The sun rose above the back decks this morning as we traveled towards Pavlof Bay for our station. As we made our way through the Aleutian Islands, the peaks of active volcanoes Mount Pavlof and Pavlof’s Sister became visible above the clouds. The Aleutians are part of the Pacific Ring of Fire, home to about 75 percent of the world’s active volcanoes, and the volcanic islands represent the boundary between the Pacific Ocean and the Bering Sea.
After the Pavlof Bay station we turned northwest, heading towards the Bering Sea via Unimak Pass. The Bering Sea was named after Vitus Bering, a Danish sailor who explored the area in 1728 for the Russians. The Bering Strait, which during the last ice age is thought to have allowed humans to migrate from Asia to North America, is named for the same man. Unfortunately, Vitus Bering did not make the return trip from his exploration of this region. A storm forced his ship, the Saint Peter, to seek refuge on an uninhabited Commander Island in the southwest Bering Sea. There, Bering and many of his crew died from illness and potentially scurvy, a vitamin C deficiency that was common in the early days of seafaring. Almost a year later, about half of the original crew did return to Russia on a smaller ship that theybuilt from the remnants of the Saint Peter. Those were some tough sailors!
From an oceanographic perspective, the Bering Sea is interesting because it provides a connection between the Pacific and Arctic Oceans. Warmer waters from the Pacific Ocean flow northward through the Bering Sea into the Arctic Ocean. This water is also rich in nutrients (e.g. nitrogen, phosphorous, etc.) needed for phytoplankton growth and picks up even more nutrients as it flows through the porous Aleutian Islands. As previously mentioned, sea ice also plays an important role in phytoplankton growth. During the coldest winter months, sea ice blocks light from the surface ocean. Light is one of the main requirements for photosynthesis, and so phytoplankton are largely unable to grow during that time. But when the ice begins to melt, light penetrates into the water, and nutrients that were locked up in the ice become available for phytoplankton to use.
After two stations where Kali and I have collected about 4 gallons of seawater, we have worked out many of the initial kinks in our sampling scheme. To quantify the amount of phytoplankton present at a location, we use a variety of tools and methods, and so for three or four hours after sampling we are always in a slight frenzy of activity. The most basic method we use, and yet somehow the most time consuming, is to collect water and concentrate the phytoplankton onto a very fine mesh or filter. We collect water from specialized Niskin bottles, which are mounted on a frame and lowered off the side of the ship. The bottles are open at both ends when they enter the water and a magnetic switch allows us to close them at specific depths, thus neatly trapping the water from those depths for our use once the frame is hauled back to the ship’s deck. The filters we use to concentrate the phytoplankton have holes that are less than one micrometer in diameter. The hair on most people’s heads is about 100 micrometers wide, so we are able to capture anything that is bigger than one-one-hundreth of a hair width. That’s pretty small! When you pass a sizable volume of seawater through these filters, the phytoplankton become visible as brown or green colored “stuff.” And when you have filtered as much seawater as I have over the years, just the amount of color on a filter can provide a general estimate of phytoplankton concentration!
We will ship many of these filters back to Lamont-Doherty to use in a variety of analyses. Tests for photosynthetic pigments such as chlorophyll will help us determine general concentrations of phytoplankton. Chlorophyll is common to virtually anything that photosynthesizes and is the compound that gives plant leaves their green color. However, as many New Englanders witness every year, plant leaves can also turn various shades of browns, reds, and oranges as the temperatures warm and Fall approaches. These striking color changes are due to other pigments that are involved in photosynthesis, and in the ocean they are often specific to different types of phytoplankton. Actually, the brown color on the filter shown to the right is from one of these pigments that is most common to the dominant type of phytoplankton we have seen out here so far. Analyses for these various pigments using high pressure liquid chromatography (HPLC) will therefore allow us to understand what groups of phytoplankton are present at different locations. Finally, analyses for carbon and nitrogen within the cells will allow us to determine if the phytoplankton have enough nutrients for growth. While filtering water may be the least exciting of all the methods we use, a huge amount of data can be gleaned from just a small handful of filters!
In the coming days I will introduce some of the more exciting instruments we are using to obtain data on the phytoplankton in real-time out here. We just passed Mounts Isanotski and Shishaldin, two more dramatic volcanoes in the islands, and we are en route to Unimak Pass where operations will resume early tomorrow morning. After a delicious dinner of baked halibut and a productive evening, my upper bunk calls. When I awake tomorrow morning we will officially have crossed into the Bering Sea!
(unless otherwise noted, all photos: B. Stauffer)