February 15: Day eight – processing Sargassum samples
At each science station along our path across the Sargasso Sea, the science team has been using fine mesh nets to collect clumps of Sargassum seaweed and the associated community of plants and animals that live around, inside, and under the floating seaweed habitat. Once the seaweed is brought aboard the ship in buckets of seawater, we inspect pieces of it for larger animals—fish, crabs, nudibranchs, sea anemones—anything that is large enough to pick out with forceps and separate into containers. These are the lions and tigers (the top predators) of the Sargassum world, but in reality they are small, only a few centimeters at most in size.
Next we gently tap the seaweed against the sides of plastic bins to dislodge the more persistent inhabitants, like the Sargassum crabs with sharp claws for holding on. After shaking off any animals that both hold tight and are well camouflaged, the real diversity of the Sargassum world reveals itself. What accumulates at the bottom of the bin is a soup of golden-colored shrimp, amphipods, and snails, many of which are smaller than a fingernail in length.
The seaweed pieces are then rinsed with freshwater to wash off even more animals that might have been holding on tightly to their seaweed home. All the while the team of scientists is busy with forceps and sharp eyes to separate the animals by general taxonomic groups. Crabs with crabs, nudibranchs with nudibranchs, and anything unusual or that we want to save for a second look. It’s a coordinated procedure with everyone working together to get through the buckets of samples as quickly as possible.
Since we collect only a portion of the Sargassum community, we need to quantify the amount of seaweed we are processing. To do this we use a displacement volume calculation. A large graduated cylinder is filled with a starting amount of water and then Sargassum is added to the cylinder. This causes the water line to rise, and the amount of Sargassum in the cylinder is proportional to the water displaced. If you start with 200 milliliters of water and then put a bunch of Sargassum in until the water line rises to 300 milliliters, then the displacement volume is 100 milliliters, meaning 100 milliliters ofSargassum. The standard sample at each station involves sifting through about 10 liters of Sargassumseaweed.
After most of the animals have been separated from a standard volume of Sargassum habitat, we can begin to do an inventory of the abundance and diversity of the animals collected. Kathleen Sullivan Sealey is working on cataloging the different species that we’ve found. To keep the data organized, she developed the species inventory that we use for each sampling station that records the date, location, and notes about the samples. At the beginning of the expedition, Ken Smith surveyed the scientific literature and reviewed his previous survey work in the 1970’s to develop a list of species that were documented to be common and abundant in the Sargasso Sea. They will compare this historical information with what we’ve found during this expedition. What has happened to the Sargassumcommunity over the past 35 years? Do these observations change from Bermuda to the Bahamas, and if so, how? If the Sargassum community has changed, are new species appearing or other species disappearing? What does this mean for the overall ecology of the open sea?
Kathleen uses a microscope and taxonomic keys to figure out the identity of the species in the samples. She sometimes needs to count body parts, distinctive shapes, historical range and other characteristics and compare these to information from scientific publications, books, and taxonomic databases on the Internet. She also takes lots of photographs of the samples, including photos taken through a microscope, to record the findings and help the overall knowledge about this area. From the collection of the first bucket of Sargassum to the final inventory of preserved samples, this work takes about five to six hours, and the expertise of all the scientists on the team.
— Debbie Nail Meyer