CTD+

The RBR CTD measures conductivity (a proxy for salinity or salt levels of the water), temperature and pressure (a proxy for depth in seawater) in seawater; CTD stands for Conductivity, Temperature and Depth. You can typically deploy this sensor at different depths to acquire temperature and salinity profiles of the water column at single time and location. Alternatively, you can fix it to a single location, like on a fixed mooring, to measure temperature and salinity through time.

CTDs have shed light on seawater properties and dynamics worldwide, contributing to our understanding of global currents and the ocean’s role in local to global climate patterns. In Hudson Bay, CTD measurements have helped communities discern layers of freshwater and seawater in the marine environment, information that is critical to understanding sea ice ecosystems.

The units for salinity collected by the RBR conductivity sensors are mS/cm, or millisiemens per centimeter, which can be converted to the more common unit, PSU (practical salinity unit, roughly equivalent to parts per thousand of salt). Normal winter sea water in Hudson Bay is around 30PSU. Over large parts of east-Hudson Bay, layers of freshwater from 23-26PSU can be seen from 15 m to more than 25m deep, layer on top of deeper salt water (which is closer to the typical 30PSU salinity).

The units for temperature are °C. Temperature measurements are important as freshwater and seawater layers also have different temperatures than can affect mixing as well as salinity. The freezing point of freshwater is also warmer than that of seawater, so it is important to keep track of both salinity and temperature when considering how oceanography affects sea ice formation.

The “+” here means that this RBR platform also includes one additional sensor that measures either: dissolved oxygen, fluorescence, photosynthetically active radiation (PAR), acidity (pH), and turbidity.

Dissolved oxygen sensors in seawater can be used to trace mixing of water masses with unique oxygen concentrations. In bodies of water contaminated by large quantities of organic matter, like sewage, dissolved O₂ is a good indicator of microbial activity, and the suitability of water for consumption. Specifically, low O₂ concentrations in seawater reflect a greater quantity of aerobic bacteria breaking down the organic matter through chemical reactions that use oxygen.

Fluorescence sensors measures light emitted by the pigment chlorophyll-a in living phytoplankton cells. Such measurements can inform us about phytoplankton abundances and primary productivity in seawater. Through the process of photosynthesis, phytoplankton harness the energy of the sun and turn it into biomass, providing the foundation of nearly all marine and freshwater food chains.

PAR sensors measure the quantity of light in seawater. Values are higher at the surface and decrease exponentially with depth.

pH sensors measure the acidity of seawater. The numbers provided are on a logarithmic scale, and are inversely related to acidity. For example, we know that the pH of battery acid is about 1, the pH of lemon juice is about 2, and the pH of orange juice is about 3. This means that battery acid is ~10 times more acidic than lemon juice, and ~100 times more acidic than orange juice. The pH value of seawater is on average 8.3 globally. The global community is concerned that the reaction of carbon dioxide released from fossil fuels with seawater, creating carbonic acid in the ocean, will lower the pH marine environments worldwide. Fractional decreases in seawater pH, from 8.3 to 8.2, for example, can significantly impact marine life.

Turbidity sensors a measure the concentrations of particles – inorganic or organic – in water. Turbidity levels are high in sediment-rich rivers and where many phytoplankton cells accumulate.

More technical information about this sensor can be found here.

This video shows how Inuit hunters deploy our sensors from RBR to capture a CTD+ depth profile.