Oshoro-maru Marine Science Department, Facility of Fishery Science, Hokkaido University
Seawater is mostly water (H2O) and composed of a mixture of various substances in ionic, colloidal, gaseous, and granular states. The major component dissolved in seawater is sodium chloride and other inorganic salt ions, which account for about 3.5% (by weight) of seawater and are commonly referred to as "salt content. Most of these major components are chlorine (Cl-), sodium (Na+), sulfuric acid (SO42-), magnesium (Mg2+), calcium (Ca2+), and potassium (K+), and the concentration and existence ratio of each ion is almost constant. On the other hand, inorganic salts include compounds composed of nitrogen (N), phosphorus (P), and silicon (Si), which are used for phytoplankton growth and called "nutrients," although the ratio of their presence is small. Other "gaseous components" such as nitrogen (N2), oxygen (O2), and carbon dioxide (CO2), "organic compounds" such as proteins and lipids, and "trace elements" such as iron (Fe) that exist in very small quantities are dissolved in seawater. In addition, seawater also contains non-dissolved substances, such as bacteria and other microscopic organisms, their remains, and mineral particles.
Figure 1 Components of seawater
塩分などの溶質：Salts and other solutes 水：Water 溶質の内訳：Solute breakdown
Each component of seawater is constantly changing slightly with the movement and mixing of seawater, biological and chemical reactions occurring in seawater, and seasonal and global environmental changes. Understanding these temporal and spatial variations, we can clarify various phenomena occurring in the ocean. Seawater samples must be collected to measure each of the components of seawater. Collecting seawater samples is called "sampling," and the method of collecting seawater samples without mixing with other layers below the water's surface has evolved along with the development of oceanographic observation techniques. This section introduces some of the methods used to collect seawater samples.
Seawater sampling methods are broadly classified into three categories: scooping up surface seawater ("bucket sampling"); continuously pumping up surface seawater using a pump equipped on the bottom of a ship ("pump sampling"); and collecting seawater below the water surface using an instrument called a water sampler ("vertical sampling").
The development of analytical techniques has made it possible to detect trace elements in seawater. It has led to innovations in water sampling methods to accurately measure slight differences in seawater composition and the amounts of trace elements. In recent years, in particular, the distribution of trace elements in seawater has attracted attention as a key to clarifying the dynamics of biological activities in the ocean, and a method has been proposed to collect seawater samples with as little contamination as possible during the sampling process. This method, called "clean water sampling," has led to improvements in the shape of sampling installments and the establishment of precautions for handling sampling installments.
Collecting water from a vessel and obtaining information through seawater analysis requires a great deal of time and effort. Therefore, underwater sensors have been developed to measure some characteristics (salinity, temperature, dissolved oxygen concentration, pH, etc.) and these are measured directly and continuously on site (in the sea). However, there are still some analytical elements for which sensors have not been developed, and for which seawater samples are analyzed to obtain high-precision information that cannot be obtained with sensors.
Bucket sampling is the simplest method for collecting surface water. Usually, a bucket with a rope tied to it is thrown into the sea to scoop up surface water while the vessel is stopped, such as during CTD observations. Water should be collected, avoiding the outlets for wastewater flowing from the ship, engine cooling water, and sewage discharged from the ship's hull. The trick to collecting clean surface water is to throw the bucket as far away from the ship as possible and lift it on board without allowing it to come in contact with the hull. If the bucket comes in contact with the hull, the seawater may be contaminated by hull paint or algae growing on the hull's surface.
Figure 2 Bucket sampling
By using a pump, surface seawater can be collected continuously.
The seawater supplied to onboard laboratories and other places necessary for research is called "research seawater." It is distinguished from "miscellaneous seawater" used for cooling ship engines, washing hulls, and sanitary purposes by the seawater intake, pumps, and piping on the ship's bottom. In addition, several considerations are made in the pumps and piping used for intake of seawater for research purposes in order to suppress changes in liquid quality. While the quality of seawater for miscellaneous use is changed by adding drugs or killing microorganisms through electrolysis at the time of seawater intake to prevent organisms from adhering to the piping, seawater for research use is taken without changing the composition of the seawater as much as possible. For this reason, pipes with resin-coated inner walls are used to prevent corrosion inside the pipes, and pipes are cleaned periodically to remove contaminants deposited inside. The "research seawater" thus pumped can be collected at any time during the voyage, regardless of the ship's navigation or weather conditions, so that information on the sea surface (water temperature, salinity, etc.) along the route can be continuously measured and recorded by feeding the surface seawater monitoring system (Figure 3, 4).
