13.1: Special Characteristics of Coastal Oceans

Because the coastal oceans are shallow, water movements are affected by the seafloor and the coastline. In addition, rivers discharge freshwater into coastal waters. As a result:

• The salinity and temperature of coastal waters are more variable than those of open-ocean waters.
• Coastal currents are generally independent of the open-ocean gyre currents.
• The mixed layer has more sources of nutrients in coastal waters than in the open ocean.
• Benthos of the coastal oceans are more diverse, abundant, and commercially valuable than open-ocean benthos.

These factors make the coastal oceans different and generally more variable in physical and biological characteristics, than the open oceans. Some of the ways in which these factors affect the coastal oceans are discussed in this section. However, you should remember that each coastal area has its own unique characteristics, and this chapter provides only a general overview and some examples of coastal and estuarine processes in some specific locations. 

Salinity

Freshwater from rivers reduces the salinity of coastal waters. Where river discharges are large, low-salinity water spreads as a surface layer over higher-salinity ocean water unless vertical mixing is intense. Consequently, there is often a halocline a few meters below the surface near mouths of major rivers. The area affected by the halocline and the halocline’s strength depend on the rate of discharge of low-salinity water from the river and on the extent of wind-, wave-, and tide-induced vertical mixing of the water column (Fig. 13-2). Haloclines inhibit upwelling and vertical mixing.

Figure 13-2. Salinity distributions in the coastal-ocean water column. (a) Near major rivers, discharges of low-salinity river water over higher-salinity ocean water create a halocline. Where winds transport surface waters offshore, the layer of low-salinity water may spread farther and be thinner. Lower-volume river discharges produce thinner, low-salinity layers that do not extend far from the river mouth. (b) Where winds are strong and river discharge is relatively small, the low-salinity river water may be completely mixed with ocean water, resulting in no halocline. (c) In areas of strong evaporation and low rainfall, warm, high-salinity surface water is produced that may be dense enough to sink all the way to the seafloor. (d) Conversely, this warm, high-salinity water may be warm enough that its density is lower than the lower-salinity but cooler offshore water. In this case, the warm, high-salinity water forms a higher-salinity surface layer overlying a halocline in which salinity decreases with depth.

Low-salinity water discharged by rivers mixes with ocean water primarily by wind action and is transported away from the river mouth by coastal currents. Chapter 8 explained that coastal currents generally flow parallel to the coastline and are often separated from the open-ocean gyre currents by sharply defined fronts. Thus, mixing of coastal water with water from the open oceans is slow, and residence times of water in the coastal ocean can be long.

The residence time of freshwater in the coastal water mass can be calculated if the volume input of freshwater from rivers, the average salinity in the coastal waters, and the volume of the coastal water mass are known (CC8). For example, the residence time of freshwater in the coastal oceans of the northeastern U.S. from Cape Cod, Massachusetts, to Cape Hatteras, North Carolina, is about 2½ years. The long residence time has implications for the use of this area for ocean waste disposal. Although the volume of coastal water is large, the quantities of potential contaminants released into the northeastern U.S. coastal ocean in discharges from cities (primarily treated sewage wastewater and storm water runoff) and industries are also large. For potential contaminants not removed by sedimentation or decomposition (Chap. 16), an amount equivalent to 2½ years of discharges is retained within this region.

Salinity varies seasonally with river flow in many coastal areas. In middle and low latitudes, salinity is usually lowest in the local rainy season. At high latitudes, salinity is always lowest in late spring or summer when snowmelt swells rivers and seasonal sea ice melts.

Salinity extremes occur in marginal seas, lagoons, or other bays with restricted connections to the oceans. In locations such as the eastern Mediterranean and the Red Sea, evaporation is high and rainfall low, so surface salinity is high—in some areas exceeding 40. In these regions, the mixed layer is usually thicker because vertical circulation is enhanced by the formation of surface waters with relatively high density due to high salinity. The mixed layer often extends to the seafloor and consists of warm, high-salinity water. The vertical circulation helps to recycle nutrients from relatively deep waters into the photic zone. Consequently, many of these regions have moderately high productivity despite their limited inputs of nutrients in runoff from land. However, if this body of water is not completely separated from the ocean, lower-salinity surface water from the open ocean may flow as a surface layer over the high-salinity water created by evaporation in another part of the basin (Fig. 13-2c). In some basins with high evaporation and low rainfall, higher-salinity but warmer surface water has lower density than the lower-salinity but colder open-ocean water. Thus, a warm, high-salinity surface layer is formed (Fig. 13-2d).

Salinity is lowest where river outflow is large and evaporation is low. Such conditions generally occur in middle- and high-latitude locations. These include many fjords and the Baltic Sea. The huge monsoon flows of the Ganges and Brahmaputra Rivers lower the surface salinity of the entire Bay of Bengal (Fig. 7-15).

Temperature 

Seasonal changes in solar intensity and air temperature cause larger temperature changes in shallow coastal waters than in the open ocean. Temperature variations are particularly large in enclosed marginal seas, fjords, and other embayments that have restricted exchange with the open ocean. 

