to the Suess effect. Second, the long-term changes in δ13C in each season (spring, summer, fall, and winter), it is clear that the changes are larger in summer and smaller in winter. It is known that land-derived nutrients are an important source of nutrients from summer to autumn in the shallow waters of Toyama Bay below 100 m depth and contribute to phytoplankton and zooplankton growth (Guo et al., 2019). In addition, the river flow of the Kurobe River is large from spring to summer, suggesting that is strongly influenced by snow melt water. The large decreasing trend of 13C during the summer season is considered changes in terrestrial water and snow melt water. The average precipitation in winter did not change from year to year, the total snowfall showed a decreasing, suggesting a change from snowfall to rainfall. In groundwater and submarine ground water in the Katakai River fan in eastern Toyama Prefecture, it caused to decrease dissolved constituents, lower pH, and increase DIC flux (Katazakai and Zhang, 2021). If DIC is supplied in sufficient amounts compared to phytoplankton growth, δ13C will decrease due to preferential utilization of 12C, suggesting that the increase in terrestrial DIC supply may be one of the reasons for the decrease in δ13C observed in this study. These results suggest that the δ13C of zooplankton reflects changes in carbon cycle, and using high frequency zooplankton samples will be an important indicator for understanding the effects of climate change on marine ecosystems.