'Invisible forest' of phytoplankton thrives in a warming climate

Phytoplankton, the microscopic flora that drive nearly half of the world’s primary production, are thriving in an “invisible forest” beneath the surface of our warming oceans.

These tiny organisms, responsible for converting sunlight into living cells through photosynthesis, are now revealing crucial insights into some of today’s most pressing environmental concerns, such as ocean acidification, biodiversity loss, and disruptions to the global carbon cycle.

Phytoplankton’s response to climate variability

Researchers from the University of Exeter have explored the behavior of phytoplankton within the ocean’s surface and the unique subsurface layer, while investigating the potential impacts of climate variability on these lifeforms.

According to the study, recently published in the journal Nature Climate Change, phytoplankton in surface and subsurface communities are responding to environmental variations in different ways.

Dr. Johannes Viljoen from Exeter’s Penryn Campus in Cornwall emphasized the importance of understanding these trends.

“Phytoplankton are the foundation of the marine food web, and play a key role in removing carbon dioxide from the atmosphere,” said Dr. Viljoen.

“Deep-living phytoplankton, which thrive in low-light conditions, respond differently to ocean warming and climate variability compared to surface phytoplankton.”

Subsurface phytoplankton growth

Significant increases in the total “biomass” of subsurface phytoplankton have been observed over the past decade, an adjustment believed to be a response to oceanic warming.

In contrast, the surface phytoplankton’s chlorophyll concentration (responsible for their green hue) has decreased, though the total biomass remains steady.

The analysis of 33 years of data from the Bermuda Atlantic Time-series Study (BATS) in the Sargasso Sea reveals a notable trend: the depth of the “surface mixed-layer” has become shallower due to rapid ocean warming.

“Our study highlights the limitations of satellite observations and underscores the urgent need for improved global monitoring of phytoplankton below what satellites can see,” said Dr. Viljoen.

The vital role of phytoplankton

Dr. Bob Brewin, a co-author of the study, explained the broader implications of the research.

“Changes at the base of the food web can have cascading effects on marine life, from tiny zooplankton to large fish and marine mammals. So the future of phytoplankton will have major implications for biodiversity, as well as climate change.”

According to Dr. Viljoen, continued monitoring of these deep-living phytoplankton will help scientists better understand ongoing changes in the ocean that might otherwise go unnoticed.

Changes in the distribution of phytoplankton

While satellite observations regularly provide valuable information about surface phytoplankton, they cannot detect the hidden subsurface community.

This revelation stresses the limitations of current monitoring technologies and calls for enhanced global tracking of underwater phytoplankton.

“Marine phytoplankton are photosynthetic, single-celled microscopic organisms that form the base of the marine food-web,” wrote the researchers. “They are essential in the regulation of global ocean biogeochemical cycles and climate.”

“Physical and chemical changes in the ocean are modifying the distribution, phenology, abundance and composition of phytoplankton, impacting higher trophic levels and biological carbon export.”

Future implications for marine ecosystems

As ocean temperatures continue to rise, understanding how different phytoplankton communities adapt will be crucial for predicting shifts in the marine food web.

“Considering the relevance of subsurface communities, it is essential to improve our understanding of how phytoplankton vertical structure may be impacted by upper-ocean warming and stratification as a result of climate variability,” noted the study authors.

Changes at the base of the food chain can trigger cascading effects, influencing everything from the smallest zooplankton to larger marine mammals and fish populations.

Climate mitigation efforts

Moreover, the ability of phytoplankton to absorb and sequester carbon dioxide has significant consequences for future climate change mitigation efforts.

If phytoplankton communities are disrupted by warming seas, the ocean’s capacity to act as a carbon sink may diminish, exacerbating atmospheric carbon levels.

To address these challenges, scientists are calling for enhanced monitoring systems that go beyond surface-level observations, using more sophisticated technologies to track subsurface phytoplankton populations.

This approach will provide deeper insights into how these organisms are responding to rapid environmental changes, offering valuable data for future conservation and climate strategies.

The study is published in the journal Nature Climate Change.

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