14 November – Data !

While many of the blog posts so far have been about the inclement weather and seas we've experienced on this trip, we have been able to occupy several stations (roughly half of our initial cruise plan – which in my experience is par for the course here in the Southern Ocean, particularly in the Drake Passage which is a pretty rough area even for down here). I thought I'd take this post to talk about the purpose of our project here on the ship and discuss some of the data we're collecting.

One of the objectives of our group's project is to collect salps, a gelatinous zooplankton that can be found in many of the world oceans, but is particularly abundant (at times) here off Antarctica. Salps and krill can both be found in very large populations here, and often you'll either find lots of salps OR lots of krill as both salps and krill feed on the same food (phytoplankton).  Krill are small crustaceans that are the base of the food web here in Antarctica. All the much larger animals that people associate with Antarctica (penguins, seals, whales) depend on krill (or animals that eat krill) as their food source. We know that when there aren't as many krill in the ocean (their populations vary from year to year), the larger animals don't do as well (in terms of reproduction or individual health).

These are Antarctic krill (Euphausia superba) which is often the most abundant zooplankton in the ocean here in Antarctica.  They are eaten by lots of different animals (including humans) so it is important for us to be able to measure how many krill there are in parts of Antarctica so we can understand how much food is available for the seals, penguins, flying seabirds, and whales that live here

One of the things that other scientists have studied is the relationship between the amount of winter sea ice here and the abundance (or not) of krill. When there is a lot of sea ice, then you often have lots of krill. When there isn't as much sea ice, then there tends to be more salps and less krill. This change in the dominant zooplankton in the ocean here will likely have an impact on all the other parts of the ecosystem (i.e. penguins, seals, flying seabirds, and whales) as we don't think any of these animals eat salps [although I don't think we know this for a fact. Salps are really delicate animals so trying to find them in the stomachs or scats of seals or penguins would be very difficult, whereas parts of krill can be found in both].

So we came down here to study the salps and see if there are environmental factors that can be related to the abundance and distribution of these animals. We want to see if we can use acoustic echosounders (an instrument that is, essentially, a very fancy fish-finder) which we use to measure aggregations of krill in the ocean, can also be used to study salps. And as part of that, we want to determine how much sound an individual salp (or any other zooplankton) will scatter, as we can use that along with echosounder data to estimate how many animals are in the ocean underneath our ship.

That may be kind of confusing, so I'll try to describe it a little more simply.  We use acoustic instruments (like a boat's depth sounder or a submarine's SONAR system) to send short pulses of sound into the ocean, we then listen for the echoes of these “pings” and depending on how long it takes the echoes to come back, we know how far below the ship the scatterers are.  We can produce colorful figures called echograms that show what's going on underneath us as we travel around. 

These are two echograms from the ship's Acoustic Doppler Current Profiler (ADCP) which show what is happening underneath our boat. Time (or distance) is the horizontal axis, and depth is the vertical axis.  Areas that produce more scattering are in yellows and reds, while blues represent water that is “empty”.  The ADCP is primarily designed to measure current velocities, but since it is mounted on the hull of the ship, it is always running. We have a towfish with another acoustic system to use on this cruise, but so far it's been too rough to deploy.  From this echogram, we can identify scattering patches that are probably krill aggregations in the upper part of the water column, although we can't be sure that's what they are from acoustics alone.

In order to identify what we see with the acoustic systems, we deploy net systems. On this trip we have a MOCNESS system and an IKMT net. Both have advantages and disadvantages to them in terms of ease of deployment and sampling efficiency; but they both collect samples of animals in the ocean so we can have a better idea of what's going on underneath us.

 IKMT recovery  caption: The IKMT net is brought back onboard the ship. The mouth opening of this net is about 5 ft x 5 ft. The metal fin at the bottom helps to keep the net going downward while it is towed. We also have a small flow-meter (little white propellor) which helps us measure how much water we have filtered through the net.

Once the net(s) have been deployed, we have to process their cod ends (cod ends are the buckets at the end of the net that hold the catch). Samples from the MOCNESS are split between the Bucklin and Warren groups on the ship. We preserve ours in formalin and back in NY we'll identify and count all the animals that we found. The Bucklin group preserves their animals differently so they can run genetic analyses on the animals they find.  We've been able to do some deep tows with the MOCNESS this trip (most down to 1000 m, and one down to 2500 m – that's more than a mile beneath the ocean surface), but we have yet to catch a huge amount of anything.

 One of the cod end catches from a deep MOCNESS tow. Seen here are shrimp, a deep-sea jellyfish, some small fish, and lots of smaller things.

We deploy the IKMT net to catch animals that are closer to the surface and we try to do the tows quickly enough that the animals are still alive when we get them on deck.  We then put them in aquariums we have on the ship to keep them happy, and proceed to do experiments on the animals.

 These are Antarctic krill (Euphausia superba) a large (4-5 cm in length) crustacean that is often the most abundant animal in the ocean here. These animals are a major food source for whales, seals, flying seabirds, and penguins. Additionally, they are also commercially fished. You may see bottles of krill oil capsules in health food stores. Several companies fish krill down here, then process it and sell it as a dietary supplement. There are likely some issues with the location (and size) of the fishery here and whether or not the fishers are competing with the native animal populations for the same resource.

One of the experiments we do on the animals is to measure their density (the mass of the animal relative to its volume). This is important to know for us to properly interpret our acoustic data in that animals that are just a small bit more (or less) dense than their surrounding seawater (or other animals) will scatter different amounts of sound.  So everytime we get live animals in our nets, two members of my team shut themselves inside a refridgerated shipping container we have chained to the deck of the ship and we make measurements of the density of individual animals relative to seawater.  So far we've made these measurements on over 100 individual zooplankton, ranging from krill, to shellless snails, to larval fish, to amphipods (another type of crustacean), and even parts of jellyfish.

Some of the equipment that we use to measure animal density. We do these experiments inside a refrigerated van to try and keep the temperature of our seawater as close to the ambient temperature that the animals normally live in. Unfortunately, we humans aren't as acclimated to the cold as the krill and other zooplankton are.  So we can only stay in the van for about 60-90 minutes at a time, before we have to leave and thaw ourselves out.

 Here are some of the data we've collected so far on this trip. The different color/shapes of the symbols correspond to different types of animals. The vertical axis is the ratio of the animals density to that of seawater. Most of our animals are between 1-5% more dense than seawater. This information is used by us to more accurately estimate animal populations from acoustic echograms.

If you look at the graph above, you'll see that I have a category (red diamond) for salps. But that there are no data points in our graph for them. That's been our biggest challenge so far this trip. We haven't caught any salps so far.  There are several possible reasons for this: salps are patchily distributed in the ocean so we may not have sampled in an area where they are; this cruise is a month earlier in the year than my cruise here last year (in December) so we may be too early to find salps; this could be a poor year for salps in terms of their health and reproduction; and it may just be too early in our cruise in that we haven't had enough stations yet.  Nevertheless, with or without salps, we are collecting data down here (on other types of zooplankton) that will be useful in trying to better monitor and understand the Antarctic ecosystem.

Sorry that was so long-winded. We scientists tend to ramble on and on about our research. We will now return to our normal blog-content (i.e. cookies).

Joe