The Mesocosms at IUI: Exotic vacation real estate for the travelling copepod

Remember all the zooplankton we collected in the Gulf of Eilat? For the past few days, they have been living in a different home: a tank at IUI filled circulating seawater. There are five experimental mesocosms inside this tank, where our brave copepods are swimming, eating, and (if all goes according to plan) pooping.

The copepods' new home/jail.

A mesocosm is an experimental system used to examine parts of the natural environment with more controlled conditions. In this experiment, zooplankton swim around inside of nylon bags through which dissolved oxygen can permeate. The bags' placement within the seawater tank also allows for regulation of temperature. By delineating ecological interactions and better representing realistic conditions, they function somewhat like very simple, very temporary terrariums. As a link between field sampling and more controlled laboratory experiments, these mesocosms can help better evaluate the relevance of prior data supporting the hypothesis.

We added phytoplankton feeding solutions to each bag, and mixed local dust into some of them as well. At the end of the experiment, we will flush/repatriate the zooplankton back into the sea, and collect all of the sediment left behind in the bags. Fecal pellets picked out of sediment samples from the different mesocosms will be analyzed for their carbon content, and we will see if there are differences between pellets from the "dusty" bags and those from the "control" bags.

Eyal provided the four wider mesocosms with cylindrical bags; we will also use these in a similar experiment in Haifa. A UCSC engineering student named Gilberto designed the taller mesocosm in the middle, which uses a conical bag and includes a removable sediment trap at the bottom. 

Gilberto's technical drawing of the taller mesocosm, and closeups of the finished product before use.

The plastic bottle functions as a sediment trap which can be opened and closed with a ball valve for easy retrieval. It is stabilized with a plexiglass holder.

Every day, I measure water quality parameters inside each bag, and take samples to monitor zooplankton concentrations. I also sample for nutrients and trace metals. We built some brick "towers" for easier access to the taller mesocosm in the middle of the tank; this way I remain (mostly) dry throughout the sampling process. It's fun to get wet, but less fun when you need to sit in the microscope room immediately after sampling to count zooplankton! 

Mesocosms and miniature "towers" for improved access while sampling.

Sampling and counting zooplankton from each mesocosm every day allows me to monitor copepod concentrations over the course of the experiment. I pour a liter of mesocosm water through a filter through which copepods cannot pass, providing a sample which I then examine under a microscope. The number of copepods I count reflects their concentration per liter within the mesocosm.

A microscope photo of a filtered mesocosm sample, showing a copepod (right) and nauplius (left).

Panoramic view showing several experiments at IUI. The seawater tank holding the mesocosms is right next to the coral tanks where Karen works.

In a couple of days, we will dismantle the mesocosms and collect sediment samples from them. Here's to hoping that we find some fecal pellets!