Historical changes of the supply of silica in the coastal areas and its effects on community structure in tidal flat ecosystems (Third year)

Department of Urban and Environmental Engineering Faculty of Engineering　Kyushu University

Asst. Prof. Megumu Fujibayashi

I would like to offer my sincere gratitude regarding my selection for EMECS Grants-in-Aid for young researchers. This is the third time I have been selected for this program, following the previous year and the year before last. I am interested in the dynamics and roles of fatty acids in aquatic ecosystems, and recently I have been focusing on eicosapentaenoic acid (EPA), a highly unsaturated fatty acid, which is synthesized mainly by diatoms and supplied to the ecosystem through the food chain. EPA may be a limiting factor for the diversity and biomass of aquatic animals. Therefore, I conduct research on the effects of EPA on ecosystem health in lakes and bays. In addition, I have been working on the dynamics of dissolved silicon (DSi), an essential nutrient for diatoms, which are the producers of EPA.

In the previous research projects, I investigated the effects of reservoirs, sewage treatment plants, and in-channel vegetation on DSi dynamics in the Zuibaiji River, which flows through Fukuoka Prefecture. The results suggest that the DSi load to the Imazu tidal flat in the estuary of the Zuibaiji River basin has decreased since the 1970s, when the Zuibaiji reservoir began operation. Furthermore, core samples taken from two sites on the Imazu tidal flat showed decreased productivity and diversity of benthic diatoms on the Imazu tidal flat after the reservoir operation started. In the third year of this research project, I aim to quantitatively evaluate the balance of DSi in the Zuibaiji River watershed. I will focus on rice paddies, which account for the majority of land use in the study area. In addition, we will examine the effects of changes in DSi loading on the Imazu tidal flat on the production of benthic diatoms and the secondary production of benthic animals. In the future, the goal is to develop watershed management that takes ecosystem conservation, based on the impact of changes in DSi load on the ecosystem structure of estuarine areas.

This is the third year of this project, and with the support of advisors and collaborators, I have been able to advance new areas of technology and analysis, such as water balance calculation, land use analysis using GIS, and diatom valve analysis. I would like to express my gratitude for this wonderful opportunity.

Relationships in seasonal fatty acid composition between oysters and microalgae

Faculty of Agriculture、 Kagawa University

Doctoral ResearcherMasatoshi NAKAKUNI

I am a postdoctoral research associate at the Faculty of Agriculture, Kagawa University. The theme of this research is “Relationships in seasonal fatty acid composition between oysters and microalgae”. Essential fatty acids such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are important for fish growth and are known to improve the growth rate of fish. In marine ecosystem, essential fatty acids are produced by microalgae, and therefore microalgae is the base of fatty acid source on the food chain. Fatty acid composition in oysters may change with microalgae fatty acid composition changing with season. However, there is limited knowledge of how the fatty acid composition of microalgae is transferred to that of oysters under in situ environments. Therefore, this study will determine from field observations how much microalgae have fatty acids and how they are passed on to oysters. Even if the amount of food is high, if the food does not contain sufficient nutrients, the organisms that eat the food are not expected to grow well. In this study, I will evaluate prey quality, which has not been well verified, and follow changes in predator nutrients in more detail by analyzing fatty acid compositions.

Studies on the distribution and growth characteristics of nanoplanktonic diatoms, the food for bivalve larvae, in the Seto Inland Sea, Japan

Coastal Productivity Research Group, Coastal and Inland Fisheries Ecosystems Division, Environmental Fisheries Applied Techniques Research Department, Fisheries Technology Institute, National Research and Development Agency, Japan Fisheries Research and Education Agency

ResearcherTomohiro Okamura

I would like to appreciate to be selected for the “EMECS Grants-in-Aid for young researchers”. I belong to the Coastal Productivity Research Group, Coastal and Inland Fisheries Ecosystems Division, at the Fisheries Technology Institute, Japan Fisheries Research and Education Agency. Our research focuses on changes in primary production and related environmental factors in coastal areas.

