The Gould, a gale, and a bit more on SAM

The Laurence M. Gould departed for Punta Arenas last night, taking Colleen with it and leaving Jamie and I on our own until reinforcements arrive in two weeks (you can check out Jamie’s blog here for more on what we’re up to this season).  That should work out fine although we’ll be very busy on sampling days – when and if we get sampling days.  We were supposed to get out today but the weather isn’t cooperating.

Shortly after the Gould departed the wind started to increase.  Right now the Gould is getting 50 kt winds at the southern edge of the Drake Passage (sorry Colleen!), we’re getting a steady 35 kt wind the blew all night and should last through today.  I’m nervous about what that will do to our sampling plan.  So far the land fast ice where our ice station is has held together; it’s a nearly a meter thick and pretty well anchored to the land.  Sometime this season it’s going to give out though, and I’m hoping that we can sample from it a couple more times before that happens.

The flip side is that when the ice goes away we’ll be able to start using the zodiacs to sample at our regular stations, at least until the ice blows back in.  The worst case scenario is being in the awkward position of too much ice for the zodiacs, but no solid land fast ice from which to sample.  To get an idea of how fast things can change compare the ice conditions in the following pictures to the conditions when the Gould departed:

The fast departure of the ice underscores an important ecological concept that is central to this region.  The timing of the switch from ice covered to open water conditions has a major impact on the strength and timing of the spring phytoplankton bloom; the annual ecological event from which everything else derives (think of it like a burst of new green grass in the Serengeti).

In the springtime Antarctic phytoplankton are limited in growth only by the absence of light.  Nutrients have been replenishing all winter, there are no grazers around (yet), and the phytoplankton are relatively indifferent to temperature.  Right now at Palmer Station we have nearly 18 hours of daylight, what keeps the phytoplankton bloom from exploding right now is the ice.  Only 6 % of the light that hits the surface of the fast ice in Arthur Harbor is making its way down into the water.  That’s enough to support the growth of specialized ice algae and low-light adapted phytoplankton just below the ice, but not a major bloom deeper in the water column.  At just 10 m depth only about 0.01 % of the light that hits the surface remains; it is essentially totally dark.

So as soon as the ice departs the phytoplankton are primed to start growing.  In Arthur Harbor the wind is driving the ice away, does this mean a bloom is about to start?  Not necessarily.  For phytoplankton, what the wind gives it also takes away.  A strong wind induces strong vertical mixing in the water column.  This impact of vertical mixing on phytoplankton has been studied in places like the North Atlantic for a very long time.  Some phytoplankton can swim, but none can swim fast enough to outpace vertical mixing.  Under a stiff, sustained wind phytoplankton in the surface are mixed deep into the water column.  If they don’t go too deep that’s fine.  Below a certain point they can’t photosynthesize enough to meet their metabolic demands (we usually take this to be the 1 % light level), but like all organisms they have energy stores and can wait to get mixed back above this depth.  Pushed deep enough however, at what we call the critical depth a phytoplankton cell has insufficient energy stores to make it back to the surface.  Under these conditions, although phytoplankton may be growing at the surface, the formation of the bloom will be suppressed.

So what does this have to do with timing?  It’s no surprise that the strongest storms happen in the winter.  In low sea ice years, with less land fast ice and an earlier retreat of both land fast and pack ice, the surface of the Antarctic ocean is exposed to late winter storms and strong mixing.  Phytoplankton that have been overwintering safely in the stable water column below the ice start to grow, but are constantly mixed down below the critical depth.  Eventually this stock of phytoplankton is depleted (or much reduced), leaving insufficient numbers to initiate the bloom when conditions finally calm down.  This idea has been explored in a number of studies, including this great 1998 paper led by Kevin Arrigo at Stanford and this 2006 study led by Hugh Ducklow at the Lamont-Doherty Earth Observatory.  This latter study is particularly interesting because it implicates the Southern Annual Mode (SAM) in determining the strength of the spring bloom.  As the plot at right shows it’s clear that SAM isn’t the only thing that determines ice duration, extent, and the strength of the bloom, but it has a clear and logical role.

More recent studies have extended the link between sea ice and SAM to higher trophic levels, including krill.  One of my favorite Palmer LTER papers is this 2013 paper by Grace Saba et al., which does a great job of illustrating the link and exploring the idea in the context of climate change.  A negative phase in the SAM during the winter and springs leads to low wind and high ice conditions (a double bonus for phytoplanton).  These conditions set the stage for a strong bloom and good krill recruitment (a large number of juvenille krill being “recruited” to the sexually mature, adult size class).  A positive SAM during the winter and spring leads to low ice, high wind, and a taxonomically different and overall smaller phytoplankon bloom.  This leads to fewer krill with a direct negative impact on penguins, seals, seabirds, and whales.

This post is getting long (this is what happens when a sampling day gets weathered out) so I want to end by wrapping it back around to the current season.  As I described in a previous post things are a little different this year.  The SAM index has generally been positive with some dips into the negative.  Only for the month of October was the mean SAM negative, and not very.  Despite this there is a definite positive sea ice anomaly.  This seems to be driven by the strong, persistent El Niño in the equatorial Pacific that shows no sign of abating any time soon.  Regardless of SAM, ice conditions are good this year, in a few weeks we’ll see what that means for the spring bloom when the ice clears out for good!