Phytoplankton- the base of the food web

 In changing polar oceans

In polar regions, phytoplankton - the primary producers in the ocean - follow a seasonality in their growth in sync with the availability of sunlight. The completely dark winters inhibit photosynthesis but support deep convective mixing of the water column that replenishes nutrients in the upper layers. In the spring, when the sun returns, photosynthetic activity starts. With ample nutrients and sunlight available, spring experiences a bloom in phytoplankton growth. Such blooms are usually rich in diatoms - the big phytoplankton. In summer, when the polar regions experience full daylight, heating warms up the upper layers and stratifies the ocean. When the stratification inhibits the supply of nutrients from the deeper waters, the bloom starts dying. Upwelling due to winds and oceanic currents can bring nutrients to the well-lit upper water column, thereby supporting additional phytoplankton growth. However, the later episodes of productivity are mainly dominated by mixotrophs- who can switch between autotrophic or heterotrophic modes of nutrition. In polar regions, dinoflagellates, which are smaller than diatoms, dominate this category. By autumn, as the light starts decreasing, photosynthesis drops. This phenology of phytoplankton in polar regions is evolving both seasonally and spatially in response to the ongoing warming, sea ice decline and other climatic changes.

Schematic diagram showing major seasonal factors influencing phytoplankton growth in the polar oceans and the advanced measurement methods

Field-based and remote sensing are the two main scientific approaches to study phytoplankton. Researchers use chlorophyll-a, the primary pigment that absorbs light for photosynthesis, as a marker to estimate the biomass. Chlorophyll-a can be measured from water samples and by using in-situ or satellite-borne sensors. While remote sensing shows the big picture, in-situ measurements are vital for understanding the dynamics and processes at the local level.

However, field-based studies need frequent physical access which is difficult, especially in polar winters. In that case, moored chlorophyll sensors at discrete depths are helpful as once deployed in the ocean, mooring can be left at the location for a longer period to record data for all seasons. Moreover, moored sensors at the subsurface capture the evolution of deep chlorophyll maximum, which otherwise goes unaccounted or underaccounted by satellite observations.

India has a subsurface mooring- IndARC in Kongsfjorden- an Arctic fjord. Fjords are high latitude estuaries created by the calving of glaciers at the head and merging with the ocean at the mouth. Using the mooring data, Archana Singh and the team from NCPOR investigated the seasonal subsurface chlorophyll-a from July 2015 to July 2016. The spring bloom in the fjord was observed during April-May, which has been the feature since 2006 when the fjord became almost sea ice-free. The researchers observed significant subsurface chlorophyll-a, fuelled by wind-induced mixing. High wind episodes disturbed ongoing subsurface primary production, but the mixing replenished nutrients for further production. This kind of refuelling was more visible in summer when subsurface nutrients started depleting. In winter, nutrients are brought up to the upper water column not only by convective mixing but also get advected by episodic Atlantic water inflow into the fjord. As sea ice melting leaves the upper water column vulnerable to the winds, which are also gaining speed in the Arctic, such intraseasonal variability in primary production may become a part of the seasonal cycle along with the shift in spring bloom timing.

Argo floats equipped with chlorophyll sensors are another way of in-situ sensor-based measurements with the advantage of continuous vertical profiling. The floats can profile vertically up to around 2000m depth and come to the surface periodically to transmit the data to satellites. Recently, a team of researchers led by Prince Prakash, NCPOR used bio-argo data from the Indian sector of the Southern Ocean. They captured seasonal variation of surface and subsurface chlorophyll-a in the frontal regions from 2014 to 2016, where deep chlorophyll maximum was seen to be prominent. With satellites focussing on the surface, these studies showcase the need of combining the in-situ and satellite observations to get a more accurate picture of primary productivity.

A hyperspectral radiometer is another way in which phytoplankton can be measured in situ. Using free-falling profiling, the instrument measures wavelengths of light absorbed or reflected by phytoplankton and other optically active constituents in the water column. The bio-optical properties measured by the radiometer are then fed into algorithms to calculate the phytoplankton biomass and other coloured substances. Of late, Sarat Chandra Tripathy and the team from NCPOR did hyperspectral radiometer profiling and water sampling measurements in two nearby Arctic fjords-Kongsfjorden and Krossfjorden- in the summer of 2016. They observed higher turbidity and coloured dissolved organic matter in Kongsfjorden, which is contributed by high glacial meltwater input. They explained that higher amounts of optically active substances in Kongsfjorden limit light penetration leading to lesser phytoplankton growth in the water column. The coloured matter in Kongsfjorden was a mixture of indigenous and those advected by the Atlantic water, while it was mainly indigenous in Krossfjorden. The researchers found the presence of non-siliceous phytoplankton in the fjords dominated by microphytoplankton at the surface. But the photosynthetic efficiency of microplankton was not as expected. When they looked into it, they found that the pigment packaging effect reduced the efficiency by intracellular overlapping of chloroplasts- the photosynthetic organs. Thus, hyperspectral profiling provides both chlorophyll-a measurements as well as the optical information around the pigment

Zooming out to a global scale, an NCPOR team led by Rajani Kanta Mishra investigated a 14-year time series of chlorophyll-a and phytoplankton composition in the Arctic Ocean, the Southern Ocean, and the rest of the global ocean. Using satellite and model data from NASA, the team reported an increase in chlorophyll-a and diatoms in the polar oceans from 2002 to 2015. However, during the same period, both chlorophyll-a and diatoms decreased in the rest of the global ocean. They explained that an increase in temperatures during the period would have stratified the water column and inhibited nutrient replenishment at the surface from the deeper water. Though this is happening everywhere, the polar oceans also experience an increase in underwater light due to thinning of sea ice. This, together with increasing nutrients by upwelling and mixing from high winds, could have been the reason for an increasing trend of chlorophyll-a in the polar oceans.

Community wise, the researchers found that coccolithophores-the calcium carbonate shelled phytoplankton- decreased everywhere. This indicates a response to acidifying oceans, which makes the shells weaker. On the other hand, smaller-sized phytoplankton- cyanobacteria increased everywhere. Cyanobacteria, in general, dominate the globe with Prochlorococcus and Synechococcus at the top of the list as they can survive in light and nutrient-deprived areas. With the ongoing warming, cyanobacteria - the fittest may influence the polar oceans in future.

Another recent study from NCPOR led by P.V. Bhaskar investigated phytoplankton communities at a regional level in the Arctic. They collected phytoplankton samples in summer from the coastal locations around Svalbard. As summer experiences high glacial melt and suspended biomass that limit photosynthesis, mixotrophs that can use both autotrophic and heterotrophic modes of nutrition dominate the water column. In their data, the team found a high abundance of mixotrophs dominated by dinoflagellates. In addition, they identified a smaller phytoplankton Synechococcus that grows well in low-lit conditions of highly turbid coastal waters of Svalbard. The researchers hypothesize that in the wake of increasing glacier melting and turbidity in Arctic waters, mixotrophs that can graze on bacteria and Synechococcus may play an important role in modulating the microbial loop.

As phytoplankton are evolving to adjust to the changing polar regions, the measurement techniques are also upgrading. Though each method has its pros and cons, researchers need to combine the data to find out the future trajectory of phytoplankton.

 Science Updates Team