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The ice edge bloom

11 Jan, 2015by Kenneth Johnson 10624 Comments

I wrote about the spring bloom seen by our profiling floats in my last post. Those floats were all in open water. Massive phytoplankton blooms along the ice edge are a major feature of Southern Ocean waters. Our floats operate under ice and, as the ice melts back, these float can observe the ice edge blooms. In open waters, the large accumulation of phytoplankton started around October 1 as the water column warmed and deep mixing slowed (although, remember phytoplankton growth rates were high before then – the phytoplankton concentration just was not increasing as it was diluted by deeper mixing). The blooms seen by floats that are far from the ice edge start later, as can take several months for the ice to melt back to the float locations. The ice edge blooms can result from several factors: a shallow mixed layer may form due to warm, fresher water that minimizes vertical mixing, iron may be released from melted ice and allow increased growth, or more sunlight allows faster growth.

The ice edge bloom is nicely resolved by float 9091 (near 64 South) in the Pacific sector of the Southern Ocean. 9091 just emerged from the ice and began reporting around New Years Day. Although the float couldn’t reach the surface before that, it was still profiling and storing data. Once it could reach the surface, all the stored profiles were transmitted and they are now available on the SOCCOMViz web site. Here are a couple of plots made with SOCCOMViz that compare float 9091 (black dots) and float 9092 (blue dots), which was around 60 South and just north of the ice edge most of the winter.

The first plot shows temperature in the upper 30 m versus time. Float 9091 saw a chilly -1.8C, the freezing point of seawater. Float 9092 only cooled to -0.7C and it started to warm up a month earlier than 9091. Hence it probably was never under ice.

The next plot shows chlorophyll in the upper 30 m versus time. The concentrations start increasing a bit earlier for 9092, but 9091 reaches higher values: about 6 ug/L for 9091 versus 1.5 ug/L for 9092.

What is driving the larger values? Before we consider that, let’s look at two other floats with ice edge blooms. Hannah and Dan were been busy deploying floats during December from the Polarstern in the Atlantic sector. The last two that have been deployed are 0511 at 65 South and 9094 at 67 South, both deployed in ice covered waters. What do they show? The following plot shows vertical chlorophyll profiles in the upper 200 m for 9091 (black), 9092 (blue), 0511 (red) and 9094 (green). Wow, 0511 is showing an enormous bloom with chlorophyll values up to 40 ug/L. The chlorophyll seen by 9094 is much lower, but just starting to increase, as the ice melts back to it. That makes sense as it is further south.

Why the big difference between the Atlantic and Pacific and what does that say about the cause of the bloom? That requires one more plot of the salinity profiles for all four floats with the same color scheme as for chlorophyll. The two Atlantic floats are seeing fresher and shallower surface layers. We haven’t seen the depth of the winter mixed layer with these floats, but I bet it was at least 60 m. In that case the salinity decrease near the surface in the Atlantic is much greater than in the Pacific, maybe 0.6 PSS (Practical Salinity Scale) in the Atlantic versus 0.2 in the Pacific.

So the mixed layer depths are somewhat shallower in the Atlantic and the amount of melted ice in the water is much greater. That might point to a larger contribution by iron from the melted ice in the Atlantic, relative to the Pacific.

Finally, here’s a challenge. The bloom is just starting at 9094. Use SOCCOMViz to see how chlorophyll is changing. A new profile will arrive every 10 days. How high will chlorophyll go for that float? The winner must post a blog comment that has a chlorophyll value within 2 ug/L of the final value reached by 9094. But you have to post your entry by January 30. And only one entry per person, please. And you have to justify your prediction. In the case of ties, the best explanation (I'm the judge) will be the tie breaker. The winner will have their name painted on a SOCCOM float. And just to complicate things, a single point "flyer" value won't get counted. There must be a clear peak value. So students, oceanographers, citizen scientists, what's your guess???

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