Surreal illustration of copepods in Tokyo Bay, symbolizing climate change impact.

Decoding Tokyo Bay's Copepods: How Climate Change is Rewriting Marine Life

Lena Voss in Climate & Environment August 2025 • 4 min read.

"Uncover the complex shifts in copepod behavior and what it means for coastal ecosystems facing environmental stress."

Imagine a world where even the tiniest creatures are feeling the heat—literally. In the bustling waters of Tokyo Bay, Japan, a fascinating drama is unfolding among copepods, tiny crustaceans that play a crucial role in the marine food web. These creatures, like many others, have adapted to survive through diapause, a state of dormancy triggered by environmental cues.

But what happens when those cues start to change? A recent study led by Aiko Tachibana, Hideaki Nomura, and Takashi Ishimaru delved into this question, analyzing three decades of zooplankton data from Tokyo Bay. Their findings reveal some unsettling truths about how climate change and human activities are disrupting the life cycles of these copepods, with potentially far-reaching consequences for the entire ecosystem.

The study focuses on three dominant copepod species: Acartia omorii, Centropages abdominalis, and Labidocera rotunda. Each has unique seasonal patterns and responses to environmental changes. By examining their phenology—the timing of biological events like reproduction and dormancy—the researchers uncovered significant shifts linked to warming temperatures and altered water conditions.

What Did the Tokyo Bay Copepod Study Reveal?

Surreal illustration of copepods in Tokyo Bay, symbolizing climate change impact.

The research team meticulously analyzed monthly zooplankton samples collected from 1981 to 2010. This long-term data set allowed them to identify trends and correlations between copepod populations, climatic indices, and hydrographic conditions. The results paint a picture of a marine ecosystem under considerable stress, adapting in complex and sometimes unpredictable ways. Here’s a breakdown of the key findings:

Overall, the study noted that there was a decrease in the abundance and increasing resting egg periods for Acartia omorii and Centropages abdominalis, and the reverse for Labidocera rotunda. Researchers used time series analysis and other methods to determine these trends and their possible impacts to coastal areas.

Changing Abundance: The abundance of Acartia omorii and Centropages abdominalis decreased over the study period, while Labidocera rotunda saw an increase.
Shifting Phenology: The timing of seasonal events, such as peak abundance and the start and end of resting egg periods, shifted for all three species.
Climate Connection: Wintertime warming, influenced by the Pacific Decadal Oscillation (PDO), was linked to earlier initiation and delayed termination of resting egg periods in cold-water species.
Hypoxic Waters: Increased stratification of the water column led to the formation of hypoxic bottom water layers, affecting the duration of resting egg periods.

Let's break this down even further to help understand what the study showed about each copepod and its role in the changing environment. The differences in each animal type show how dynamic and diverse each of these creatures are:

Why This Matters?

This study underscores the vulnerability of coastal ecosystems to the combined pressures of climate change and human activities. The shifts in copepod phenology and abundance have ripple effects throughout the food web, potentially impacting fish populations, marine mammals, and even human fisheries. By understanding these complex interactions, we can better predict and mitigate the ecological consequences of environmental change in our coastal areas.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.1002/lno.11030, Alternate LINK

Title: Impacts Of Long‐Term Environmental Variability On Diapause Phenology Of Coastal Copepods In Tokyo Bay, Japan

Subject: Aquatic Science

Journal: Limnology and Oceanography

Publisher: Wiley

Authors: Aiko Tachibana, Hideaki Nomura, Takashi Ishimaru

Published: 2018-09-24

Everything You Need To Know

What are the key findings of the Tokyo Bay copepod study regarding changes in copepod populations and their environment?

The study revealed a decrease in abundance for Acartia omorii and Centropages abdominalis, while Labidocera rotunda increased. Shifting phenology, such as changes in peak abundance and resting egg periods, was observed across all three copepod species. Winter warming, influenced by the Pacific Decadal Oscillation, correlated with altered resting egg periods in cold-water species. Increased water stratification led to hypoxic bottom water layers, further impacting resting egg durations.

How does climate change affect the copepod diapause and what are the implications for marine ecosystems?

Copepods enter diapause, a dormancy state, triggered by environmental cues. The research showed that warming temperatures and altered water conditions are disrupting this process for copepods like Acartia omorii, Centropages abdominalis and Labidocera rotunda. This disruption affects their life cycles, potentially destabilizing the entire marine ecosystem. Without diapause, these species could suffer in harsh environmental conditions, leading to population declines and further ecosystem imbalances.

What is the Pacific Decadal Oscillation (PDO), and how does it influence copepod phenology in Tokyo Bay?

The Pacific Decadal Oscillation (PDO) is a long-lived El Niño-like pattern of Pacific climate variability. Research indicates that wintertime warming, influenced by the PDO, is linked to earlier initiation and delayed termination of resting egg periods in cold-water copepod species. This means that Acartia omorii and Centropages abdominalis resting periods are increasingly altered by changes in the PDO's cycles.

How does increased water stratification and hypoxia impact copepod resting egg periods, and what does it mean for these marine species?

Increased stratification of the water column leads to the formation of hypoxic bottom water layers, which are areas with low oxygen levels. For copepods like Acartia omorii, Centropages abdominalis, and Labidocera rotunda, this means their resting egg periods are affected. Hypoxic conditions can reduce the viability of resting eggs, disrupt normal development, and alter hatching success. This can shift population dynamics and impact the food web. More research is needed to understand the long-term consequences of hypoxia on copepod populations and the broader marine ecosystem.

Why are changes in copepod populations and phenology significant for overall marine life and the food web?

Changes in copepod populations affect marine life because these tiny crustaceans are a crucial part of the marine food web. If the abundance and phenology of copepods like Acartia omorii, Centropages abdominalis and Labidocera rotunda shift, it has ripple effects throughout the ecosystem, potentially impacting fish populations, marine mammals, and human fisheries. Shifts in the timing and duration of copepod resting egg periods can disrupt the food supply for other marine organisms, leading to declines in their populations and altering the structure of the ecosystem.