Tropical storms could help fight low oxygen levels in oceans

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The destructive power of tropical cyclones, known in different regions as hurricanes or typhoons, is well-documented, especially in terms of the havoc they wreak on land.

Yet, a new dimension to these natural phenomena has been uncovered by researchers, highlighting a less understood but equally important aspect: their impact on the ocean’s biology and chemistry.

In a study published in Frontiers in Marine Science, Wen Yang and colleagues from East China Normal University have delved into how tropical cyclones influence the oxygen and nutrient dynamics in the East China Sea, a region frequently visited by such storms.

With about 46% of tropical cyclones in the East China Sea reaching the mainland’s east coast, understanding these effects is crucial for a comprehensive understanding of the marine ecosystem’s health and productivity.

The study is based on the concept that the disturbance caused by tropical cyclones promotes the mixing of ocean waters.

This process not only introduces oxygen to deeper waters, helping mitigate low oxygen levels, but also distributes nutrients across a wider surface area, fostering the growth of primary producers like phytoplankton.

Phytoplankton, the ocean’s primary producers, form the base of the marine food web, and their productivity is a key factor in the ocean’s overall health and carbon cycling.

The research team combined historical data on tropical cyclones’ paths and speeds with satellite imagery to analyze chlorophyll concentration changes in the ocean surface. Chlorophyll is a good proxy for phytoplankton activity and, by extension, primary productivity.

Their findings show an initial decrease in chlorophyll levels before the cyclone, followed by a significant increase, particularly in offshore areas after the storm’s passage.

This pattern suggests an enhancement in phytoplankton productivity due to the cyclone-induced mixing of water columns, bringing nutrients up to the surface layers where sunlight can fuel photosynthesis.

Furthermore, the study utilized modeling to explore the dissolved oxygen budget in the ocean, accounting for the cloud cover during cyclones that can obscure satellite observations.

The results indicate a net gain in oceanic oxygen following tropical cyclone events, driven by factors such as cyclone intensity and the storm’s proximity to nutrient-rich estuaries like the Changjiang.

This increase in oxygen and primary productivity has significant implications for the marine ecosystem, potentially boosting the food web and enhancing overall ecosystem function.

The findings are especially relevant in the context of global climate change, which is expected to increase the frequency and intensity of marine heatwaves and possibly tropical cyclones.

While the terrestrial impacts of these storms are often devastating, their role in stimulating marine productivity presents a complex, paradoxical effect of climate phenomena on different ecosystems.

This research not only sheds light on the intricate ways in which natural disturbances like tropical cyclones affect marine ecosystems but also underscores the importance of considering both terrestrial and marine impacts in our understanding of climate change.

The study’s insights into the net positive effects of tropical cyclones on ocean oxygenation offer a silver lining, suggesting these powerful storms play a crucial role in counterbalancing the negative impacts of deoxygenation and hypoxia in the world’s oceans, conditions that are exacerbated by human activities.

The research findings can be found in Frontiers in Marine Science.

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