The Johnson Sea-Link II deploys a larval trap. In the background, juvenile mussel settlement cages rest on the mussel bed. These larval traps allowed graduate student Shawn Arellano to measure the abundance of mussel larvae in the area.
Trapping Larvae at the Brine Pool
Student Report
February 12, 2003
Jule Schultz, Master's Candidate
Oregon Institute of Marine Biology
University of Oregon
Shawn Arellano, a doctoral student at the Oregon Institute of Marine Biology, is studying the ecology of mussels that live 2,100 ft below the surface of the Gulf of Mexico in an area known as the Brine Pool. Here, thousands of these bivalves (Bathymodiolus childressi) live crowded around a dark "lake" of super- salty (hypersaline) water, up through which bubbles methane. The mussels prosper in this unique environment because they harbor symbiotic chemosynthetic bacteria, which use the methane to produce energy for their mussel hosts.
The density of mussels decreases away from the Brine Pool, as does the number of newly settled mussels. Mussels settle on hard substrata (usually other mussel shells) and attach themselves with stringy bissel threads. Shawn is investigating whether mussel larvae, which are tiny, free-swimming bivalve babies, preferentially settle in areas where other mussels already exist. Specifically, she wants to know how the number of settlers on her experimental, mesh recruitment panels correlates with larval abundances. To do this, she needs to know if mussel larvae are present at greater densities near the Brine Pool than away from it. Today, I helped deploy larval traps that aim to help her answer these questions.
The larval traps consisted of vertical PVC pipes held on the ocean bottom with 5-lb circular weights. Formalin, a dense liquid that is deadly when concentrated, was placed inside the PVC pipe. Larvae, brought to the area by deep ocean currents, will fall into the traps, preserving them so that Shawn can count them when they are retrieved in six months.
The traps were placed in the box on the outside of the Johnson Sea-Link II submersible for transportation to the ocean floor. Soon, we descended to the Brine Pool and got to work. From the back of the submersible, I directed the pilot where to place the traps. The mechanical arm gently lifted each trap from the box and set it beside a recruitment panel. The cellophane, which prevented formalin from leaking out on the way down, was removed by lifting a rope that popped a rubber band attached to the cellophane. This painstaking process was performed for nine traps: three near the mussel bed next to the Brine Pool, three in the mussels away from the Brine Pool, and three in a sandy area without mussels even farther away from the Brine Pool.
Readings were taken above each larval trap using a current meter attached to a stick, which was delicately maneuvered using the submersible's arm. After these tasks were completed, it was time to ascend to the surface. We grabbed the current meter that had been accidentally left on the bottom from a previous dive, and floated upward through a bioluminescent light show.
We reached the surface in about 30 min and were lifted safely back on the ship. I jumped out of the submersible, glowing from my experience. Using some of the most advanced technology available to study this wonderful and unique ecosystem was truly an experience I will never forget.
Linking the Shallow and the Deep
Student Report
February 12, 2003
Craig Everroad, Doctoral Candidate
Oregon Institute of Marine Biology
University of Oregon
Today was an extremely busy and exciting day that included two dives at the Brine Pool. In the morning, I was aboard the Johnson Sea-Link II for my first submersible dive, and upon my return to the surface, I was quickly pressed into service. I was given the opportunity to direct the second CTD cast of the cruise and so had little time to reflect on my first trip to the bottom of the ocean.
The calm seas and blue skies were good omens as the Johnson Sea-Link II descended into the deep to deploy equipment, collect samples, and explore. We were out to collect samples of the invertebrates Bathynerita, Acesta and Lamellibrachia, and we promised a colleague we would bring back bacterial mat samples from the sediment near the Brine Pool. Although we experienced a few mechanical setbacks, the dive was successful. We brought back numerous specimens to the delight of the deep-sea biologists aboard the NOAA Research Vessel Seward Johnson, including the first bacterial mats collected on this cruise.
As a phytoplankton ecologist, I am primarily interested in the upper part of the ocean called the photic zone. Since light is strongly attenuated by seawater, my area of expertise quickly fades with the growing darkness of the deep ocean. Fortunately, I had the opportunity to work with scientists interested in the life cycle of benthic molluscs. Despite their apparent independence from the surface (and sunlight) at depth, these molluscs reproduce on an annual cycle and their larvae are known to enter the water column and eat the very phytoplankton I study. One area of interest has been the diet and distribution of these larvae, but hundreds of meters of seawater seem to separate the predator from the prey. Thus, I was given a unique opportunity to look at the distribution of phytoplankton in the deepest waters above the Brine Pool, and the tool at my disposal was the CTD.
"CTD" stands for conductivity, temperature and depth, and is the name of a probe used to vertically profile the water above a particular spot in the ocean. Not only does a CTD continuously record this data as it is lowered and then raised through the water, it also includes a number of closable bottles that can be used to collect water samples from anywhere along the probe's path. On this cast, I focused on the deeper water to see if phytoplankton were present. Though phytoplankton grow only where there is light, they can sink or be carried downward by currents and provide food to the organisms who make their living deep in the ocean. If these plankton are in fact present at depth, they not only provide an important link between the ecology of the deep sea and the surface, but they are also a nutritious snack for deep-sea organisms.
