12 Dec - Ship-board Experiments

Part of our work on this cruise is to find material properties of salps and krill. These material properties include, among other things, the densities of such organisms. Density is important to know if you want to differentiate organisms from one another in acoustic scatter data. We are using different techniques to find the density of krill and the density of salps. Both rely on the principle that when an animal is placed into a liquid that is less dense than it, the animal will sink. When the animal is in liquid denser than it, the animal will float at the surface. Finally, if the animal is placed in liquid that is the same density as it, the animals will (you guessed it) become neutrally buoyant and float!

To find the density of salps, we start with a known amount of seawater, in which salps are negatively buoyant, or sink. We then add hyper-saline solution (i.e. saltier and therefore denser than seawater) very slowly using a burette, a long glass tube with increments of volume marked. This process of adding one substance slowly in measured increments is called titration. As soon as the salp starts to lift off the bottom of the container and become neutrally buoyant, you know that the density of the solution in the container is the same as the density of the salp. Using volume and salinity of the ending solution in the container with the salp, you can calculate density! 

Here you can see Katie doing a density experiment on a salp. Notice the burettes (the long tubes), the bottles holding the seawater and hyper-saline solution used to add to the container holding the salp, and the YSI instrument used for measuring salinity.

Krill are generally denser than salps, so mixtures of glycerin are used instead of seawater and hyper-saline solution. A glycerin solution of known density is prepared and placed into a beaker or graduated cylinder. A krill is placed into solutions, one by one in order from least dense solution to more dense. We note in which solution the krill becomes neutrally or positively buoyant, and can therefore approximate the density of the animal based upon which solution made it float rather than sink (i.e. was the same or of greater density). 

Here are the different solutions of glycerin in order from least to most dense, as well as the glycerin used to make the solutions.

Size of the organism and temperature of all the different solutions used are important to both methods. Therefore, we measure both and record all of the data.

Using the refined densities of salps and krill, or G-values as they are called in acoustics, scientists like Joe can refine their interpretation of acoustic data to be more specific as to which organisms they are seeing in their acoustic data.

- Karen