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Biological oceanography

Plankton fill up ice-free summer homes

Source: Li, Y., R. Ji, S. Jenouvrier, M. Jin, and J. Stroeve (2016), Synchronicity between ice retreat and phytoplankton bloom in circum-Antarctic polynyas, Geophys. Res. Lett., 43, 2086–2093, doi:10.1002/2016GL067937.

Antarctic coasts

Despite the dark winters and freezing cold conditions, the coastline of Antarctica is a hotspot for growth of phytoplankton, the tiny, photosynthesizing organisms that are the primary producers of the ocean. Phytoplankton are the food source for larger zooplankton and krill, which in turn provide food for fish, whales and seabirds. In winter much of the water around Antarctica is covered in ice. Large gaps in the sea ice, known as polynyas, are formed when strong winds blow the ice away from the edge of the continent.

Sea ice acts like a cap on the ocean, preventing any sunlight from entering so as the ice retreats to form a polynya, sunlight can spur blooms of phytoplankton.

Timing is everything

The timing between ice retreat and phytoplankton blooms is a delicate balance, and small mismatches in conditions of the physical and biological environments can have major effects on food webs. It’s like if you showed up for dinner an hour later than planned, only to find that the restaurant was closed. Small variations in timing of phytoplankton blooms may strongly impact the food source for their predators, and these effects can cascade up to higher trophic levels.

The disappearance of ice and start of phytoplankton blooms are closely related in many parts of Antarctica, but there are very few measurements in these locations to explain why they are synchronized. A common explanation among scientists is that when the ice melts the ocean is exposed to sunlight, warming the surface waters and making the uniform layer in which phytoplankton live closer to the surface where they can access more sunlight.

Recent observations in a handful of locations suggest that the explanation is not so simple, and other changes in the phytoplankton surroundings could be playing a role. With this in mind, a group of researchers from the U.S. set out to determine exactly how synchronized ice cover and phytoplankton blooms in polynyas are across Antarctica, and find what links the two.

Looking from above

Working in Antarctica, particularly in early spring when phytoplankton blooms occur, is extremely challenging, so there are very few direct measurements of phytoplankton blooms in polynyas. However, we have satellite images of sea ice cover and ocean color, which are used to estimate the concentration of phytoplankton at the ocean surface (the greener the ocean, the more phytoplankton there are). The satellite data go back to 1997, so the researchers had 18 years of data to work with. They found polynyas in 50 locations along the Antarctic coast (Figure 1), and focused on the time right after the polynyas open up, rapidly increasing the light available for growth. The authors defined the day of ice-adjusted light onset as the first time in the spring when the amount of light available each day exceeds 8 hours, and then day of bloom initiation as the first day the net phytoplankton growth rate was positive.

grl54080-fig-0001

Figure 1 from Li et al. 2016, showing the location of 50 coastal polynyas around the coast of Antarctica.

Synchronized swimming

During summer months, days get longer more quickly at higher latitudes, so it might be expected that the day of light onset might occur earlier in polynyas closer to the south pole, and later in polynyas closer to the equator. However, as Figure 2a shows, there is a lot of variation in the day of light onset at different latitudes due to differences in local sea ice cover. Light onset happens between late October and December on average, but the day of light onset varies a lot from year to year in individual polynyas, and some polynyas show much larger variations than others. The bloom initiation day (Figure 2b) resembles the light onset day, suggesting that the timing of the two events is closely coupled.

Figure 2 from Li et al. (2016). The top panel shows day of year of light onset (0 is January 1st), as a function of latitudes and the lower panel shows bloom initiation day as a function of latitude.

Figure 2 from Li et al. (2016). The top panel shows day of year of light onset (0 is January 1st), as a function of latitudes and the lower panel shows bloom initiation day as a function of latitude.

To test how well light onset and bloom initiation are synchronized, the researchers calculated the correlation between light onset day and bloom initiation day at each of the 50 polynyas (Figure 3). Less than half of the polynyas show significant correlation and there is a lot of spatial variation, with more significant correlations in the western Antarctic than in the eastern Antarctic. The authors suggest that variation in local processes, particularly the stronger winds in East Antarctica could be responsible for the differences. The strong winds create a sea ice factory, where sea ice is formed and blown away from the coast rapidly, causing more mixing at the ocean surface, weakening the warm layer that traps the phytoplankton at the surface which can slow development of a bloom. To support this hypothesis, a comparison between the synchronicity (from Figure 3) and the rate of sea ice production shows that regions with high sea ice production are less likely to be synchronized (Figure 4).

Figure 3 from Li et al. (2016), showing significant correlation between light onset day and bloom initiation day at each polynya.

Figure 3 from Li et al. (2016), showing significant correlation between light onset day and bloom initiation day at each polynya.

This result highlights that light onset is not the only factor driving phytoplankton growth, and a combination of environmental conditions need to be aligned to spur a phytoplankton bloom. Because local winds and other physical conditions are important, changes in these conditions due to climate variability could have very different outcomes in different polynyas. Understanding what drives growth in polynyas is an important step toward understanding and predicting variability and future changes in Antarctic ecosystems. Future changes in global climate may change the timing of polynya formation and sea ice production, which has the potential to upset the delicate balance that allow phytoplankton to flourish in polynyas and sustain Antarctic food webs.

Figure 4 from Li et al. (2016) showing the relationship between synchronicity as shown in Figure 3, and the rate of sea ice production in each polynya. East Antarctic polynyas tend to have higher sea ice production due to local winds and lower synchronicity.

Figure 4 from Li et al. (2016) showing the relationship between synchronicity as shown in Figure 3, and the rate of sea ice production in each polynya. East Antarctic polynyas tend to have higher sea ice production due to local winds and lower synchronicity.

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