13.2: Nutrient Supply to the Coastal Photic Zone

The most productive areas of the oceans are in coastal regions (Fig. 12-13). With only a few exceptions, coastal waters have higher primary productivity than the adjacent open-ocean waters because a variety of mechanisms supply or resupply nutrients to the photic zone. The most important nutrient supplies are provided by river inputs and coastal upwelling.

Some rivers discharge large quantities of nutrients into coastal waters, where currents carry water parallel to the coastline so that it mixes only slowly with offshore waters. Consequently, nutrients discharged by rivers tend to be distributed along the coast in the direction of the prevailing coastal current. The influence of river-borne nutrients and coastal-current transport can be seen in the high primary productivity near the Amazon River mouth in Brazil (Fig. 13-5).

Figure 13-5. This false color–enhanced satellite image of the Atlantic coast of South America depicts the elevated (red colors) chlorophyll concentrations associated with the nutrient inputs in the outflow of the Amazon and Orinoco Rivers. The river outflow is transported north along the coast by the coastal currents.

River-borne nutrients are important in many areas, but in other areas, much larger quantities of nutrients are supplied to the coastal photic zone by wind-driven coastal upwelling (Chap. 8). In addition, various other physical mechanisms mix nutrients into photic-zone waters of the continental shelf.

Wind-Driven Coastal Upwelling

Coastal upwelling occurs when wind-driven Ekman transport (Figs. 8-4, 8-6) moves the surface water layer offshore. Coastal upwelling brings offshore water from depths of 100 m or more to the surface to replace the mixed-layer water that is transported offshore. If the water raised to the surface has high nutrient concentrations and replaces nutrient-depleted surface water, and if the upwelling occurs where photosynthesis is not light-limited, the nutrients enhance primary productivity. In coastal-ocean regions where these conditions are met and where winds that cause the upwelling are persistent, primary productivity is high and remains high while upwelling persists (Fig. 13-6). The term “coastal upwelling region” is usually applied to such areas. Ekman transport also causes upwelling in other areas where the upwelled water originates from above the permanent thermocline. This water is typically poor in nutrients, and the upwelling has little effect on productivity.

Figure 13-6. Surface water chlorophyll concentrations in the Indian Ocean between Indonesia and northwestern Australia (June 1981). High concentrations are shown in red and are associated with high rates of primary production. The high productivity off northwestern Australia occurs because winds cause offshore Ekman transport and coastal upwelling. Note the complex local variations along the coast. The productivity is much lower near Indonesia, where there is no major coastal upwelling.

Nutrient-rich water from below the permanent thermocline generally does not penetrate far onto the continental shelf off middle latitude east coasts because of the swiftly flowing, deep western boundary currents of the ocean gyres (Fig. 8-10). In contrast, nutrient-rich water does penetrate onto the continental shelf off the west coasts of the continents. Hence, high-productivity coastal upwelling regions are concentrated off the west coasts of the continents, notably off California, Peru, and the western Africa coast both north and south of the equator (Fig. 12-13).

The intensity of upwelling varies locally in coastal upwelling regions (Fig. 13-6). It is especially strong and persistent off coasts where prevailing winds produce offshore Ekman transport in regions with dry or desert climates, such as California and Peru. In some other coastal regions, winds cause offshore Ekman transport, but river inputs of freshwater are substantial. In these areas, the low-salinity water discharged by the rivers spreads to form a low-density, relatively shallow surface layer. Because this layer “slides” relatively easily over the higher-density water below it, offshore Ekman transport tends to spread it farther offshore to be replaced by additional low-salinity water from the river. The low-salinity, low-density surface layer inhibits upwelling. If Ekman transport is strong and river flow rate relatively low, upwelling does occur, but the upwelled water may be low-nutrient shelf water from below the shallow halocline rather than high-nutrient water from below the deeper permanent thermocline. Upwelling of nutrient- rich water from below the permanent thermocline can occur only if the winds and offshore Ekman transport are persistent and strong, and if river input remains relatively low.

For coastal upwelling to occur, winds must blow in the appropriate direction long enough to initiate Ekman transport, which then must move sufficient surface water offshore to bring nutrient-rich waters to the surface. This process may take many hours or several days, depending on the wind strength and direction and the depth of the thermocline. In locations where winds are variable in strength and direction, upwelling may be sporadic and highly variable from day to day and from year to year. Coastal upwelling is generally most consistent in the trade wind region because trade winds are less variable than winds at other latitudes (Chap. 7).

