Oral

IAPSO

P02 - Physics and Biogeochemestry of Semi-Enclosed, Shelf Seas and Coastal Zones

Secular variations in carbon stable isotope of deep-sea zooplankton: implications for regional climate change impacts on a coastal ecosystem

Global warming is altering water and material cycles, raising significant concerns about the impact on marine ecosystems necessitating the accumulation of knowledge through historical Marine ecosystem data to adapt to its accelerating pace. Toyama Bay, located in the central part of the Sea of Japan, where global warming is progressing rapidly, is characterized by a unique marine environment with a three-layered water column (freshwater, Tsushima Warm Current, and Japan Sea Proper Water) within a 1,000-meter depth and is connected to steep 3,000-meter class mountains. This rapid hydrological cycle resulting from this unique topography provides an ideal model system for investigating the impact of land-to-ocean water and material transport. Recent research in this region suggests a decrease in nutrient flux from land and a decline in primary productivity in coastal areas attributed to shifts from snowfall to rainfall. These environmental changes possibly propagate through the coastal food web. However, due to the significant cost and effort required for consistent sampling and biogeochemical analysis, historical data are limited, resulting in a substantial knowledge gap in understanding how these changes affect the food web. To address this gap, we focused on zooplankton readily and consistently collected from a deep-sea water utilization facility. Zooplankton is a key intermediary in energy and material transfer to higher consumers and integrators of environmental information for approximately two weeks. This study aims to evaluate the impact of these environmental shifts on coastal ecosystems and contribute to a broader understanding of ecosystem responses. Samples were collected regularly from 384 m depth using a 100 ��m mesh net attached to the facility. Zooplankton were categorized into small copepods, large copepods, chaetognathas, and amphipods, and their carbon and nitrogen stable isotope ratios (��13C and ��15N) were measured. Isotopic analysis indicated that four groups functioned as primary or secondary consumers. Furthermore, a significant decrease in ��13C values (1.08-1.95��) was observed over a decade (2010-2020). While global ��13C decreases are widely observed, primarily attributed to the 13C-Suess effect and phytoplankton species, our findings show a larger change, potentially reflecting regional influences. Seasonal analysis revealed substantial ��13C fluctuations in spring and summer, coinciding with snowfall patterns, suggesting a strong influence of snowmelt. Assuming that the observed changes were influenced by both global and regional factors, we estimated the contribution of regional factors by reported rates of change in small pelagic fish within the Japan Sea. The estimation revealed that regional hydrological cycle changes account for approximately 30% of the observed ��13C decrease. These results underscore the significant impact of climate change on coastal ecosystems, emphasizing the necessity for ongoing monitoring and research.