Natasha Christman: Casting the CTD

A significant portion of my research project this summer investigates the characteristics of the Bellingham Bay hypoxia. We aim to create a solid description of the water column throughout the affected region, an endeavor which requires examining the biological makeup of the ocean layers and their relationship to the observed hypoxic conditions. Some of the information we are gathering helps us map the bottom water dissolved oxygen concentration, the depth at which the water transitions into hypoxia conditions, and where the oxycline, or the significant window of change in oxygen concentration, occurs.

As straightforward as this data collection might sound, there is significant material and method behind this task. The water in the bay we are focusing on is generally no deeper than 30m, but still beyond the reasonable limits of a scuba diver to take instruments down to the bottom and record the data. So to measure some of the basic but essential data like temperature, salinity, and dissolved oxygen concentrations, we send down a probe to do the work.

In our last cruise, we utilized a piece of equipment called the CTD sensor (Conductivity, Temperature, and Depth) to record this data. The CTD is a metal wheel featuring a myriad of sensors for picking up a variety of data and hosting Niskin bottles that when directed, will snap shut to collect samples from a specific section of the water column. Our particular CTD has six of these bottles, three on each side with the sensors in between.

The CTD on the deck of the research vessel. The green rope helps to secure it in transit.

At each of the research sites along our transects of Bellingham Bay, the CTD is carefully lowered into the water by cable and directed off the platform by two people on the ship deck. The cable hooks the equipment's sensors to a computer on board the ship. There, real-time data from the sensors are received, and we can monitor the probe's descent to the bottom and control its functions.

The CTD heading into the water.

The computer lies on top of a deck unit, which features a direct connection with the CTD's Niskin bottles. After the unit scales all the way to about 1-2 meters from the bay's bottom, at least one of the bottles is directed to snap shut. This captures a sample of water from that specific depth that we can bring on board and store for future examination in the lab. This ability to collect samples at depth is crucial for our research; water samples can be directly analyzed for chlorophyll and nutrients, both of which give us information as to what the biological characteristics of the water are at those points. Then we can relate some of this data to other aspects of the water column, such as the dissolved oxygen levels, and we can continue to develop our understanding of the make-up of the Bellingham Bay hypoxia.

The green "FIRE" button used to direct the Niskin bottles also makes this
portion of the research very dramatic. The toggle of the left lets you select which
 of the six bottles is deployed, and on the right, the unit keeps track of which are closed.

So later, when transferred from the vessel's computer and onto laboratory computers, we can graph the data collected from the cruise. For example, here is a graph of the temperature and salinity of the water column at station BB3 on a previous cruise.

Collected as part of the Northwest Indian College Bellingham Bay hypoxia study.

From the surface to a depth of about 5 meters, there is a significant change in the temperature and salinity of the water that then remains relatively constant throughout the rest of the sampling. This shows a unique characteristic of the bay: the stratification of the water due to the freshwater input from the Nooksack river. In the north side of Bellingham Bay, the Nooksack river drains into the larger saltwater body. The different densities of freshwater and saltwater mean that the two largely do not mix, resulting in a fairly defined layer of water on top that has different properties than the water underlying (the pycnocline). The gradient of temperature is called a thermocline, while the gradient of salinity is called the halocline.

Understanding this process is another important part to our research of the hypoxia of Bellingham Bay and can be helped understood by the data collected on the research cruises. Some properties of the water, such as the stratification seen here, can be observed by simple analyses like the graphing of temperature and salinity data. However, more complicated and intricate properties of the water body like the gradient of dissolved oxygen (oxycline) must be investigated by combining many different sets of data to formulate a defined model of the hypoxia. Thus, measurements such as chlorophyll, nutrients, and dissolved oxygen are taken and analyzed.

Later experiments with respiration and further research on the chemical and biological characteristics of the bay will hopefully shed more light onto this summer's project. This week, we start outlining the details of those experiments and investigate more of these components!