In many regions, such as off the California coast, climatic winds and coastal currents change direction seasonally (Fig. 8-15) and upwelling is also seasonal. For example, upwelling occurs during summer and fall off the California coast, when winds blow from the north. Seasonal upwelling may occur each year, but the timing, intensity, and persistence of upwelling vary from year to year. These variations are important to major fisheries that depend on the upwelling.

Coastal upwelling often extends farther offshore and is stronger and more persistent near capes (a headland, peninsula, or promontory that extends out into the ocean) (Fig. 13-7). Fishers have long known about this phenomenon because capes have especially abundant fish, seabird, and often marine mammal populations. The reason is still not totally understood, but the phenomenon appears to be the result of an interaction of coastal currents with the undersea topographic ridge that usually extends onto the continental shelf as an extension of the cape. The coastal current flows parallel to the coastline, and as it meets the ridge, it is deflected first offshore and then back toward the shore as it passes up and over the ridge. This deflection sets up a complex eddy circulation similar to the mesoscale eddies discussed in Chapter 8. Upwelling is especially intense immediately downstream of the ridge, but upwelled water is also entrained in the eddy circulation and carried farther offshore.

Figure 13-7. These false color–enhanced satellite images show the elevated (red colors) chlorophyll concentrations associated with the interaction of coastal currents and capes. (a) The complex eddies that form at capes and that support enhanced primary productivity are clearly seen in this monthly composite image of the coastal region off Baja California. (b) Upwelling is especially strong and extends farther offshore at each of the several capes on the western African coast. (c) This image shows the intense upwelling that sustains the abundant marine species populations of the California coastal zone. This, and other eastern boundary current coastal upwelling zones, are difficult to capture from satellites since they are often obscured by the marine layer clouds and fog that you can see offshore in the image and that characterize these zones.

Biology of Coastal Upwelling Zones

In most highly productive coastal upwelling regions, upwelling persists for several months or more each year. During this period, continuous, although variable, onshore–offshore transport processes are established (Fig. 13-8). Surface layer water flows offshore due to Ekman transport and is replaced by cold, nutrient-rich water that flows inshore near the seafloor. As the upwelled cold water is transported offshore, it is progressively warmed by solar heating and mixed with warmer offshore surface water.

Figure 13-8. In coastal upwelling regions, the distribution of water properties and biota is related to the upwelling circulation. Upwelled water, high in nutrients, supports only low levels of primary production at first, but as it moves offshore as the surface layer, it supports a diatom bloom that depletes the nutrients, after which flagellates become dominant. Zooplankton and fish species are distributed to take advantage of this distribution of food. Many species of plankton use the upwelling circulation by changing their depth to be transported either onshore or offshore. Many fish and invertebrate species also use the circulation by placing their eggs and larvae at the right place and depth so that they are transported to a location where the appropriate food source is available at the different stages of their life cycle.

Although the newly upwelled water near the coast has high nutrient concentrations and adequate light, phytoplankton growth does not begin immediately. There may be several reasons for the time lag, including the following:

• Newly upwelled water contains few viable phytoplankton or other primary producer cells.
• Dissolved micronutrient organic compounds may be lacking in newly upwelled water.
• Dissolved trace metals in the newly upwelled water may be in a more toxic ionic form because the concentration of dissolved organic compounds is low.
• Dissolved nutrient trace metals, such as iron, may be in an ionic form unavailable to some species of primary producers, again because the concentration of dissolved organic compounds is low.

The toxicity and biological availability of metals in solution are known to be changed by chelation, a process in which the metal atom or ion is surrounded by an organic molecule or molecules and stable bonds are formed with part of the organic molecule structure. Generally, toxicity is decreased by chelation, and the metal may become more readily available for uptake by living organisms. In laboratory experiments, the addition of a simple organic compound known to be a chelating agent to newly upwelled water reduces the lag time. Therefore, either toxicity or unavailability of metals must account for at least some of the lag time in phytoplankton blooms. In newly upwelled water, a variety of small flagellate phytoplankton species and cyanobacterial communities are apparently less affected by the factors that create the lag time. These species are the first to grow and reproduce actively in the newly upwelled water. In doing so, they are thought to synthesize a variety of organic compounds that they then, at least in part, release to solution. These compounds must include chelating agents and/or the micronutrients needed by other phytoplankton species, which then allows those species to grow.

