| Seabird Data units |
| SALINITY
refers to the amount of dissolved solids in the seawater and is
measured in parts per thousand by mass. The dissolved solids consist
of positive ions [mainly sodium (Na+)] and negative ions
[mainly chloride (Cl-)]. Other ions present in seawater
are magnesium, sulfur, calcium and potassium, and over fifty other
elements occur in trace amounts. Most seawater has a salinity
between 33 and 37 ppt.
DENSITY is related to salinity; as the salinity increases, so does the density. Minute changes in density are of significance to marine scientists. Consequently, ocean density is measured in micrograms, i.e. 1.023611 g/cm3. In order to simplify reporting, density is converted to a sigma t (st) value. To derive sigma t from density, divide the density by the density of pure water (1.000000 g/cm3) to remove the units, then subtract 1 and multiply by 1000. Thus, a density of 1.023611 converts to a sigma t value of 23.611. DISSOLVED OXYGEN refers to the amount of oxygen gas dissolved in the seawater. It is measured in milligrams per liter. pH measures the acidity or alkalinity of the water. It is a unit-less number related to the concentration of H+ ions. PAR is "photosynthetically active radiation". It is measured in micro-einsteins/sec/meter2. An einstein is a unit of photon energy; the energy of a mole of photons. It depends on wavelength. The formula is "NAhn" where NA is Avogadro's number, h is Planck's constant, and n (nu) is the frequency of the light. chl a is the concentration of Chlorophyll a, measured in micrograms per liter. |