Elucidation of the effects of changes in the feeding environment on bivalve larvae and development of an index to evaluate the larval settlement.

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”.
In the Seto Inland Sea, the production of bivalves such as asari clam have been decreasing. Various factors have been pointed out as reasons for this decrease, and changes in the feeding environment is 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, we conducted filed observation to test our hypothesis that “Changes in the feeding environment during the larval stage affect the physiological activity and settlement of bivalve larvae”. As a result, it was confirmed that oyster larvae that occurred during the periods when the nanoplanktonic diatoms Cyclotella sp.A and Skeletonema menzelii were dominant (i.e., oyster larvae that appeared to have fed mainly on these diatoms) tended to have high physiological activity and achieve successful settlement. On the other hand, the larvae occurred when Chaetoceros tenuissimus was dominant tended to be less physiological activity and fail to settlement. Thus, although the results of field study supported the hypothesis, analysis of field data alone could not exclude the effects of fluctuating water temperature and salinity on larvae, and it was not able to genuinely evaluate the effects of each nanoplanktonic diatom on the larvae. In this study, in order to investigate the effects of different diets on the physiological activity, growth and survival of the oyster larvae, we will feed each of above three nanoplanktonic diatoms to oyster larvae under rearing conditions with uniform water temperature and salinity. Last year’s field observation also confirmed that oyster larvae with high physiological activity tended to have a high success rate in settlement. Therefore, we considered that it is possible to evaluate whether the coastal areas are suitable for bivalve production by investigating the physiological activity of the larvae that occurred there. This year, we will continue to accumulate the data on the physiological activity and success or failure in settlement of oyster larvae, and verify whether the physiological activity of bivalve larvae is an effective indicator for assessing bivalve production areas.

The results of this study will lead to the presentation of environment conditions for sustainable bivalve production and the development of methods for assessing the coastal areas. We believe this study will contribute to maintenance of a healthy coastal areas in which bivalve resources can be used sustainably.

Spawning and migration ecology of anadromous Japanese grenadier anchovy in the Ariake Sea (3)

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　in Asia. 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 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 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

Asso. Prof.Tomonori Isada

Firstly, I am deeply grateful to the International EMECS Center for its Grants-in-Aid for young researchers. I am studying marine phytoplankton and seagrass dynamics in Akkeshi-ko estuary and Akkeshi Bay, 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.

Development of a Shizugawa Bay Digital Twin using an integrated watershed-ocean-ecosystem model (part 2)

School of Environment and Society, Tokyo Institute of Technology

Assoc. Prof.Takashi Nakamura

I would like to thank you again for selecting me for the EMECS Grants-in-Aid for young researchers, and, I am grateful to continuously study the coastal ecosystem in Shizugawa Bay, Minamisanriku, Miyagi with the great support by International EMECS Center.
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. 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 including the dynamics of marine resources. In addition, this digital will be able to numerically seek the optimal sustainable use of marine resources and aquaculture in the present and the future under climate change.
One of the interesting phenomena of oyster farming in Shizugawa Bay is that oyster yield increased when the number of oyster rafts was reduced with the great decision of local fishermen after the earthquake disaster. This fact indicates that understanding the negative impact of overcrowded aquaculture and estimating the optimum density of aquaculture is very important. The effects of overcrowding in aquaculture must be biological, such as competition for the availability of oyster food (phytoplankton), and physical, such as the weakening of water circulation by the drag of oyster rafts. This year I will mainly focus on modeling the effects of overcrowding by incorporating both biological and physical processes.

Elucidating the Carbon Budget in Coastal Areas: A Case Study of Toyama Bay (part 2)

Faculty of Science, Academic Assembly University of Toyama

Asst. Prof. Hidetaka Kobayashi

From the perspective of carbon dioxide uptake in the ocean, coastal areas are regions where nutrients necessary for phytoplankton growth are abundantly supplied, leading to high primary production. Through gas exchange between the atmosphere and the ocean, coastal areas are expected to function as sinks for carbon dioxide. This project aims to elucidate the formation and variability mechanisms of the ocean biogeochemical cycle in Toyama Bay through both model experiments and observational data.

We construct an ocean biogeochemical cycle model to quantify the budget of nutrients and carbon related to the ecosystem. Toyama Bay receives abundant nutrient supplies from riverine and groundwater inputs from the land. It has been shown that up to 20% of the plankton bloom in the bay is supported by nutrient supply from terrestrial sources. By constraining these input amounts through observations and incorporating them into the model, we aim to evaluate the impact of terrestrial nutrient supply on the coastal ecosystem of Toyama Bay.

We have advanced the construction of the model, calculated the climatological distribution of the physical and biogeochemical fields, and examined the differences in results concerning freshwater input from the Jinzu River and the spatial resolution of the model. Additionally, we collected and organized existing observational data in Toyama Bay and compared the monthly variations in biogeochemical fields between the model and observational data.

