Water Column Profile
Glossary Terms
A water column profile is a cross-sectional view of the sea. The profile describes changes in the physical, chemical, and biological features of seawater with depth, and serves as a foundation for understanding ocean and coastal processes such as the currents and productivity in the sea. Some of the basic features of the water column profile — temperature and salinity, dissolved oxygen content, and nutrient content — all play an important role in supporting different types of marine life.
The water column profile for the area off Kahe Point was studied extensively in the 1980s as part of the proposed (but never built) 40 megawatt (MW) Ocean Thermal-Energy Conversion plant. Although Kahe Point is just outside the southern extent of the Wai‘anae moku, information and data collected at Kahe Point can be considered relevant to the Wai‘anae coast. Changes in temperature, salinity, oxygen, and nutrients with depth were measured from surface water to depths of 900 meters (2,950 feet; Noda and others 1981).
Temperature and Salinity
The temperature and salinity (salt content) of seawater are important attributes of the water column profile. Temperature and salinity gradients define different layers in the water column and indicate the boundaries of different water masses off the Wai‘anae coast. This helps scientists determine water movement near the Hawaiian Islands.
The mixed layer of the ocean off the Wai‘anae coast extends from the surface to depths of about 30 to 60 meters (100 to 200 feet). In the mixed layer, temperature is nearly uniform with depth. Below the mixed layer is the thermocline, the layer in which seawater temperature declines rapidly with depth. In the thermocline, seawater temperature decreases from about 24 degrees centigrade at a depth of 60 meters (200 feet) to 15 degrees centigrade at over 200 meters (650 feet). Below this depth, temperature decreases gradually. At 900 meters (2,950 feet) depth, seawater temperature off Kahe Point is about 4 degrees centigrade.
Surface water salinity off Kahe Point is about 34.8 parts per thousand (ppt), typical of the Pacific central water mass. This low-salinity warm surface layer grades into the underlying Pacific intermediate water mass, which is characterized by a maximum salinity of 35.1 parts per thousand (ppt) at 180 meters (600 feet) and minimum of 34.2 ppt at 460 meters (1,500 feet). At 900 meters (2,950 feet) depth, seawater salinity off Kahe Point is about 34.4 ppt.
Dissolved Oxygen
There is about 100 times less oxygen in the entire ocean than in the earth's atmosphere, yet this is a vital resource for animals in the ocean. Higher concentrations of dissolved oxygen correspond to higher levels of photosynthesis in the ocean. Colder water can also dissolve higher levels of oxygen. These are the primary reasons why there is a lower concentration of oxygen in Hawaiian waters than much of the world's oceans. An oxygen minimum of one milliliter per liter of seawater occurs at a depth of 680 meters (2,240 feet). The oxygen minimum zone is caused by the settling and decomposition of plankton and other organic matter from the surface mixed layer. Off Kahe Point, the dissolved oxygen content of the surface water is around 4.8 milliliters per liter of seawater.
Nutrients
Nutrients are taken up by marine plants, phytoplankton, and marine algae for primary production. Nutrients commonly measured in seawater include silica and inorganic and organic forms of nitrogen (nitrate and nitrite, and dissolved organic nitrogen) and phosphorus (phosphate, dissolved organic phosphorus). Nutrient concentrations in seawater off the Wai‘anae coast are likely to vary with the time of year and location.
Land-based sources of nutrients from streams and surface water runoff cause localized increases in nutrient concentrations in coastal waters. The uptake of nutrients by marine plants and decomposition of marine life in the sea also contribute to variation in nutrient concentrations found in the water column. In general, surface waters of the Hawaiian Islands, and in particular off dry leeward sides of the islands such as Wai‘anae, have low nutrient concentrations. Primary production is generally considered to be limited by the availability of nitrogen and micronutrients such as iron.
Reference Cited
Noda, E.K., P.K. Bienfang, W.J. Kimmerer, and T.W. Walsh. 1981. OTEC Environmental Benchmark Survey, Kahe Point, Oahu, Final Report. DOE/NBM-2016055. Prepared under Contract W-7405-ENG-48 for U.S. Department of Energy. 116 pp.