Figure 3 Measurement section of the surface monitoring system of the Oshoro Maru (Hokkaido University)
クロロフィル・濁度センサ： Chlorophill and turbidity sensor 水温・塩分センサ： Temperature and salinity sensor 流入口：inlet 研究用海水：Research water
The surface monitoring system consists of a combination of a measurement unit, which is a container filled with surface seawater and equipped with various sensors, and a control unit that records the acquired data or distributes the data to various key locations. The measurement unit is located near the sink in the laboratory and is constantly supplied with seawater for research purposes. The tank is filled with seawater, and two underwater sensors inserted into the tank measure water temperature, salinity, chlorophyll, and turbidity, respectively. The tank is designed so that research seawater is supplied from the bottom of the tank and overflows from the top, preventing the generation of air bubbles that could cause abnormal sensor readings. In addition, the tank is made of light-shielding and non-reflective material so as not to affect the optical sensors (chlorophyll and turbidity). The information acquired by the measurement unit is stored in a PC for data recording via a cable connected to the sensor, and is displayed sequentially by dedicated software.
Figure 4 SST observed by the surface monitoring system (SST: Sea Surface Temperature)
On the other hand, seawater collected by the ship-bottom pumping method contains metallic elements eluted from the hull and piping, and thus cannot be used as a test water for the analysis of trace metals in seawater. Therefore, a towing body method is used to continuously collect seawater over a wide area with little contamination from the vessel's hull.
A boom is used to lower the towing structure away from the hull of the vessel, and the vessel is then allowed to navigate, pumping seawater overboard from the end of the towing structure. In this way, seawater samples can be collected without contact with the hull (Figure 5).
Figure 5 Conceptual diagram of water sampling of a towing body
ポンプ：pump 採水ホース：water sampling hose 曳航ブーム：towing boom 曳航体：towing body
Vertical Water Sampling
Collecting seawater from an arbitrary depth in the ocean is called "vertical water sampling," and the equipment used for this purpose is called a water sampler. The water sampler is attached to a wire rope with the lid open, lowered to the desired depth, and then the lid is closed to collect seawater at that spot. A weight called a messenger is dropped from the vessel along the wire rope, which activates the trigger of the water sampler to close the lid (Figure 6).
Figure 6 Water sampling by messenger
上蓋： upper lid 下蓋： bottom lid ゴムチューブ：rubber tube 採水筒：bottole トリガー：trigger ワイヤロープ：wire rope メッセンジャー：messenger
By having multiple water samplers attached to a wire rope at arbitrary intervals, seawater can be sampled from several different depths in a single operation. As soon as the messenger activates the trigger, the next messenger starts falling toward the lower level, so that the lids are closed one after another from the upper-level sampler to the lower-level sampler (Figure 7).
Figure 7 Multi-layer water sampling using a series of water samplers
水深：depth メッセンジャー：messenger リリースピン解除：release pin disengaged a) Attach multiple water samplers to the wire rope at desired intervals and lower them into the water. b) Each water sampler is left with its lid open and a messenger hanging from the trigger. c) When the messenger arrives from the upper layer, the lid of the water sampler closes, and at the same time, the messenger begins to fall toward the lower layer.
Several types of water samplers have been invented over time, but the basic structure remains the same in that it consists of a "sampling bottle" that stores seawater and a "lid" that closes the sampling bottle.
The well-known water sampler that has been widely used since the early 20th century is the Nansen bottle, which is made of metal (brass). This device has a mechanism that closes the upper and lower valves when the water sampler is tipped over, and when used in combination with a tipping thermometer*, it was widely used for oceanographic observation because it could simultaneously measure the depth of water sampling and the temperature of the water at the site. Toward the end of the 20th century, as analytical items became more diverse and analytical accuracy improved, improvements were required in water sampling methods. The Nansen sampler, which was made of metal (brass), heavy, and limited in its capacity to collect seawater, was replaced by a newer type of sampler. Currently, the most widely used type is the Niskin bottle, which has both a water sampling bottle and lid made of polyvinyl chloride and is available in a wide range of sizes from 1.2 to 30 liters (Figure 8).