The CTD Cast: How Oceanographers Read a Column of Water

Lower a metre of instruments over the side of a research vessel on three kilometres of wire, and for the next few hours the ocean will confess its structure. The instrument is a CTD — conductivity, temperature, depth — and the act of lowering it from surface to seafloor and back is called a cast. Almost everything oceanography knows about the interior of the sea, from the anatomy of a marine heatwave to the layered structure we call stratification, arrived first as a CTD cast. It is oceanography's oldest and most trusted cut through the water column, and it remains the reference standard against which every float, glider and satellite product is calibrated.

The three letters

C — conductivity

Pure water barely conducts electricity; seawater, rich in dissolved salt, conducts well — and the more salt, the better it conducts. A CTD measures conductivity through a small cell, either by inducing a current in seawater flowing through a loop or with electrodes, and converts it to salinity. The conversion is not trivial (it depends on temperature and pressure too), but it is so well standardised — the Practical Salinity Scale of 1978 — that salinity values from any modern instrument anywhere in the world are directly comparable. Salinity is reported as a dimensionless practical salinity value near 35 for typical ocean water; for context and the wider chemistry, see our seawater chemistry primer.

T — temperature

A high-precision platinum thermometer reads the water to a few thousandths of a degree. That precision matters more than it sounds: density, sound speed and the fate of entire ecosystems turn on tenths of degrees, and climate signals on hundredths. Temperature sensors respond in fractions of a second, fast enough to resolve thin layers only metres thick.

D — depth (via pressure)

There is no depth gauge in the sea — you measure pressure with a strain-gauge sensor and divide by water density (plus gravity). Pressure is the one measurement that is easy and exact: one decibar of pressure is almost exactly one metre of depth, a happy coincidence of units.

To these core three, modern packages bolt on oxygen optodes, chlorophyll and turbidity optics, pH, nitrate, transmissometers measuring water clarity, and altimeters that stop the package a safe distance above the mud.

The rosette: the CTD's better half

A CTD measures; it cannot collect. So oceanographers mount it in the centre of a rosette — a ring of twelve to thirty-six open-ended cylinders called Niskin bottles, each holding a few litres. The package descends with every bottle open, profiling on the way down; on the way up, an operator watching the real-time trace fires bottles one by one at chosen depths, snapping them shut on water from the oxygen minimum, the chlorophyll maximum, the bottom boundary layer.

That water is the true payload. Back in the wet lab it is divided among a small crowd of analysts: dissolved oxygen here, dissolved inorganic carbon there, nutrients, chlorophyll filters, eDNA samples for biodiversity, isotope samples for tracer work. The bottles also exist to serve the CTD itself: lab measurements of the bottled water calibrate the electronic sensors, correcting the slow drift every sensor suffers. A world-class CTD section is a duet — electronics for resolution, bottles for truth.

How to read a profile

When a cast comes home, its plot — properties against depth — reads like a weather map of the vertical ocean.

• The mixed layer. The top tens of metres, stirred by wind and cooled at night, hold nearly constant temperature and salinity. Its depth tells you how hard the ocean has been worked by recent wind.
• The thermocline (and halocline). Below the mixed layer, temperature falls rapidly through the main thermocline; in rainy or estuarine regions salinity changes faster than temperature and sets the density structure instead. The strength of this jump is stratification itself — the barrier that decides whether nutrients from below can ever reach the sunlit layer, and the variable that marine heatwaves crank tighter.
• Water masses. In the deep ocean, temperature–salinity pairs fingerprint great water masses — Antarctic Intermediate Water's cool, fresh tongue, the salty warmth of subtropical mode water — transported thousands of kilometres from their birthplaces. A single deep cast reveals which "waters" a location hosts, like reading the strata of the sea.
• Fine structure. Thin inversions and steps betray double-diffusive convection, internal-wave breaking and intrusions — the small-scale mixing that moves heat and nutrients between layers.

