Ocean Stratification and Implications 

With the ongoing warming, our oceans are increasingly stratifying with the lighter water above the heavier. Recent research has highlighted that with increasing stratification, temperature signals in the ocean may get lost. Increased stratification reduces the upper mixed layer depth in the ocean, which is important for the ocean-atmosphere interactions. A continued stratification and thinning of the layer will eventually make the mixed layer depth too small to hold oceanic signals for long. Ultimately the oceanic signals would be difficult to delineate from the noise.

At NCPOR, scientists are studying different aspects and implications of stratification in the oceans. In the polar oceans, warming leads to sea ice melting, which adds nutrient-deficient freshwater to the surface, thereby strengthening stratification. Stratification also restricts mixing inhibiting replenishment of nutrients from the deeper water to the surface.

Increased Stratification in the Arctic Ocean
A reference from the past

Recently, a group of researchers set out to find whether such an increase in stratification happened earlier in the Arctic Ocean. The team led by Manish Tiwari and Padmasini Behera from the NCPOR used a 507-meter sediment core from the Yermak Plateau in the Fram Strait. The Fram Strait is one of the pathways through which Atlantic water enters the Arctic Ocean. From the core, they used sections dated from 3.4 to 2.6 million years ago, which included the mid-Pliocene warm period when carbon dioxide concentration, atmospheric temperature and sea level were higher than that of today.

The researchers estimated past productivity using calcium carbonate and barium concentration which are the proxies preserved in sediments. They found an increasing trend in surface ocean productivity during interglacials - the globally warmer periods separating consecutive colder periods. According to them, the high productivity was caused by the warming, which resulted in more open surface water with sufficient light and nutrients fuelled by the meltwater and river discharge. However, with the continued stratification, nutrients got depleted in the surface. Thus,  nutrient utilization and depletion are useful to track surface stratification.

The team estimated nutrient utilization from the nitrogen isotopes of the organic matter in the sediment. Phytoplankton - microscopic plants in the ocean - utilize nutrients with the lighter nitrogen-14 isotope preferentially over those with the heavier nitrogen-15. Subsequently, the sinking organic matter contains a higher amount of lighter isotope and is depleted in the heavier one. However, during spring and summer, surface stratification strengthens and nutrients become limited. In this dearth condition, phytoplankton are left with no choice but to utilize nutrients with the heavy isotope leading to the settling of isotopically heavier organic particles to the bottom.

From the present sediment core, the researchers observed heavy isotope-enriched nutrient utilization during interglacials indicating enhanced surface stratification upon warming and melting. ‘Further, the Bering Strait and the Canadian Arctic archipelago - the other two Atlantic water pathways to the Arctic were relatively closed during the interglacials, which accelerated the Atlantic water inflow through the Fram Strait thereby increasing the sea ice melting and stratification’, say the researchers. These events followed the eccentricity cycle, which is the cyclic departure of the Earth from its circular orbit influencing the incoming solar radiation. On the other hand, during glacials - the colder periods with glacier advances - nitrogen isotopic ratio and stratification were opposite to that of the interglacials.

The story of interglacials matches well with the modern Arctic Ocean stratification giving climate scientists a reference from the past and a clue to the future.

Schematic showing implications of warming and sea ice melting on ocean stratification, which in turn affect nutrient mixing and marine life.

Stratification and Microbial Distribution

In the Arctic Ocean

Changes in the Arctic Ocean are more prominent in the marginal ice zones, where the sea ice meets the ice-free open ocean. The marginal ice zones contain low-saline meltwater ponds on the sea ice and a deep chlorophyll maximum in the subsurface. While going deeper, warm-saline Atlantic waters reside in the middle layers and cold-saline Arctic waters in the deepest layer. How do microbes differ in these distinct water mass layers?

To answer, P. V. Vipindas, NCPOR and team collected water samples from the surface to around 3000m depth in the marginal ice zones of the Beaufort Gyre region in the central Arctic Ocean. They analyzed the samples for microbial composition and environmental parameters.

Overall, microbial diversity was low in the upper layer and increased while going deeper, with the maximum diversity in the deep Arctic waters. For the low upper diversity, the researchers reasoned that the layer was diluted by the nutrient-deficient freshwater from the melted sea ice, which also strengthened the stratification, inhibiting nutrient supply from below. In addition, an increase of meltwater within the Beaufort Gyre region produced a density gradient between the basin and shelf resulting in a westward flow over the shelf and slope. The flow blocked the high-nutrient shelf water to reach the central Arctic Ocean inhibiting production and bacterial growth, say the researchers.

Of the total 314 bacterial genera identified from the melt ponds and open seawater, Bacteroidota was dominant in the melt ponds and Proteobacteria in the open seawater. Overall, the team observed different microbial compositions in the melt ponds and open seawater. The genera Actinobacteria, Bacteroidia and Gammaproteobacteria majorly dominated the melt ponds composition, whereas the middle Atlantic and deep Arctic water layers had common bacterial communities. The team found that the total chlorophyll-a was the major influencing factor for microbial diversity in the surface and deep chlorophyll maximum. However, the microbial community in the middle and deeper layers were shaped by the inorganic nutrients.

Using a mathematical model, the researchers tracked the source of microbial communities in the different layers. Surface water contributed 21 per cent to the communities of the deep chlorophyll maximum. However, the deep chlorophyll maximum layer contributed only 3 per cent to the composition of the deeper layers. The researchers observed maximum dispersion of 37 per cent from the middle Atlantic water to the deep Arctic water.

As microbes respond quickly to the changes, they can be used as a proxy to understand immediate changes in the biosphere upon increasing ocean stratification. 

Stratification in An Upwelling Region

Impact on plankton community

Seychelles-Chagos thermocline ridge in the tropical Indian Ocean is an upwelling region created by wind-imparted churning of the water column above its elevated bathymetry. Due to the upwelling, the surface and subsurface layers remain nutrient rich throughout the year supporting a wide diversity of plankton. However, what happens when an upwelling region faces stratification?

A team led by V. Venkataramana from NCPOR studied the plankton community structure using a 4-day time series in June 2014 when the Seychelles-Chagos thermocline ridge region was stratified. The physical observations indicated that the stratification was a result of high incoming solar radiation and air temperature during the period. They observed relatively warm water from the surface up to 40m and colder temperatures below it.

The researchers collected water samples at 6-hourly intervals to measure nutrients, phytoplankton and zooplankton composition. The researchers found low nitrite concentrations in the upper ocean due to stratification. In these nitrogen-limiting conditions, total chlorophyll-a biomass- an indicator of total phytoplankton biomass - was low, and smaller communities of picometer size were dominant in the surface and subsurface layers. The team found that the picophytoplankton contributed around 72 per cent of the total chlorophyll-a biomass. Picophytoplankton, mainly Prochlorococcus and Synechococcus, thrive in low nutrient oceans as they use both new as well as regenerated nitrogen, say the researchers. Similarly, the zooplankton community, which feeds on the phytoplankton, was dominated by small copepods, especially poecilostomatoids. The researchers emphasized that the smaller plankton provide limited food to the higher trophic levels in the stratified conditions. In this way, stratification affects energy transfer from the atmosphere to the ocean via the food web and carbon cycling.

As projected, ocean stratification will continue to rise with the ongoing increase in carbon emissions, warming, and melting of glaciers and ice sheets. The consequences include reduced vertical mixing and exchanges of nutrients, heat, and carbon. And, in the long run, stratification may disturb the global heat distribution by density-driven ocean circulation, resulting in an uneven heating and cooling on the Earth.