Researchers have found significant changes in the growth patterns of deep-sea phytoplankton due to global warming, as revealed by a 33-year study near Bermuda. These microscopic algae, which are important for marine food chains and carbon dioxide absorption, respond differently depending on depth; Biomass increases in deeper layers and chlorophyll decreases in surface layers.

Beneath the ocean surface, tiny algae known as phytoplankton are thriving in response to global warming. This discovery was the result of a new study published in the journal. Nature Climate ChangeIt includes an unprecedented long-term (over 30 years) analysis of these microscopic plants that are invisible to ocean monitoring satellites.

Phytoplankton forms the basis of the marine food web. They are consumed by slightly larger zooplankton (microscopic animals), which are in turn eaten by smaller fish, which are then eaten by larger fish and other fish in the food chain. As a result, any changes in phytoplankton populations can have cascading effects throughout the marine ecosystem, affecting all organisms, right down to large predators such as sharks and whales. Understanding how phytoplankton adapt to climate change is therefore vital to predicting the future health of our oceans.

Stratification and layers of phytoplankton

More than 70% of the global sunlit ocean is permanently or seasonally divided into at least two layers. Similarly, these microscopic organisms exist in two distinct layers: surface phytoplankton in the well-lit, turbulent upper ocean and those living deeper where light is low but nutrients are abundant.

Surface phytoplankton is easy to monitor with satellites that can detect ocean color by color and observe large areas in real time. However, even in the clearest waters, these satellites can only capture the state of phytoplankton at an altitude of about 50 meters. Deep-sea phytoplankton are not routinely monitored by satellites, and we still know very little about them.

This is a serious limitation. Deeper phytoplankton constitute a significant portion of the total phytoplankton biomass (estimated to be approximately 10%-30%). Despite the low light, the bottom-up supply of nutrients means that they produce much of the new biomass created through photosynthesis in the oceans, and their “blooms” (sudden increases in total biomass) can outlast their surface counterparts.

Dynamics of surface and deep phytoplankton

To examine both layers of phytoplankton, we used 33 years of ship data from a location near Bermuda in the Sargasso Sea, a normally calm region in the middle of several major currents in the North Atlantic. It is one of the few places in the world where this data has been collected regularly over such a long period of time.

We then used a new two-layer modeling tool to analyze the ocean surface and subsurface separately. We find that deep-sea phytoplankton increased their total biomass in response to warming in the North Atlantic, especially given the acceleration of warming in the last decade.

Meanwhile, surface phytoplankton have decreased chlorophyll levels and appear less green. This may be because warmer surface waters tend to mix less with those below, keeping them in brighter conditions for longer periods of time as they acclimate to higher light levels near the surface. This may also be because greener phytoplankton species are being replaced by species better adapted to brighter conditions at the nutrient-poor surface. These species typically produce less chlorophyll, resulting in less green phytoplankton.

The importance of monitoring underground phytoplankton

These changes could have far-reaching consequences for marine ecosystems and how oceans can remove carbon dioxide from the atmosphere. We hypothesize that the deep-sea phytoplankton community may support a food web distinct from the surface community and may contribute a significant fraction of the organic matter that sinks to the ocean depths as “marine snow.”

That’s why monitoring this “invisible forest” of phytoplankton beneath the surface is so important, because it is hidden from satellites and these changes may otherwise go unnoticed.

The next step is to use floating ocean robots powered by satellite data to monitor phytoplankton at great depths. These robots already exist and provide valuable data from below the ocean surface, where satellites cannot reach. Other technologies include lidar satellites that provide deeper visibility. But like robots, they haven’t been around long enough to fully capture long-term trends in deep-sea phytoplankton.