Background Information on Seawater
SEAWATER
Seawater is a solution of salts of nearly constant composition, dissolved in variable amounts of water. There are over 70 elements dissolved in seawater but only 6 make up over 99% of all the dissolved salts.
Oceanographers use salinity - the amount (in grams) of total dissolved salts present in 1 kilogram of water - to express the salt content of seawater. Normal seawater has a salinity of 35 grams/kilogram (or litter) of water, also expressed as 35‰, or 35 parts per thousand.
Seawater varies in salinity from place to place, ranging between 34‰ and 37‰ in open ocean areas
Dissolved Gases in Seawater
Seawater also contains small amounts of dissolved gases (nitrogen, oxygen, carbon dioxide, hydrogen, and trace gases.) Water at a given temperature and salinity is saturated with gas when the amount of gas entering the water equals the amount leaving during the same time. Surface seawater is normally saturated with atmospheric gases such as oxygen and nitrogen. The amount of gas that can dissolve in seawater is determined predominantly by the water’s temperature and salinity. Increasing the temperature or salinity reduces the amount of gas that can be dissolved.
Dissolved Oxygen (DO): Dissolved oxygen is a measure of the soluble oxygen present within the sample. DO is necessary for the life supporting capacity of a body of water. A value of greater than 6 is considered necessary to support a stable aquatic community. Many aquatic creatures (fishes especially) are capable of greatly varying their respiration rate to adjust to a range of temperatures and oxygen levels. DO is measured in mg L-1.
Carbon Dioxide (CO2): Carbon Dioxide dissolves in water and combines with other chemicals in the water to form various compounds. These compounds affect the chemistry of the water.
When a small amount of CO2 enters the water it combines with the water to form carbonic acid:
CO2 + H2O <--> H2CO3
Carbonic acid (H2CO3) may dissociate and form bicarbonates:
H2CO3 <--> HCO3- + H+
Bicarbonate (HCO3-) may be taken up and used by photosynthesizing plants. Therefore carbon dioxide in water is not measured as CO2 concentration but rather is reflected in the amount of bicarbonate and carbonate in the water.
Nutrients in Sea Water
Nutrients measured in marine waters include phosphorous and nitrogen. These nutrients exist in dissolved and particulate form (attached to matter). Dissolved nutrients are more readily available to be taken up by plants and animals (referred to as bioavailable). Because nitrogen and phosphorus are found in the environment in a number of forms, water quality scientists measure them in different ways. In marine waters, nitrogen is more readily recycled than phosphorus, and is therefore more readily available.
Nitrogen: Nitrogen is an element required for life and is usually the most significant nutrient in estuarine and marine waters. Nitrogen present in water may be bound up in plant or animal tissue, in which case it is referred to as "organic" nitrogen. Organic nitrogen includes all organic compounds such as proteins, polypeptides, amino acids, and urea. Organic nitrogen is not immediately available for biological activity. Therefore, it does not contribute to furthering plant proliferation until decomposition to the inorganic forms of nitrogen occurs. Organic nitrogen eventually breaks down into "inorganic" forms; oxides of nitrogen (NOx) or ammonia (NHx). These inorganic forms are bioavailable forms of nitrogen found dissolved in water.
Ammonia (NH3 & NH4) represents the end-product of protein metabolism in fish and other aquatic animals. Ammonia is rapidly oxidized by certain bacteria, in natural water systems, to oxides of nitrogen (NO2 or NO3) --a process that requires the presence of dissolved oxygen. The ammonia forms of nitrogen are very soluble in water.
Phosphorus: Phosphorus is essential to the growth of organisms just as nitrogen is. Phosphorus is normally available to cells in the form of phosphates (PO4-3). These phosphates are soluble in water. Soluble phosphorus (PO4-3) is a measure of the biologically available dissolved form of phosphorus readily available for uptake during photosynthesis. Total phosphorus includes dissolved, particulate and organically bound phosphorus. Both are measured in mg L-1.