What instrument is commonly used to collect water samples at various ocean depths?

The Niskin bottle is a widely recognized instrument specifically designed to collect water samples from different depths in the ocean. This device operates by being deployed underwater, where it can be triggered to close at specific depths. The design of the Niskin bottle, typically made of plastic or metal, allows it to capture water in a sealed container, preventing contamination from external factors as it is retrieved. This capability makes it particularly valuable in oceanographic studies, allowing researchers to analyze the chemical, physical, and biological properties of seawater at various layers in the water column.

While other instruments listed may have roles in oceanography, they do not serve the same function of collecting water samples in a systematic manner necessary for depth-specific analysis. A CTD device, for instance, is utilized to measure conductivity, temperature, and depth but does not collect water samples itself. A "water sampler" could refer to various types of devices for collecting samples, but it lacks the specific design and purpose associated with a Niskin bottle. A hydrophone is a device used for recording underwater sounds and is not related to water sampling.