It’s been a busy few days as we wrap up ice sampling and make the transition to sampling by boat at the regular Palmer LTER stations. This afternoon we’ll break down the ice removal experiment we started over a week ago. On Monday we went out for the final sampling at our ice station – though if the ice sticks around for a couple more weeks we’ll try to go out one final time to see how the spring ice algal bloom is developing. The heavy snow cover on the sea ice has delayed the start of the bloom, however, things are starting to happen. During our last sampling effort we lowered a GoPro camera underneath the ice to take a look.
You’ll notice a couple of interesting things about the underside of the ice. First, it’s extremely rough. Landfast sea ice often looks like this; the ice forms from many small flows being compressed together against the shoreline during the fall. As a result there is a lot of “rafting” of small ice floes atop one another. This can present some real challenges when selecting a sampling spot. The first couple of holes that we tried to drill exceeded what we knew to be the mean thickness of the sea ice. It took a few tries to find a representative spot.
The amount of ice algal growth in McMurdo Sound sea ice in mid-October of 2011, covered by only a few centimeters of snow, is much greater than in the Arthur Harbor sea ice, covered by 30 cm of snow, despite that fact that it is already mid-November and Arthur Harbor is much further north than McMurdo Sound.
You’ll also notice that the ice has a distinct green color, concentrated on the lower (or higher, in the video) rafts. That’s the start of the ice algal bloom. If the ice was snow free the bloom would have developed by now into a thick carpet. You can contrast the video above with the image at right of sea ice sampled from McMurdo Sound roughly three weeks earlier in the season (in 2011). Although much thicker that ice was covered by only a few centimeters of snow. If the Arthur Harbor ice sticks around for a couple more weeks it will develop some good growth (unless the krill come along and graze the algae down). You might be wondering why, if the algae are limited by the availability of light, they are concentrated on the deeper rafts further from the light. I’m not entirely sure, but I have a hypothesis. I’ve been searching for a literature reference for this and haven’t located one yet, but I recall hearing a talk from an expert on the optical physics of sea ice describing how the sunlight that manages to penetrate sea ice reaches a maximum some distance below the ice. This might seem counter-intuitive, but makes sense if you consider the geometry of the floes that coalesced to make the ice sheet.
One hypothesis for the vertical distribution of ice algae – and I have to caution that this is just an idea – is that the refraction of light (indicated in this schematic by yellow lines) as it passes through sea ice sets up a light maximum that is some distance below the bottom of the ice floes (white boxes). Algae and phytoplankton (indicated by green) would preferentially inhabit this zone.
As you can observe in the video the light is largely penetrating the ice around the edges of these floes. The rays of light enter the water at an angle, and intersect at some distance below the ice determined by the mean size of the floes and (I’m guessing) the angle of the sun. The depth where this intersection happens is the depth of greatest light availability. Above this depth the water is “shaded” by the ice floes themselves. In our case I think this depth corresponds with the depth of those deeper floes. Unfortunately our crude hand-deployed light meter and infrequent sampling schedule are insufficient to actually test this hypothesis. We’d need a much higher-resolution instrument that could take measurements throughout the day. Something to think about for the future.
In the meantime Rutgers University undergraduate Ashley Goncalves, spending her Junior year with the Palmer LTER project at Palmer Station, made this short video that describes the process of collecting water for our experiment from below the ice in Arthur Harbor. Let the boating begin!
