Garis besar topik

    • Keiri Imai, Kenji Oguma, and Koki Sawada　

      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.

    • 海水の組成

      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　

      塩化ナトリウム：Sodium chloride　塩化マグネシウム：Magnesium chloride　硫酸マグネシウム：Magnesium sulfate　硫酸カルシウム：Calcium sulfate　塩化カリウム：Potassium chloride　その他：Other　その他の塩(重炭酸、臭素、ホウ素など)：Other salts (HCO3−, Br−, B, etc.)　栄養塩類(リン、窒素、ケイ素)：Nutrients (P, N, Si)　有機化合物(タンパク質、脂質、炭水化物など)：Organic compounds (protein, fat, carbohydrate, etc.)　気体成分(窒素、酸素、二酸化炭素、アルゴン、水素など)：Gaseous components(N2, O2, CO2, Ar, H2, etc.)　微量元素(リチウム、ヨウ素、モリブデン、亜鉛、鉄など)：Trace elements (Li, I, Mo, Fe, etc.)



    • 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.