Bivalve, a filter feeder, provides us not only food, but also clean and healthy coastal environment. However, In Seto Inland Sea, production of asari clam and other bivalves has been decreasing since the mid-1980s. Various factors have been pointed out as reasons for this decrease, and changes in the feeding environment are one of them. Our previous study has shown that < 10 μm sized diatoms (nanoplanktonic diatoms) are important food for bivalve larvae and their biomass affects the larval settlement which could influences bivalve resources. Last year, based on the hypothesis that “lack of larval food decreases bivalve resources”, we investigated the abundance of nanoplanktonic diatoms in various areas of Seto Inland Sea, where bivalve production varies. The results indicated that biomass of certain nanoplanktonic diatoms, which were considered an important food for asari larvae, were low in the area where asari clam resources was decreased and supported our hypothesis. However, effects of each nanoplanktonic diatom biomass on bivalve larvae is unknown. In this study, we will investigate the effects of each nanoplanktonic diatom abundance on the vitality and settlement of bivalve larvae in the field. In order to improve the production environment of bivalves, the bloom formation mechanism of nanoplanktonic diatoms needs to be clarified. Our previous study also indicated that nanoplanktonic diatoms formed a bloom after a bloom of >10 μm sized diatoms (medium-sized diatoms), so we hypothesized “nanoplanktonic diatoms utilize the organic matter derived from medium-sized diatoms”. In this study, we will test this hypothesis in laboratory to reveal the bloom formation mechanism of nanoplanktonic diatoms.

The results of this study will provide insight into the environmental conditions suitable for bivalve production and also provide the basic data for analyzing the causes of decrease in bivalve production. We believe this study will contribute to the sustainable use of bivalve resources and the maintenance of a healthy environments in coastal areas around the world.

Spawning and migration ecology of anadromous Japanese grenadier anchovy in the Ariake Sea

Atmosphere and Ocean Research Institute、 University of Tokyo

Asst. Prof.Hikaru Itakura

I’m Hikaru Itakura, working at Atmosphere and Ocean Research Institute (AORI), University of Tokyo. I received my PhD from University of Tokyo in 2014, and then worked at Kobe University and University of Maryland, USA as a JSPS Research Fellow. My research topic is understanding response mechanisms of fisheries resources to environmental change, with primary interest of how diversity in migration and life history contribute to resilience in exploited fishes under environmental change. I have been studying effects of climate change and habitat modification on distribution, migration, and growth of migratory fishes such as freshwater eels and striped bass that migrate between marine and freshwater habitats.

In this research project, I’m studying about spawning and migration ecology of anadromous Japanese grenadier anchovy (Coilia nasus), to contribute to the conservation for enclosed coastal seas and resources management of this species. The enclosed coastal seas including estuaries and inner bays are an important ecosystem with high production and biodiversity around globe. In particular, the estuarine turbidity maximum (ETM) formed in brackish water is essential habitats for many aquatic organisms. Fishes of genus Coilia (family Engraulidae), which are widely distributed in East Asia, strongly depend on such enclosed coastal seas, especially ETM zones, and migrate between the sea and rivers. Thus, they have a potential to be an indicator species for conservation of health of ETM zones and connectivity between rivers and enclosed coastal seas. In Japan, Japanese grenadier anchovy is distributed only in the inner part of the Ariake Sea and its inflowing rivers, such as the Chikugo River, and adult anchovies migrate upstream for spawning up to the tidal freshwater area in early summer. Owing to such limited occurrence in space and time, they are prized as a summer feature in the local area. However, the fisheries catch of the species has continuously decreased over the past decades. Despite this situation, ecological knowledge of Coilia are scarce. Therefore, I’m exploring their potential spawning rivers, nursery grounds within rivers, migration patterns, and spatial distribution in the Ariake Sea using otolith stable-isotopic ratio and environmental DNA analyses, in order to clarify environmental characteristics that the fish prefer along their life history. In addition, I’m examining the degree of natal homing (contribution of each river stock to the entire population of the fish) to understand population dynamics of the anchovy in Ariake Sea so that the results will contribute to the conservation of enclosed coastal seas and appropriate resource management of Coilia in East Asia based on ecological information of this symbolic fish.

Dynamics of Transparent Exopolymer Particles (TEPs) in the eelgrass meadows of Akkeshi-ko estuary, Japan

Akkeshi Marine Station Field Science Center for Northern Biosphere Hokkaido University

Assoc. Prof.Tomonori Isada

Firstly, I am deeply grateful to International EMECS Center for its Grants-in-Aid for young researchers to me. I am studying marine phytoplankton and seagrass dynamics in Akkeshi-ko estuary and Akkeshi Bay, which is located in the eastern part of Hokkaido, Japan, to better understand the carbon cycle in the coastal marine ecosystem.

Macroalgae beds and seagrass meadows in estuarine and shallow coastal areas have a significant role in carbon burial and sequestration to mitigate climate change as blue carbon. Especially, Japan is considered to be a major blue carbon area because Japan has the sixth longest coastline in the world. It is therefore important to understand the carbon cycle of macroalgae beds and seagrass meadows in estuarine and shallow coastal areas.

