Hawaii FEP - Western Pacific Fishery Council

hydrogen sides and a negative charge on the oxygen side of the molecule. This chargeasymmetry allows water to be an effective solvent, thus the ocean contains a diverse array ofdissolved substances. Relative to other molecules, water takes a lot of heat to changetemperature, and thus the oceans have the ability to store large amounts of heat. When waterevaporation occurs, large amounts of heat are absorbed by the ocean (Tomczak and Godfrey2003). The overall heat flux observed in the ocean is related to the dynamics of four processes:(a) incoming solar radiation, (b) outgoing back radiation,(c) evaporation, and (d) mechanical heattransfer between ocean and atmosphere (Bigg 2003).The major elements (> 100 ppm) present in ocean water include chlorine, sodium, magnesium,calcium, and potassium, with chlorine and sodium being the most prominent, and their residue(sea salt–NaCL) is left behind when seawater evaporates. Minor elements (1–100 ppm) includebromine, carbon, strontium, boron, silicon, and fluorine. Trace elements (< 1 ppm) includenitrogen, phosphorus, and iron (Levington 1995).Oxygen is added to seawater by two processes: (a) atmospheric mixing with surface water and(b) photosynthesis. Oxygen is subtracted from water through respiration and bacterialdecomposition of organic matter (Tomczak and Godfrey 2003).3.2.4 Ocean LayersOn the basis of the effects of temperature and salinity on the density of water (as well as otherfactors such as wind stress on water), the ocean can be separated into three layers: the surfacelayer or mixed layer, the thermocline or middle layer, and the deep layer. The surface layergenerally occurs from the surface of the ocean to a depth of around 400 meters (or lessdepending on location) and is the area where the water is mixed by currents, waves, and weather.The thermocline is generally from 400 meters –to 800 meters and where water temperaturessignificantly differ from the surface layer, forming a temperature gradient that inhibits mixingwith the surface layer. More than 90 percent of the ocean by volume occurs in the deep layer,which is generally below 800 meters and consists of water temperatures around 0–4° C. Thedeep zone is void of sunlight and experiences high water pressure (Levington 1995).The temperature of ocean water is important to oceanographic systems. For example, thetemperature of the mixed layer has an affect on the evaporation rate of water into theatmosphere, which in turn is linked to the formation of weather. The temperature of water alsoproduces density gradients within the ocean, which prevents mixing of the ocean layers (Bigg2003). See Figure 4 for a generalized representation of water temperatures and depth profiles.The amount of dissolved salt or salinity varies between ocean zones, as well as across oceans.For example, the Atlantic Ocean has higher salinity levels than the Pacific Ocean due to inputfrom the Mediterranean Sea (several large rivers flow into the Mediterranean). The average saltcontent of the ocean is 35 ppt, but it can vary at different latitudes depending on evaporation andprecipitation rates. Salinity is lower near the equator than at middle latitudes due to higherrainfall amounts. Salinity also varies with depth causing vertical salinity gradients often observedin the oceans (Bigg 2003). See Figure 4 for a generalized representation of salinity at variousocean depths.38