Science Results : Daily Update

Daily Update | Current SEA Research

June 19, 2010

By Giora Proskurowski

In addition to surface net tows focused on plastic there is a lot of other information we record so that we can place our plastics data within a scientific description of the ocean environment from where they came. The primary oceanographic tool, and one of the most useful sources of data, is the CTD. This is a sensor that measures the conductivity (salinity), temperature, and depth (CTD) of the water through which it is lowered. This information gives us a snapshot of the density structure of the water column, providing a framework to evaluate additional observations. We do a CTD cast every day, and will be increasing the frequency of these casts to several times a day when we begin our westward leg.

On our current CTD package we also have sensors that measure the relative amount of chlorophyll-a, the oxygen levels, the water clarity, the amount of light, and the amount of colored dissolved organic matter (CDOM). Because we are primarily interested in the physical forcing that may distribute plastic in the upper water column, we are most interested in just the upper layer of the upper ocean.

The ocean water column can be broken into three levels: starting at the top, the "mixed layer" is a layer of uniform density that is easily mixed by wind, saturated with sunlight, relatively uniform in temperature, and low in nutrients. Below is the "thermocline", where temperatures begin to cool, and a zone where life flourishes in the gradients between light and nutrients. Extending from the base of the thermocline (somewhere around 500 meters depth or below) to the seafloor is "deep water", rich in nutrients, but cold, dark and sparsely populated.

Although we have several thousands of meters of wire on the winch that we lower the CTD package on, for this trip we're just lowering to 200 meters, and we are really only interested in the top 50 meters or so. The mixed layer depth has been around 20 meters for the bulk of this expedition. Our idea is that within this layer plastic fragments can be easily distributed, and that what we measure in our surface nets is just a fraction of what is actually present in the upper ocean.