Two casts ten kilometres apart over the same shelf can look like different planets: one showing cool, salty, nutrient-rich upwelled water tight against the coast, the other warm stratified water offshore. That difference is the story a kelp forest experiences.

Quality: the unglamorous heart of CTD work

CTD practice is a discipline of corrections. Temperature and conductivity sensors respond at slightly different rates, so a sharp gradient momentarily scrambles the salinity calculation — software must align the responses in time. Conductivity cells foul with slime and drift between cruises, so every profile is checked against bottle salinities analysed on a laboratory salinometer. Deep casts are corrected for the thermal expansion of the pressure case itself. This calibration chain — traceable to international standards, cruise after cruise — is what lets a cast from 1972 speak meaningfully to a cast from 2026. Long-term ocean change lives in differences of hundredths of a degree; without metrology that careful, the signal would be noise. The same philosophy anchors the accuracy statements in our impact methodology.

The lineage: from Nansen bottles to global grids

The cast's ancestors were reversing thermometers hung on a wire with water bottles that flipped and trapped their sample — the gear of the great 1920s Meteor expedition that first sounded the Atlantic in depth. Electronic CTDs arrived in the 1960s–70s and turned wire-time into real-time data. The 1990s World Ocean Circulation Experiment weaved thousands of casts into the first global deep-ocean survey; its successor, the GO-SHIP program, reoccupies those lines roughly every decade — oceanography's great longitudinal study, tracking how heat and carbon penetrate the deep sea. Against this backdrop, Argo floats now deliver daily global coverage of the upper two kilometres — but every new float type still earns its place by matching shipboard CTDs, cast by cast.

CTDs beyond the big ships

The instrument has democratised. Hand-lowerable CTDs serve small coastal programs and aquaculture farms; cheap DIY profiler kits have opened the practice to schools and citizen-science fleets — including oyster farmers logging estuary stratification on their lunch runs. Diver-carried instruments ride along on monitoring swims; moored CTDs sample a fixed depth for years; and the workhorse of our restoration monitoring — a compact CTD on the survey frame of our Drifter vessels — ties every reseeding operation to the temperature and salinity history of its site. Wherever a kelp forest stands or falls, there is a cast behind the decision.

Key takeaways

• A CTD measures conductivity (→ salinity), temperature and pressure (→ depth) with precision traceable to world standards; a cast is one lowering-and-raising through the water column.
• Rosette Niskin bottles collect water at chosen depths for chemistry, biology and sensor calibration — electronics for resolution, bottles for truth.
• Profiles reveal the mixed layer, thermocline and water masses; the strength of the density steps is the stratification that governs nutrients, heatwaves and kelp.
• Decades of careful calibration make century-scale ocean change measurable; shipboard CTDs remain the reference for floats, gliders and satellites.
• The practice has spread from global expeditions to farm skiffs and reseeding vessels — the water column is readable almost anywhere.

FAQ

What does CTD stand for?

Conductivity, Temperature, Depth — the three core measurements. Depth is derived from pressure; salinity is computed from conductivity, temperature and pressure together.

How accurate is a CTD?

Shipboard research CTDs resolve temperature to a couple of thousandths of a degree and salinity to a few thousandths (practical salinity), with pressure good to well under a metre of depth. Achieving those numbers year after year requires bottle calibration and careful lab standards.

Why collect bottles if the CTD measures electronically?

Sensors drift, and many variables — dissolved oxygen, carbon, nutrients, DNA — cannot be measured by the core CTD at all. Bottled water analysed in the lab both adds those variables and pins the electronic record to trustworthy references.

How deep do CTD casts go?

Full ocean depth — more than 6,000 metres — on deep-ocean expeditions, limited only by wire length and pressure rating. Coastal monitoring casts are typically tens to a few hundred metres.

Can non-scientists use CTDs?

Yes. Compact and low-cost profilers have put CTD-quality measurements in the hands of fishers, farmers, schools and dive groups, and much of that data feeds shared coastal observing programmes — the same open spirit as Argo's data policy.