This project focuses on the Transparent Exopolymer Particles (TEPs) dynamics in eelgrass meadows in Akkeshi-ko estuary. TEPs consist of acid polysaccharides and are sticky particles with > 0.4 µm diameter, resulting in particle aggregation like marine snow. TEPs significantly contribute to the carbon flux into deep waters. TEPs stem from dissolved organic matter (DOC) released by aquatic organisms. Previous studies showed that phytoplankton is the main producer of DOC and TEPs in coastal and open oceans. However, little is known about the concentration of TEPs and their controlling factors in seagrass meadows. Since seagrass produces a large amount of DOC, TEP also could be produced in the seagrass meadows. Although seagrass’s root and leaf litter are the main fraction of carbon burial and sequestration, the evaluation of carbon storage by TEPs in the seagrass meadows is required. Last year, we found a higher contribution of epiphytic diatoms to TEP production in eelgrass beds. Next, we aim to investigate the epiphytic diatoms in eelgrass beds in more detail.

Development of a Shizugawa Bay Digital Twin using an integrated watershed-ocean-ecosystem model

School of Environment and Society, Tokyo Institute of Technology

Assoc. Prof.Takashi Nakamura

My name is Takashi Nakamura of Tokyo Institute of Technology. I would like to thank you for selecting me for the EMECS Grants-in-Aid for young researchers. I have been conducting research and modeling coastal ecosystems in tropical and subtropical regions, especially on corals, seagrasses, and mangroves. I became involved in Shizugawa Bay when Professor Haruko Kurihara of the University of the Ryukyus, who studies the effects of ocean acidification on organisms, invited me to do research on oysters in Shizugawa Bay. I also collaborate with Associate Professor Takashi Sakamaki of Tohoku University, who has long-term research experience on Shizugawa Bay. Shizugawa Bay is an interesting site with various attractions as a research subject as described below, but what attracted me the most was the seafood of Shizugawa Bay.

As for the content of the research for which we received the grant, it is known that semi-closed bays, such as Shizugawa Bay, have a complex flow and water quality environment due to various influences such as the inflow of freshwater from land, nutrients, organic matter, sediment, biochemical reactions in the bay, and open-ocean water intrusion. For sustainable utilization of marine resources and aquaculture, it is essential to understand these processes and to understand the bay environment with a high degree of accuracy. Furthermore, global warming and ocean acidification associated with recent increases in atmospheric CO2 concentrations may enhance stratification, especially in summer, and worsen hypoxia and acidification in the bottom layer. And it has been suggested that the progress of acidification may have serious impacts on marine resources, especially calcifyers, such as oysters. Based on this awareness of such issues, this study aims to develop a digital twin of Shizugawa Bay using an integrated watershed-ocean-ecosystem model to fully reproduce the bay environment and ecosystem. The aim is to fully reproduce the bay environment and ecosystem, including the dynamics of marine resources, through numerical simulation, and to search for the optimal sustainable use of marine resources and aquaculture in the present and in the future under climate change.

Elucidating the Carbon Budget in Coastal Areas: A Case Study of Toyama Bay

Faculty of Science, Academic Assembly, University of Toyama

Asst. Prof. Hidetaka Kobayashi

My name is Hidetaka Kobayashi and I am at the Faculty of Science, University of Toyama.　I have been working at the laboratory of environmental and analytical geochemistry since 2022.

The ocean is a vast reservoir of carbon on Earth, absorbing about 30% of the carbon dioxide emitted by human activities, making it a carbon sink as important as, or even more important than, terrestrial systems. Understanding the variability of carbon cycle within the ocean is critical because it contributes to the long-term variability of atmospheric carbon dioxide concentrations.

I have studied past changes in the ocean carbon cycle by mathematically representing the processes involved and constructing numerical models that were incorporated into computer programs. Through numerical modeling, I have investigated how the transport of carbon in seawater, influenced by factors such as ocean temperature, circulation, and phytoplankton photosynthesis, affects atmospheric carbon dioxide concentrations. I have shown that these factors interact and combine in complex ways to produce significant variations.

From the perspective of carbon uptake by the oceans, coastal areas are regions where an abundant supply of nutrients promotes the growth of phytoplankton, resulting in active photosynthesis in marine ecosystems. Consequently, these coastal regions are expected to act as carbon dioxide sinks through atmosphere-ocean gas exchange. In this study, I aim to elucidate the ocean biogeochemical cycle in Toyama Bay, with a focus on carbon, through both numerical modeling and sample analyses obtained from observations.

A newly constructed ocean biogeochemical cycle model will be used to quantify the balance of dissolved substances such as nutrients and carbon that can be associated with the ecosystem in Toyama Bay. In addition, Toyama Bay receives a rich supply of nutrients from river water and submarine groundwater discharge originating from the terrestrial environment. I will use observations to constrain these inflow rates and incorporate them as inputs to the model. This will allow the assessment of the impact of terrestrial-derived nutrients on the coastal ecosystem in Toyama Bay.