Oceanic intrusions move tiny sea life to the deep layers

The ocean plays a vital role in Earth’s carbon cycle, acting as a vast sink that absorbs atmospheric carbon dioxide. Understanding how carbon moves from the ocean’s surface to its depths is crucial for modeling climate change. New research from Brown University, Woods Hole Oceanographic Institution, and the Marine Biological Laboratory sheds light on a previously overlooked mechanism: oceanic intrusions.

What are oceanic intrusions?

Oceanic intrusions are fascinating and complex phenomena that occur when water masses of different densities, temperatures, or salinities come into contact within the ocean.

These differences in water properties can be due to various environmental factors, such as solar heating, evaporation, or the influx of fresher water from rivers or melting ice. When these differing water masses meet, the denser water tends to slide under the lighter water, creating a layered effect in the ocean’s subsurface.

These intrusions are not trivial in scale; they often stretch horizontally across many kilometers. Imagine vast, underwater rivers weaving through the ocean, sometimes extending over tens or even hundreds of kilometers.

But their impact isn’t just lateral. Vertically, these currents can push or pull water hundreds of meters below the surface. This vertical movement is particularly significant because it acts against the usual stratification of the ocean, where lighter, warmer water sits above cooler, denser layers.

Significance of oceanic intrusions

The mechanism of oceanic intrusions is pivotal for the transport of microscopic organisms. These organisms, primarily phytoplankton, bacteria, and other microorganisms, typically inhabit the upper layers of the ocean where sunlight penetrates, enabling photosynthesis.

As the intrusions occur, they sweep these microorganisms along, carrying them from the sunlit, productive upper layers to the darker, nutrient-poor depths of the ocean.

This transport is crucial for several reasons. Firstly, it aids in the redistribution of nutrients and organic materials throughout different layers of the ocean, which can affect local ecosystems profoundly. For instance, as these microorganisms are transported deeper, they can introduce organic carbon to regions where it is otherwise scarce, impacting the local food chains and microbial dynamics.

Furthermore, this process plays a significant role in the global carbon cycle. The ocean is a major carbon sink, and the movement of carbon from the surface to the deep ocean (a process known as the biological pump) is essential in controlling atmospheric carbon dioxide levels.

Oceanic intrusions help facilitate this by carrying carbon-rich organisms into the deep ocean where they eventually decompose, locking away carbon from the atmosphere.

Oceanic intrusions and microbes

An international team of researchers conducted extensive fieldwork in the subtropical Mediterranean Sea. Employing Conductivity-Temperature-Depth (CTD) sensors, water sampling techniques, and computer modeling, they investigated the role of intrusions in transporting surface microbes to the deep ocean. Here are some of the key findings from the study:

Year-round transport

Oceanic intrusions are a dynamic and ongoing phenomenon, occurring throughout all seasons, which contrasts sharply with previous models that primarily associated carbon transport with seasonal patterns.

Traditionally, scientists believed that carbon transport via ocean currents was mostly a seasonal occurrence, linked to specific climatic conditions that vary throughout the year, such as temperature changes or seasonal winds that might influence ocean currents.

However, the discovery that intrusions happen year-round introduces a new understanding of how carbon and other nutrients are continuously cycled between ocean layers. This revelation is significant as it suggests that the ocean’s role in carbon storage and atmospheric carbon regulation is more constant and reliable than previously thought, providing a stabilizing effect on global carbon levels regardless of seasonal fluctuations.

Biomass-rich origins

Intrusions typically begin in regions where microbial life is abundant, particularly where surface-dwelling microbes like phytoplankton thrive. These areas are often nutrient-rich, illuminated by sunlight, and support a diverse array of microorganisms.

The initiation of intrusions in these biomass-rich zones is crucial because it ensures that a significant amount of organic material and microorganisms are available to be transported deeper into the ocean.

The presence of such high concentrations of microbes at the origin points of intrusions enhances the efficiency and impact of this transport mechanism, carrying substantial amounts of carbon and nutrients into the deeper, less hospitable parts of the ocean where fewer life forms can survive and compete for these resources.

Microbial hitchhikers

The composition of microbial communities found within oceanic intrusions offers compelling evidence of their role in transporting surface-dwelling microorganisms to deeper waters. Studies have shown that these communities closely resemble those found at the ocean’s surface, including both photosynthetic organisms and the bacteria that depend on them for food.

The similarity indicates that intrusions effectively capture and transport these communities from the upper, light-filled layers down to darker, deeper layers. This transport is critical because it extends the influence of surface microbial communities into the deep ocean, affecting deeper ecosystems by introducing new sources of energy (in the form of organic carbon) and altering the microbial dynamics.

The transport process not only helps in distributing life-sustaining elements but also plays a crucial role in deeper oceanic chemical processes, such as the decomposition and subsequent storage of carbon, impacting overall ocean health and the global carbon cycle.

Study implications

Oceanic intrusions provide a continuous carbon transport mechanism, supplementing the traditionally understood method of sinking particles.

Moreover, the influx of microbes to deeper waters diversifies available food sources, impacting deep-sea food webs and microbial biodiversity. This research reveals a significant new factor in the ocean’s carbon cycle. It underscores the complex interplay between physical ocean currents and ocean biology.

“We found that because these organisms are so small, they can be swept up by ocean currents that then bring them deeper than where they grow. It’s often a one-way trip for these organisms, but by taking this trip, they play a critical role in connecting different parts of the ocean,” said Mara Freilich, assistant professor at Brown University.

The study opens numerous avenues for further exploration. Researchers will strive to determine the global prevalence of intrusion-based carbon transport, its role in various ocean regions, and how it could influence the ocean’s response to a changing climate.

Integrating this new knowledge into climate models will enhance our understanding of the ocean’s role in regulating the Earth’s climate system.

The study is published in the journal Proceedings of the National Academy of Sciences.

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