Figure 13-3. Temperature distributions in the coastal-ocean water column. (a) In high-latitude regions where ice is forming or melting throughout the year, the water column is uniformly at or near the freezing point. (b) In mid-latitude regions, surface waters are warmed in summer, producing a shallow seasonal thermocline. In winter, the surface water is cooled and sinks, and the water column is generally well mixed by this convection and by winds. In some areas near freshwater inputs, low-salinity runoff may produce cool but low-salinity surface water and a shallow pycnocline that is both a halocline and a thermocline in which temperature increases with depth. (c) In low-latitude regions where freshwater inputs are limited, the temperature of the water column above the permanent ocean thermocline depth is usually uniform and high.

In high latitudes, coastal water temperatures vary little between seasons (Fig. 13-3a). Sea ice in these regions melts during summer and re-forms during winter. This process maintains the water temperature at the freezing point of seawater, usually above –2°C (Chap. 5, CC5).

Seasonal temperature changes in coastal waters are most evident in mid latitudes (Fig. 13-3b). In these regions, the surface water layer becomes warmer and less dense as solar heating increases in the spring, and reaches a temperature maximum in the summer. In addition, in many locations, mixing by wind and waves decreases during summer. As a result, a shallow thermocline is formed and the mixed-layer depth is reduced to about 15 m (Fig. 13-3b). This seasonal thermocline breaks down in winter when surface water is cooled and convection and wind mixing increase. A uniformly cold mixed layer is produced that extends to the top of the permanent thermocline (about 100 m) or to the seafloor, if it is shallower. Because most coastal waters are less than 100 m deep, the mixed layer extends to the seafloor in most of the temperate coastal region during winter.

In tropical regions, coastal water temperature is uniformly high year-round, and the water column is often isothermal to the seafloor, particularly in shallow lagoons (Fig. 13-3c). In deeper coastal waters, the depth of the mixed (or isothermal) layer is determined by physical processes, including wind-, tide-, and wave-induced mixing.

Coastal-ocean water temperature in middle latitudes is usually lower in summer and higher in winter than air temperatures over the adjacent coastal land. This difference affects the climate of coastal regions (Chap. 7) by moderating temperature variations.

Waves and Tides

Tidal-current speed increases as the tide wave enters shallow coastal waters and its energy is compressed into a shallower depth (Chap. 10). Consequently, tides are an important contributor to vertical and horizontal mixing in the coastal zone.

As tides ebb and flood, they cause turbulence that enhances vertical mixing. Tides also mix water across the shelf. Near-bottom tidal currents are slowed by friction with the seafloor. In addition, tidal currents may be different above and below a pycnocline when one is present. Thus, water in different layers may move different distances across the shelf during a tidal cycle. Turbulent mixing between layers transfers water from one layer to another.

Wind-driven waves create turbulence and vertical mixing. In the coastal zone, vertical mixing is enhanced where waves move into shallow waters and break. Langmuir circulation and internal waves (Chap. 8) also contribute to vertical mixing.

Turbidity and the Photic Zone

The photic zone extends to the seafloor in many areas of the coastal oceans. In these areas, kelp and other macroalgae (Fig. 13-4) can live and photosynthesize on the seafloor. However, the photosynthetic community of the shallow seafloor is often dominated by species of single-celled algae similar to phytoplankton. These algae encrust rocks, dead coral, and other organisms that live on the seafloor. A number of such species live inside the tissues of corals, clams, and other benthic animals in a symbiotic relationship (Chap. 14).

Figure 13-4. Benthic algae. (a) Various species of green, brown, and red macroalgae in a tide pool on the California coast near San Francisco. (b) A red calcareous macroalga (phylum Rhodophyta, Philippines). (c) The upper fronds of kelp (Macrocystis sp.) floating on the water surface in Monterey Bay, California. (d) A coral reef green macroalga called “grapeweed” (Caulerpa racemosa, Papua New Guinea). (e) An encrusting red alga (phylum Rhodophyta, Hawaii) covering a platelike hard coral.

The photic zone is generally shallower and more variable in depth in the coastal oceans than in open-ocean waters because the concentration of suspended particles, and hence turbidity, is higher. Turbidity is higher primarily because of the proximity of river discharges that can carry large quantities of suspended sediment. The finest particles are continuously resuspended and transported in the coastal zone by waves and currents, so their concentration remains high. Once they have been transported to the lower-energy deep oceans, these particles sink below the photic zone.

The concentration of suspended sediment in coastal waters is variable because it depends on such factors as location in relation to river discharges, river discharge rates, intensity of wave resuspension activity, and the speed of coastal currents. Each of these factors varies spatially and temporally. Similarly, phytoplankton concentrations are highly variable in the coastal zone. As a result, the photic-zone depth can vary substantially over short distances and time periods. Populations of benthic algae, particularly in the relatively deep waters of the mid-shelf region, vary accordingly. High turbidity also affects the health and distribution of coral reefs by reducing the light available for photosynthesis by their symbiotic algae, the zooxanthellae.

Currents

Because coastal currents (Chap. 8) are dominated by local winds, they vary seasonally and on shorter timescales in response to weather systems. Coastal currents are generally steered by their interaction with the seafloor, so they flow parallel to the coastline. However, interactions between currents and the seafloor or shoreline can also cause permanent or semipermanent eddies to form.

Nutrients

The supply of nutrients to the coastal photic zone is generally greater and more variable than the supply to open-ocean photic zones. The sources of, and variations in, nutrient supply are important to the biological characteristics of coastal waters and are discussed in detail in the next section.