For oceanographic studies and to sample larger volumes of water from the deep sea, we use a CTD Rosette Water Sampler. This instrument is equipped with sensors for conductivity, temperature, and depth (CTD) and a carousel of water samplers, which explains the name ‘rosette’. Other instruments like turbidity meters, oxygen sensors or acoustic current meters are added to the device to enhance the data set retrieved during a single CTD haul.
On board the research vessel, the instrument is lowered to a determined depth or to a few meters above the ocean floor, generally at a rate of about 0.5 m per second. A conducting wire cable attached to the CTD allows instantaneous uploading and real time visualization of the collected data on a computer screen. The water column profile of the downcast is often used to determine the depths at which the CTD-Rosette will be stopped on its way back to the surface to collect the water samples.
Although we can collect water samples with CTD Rosette Water Sampler during our annual expeditions to HAUSGARTEN observatory, we cannot occupy our study site year round to obtain information on seasonal changes. In an attempt to constrain the seasonal dynamics of nutrient cycles we use autonomous devices that act as remote access water samplers. These devices are integrated into our long-term annual moorings and are typically operated at shallow depths (<100 m). The samplers can be programmed for 48 discrete sampling events of 500 ml giving approximately weekly resolution over the course of an annual deployment. The samples are preserved during collection by mixing seawater with pre-determined fixatives. Currently we use these samplers to collect information on the seasonal cycles of dissolved inorganic and organic nutrient concentrations and prokaryotic and eukaryotic community composition.
For smaller water samples, we developed a water sampler for our Autonomous Underwater Vehicle (AUV). It consists out of an assembling frame with two revolving magazines, which carry eleven sample containers each. The volume of each sample container is 220 ml, giving the sample collector an overall capacity of 4.8 l sample material. In order to gather a large number of samples during a single AUV deployment, the volume of each sample had to be as small as possible, but big enough to meet the minimum demands of basic biogeochemical methods. A sample volume of 220 ml turned out to be the best compromise between scientific requirements and technical limitations.
Every container has an intake and outlet valve, allowing free flushing and filling of the containments when both valves are open. Flushing, filling and the final collection of a sample are processes driven by the motion of the vehicle. Two funnels, sticking out the vehicle's nose, “catch” the water and channel it to the sampler via tubes. At a pre-programmed time the valves of the water sampler are closed mechanically to isolate the sample. The instrument is depth rated to 4000 m.
Wulff, T., Lehmenhecker, S., Hoge, U. (2010): Development and Operation of an AUV-based Water Sample Collector. Sea Technology, December 2010: 15-19.