Plankton communities and incubation experiments on GOMECC-3

Author: Mrunmayee Pathare

The microscopic phytoplankton that we study.

Our lab comprises some of the biological sampling being conducted aboard the Ronald Brown on the GOMECC-3 cruise. We study tiny ocean organisms called plankton which range in size from microscopic phytoplankton that use photosynthesis to produce energy, to millimeter sized copepods that can be seen by the naked eye “jumping” to catch their prey.

Phytoplankton form the base of ocean food webs, they are the tiny plants of the ocean, floating in the water column turning carbon dioxide into energy. Phytoplankton fix organic carbon found in the atmosphere and dissolved in the water into energy that is transferred through the food web by bigger organisms eating the smaller organisms. Most of these tiny organisms are eaten, but those that are not eaten fall to the ocean floor, drifting thousands of meters down the water column to be decomposed by bacteria. Phytoplankton fix 45 gigatons of inorganic carbon per year, and are an integral part of the mechanism removing CO₂ from the ocean (fixing it), and turning it into food that gets passed up through the food chain, or falls to the sea floor as marine snow.

A copepod predator that eats prey plankton.

On this cruise, we will be looking at the plankton communities in the top 5 meters of the Gulf of Mexico and who is eating whom. We are conducting a 24-hour incubation on a series of light and dark bottles containing seawater sampled by the CTD. Some of these bottles will contain only phytoplankton and small grazers, and some of them will contain phytoplankton and copepods. This set up will give us a snapshot of predator-prey dynamics at the base of the food chain (who is eating whom), how carbon moves through the base of the food chain in different conditions within the Gulf of Mexico (how much is being eaten and how it changes in different parts of the Gulf of Mexico). We also have some oxygen optodes fixed inside these bottles that will let us measure the amount of respiration taking place in the bottles during their incubation.

Gluing the optodes (tiny orange circle on the forceps) inside the bottles took a surprising amount of contortion and skill!

To simulate the environment that we are taking these little critters from, we rigged up an incubation tank on the back deck of the ship. We had to get creative with the materials and the location, and then strap it down securely so it won’t move when the Gulf decides to throw bad weather at us.

Incubation tank is strapped down safely and all ready to go. Some murmuring was heard about using the tank as a pool when there are no plankton bottles chilling in it.

Plankton soup is (not) on the menu! Black screen on the mesh bags holding the bottles is to control the amount of light available to the plankton.

The tank simulates the natural environment of the ocean and there is sea water constantly trickling through a hose to keep up the circulation and make sure the water inside the bottles doesn’t turn into plankton soup or get the photosynthesizing plankton fried by the sun.

We are conducting a total of 8 of these incubations over the course of the cruise, and although the results will be analyzed after we return from the cruise we are very excited to study the plankton communities of the Gulf of Mexico and contribute to the better understanding of carbon fate and transport.