In Toyama Prefecture, significant crustal deformation was observed due to the Noto Peninsula Earthquake in 2024. Changes in groundwater levels and quality have been confirmed, raising concerns about changes in the flow and quality of groundwater from land to coastal shallow sea areas. Submarine slope failures have also been identified, suggesting changes in submarine groundwater discharge points. These changes in nutrient supply to coastal waters are likely to induce variations in primary production and carbon dioxide fluxes in coastal areas. Therefore, this year, we plan to conduct surveys of terrestrial and submarine groundwater discharge and observations in Toyama Bay. By comparing with past observational data, we aim to understand the impact of the Earthquake and evaluate its impacts on the coastal ecosystem using models.

The ecology of green Noctiluca scintillans and environmental shifts during its red tide occurrences: a study in the Inner Gulf of Thailand

Seto Inland Sea Regional Research Center, Kagawa University

Assistant ProfessorMasatoshi NAKAKUNI

I am Masatoshi Nakakuni, a specially appointed assistant professor at Kagawa University. I am deeply grateful for being selected for the EMECS Young Researcher Support program. My research project is titled “The ecology of green Noctiluca scintillans and environmental shifts during its red tide occurrences: a study in the Inner Gulf of Thailand”. This study focuses on Noctiluca scintillans, a species of dinoflagellate. Noctiluca scintillans inhabits coastal waters worldwide and frequently forms red tides. These red tides bring enormous biomass to coastal areas, causing water discoloration and foul odors, which adversely affect various industries including fisheries, aquaculture, and tourism. This issue is not limited to the inner Gulf of Thailand but is also observed in other Southeast Asian coastal areas, necessitating urgent measures to mitigate the damage.

In the Gulf of Thailand, green Noctiluca blooms are concentrated in the inner coastal areas from February to April, with the affected area expanding to the central part of the inner gulf from May to September. These areas coincide with regions where hypoxic water masses form, suggesting that green Noctiluca red tides may be a contributing factor to the formation of hypoxic water masses in the inner Gulf of Thailand.

However, scientific evidence directly linking green Noctiluca red tides to the formation of hypoxic water masses in the inner Gulf of Thailand is still lacking. One reason for this knowledge gap is the insufficient understanding of the ecological characteristics and composition of green Noctiluca during its bloom development in natural environments.

To address this, our research aims to investigate the basic composition characteristics of green Noctiluca in its natural environment and conduct high-density observations from the pre-bloom season to the post-bloom season. This approach will allow us to provide essential field observation data for modeling the red tide occurrence mechanism of green Noctiluca and attempt to elucidate the process of red tide development using the collected dataset

Primary production on tidal flats reflecting tides and submarine groundwater discharge: machine learning and continuous observation approach

Prefectural University of Kumamoto

Tatsuya Ozaki

I would like to offer my sincere gratitude regarding my selection for EMECS Grants-in-Aid for young researchers. I belong to the doctoral course of the Graduate School of Environmental Symbiosis、 Prefectural University of Kumamoto、 and I am working on quantifying the primary producers of tidal flats in the Midori river tidal flat、 which has the largest tidal flat area in Japan. Tidal flats are hotspots of ecosystem services with high biological production. Tidal flats bordering land are vulnerable to development、 and approximately 16% of the world’s tidal flats have been lost since 1984. Conservation of disappearing tidal flats requires accurate quantification of the primary producers (phytoplankton and microphytobenthos inhabiting the sea bottom)、 which are fundamental to the ecosystem function of tidal flats. The main limiting factors for the primary production on tidal flats are light and tides. However、 conventional estimates of primary production ignore the impact of tides、 and there are no studies of quantifying primary production on tidal flats for longer than one year.

In contrast to the conventional in-situ incubation method in the field、 I established the laboratory experimental method using an artificial light source with light intensity equivalent to the sun、 and combined the method with continuous observation of water quality over two years to estimate the primary production with high time resolution. Generally、 the time of ebb tide on tidal flats is daytime in summer (daytime tides) and nighttime in winter (nighttime tides)、 thus the daytime tides are suitable for photosynthesis for microphytobenthos、 while the nighttime tides are unsuitable for photosynthesis. Therefore、 by using established monitoring methods to precisely quantify the primary production on tidal flats over a long-term、 I will verify that the primary production and relative importance of microphytobenthos increases with increasing time of ebb tide in the daytime from the nighttime tides to the daytime tides. In addition、 while light is more intense in summer、 microphytobenthos are exposed to drought stress during ebb tides. In recent years、 nutrient supply from the submarine groundwater discharge (SGD) is reported to increase primary production in coastal areas around the world. Based on this、 I hypothesized that the photosynthetic activity of microphytobenthos increases significantly when the SGD keeps the sediment surface moist and provides abundant supply of nutrients during ebb tides. In this study、 I will test this hypothesis by measuring and comparing the photosynthetic activity of microphytobenthos in SGD area and control area during ebb tides. The results obtained in this study will be used to construct the primary production on tidal flats estimating model reflecting tides and SGD.