Presentation Information
[ACG68-P07]Contribution of ice algae to spring phytoplankton blooms during the ice melt in Saroma-ko lagoon, Hokkaido, Japan.
*Kyouko Kawanobe1, Koetsu Matugasaki2, Koichi Sakaguchi2, Koga Nishijo3, Yoshihiro Suzuki3 (1.Graduate School of Science, Kanagawa University, 2.Saroma Lake Aquaculture Fisheries Cooperative Association, 3.Department of Science, Faculty of Science, Kanagawa University)
Keywords:
ice algae,first-year ice,ice melt,microalgal blooms,Saroma-ko lagoon
Introduction
As global warming diminishes multi-year Arctic sea ice and transforms it into thin first-year ice, microalgal communities (the primary producers) are facing profound impacts that could alter the entire food web. The Saroma-ko lagoon in Hokkaido, which is located at the southernmost edge of the seasonal sea ice zone, is an ideal place to study these 'first-year ice zones'. In winter, sufficient sunlight penetrates the thin ice, enabling ice algae to proliferate throughout the ice layer. This study quantitatively assesses the contribution of these algae as a 'seed source' for the large-scale blooms that occur after the ice breaks up, in order to examine their role in primary production.
Materials and Methods
Seawater samples were collected twice weekly from March 27 to April 22, 2024, at Sakaeura Harbor in Kitami City. CTD measurements were taken to record water temperature, salinity, chlorophyll a (Chl a), and dissolved oxygen. On April 15, at the peak of the bloom, samples were taken from three depths at five lake monitoring points. On 20 February, sea ice (thickness ca. 40 cm) was sampled in 10 cm layers and seawater in 1 m layers. Additionally, sediment traps were deployed 10 km offshore from early January to late April. Samples were fixed with a paraformaldehyde-glutaraldehyde mixture and analysed using the Utermohl method with an inverted microscope. Differences in species composition were analyzed using community similarity and multiple regression analysis.
Results and Discussion
Sea ice melt began in late March 2024. During the observation period, the surface temperature of the seawater increased from -0.3 to 11 deg C, while salinity rose from 1.6 to 30. In the maximum Chl a layer, Chl a increased from 2.7 to 25 ug L-1, while nutrient concentrations (NO3- and PO43-) decreased significantly. Total algal cell counts increased twentyfold, reaching 11.5 x 103 cells mL-1 at the peak bloom.
Dominant species shifted from ice algae (Detonula confervacea ), which accounted for 42-83% during the early thaw period, to planktonic algae (56-96%) during the bloom period, with the proportion of ice algae dropping below 8% (Figure 1). Early bloom species such as Thalassiosira nordenskioeldii were replaced by four other Chaetoceros species at the peak of the bloom. Similarity analysis identified four distinct community groups: pre-bloom ice algae; early cold-water plankton; bloom-forming species; and transitional flagellates. Multiple regression analysis revealed that water temperature and salinity are significant controlling factors (p< 0.01) in this succession of communities.
Geographical comparisons showed that while planktonic algae dominated ( >90%) across the lake at the peak, ice algae were significantly more prevalent (66-94%) in the early-freezing eastern shore area compared to the lake center (6-11% in sediment samples).
These results suggest that the spring bloom in Saroma-ko lagoon is predominantly caused by planktonic algae, with the decrease in ice algae being driven by changes in water temperature and salinity. Although ice algae contribute only minimally to the bloom, they are likely serve as a crucial early food source for benthic organisms, particularly in the well-developed ice of the eastern lagoon, which is a key hub for primary production in the early spring.
Figure caption
Figure 1. Changes in algal biomass and community structure within the chl a maximum layer during the bloom period of the ice-melt season in the Saroma-ko lagoon.
The upper figure shows the cell density, total chlorophyll a concentration, and J' (Pielou's evenness index) for each algal group. The figure below shows changes in the cell density of dominant species. The dashed line represents ice algae, the solid line represents cold-water planktonic algae, and the dotted line represents bloom-forming species. Note that different scales for cell density are used *1 and *2 on the first and second axes of the Y-axis, respectively.
As global warming diminishes multi-year Arctic sea ice and transforms it into thin first-year ice, microalgal communities (the primary producers) are facing profound impacts that could alter the entire food web. The Saroma-ko lagoon in Hokkaido, which is located at the southernmost edge of the seasonal sea ice zone, is an ideal place to study these 'first-year ice zones'. In winter, sufficient sunlight penetrates the thin ice, enabling ice algae to proliferate throughout the ice layer. This study quantitatively assesses the contribution of these algae as a 'seed source' for the large-scale blooms that occur after the ice breaks up, in order to examine their role in primary production.
Materials and Methods
Seawater samples were collected twice weekly from March 27 to April 22, 2024, at Sakaeura Harbor in Kitami City. CTD measurements were taken to record water temperature, salinity, chlorophyll a (Chl a), and dissolved oxygen. On April 15, at the peak of the bloom, samples were taken from three depths at five lake monitoring points. On 20 February, sea ice (thickness ca. 40 cm) was sampled in 10 cm layers and seawater in 1 m layers. Additionally, sediment traps were deployed 10 km offshore from early January to late April. Samples were fixed with a paraformaldehyde-glutaraldehyde mixture and analysed using the Utermohl method with an inverted microscope. Differences in species composition were analyzed using community similarity and multiple regression analysis.
Results and Discussion
Sea ice melt began in late March 2024. During the observation period, the surface temperature of the seawater increased from -0.3 to 11 deg C, while salinity rose from 1.6 to 30. In the maximum Chl a layer, Chl a increased from 2.7 to 25 ug L-1, while nutrient concentrations (NO3- and PO43-) decreased significantly. Total algal cell counts increased twentyfold, reaching 11.5 x 103 cells mL-1 at the peak bloom.
Dominant species shifted from ice algae (Detonula confervacea ), which accounted for 42-83% during the early thaw period, to planktonic algae (56-96%) during the bloom period, with the proportion of ice algae dropping below 8% (Figure 1). Early bloom species such as Thalassiosira nordenskioeldii were replaced by four other Chaetoceros species at the peak of the bloom. Similarity analysis identified four distinct community groups: pre-bloom ice algae; early cold-water plankton; bloom-forming species; and transitional flagellates. Multiple regression analysis revealed that water temperature and salinity are significant controlling factors (p< 0.01) in this succession of communities.
Geographical comparisons showed that while planktonic algae dominated ( >90%) across the lake at the peak, ice algae were significantly more prevalent (66-94%) in the early-freezing eastern shore area compared to the lake center (6-11% in sediment samples).
These results suggest that the spring bloom in Saroma-ko lagoon is predominantly caused by planktonic algae, with the decrease in ice algae being driven by changes in water temperature and salinity. Although ice algae contribute only minimally to the bloom, they are likely serve as a crucial early food source for benthic organisms, particularly in the well-developed ice of the eastern lagoon, which is a key hub for primary production in the early spring.
Figure caption
Figure 1. Changes in algal biomass and community structure within the chl a maximum layer during the bloom period of the ice-melt season in the Saroma-ko lagoon.
The upper figure shows the cell density, total chlorophyll a concentration, and J' (Pielou's evenness index) for each algal group. The figure below shows changes in the cell density of dominant species. The dashed line represents ice algae, the solid line represents cold-water planktonic algae, and the dotted line represents bloom-forming species. Note that different scales for cell density are used *1 and *2 on the first and second axes of the Y-axis, respectively.