Presentation Information
[AOS26-P07]Assessing Primary Production in a Fluctuating Coastal Environment: A Six-Year Winter Survey at the Sagami River Estuary
*Koga Nishijo1, Kyouko Kawanobe1, Yoshihiro Suzuki2 (1.Graduate School of Science, Kanagawa University, 2.Department of Science, Kanagawa University)
Keywords:
biomass,Sagami River estuary,species diversity
Introduction
Sagami Bay features a complex physical environment formed by the interaction of the warm Kuroshio Current and riverine discharge. The Hiratsuka area, at the Sagami River mouth, features the steep Hiratsuka Canyon and exhibits significant variability due to topographical and hydraulic factors, complicating ecosystem understanding. Yet, high productivity here supports regional fisheries and tourism. Thus, understanding primary production structure is essential for sustainable resource use. We focused on the winter season, when water mass structure is relatively stable, to minimize short-term fluctuations and clarify baseline characteristics. From 2021 to 2026, we conducted detailed oceanographic and phytoplankton surveys during mid-winter (December–January). We analyze the impact of winter physicochemical factors on phytoplankton communities and discuss the ecological characteristics of this suburban estuarine area.
Methods
From 2021 to 2026, we surveyed thirteen points along a north-south transect extending from the Sagami River estuary (0 m) to 5,000 m offshore. At each station, a water quality profiler (AAQ126, JFE Advantec) measured physical factors (temperature, salinity, light) and vertical chlorophyll fluorescence profiles. Fluorescence was calibrated to absolute values (μg/L) using spectrophotometric Chla data (TD-700, Turner Designs) from paired samples. For community analysis, surface water samples were collected at representative points (estuary, 1,000 m, and 5,000 m offshore) and analyzed using an optical microscope. Diatoms, morphologically distinct, were analyzed separately. Cryptophyta and dinoflagellates were grouped as "flagellates."
Results and Discussion
1. Changes in Oceanographic Conditions Salinity and density trends were consistent over the five years. A low-salinity surface plume consistently extended to ~1,000 m offshore. Meanwhile, winter vertical mixing maintained a uniform structure down to 50 m deeper offshore. Stratification was pronounced in 2025 and 2026. Unlike stable salinity, water temperature fluctuated dramatically. Temperatures remained between 16.5–17.5°C (2021–2023) but exceeded 18.0°C abruptly in 2024. In 2025, temperatures dropped sharply to ~16.0°C. Low temperatures persisted in 2026, except in the deep offshore zone. These results suggest the thermal environment is influenced by Kuroshio trends and global warming.
2. Phytoplankton Biomass and Community Response Spatially, high local Chl-a (>2.0 µg/L) consistently appeared near the seabed ~1,000 m offshore, suggesting physically driven accumulation. Conversely, interannual variability was high. Biomass declined through 2023, remaining low during the 2024 Kuroshio-driven warm anomaly. However, following the sharp temperature drop in 2025, 2026 saw Chl-a concentrations increase, with cell numbers reaching 5.39 million cells/L—a ~13-fold year-over-year increase. Species composition analysis revealed this surge was due to a diatom bloom.
Summary
Long-term monitoring revealed a dual winter environment: stable salinity stratification versus fluctuating temperatures, likely influenced by the Kuroshio and global warming. Significant diatom growth during low-temperature years suggests possible changes driven by temperature drastically alter primary production. Thus, the ecosystem in this area is highly responsive to physical shifts.
Sagami Bay features a complex physical environment formed by the interaction of the warm Kuroshio Current and riverine discharge. The Hiratsuka area, at the Sagami River mouth, features the steep Hiratsuka Canyon and exhibits significant variability due to topographical and hydraulic factors, complicating ecosystem understanding. Yet, high productivity here supports regional fisheries and tourism. Thus, understanding primary production structure is essential for sustainable resource use. We focused on the winter season, when water mass structure is relatively stable, to minimize short-term fluctuations and clarify baseline characteristics. From 2021 to 2026, we conducted detailed oceanographic and phytoplankton surveys during mid-winter (December–January). We analyze the impact of winter physicochemical factors on phytoplankton communities and discuss the ecological characteristics of this suburban estuarine area.
Methods
From 2021 to 2026, we surveyed thirteen points along a north-south transect extending from the Sagami River estuary (0 m) to 5,000 m offshore. At each station, a water quality profiler (AAQ126, JFE Advantec) measured physical factors (temperature, salinity, light) and vertical chlorophyll fluorescence profiles. Fluorescence was calibrated to absolute values (μg/L) using spectrophotometric Chla data (TD-700, Turner Designs) from paired samples. For community analysis, surface water samples were collected at representative points (estuary, 1,000 m, and 5,000 m offshore) and analyzed using an optical microscope. Diatoms, morphologically distinct, were analyzed separately. Cryptophyta and dinoflagellates were grouped as "flagellates."
Results and Discussion
1. Changes in Oceanographic Conditions Salinity and density trends were consistent over the five years. A low-salinity surface plume consistently extended to ~1,000 m offshore. Meanwhile, winter vertical mixing maintained a uniform structure down to 50 m deeper offshore. Stratification was pronounced in 2025 and 2026. Unlike stable salinity, water temperature fluctuated dramatically. Temperatures remained between 16.5–17.5°C (2021–2023) but exceeded 18.0°C abruptly in 2024. In 2025, temperatures dropped sharply to ~16.0°C. Low temperatures persisted in 2026, except in the deep offshore zone. These results suggest the thermal environment is influenced by Kuroshio trends and global warming.
2. Phytoplankton Biomass and Community Response Spatially, high local Chl-a (>2.0 µg/L) consistently appeared near the seabed ~1,000 m offshore, suggesting physically driven accumulation. Conversely, interannual variability was high. Biomass declined through 2023, remaining low during the 2024 Kuroshio-driven warm anomaly. However, following the sharp temperature drop in 2025, 2026 saw Chl-a concentrations increase, with cell numbers reaching 5.39 million cells/L—a ~13-fold year-over-year increase. Species composition analysis revealed this surge was due to a diatom bloom.
Summary
Long-term monitoring revealed a dual winter environment: stable salinity stratification versus fluctuating temperatures, likely influenced by the Kuroshio and global warming. Significant diatom growth during low-temperature years suggests possible changes driven by temperature drastically alter primary production. Thus, the ecosystem in this area is highly responsive to physical shifts.