Oceanography Sampling Techniques: the CTD
and bottle Rosette

Mike Lucas
26 April 2008

The most common way of measuring ocean temperature and salinity while also taking water samples from any depth down to the ocean floor is by using an electronic conductivity (= salinity), temperature and depth (from a pressure sensor) probe (a CTD) attached to a circular stainless steel or titanium frame. Cylindrical water sampling bottles (12 or 24) made from high density PVC of 10, 12 or 20L capacity are arranged around the frame. The bottles have electronically controlled closing / opening caps at either end and a tap (or two) at the bottom of each bottle to draw water from when they are brought back on-board. The CTD rosette frame may also carry other instrumentation such as upward / downward looking ADCP as well as oxygen, nitrate and turbidity sensors, light meters to measure downward light attenuation and fluorometers to measure phytoplankton pigment (chlorophyll-a) fluorescence. More recently, fast repetition rate fluorometers (FRRF) have been added which measure active fluorescence by phytoplankton to provide a measure of physiological and growth status of phytoplankton in response to light and nutrient availability. [insert photo?]
All the electronic information is fed to main lab computers via a conducting core of the steel cable used to lower and retrieve the whole instrument through water column, sometimes to depths of 6000m (6km). At a typical lowering and recovery speed of 1m/sec, CTD stations to the ocean floor 6km down can take 4-6 hours to complete. When the CTD package is lowered, we can monitor continuous and real-time depth profiles of all the variables being measured; such as light attenuation, temperature, salinity, oxygen, nitrate, chl-a and phytoplankton FRRF parameters. This allows us to see the physical and biological structure of the water column. In the North Atlantic in summer we might typically observe warm salty nutrient depleted water in the surface to depths of ~60m. At about that depth, determined by the rate of sun-warming, we expect to observe a transition between the well-lit (euphotic) surface ocean to the dark (aphotic), cold but nutrient rich ocean interior. This transition region is marked by steep light, temperature, salinity, chl-a and nutrient gradients associated with the thermocline, halocline and nutricline. From the combined thermocline and halocline, we can derive the density transition boundary between buoyant surface waters and denser deep waters – the pycnocline.
All of these data that stream into the computers provide a wealth of information. Temperature (T) and salinity (S) data can be used to construct T/S plots to provide a unique “signature” that can identify the origin of different water masses. Differences in salinity profiles between adjacent and consecutive CTD profiles can be used to calculate geostrophic flows at different depths throughout the water column. The ADCP instrumentation provides a measure of current velocity (speed and direction) while the light meters give information on the underwater light field experienced by phytoplankton. Oxygen sensors and in situ concentrations tell us much about biological activity. Where oxygen concentrations are high in surface waters, this is indicative of active photosynthesis in the euphotic zone, where carbon is fixed by phytoplankton but oxygen is also evolved. Deeper in the water column there is usually an “oxygen minimum” layer (~200-500m) where in the absence of light, heterotrophic bacteria decompose sinking particulate organic material, but in doing so, use oxygen. Much deeper down in the water column, oxygen concentrations rise again, but this is not due to local regeneration of oxygen. Instead, these higher oxygen concentrations are associated with water that has sub-ducted from the surface at high polar latitudes and been advected horizontally through the ocean basins at depths of 2000-3000m, carrying with it the oxygen signature of the atmosphere from those far off polar locations. Chl-a fluorescence profiles typically reveal that maximum phytoplankton biomass peaks a third of the way down through the sunlit surface euphotic layer and diminishes quickly towards the poorly lit base of the euphotic layer at the thermocline / pycnocline boundary. Phytoplankton growth does of course strip the nutrients (nitrate, silicate, phosphate) from the surface mixed layer above the thermocline, although nutrient concentrations increase quickly across the thermocline / pycnocline boundary. Indeed, most nutrient regeneration occurs in the deep ocean and the upward flux of nutrients is dependent apon water column overturning in winter or wind-driven upwelling. The distribution of nutrients and light in the water column provides phytoplankton with a problem in well-stratified waters of the summer. Near the surface, there is plenty of light but they become nutrient limited, while conversely near the thermocline, nutrient concentrations are higher but there is the potential for light limitation. The competition for light and nutrients shapes phytoplankton community structure. With sufficient light and nutrients in spring and early summer, diatoms usually prevail, but as nutrients become limiting, other smaller phytoplankton (eg flagellates) take over that are better able to compete for nutrients at lower ambient concentrations. Taking advantage of nutrient diffusion across the thermocline, such phytoplankton may aggregate into a deep chlorophyll maximum layer of just a few feet to meters in band-width, while they also typically show physiological adaptations to low light availability that charecterises this region in the water column.
When the CTD arrives on deck, there is a frenzy of activity and competition amongst scientists to collect the samples they need – and there is a strict “pecking order” of priority. Water to measure oxygen (or other gaseous measurements) is collected first, followed by water for nutrient analyses. Both of these measurements are used to verify and calibrate those same measurements derived from the electronic instrumentation on the CTD. Next, water for some biological measurements is collected, usually for phytoplankton taxonomy (by microscopy) and for chl-a concentration as a measure of biomass. Then water is collected for the various biological rate measurements. These can include measurements of phytoplankton production (most commonly using 14C radio-isotope incorporation), measurements of phytoplankton nitrogen cycling (using 15N stable isotopes), measurements of heterotrophic bacterial growth rates (using 14C-glucose radio-isotope incorporation) and phytoplankton physiological processes (using FRRF).
Once this mosaic of data is assembled into a completed jig-saw puzzle, it provides us with the information we need to calculate the strength and magnitude of ocean circulation as well as the impact of the “biological carbon pump” and its effectiveness (or not) in removing the anthropogenic atmospheric CO2 thought to contribute to current rates of global warming. However, while some regions of the world oceans are relatively well sampled (e.g. North Atlantic), other oceanic regions remain poorly sampled. Future cruises to the more remote oceans (e.g Southern Ocean) will be needed to conduct the large-scale CTD surveys required to complete a global picture of information on physical and biological oceanography of use to climate change specialists, fisheries biologists and governmental policy-makers to provide the best management advice needed to preserve our “blue planet”.