Check out A Scientist at Sea: California Current Research Cruise (Part I) as well!

This research cruise is about halfway over and we find ourselves within sight of Big Sur, off the coast of southern California. Everyone is searching, frantically, for water. The dark irony of it hasn’t escaped the scientists aboard the R/V Melville—we’re in the largest body of water on the planet, but can’t find the right kind of seawater we need to start another round of our experiments.

Before studying oceanography in graduate school, I never realized how diverse the ocean is. I mean, I knew that there are a lot of different animals in the ocean. But the reason we see so much variation in biology—from krill and penguins at our poles to brightly colored corals and fish in the tropics—is because the water that is home to these organisms is so different. Heat, salt, light, murkiness, and nutrients—all these environmental factors play a role in determining what sort of organisms will thrive in a given part of the ocean. At the microscopic scale as well, we see dramatic changes in the kinds of bacteria and phytoplankton that grow as the water they live in changes its characteristics.

Generally, phytoplankton growth in the ocean is capped by the limited availability of two primary nutrients, nitrate and phosphate. Similarly to how we need protein, carbs, and fats to survive, phytoplankton ‘eat’ these nutrient molecules to keep growing and dividing. However, the nutrient that the scientists on this cruise are really interested in is iron. Phytoplankton don’t suck up iron the way they do nitrate/phosphate, but when iron levels are really low it can become the factor which limits phytoplankton growth. Diatoms in particular start growing like crazy once iron is added to seawater that was previously low in the nutrient.

One of the main goals of my lab group on this cruise is to do a series of experiments to see how different kinds of diatoms cope with changes in available iron. Some diatoms, like the needle-shaped pennates, are able to store iron for later use using a protein called ferretin. This means that the rate at which iron becomes plentiful and is once more used up can change which diatoms are able to grow there. And because some diatoms can sink faster and store more carbon in the deep ocean than others, knowing which diatoms are the ‘winners’ in different nutrient scenarios can affect our understanding of processes such as climate change and marine nutrient cycling.

What this all boils down to is waking up in the middle of the night to fill 10-liter containers with seawater. We then add iron, DFB (a chemical that binds to iron and decreases the amount available to phytoplankton), or nothing at all to the containers and see what happens to the phytoplankton community over time. To make things even more interesting, we take some of the containers that we’ve previously added iron to and, about halfway through our experiment, add DFB to the containers so the iron we previously added is suddenly unavailable for the phytoplankton to use. In this way, we can see which diatoms do well with and without iron, and look at which among them might be using ferretin to store iron for later use.

We used the boxes you see above to hold our incubation experiments. Seawater pumped through the boxes kept our experiments cool and the phytoplankton happy.

Setting up these experiments, as well as filtering the water from them (see last week’s post for more on that fascinating topic) is what I spend a lot of my time doing on the R/V Melville. I’m in the lab a lot, but I’ve also spent time playing cards in the galley, resting in the sun on the ship’s stern, and watching swarms of dolphins race the Melville from it’s bow. Last week we were traversing an eddy further away from the coast and spent days passing through a bloom of By-the-wind Sailors, a type of small jellyfish. With the sun shining, you could see them bobbing in the waves all the way to the horizon. These jellies get their name by ‘sailing’ across the ocean’s surface using a prominent, sail-like fin. And if you look closely, you can see that below the water’s surface a skirt of iridescent-indigo tentacles helps them to capture food.

When we first boarded the Melville I felt a little antsy—stuck in a small metal box doing science day-in and day-out. It’s still weird to realize I have nowhere I can go (I try not to think about it too much), but I feel like I’ve accepted (embraced? made the best of?) life on a research cruise. It’s fascinating to see how the ocean changes day-by-day, or even hour-by-hour, according to the prevailing winds and currents. Hopefully it’ll change again soon, and we’ll get the water we need. Even if it doesn’t, there’s always something new to explore—that’s part of the beauty of studying the ocean; the fact that we know so little about what’s going on beneath its surface.