10 - Small plants in the ocean

Gayatri working her magic

Small plants in the ocean known as ‘phytoplankton’ are the primary producers in the food chain of the ocean. They are almost invisible to the naked eye (find out why these marine plants are so small).

More than two-thirds of the Earth’s surface is covered by Oceans, and there are huge quantities of these tiny plants. Large enough in fact to make up approximately half of the global biosphere production!

Being the primary elements of the food chain they affect the abundance and diversity of marine organisms throughout the food-chain and play a major role in marine ecosystem functioning and fishery yields. Since they are plants they take up carbon dioxide, and hence, they are a major sink for the carbon dioxide in the atmosphere.

Considering their importance to the carbon cycle, it is vitally important and indeed our duty, to study them. That means we need to estimate their quantity and presence in the different regions of the ocean. To do so we have to ask some big questions. For example how does their concentration vary throughout the water column? How does their size vary with depth and geographic location? Which species dominate with the seasons of the year and on decadal timescales?

To answer these questions we need a better understanding of the different factors that affect phytoplankton, how are marine organisms affected by human activities and in turn how do they influence our own environment. Confused? These are not trivial questions, even for scientists.

On this cruise I am trying to answer a few of these questions in relation to the PAP site. For example, how many of these plants are present in these waters? How does their quantity and size vary as we enter the Twilight Zone? Now, I know you may wonder how we measure plants not even visible to the naked eye. Well, here's the simple answer; every plant on Earth contains the pigment 'chlorophyll a.'

The deployment of the CTD

Once we recover water samples from the Twilight Zone using the CTD Rosette (image right), we use an optical instrument that bombards the chlorophyll with a particular wavelength to make it fluoresce or glow, and the amount of fluorescence is a direct proxy of the amount of chlorophyll, and therefore phytoplankton the seawater contains.

Once that is done, we can move to answering some of the more complex questions.

By Gayatri Dudeja, PhD Student, NOC