As upwelled water moves offshore, larger phytoplankton, including chain-forming diatoms, reproduce rapidly. In the mid-shelf region, these blooms peak, and phytoplankton biomass reaches more than 100 times that present in nutrient-poor surface waters farther offshore. The blooms consume and thus deplete nutrients (nitrogen, phosphorus, and silica) in the previously upwelled water. As the nutrient-depleted water is transported farther offshore and mixed with offshore surface waters, the phytoplankton, which are now nutrient-limited and no longer actively growing, are reduced by grazers. Consequently, the larger phytoplankton are replaced by smaller numbers of flagellate species that are able to grow at low nutrient concentrations.

Zooplankton and nekton populations within the coastal upwelling ecosystem are distributed to take advantage of the local food supply. Few zooplankton or nekton are present in newly upwelled water, where primary productivity is still low. The mid-shelf area where concentrations of diatoms are highest has large populations of herbivorous fishes and zooplankton. Farther offshore, larger carnivorous zooplankton and carnivorous fishes feed on the herbivores brought to them by the offshore flow. These food webs are short in comparison with open-ocean food webs (CC15). Hence, coastal upwelling fisheries use photosynthetically produced organic matter more efficiently than open-ocean fisheries do (CC16).

Fecal pellets and detritus produced by organisms in the surface water layer of the upwelling zone fall below the surface layer into the near-bottom water or to the seafloor, where they are decomposed. Nutrients released by this decomposition are added to the water that moves inshore to be upwelled. Thus, upwelling-zone circulation ensures that nutrients are rapidly recycled.

Coastal upwelling circulation is also important to the life cycles of species in the upwelling-zone ecosystem, particularly plankton, because they are continuously transported offshore in the mixed layer. If they are carried offshore beyond their optimal location, members of these species can return by sinking through the water column so that they are carried onshore by the upwelling circulation. Many phytoplankton species form resting phases called spores or cysts, and zooplankton species lay eggs in offshore waters, where they sink into cold subthermocline water that moves onshore. Once upwelled into an area where nutrients or food is available, the phytoplankton revive and zooplankton eggs hatch.

Nekton use a similar strategy to distribute their eggs and larvae to favorable locations. Many nekton species migrate inshore or to estuaries to spawn, while others may spawn offshore. Their eggs are carried in the near-bottom waters toward shore. They hatch into larvae that are then upwelled into nutrient-rich waters, timed with the large phytoplankton bloom. The larvae may change their physical form and feeding strategy several times to exploit different food sources as they are transported offshore as plankton. A final change to the adult nektonic form occurs when an appropriate location in the upwelling circulation is reached.

Many upwelling-zone species rely on the upwelling circulation to deliver their eggs and larvae to locations where appropriate food is available at specific times during their life cycles. Consequently, in years when upwelling occurs at a different time or differs in intensity and duration, some species may be advantaged or disadvantaged in relation to others. The result is considerable year-to-year variation in the breeding success of many species. The cause of the collapse of fisheries for many species has likely been that they were overfished during one or more years when environmental factors substantially reduced reproduction.

Other Mechanisms of Nutrient Supply

Coastal upwelling and runoff from the land are the two most important mechanisms of nutrient supply to the coastal photic zone. However, there are other mechanisms of nutrient supply that depend on waves and tides, the interaction of ocean currents with the seafloor, and thermal convection.

The mixed-layer depth is partially determined by the intensity and duration of winds. Winds cause vertical mixing due to Langmuir circulation (Chap. 8) and wave action. In regions where a seasonal thermocline is formed, intense storms in summer or fall may generate waves with long wave periods and wave heights that disrupt the thermocline. This disruption mixes water from above and below the thermocline, weakens the thermocline, and transports some nutrient-rich waters upward into the mixed layer. Nutrients are also returned to the mixed layer by convection that occurs when the surface layer water cools in fall-winter and sinks, causing the seasonal thermocline to disappear (Fig. 13-9c). Where the seafloor is shallower than the permanent thermocline depth, detritus falls through the photic zone and accumulates on the shallow seafloor. As this detritus is decomposed, nutrients are released into near-bottom waters, from where they can be returned to the photic zone by vertical mixing.

