Plants of the Sea

Today we finished the 3rd of our “process stations” which are aimed at getting a more in-depth understanding of what influences the living creatures in the ocean at various locations. Some of the most abundant and most important sea creatures are phytoplankton. They are the tiny (microscopic!) photosynthetic plants that live in our seawater and form the base of the oceanic food web: phytoplankton are eaten by zooplankton such as krill, who are in turn eaten by fish, whales and penguins.

Oscar Schofield (Rutgers U.) and his team are on board to understand both the distribution of these phytoplankton and what controls their growth. There are many types of plankton, and light levels are one of the ways they differentiate their niches.  At each station his group sends optical sensors into the upper water column to collect data on light absorption and scattering, which is used to identify who is living where. By calibrating these measurements against satellites in space, we hope future satellite data can be used to predict phytoplankton community composition. We already use satellites to measure sea surface temperature and phytoplankton quantity – but being able to tell more about which types are there will help us make better predictions about the rest of the food web.

Oscar Schofield and his student Filipa Carvalho collecting seawater.

In addition to the optical sensors, more traditional methods of measuring phytoplankton are employed. This involves collecting seawater and passing it through very small (45 µm) filters. These filters are then analyzed back on land for both the quantity of phytoplankton and which species are present.

Left: Unused filter, Right: After 500 mL of seawater were filtered. Although the phytoplankton are too small to see in the seawater, here we can clearly see many diatoms are present (identified by their greenish-brown color).

Besides light, nutrient availability and grazers (zooplankton) also control phytoplankton growth. If there aren’t enough nutrients (i.e., food) like iron or nitrogen in the water not very many phytoplankton can grow. In contrast if zooplankton out number phytoplankton then they will be eaten as quickly as they can grow, keeping the population small. To understand which of these factors is most important in controlling phytoplankton blooms (large population spurts), graduate student Filipa Carvalho is doing incubation experiments that vary light, iron and grazer concentrations in clear bottles.

Recent doctoral graduate Mansha Seth-Pasricha and grad student Filipa begin an incubation experiment.

On this trip we only have time to stop at ~30 stations to collect data, so the Schofield group also has “gliders” which fill in the big gaps between these locations. The gliders are autonomous underwater vehicles (AUVs) which look like yellow torpedoes. They swim through the water, diving between the surface and several hundred meters depth. The gliders collect data on the current, salinity, temperature, oxygen levels, and biomass throughout the West Antarctic peninsula’s underwater shelf and then report it back electronically. Many oceanographers see AUVs as the future of oceanography because they can go out and collect large quantities of data for you to process from the comfort of your office. How convenient!