Figure 13-9. Seasonal variations in solar intensity, nutrient concentrations, phytoplankton and zooplankton biomass, and surface water temperature are different in middle (temperate), high, and tropical latitudes. (a) In high latitudes, there is an intense plankton bloom in the short summer when light intensity is sufficient for growth. Nutrients are not depleted, because strong vertical mixing occurs and this mixing prevents the formation of a thermocline. (b) In tropical latitudes, a strong permanent thermocline exists. This thermocline inhibits vertical mixing, and nutrients in the surface layer are permanently depleted. As a result, primary production is limited to uniformly low levels throughout the year. (c) In middle latitudes, there is a distinct seasonal cycle in which a shallow thermocline forms in summer, a spring bloom of plankton depletes the nutrients, and a smaller fall bloom sometimes occurs as nutrients are recycled.

Long-wavelength internal waves that form on the thermocline are slowed as they reach shallow water, and they eventually break on the outer portion of the continental shelf (Chap. 9). In some areas, such as the outer continental shelf next to Long Island and New Jersey, these waves may transport nutrient-rich subthermocline water into the mixed layer.

Tides in certain regions generate swift currents, particularly in bays, estuaries, and gulfs. Turbulence created by these currents can prevent a seasonal thermocline from forming. Thus, nutrients are recycled more effectively in such locations.

Where currents encounter seafloor topographic features, such as ridges, plateaus, seamounts, banks, or islands, water must flow over or around the feature, and nutrient-rich deep water can be mixed into the photic zone by complex eddies and turbulence. The increase in productivity caused by the interaction of ocean currents with islands can be seen in the increased chlorophyll concentrations around some such islands (Fig. 13-10).

Figure 13-10. Elevated chlorophyll concentrations are found around islands, including the Galápagos Islands, which are depicted in this false color–enhanced satellite image. Primary productivity is enhanced by the vertical mixing that is caused by the interaction of ocean currents with the island topography, which brings nutrients to the surface layer, and by nutrients in freshwater runoff from the islands.

When surface mixed-layer water is cooled, it becomes dense and sinks while convective motions replace it with nutrient-rich water from below. Convective mixing is extremely important in the high-latitude oceans (Fig. 13-9b). In these regions, surface waters are cooled continuously except in midsummer, so there is no thermocline, and convective mixing is nearly continuous. When light intensity is sufficient, these regions sustain intense phytoplankton blooms that are rarely slowed by nutrient depletion.

Georges Bank off the New England coast is an extremely productive fishery, primarily because of a plentiful supply of nutrients. The bank is a huge shallow shoal in the mouth of the Gulf of Maine (Fig. 13-11). Cold, nutrient-rich water flows out of the Gulf of Maine and intersects the north side of the bank, where it is deflected. Some water flows over the bank, but most forms a huge eddy, and a large fraction (approximately 10% to 30%) flows all the way around the bank. Wind waves, tidal currents, and turbulence and upwelling caused by deflection of the water mass into and around the bank continuously inject nutrients into the eddy and the water on the bank.

Figure 13-11. The water circulation around Georges Bank, New England, is characterized by two permanent current patterns. One is a clockwise-rotating fixed gyre that runs around the edge of Georges Bank. The other is a counterclockwise gyre in the Gulf of Maine. The circulation around Georges Bank causes upwelling of nutrients into the water over the Bank and also helps to retain plankton over the Bank so they are able to reproduce to high population densities.

Productivity is especially high because the gyre that circulates around the Bank enables phytoplankton to remain in the water on the Bank, where nutrient supplies are high. The residence time of water and plankton on the Bank is about 2 to 3 months, so phytoplankton may go through many cell divisions before being swept off the Bank. The phytoplankton provide abundant food for zooplankton, which also remain on the bank long enough to feed and reproduce, ensuring that their young have ample food. Fishes, in turn, exploit the abundant phytoplankton and zooplankton. Unfortunately, Georges Bank fisheries declined substantially because of overfishing, especially prior to 1994 when some fishing restrictions were put in place. The populations of commercially valuable species of fishes have recovered very slowly but not yet to their previous abundance.