Introduction to Ocean Sciences

Chapter 15: Coastal Oceans and Estuaries

Guide to Reading and Learning

Chapter 14 provides a basic understanding of the physical and chemical properties that control life in the oceans, and of the fundamental characteristics of ocean food webs. Chapter 15 extends this understanding to the coastal oceans. The coastal oceans must be considered separately for several reasons. First, the physical and chemical characteristics of the coastal oceans are much more variable than those of the open ocean. Second, seasonal variations are greater and more important to the marine ecosystem in coastal oceans. Third, life is much more abundant in the coastal oceans than in comparably sized areas of the open ocean despite the much greater depth, and therefore volume, of water in the open oceans. Did you know that more than 95% of the world’s fish and shellfish catch comes from the coastal oceans even though the coastal oceans make up about 10% of the area of the world oceans? Did you know that about half of the world seafood catch comes from a few coastal areas that represent just only about 1 percent of the total area of the world oceans?

In the first part of this chapter we begin to develop the story of the interplay between physics, chemistry, and biology in determining when and where primary production is concentrated in the oceans. First, we look at the individual factors that take part in these intricate workings, including variations of temperature, salinity, nutrient supplies, light availability, suspended sediments, tides, waves, and currents and how they vary in different parts of the coastal oceans. Following this, we describe how all of these factors interplay in a coastal upwelling zone and how they influence not only the amount of primary and secondary production and their variations in time and space but also the abundance, location, and spawning behavior of fishes and invertebrates. We learn that coastal upwelling ecosystems are naturally highly variable and that this variability means that the biological characteristics of coastal upwelling zones are also naturally highly variable. It is not unusual for a particular species of fish that live in these zones, especially during their early life stages, to experience drastic changes in their reproductive success from year to year.

Turning next to the seasonal cycles in coastal waters in general, we learn that different factors limit the primary production and fish and invertebrate populations at different latitudes. Primary production in the tropics, despite the seeming abundance of coral reefs, is much more limited than at higher latitudes. Mid latitudes have strong seasonal cycles, with most primary production concentrated in spring and limited by lack of available nutrients in summer. In polar and subpolar regions light, not nutrients, limits the primary production to just a few short weeks in the summer. However, we learn too that these patterns can vary in specific localities when other factors such as river runoff are important.

Perhaps the most important section of this chapter is the discussion of algal blooms. Toxin production in these blooms is a considerable threat to human and environmental health. However, of most concern is the ability of blooms to strip oxygen from coastal waters and to produce areas where fish and invertebrates cannot live because of the lack of oxygen. These blooms and low oxygen events, or dead zones as they are often labeled, are becoming more frequent and widespread worldwide. This increased frequency and extent is largely due to the release of nutrients to the ocean environment by humans and our agriculture. In fact, the international marine science community now considers this problem to be second only to overfishing as the major cause of human damage to coastal marine ecosystems.

The chapter closes with a section on estuaries. Most of the world’s major cities are built on estuaries, and estuaries receive the bulk of the effluents from human activities. It was once thought that wastes released in an estuary would soon be washed out to sea and diluted beyond any possibility of causing harm. However, a simple knowledge of the physical processes that occur in estuaries will convince you otherwise. In fact, you might be surprised to learn that estuaries act as effective traps for contaminants. The still-widespread notion that estuaries are conduits to the oceans for such things as sewage wastes is wrong and, historically, has led to serious degradation of many of the world’s estuarine ecosystems. Fortunately, some progress has been made but many estuaries, which are vital nursery areas for many marine species, are still heavily impacted.

Chapter 15 Essential to Know 

Critical Concepts used in this chapter

CC.5, CC.8, CC.11, CC.12, CC.15, CC.16

15.1 Special Characteristics of the Coastal Oceans

  • Characteristics including salinity, temperature, currents, sources of nutrients, and benthic species are more variable in the coastal oceans and estuaries than they are in the open oceans.


  • Salinity is reduced by river discharges that can form a low-salinity surface layer separated by a halocline from the water below. This layer can extend far out from the river mouth and its thickness varies depending on the river discharge rate and winds.
  • The residence time of river discharges of freshwater in the coastal water mass is often long (months to years).
  • Salinity can be elevated where evaporation exceeds precipitation and low where precipitation exceeds evaporation, especially in marginal seas.


  • Water temperature varies little in the tropics and polar regions but varies seasonally in mid latitudes, with generally larger variations in poorly flushed estuaries and lagoons.
  • Water column temperature is generally uniform and cold throughout its depth in polar and subpolar coastal regions as a result of the high latent heat of fusion of water. Ice formation and melting maintains a constant water temperature throughout the year.
  • Water temperature is generally uniform and warm throughout its depth in tropical coastal regions because the thermocline is deep.
  • A shallow seasonal thermocline forms in summer in mid latitudes, which limits primary production by preventing resupply of nutrients to the photic zone from sub-thermocline waters.

Waves and Tides

  • Tidal current speeds increase in the shallow waters of the coastal zone, especially in bays and estuaries with narrow openings.
  • Vertical mixing by waves is enhanced when the waves move inshore and break.

Turbidity and the Photic Zone

  • Rivers bring suspended sediments to the estuaries and oceans so that turbidity is higher and more variable than in the open oceans, and the photic zone is generally shallower.
  • Benthic animal biomass is greater in continental shelf sediments than in the deep oceans, and benthic algae may perform primary production on the seafloor in those areas where turbidity and/or depth are low enough that light reaches the seafloor.


  • Coastal currents are dominated by local winds and vary in response to weather systems and seasonal wind patterns.
  • Coastal currents generally flow parallel to the shore.


  • There are more sources of nutrient supply to the photic zone in the coastal oceans and estuaries than in the open ocean. These include: wind-driven coastal upwelling especially on the west coast of continents, fresh water runoff and rivers, and upwelling induced by eddies formed as currents flow over shallow banks and ridges and past capes.

15.2 Nutrient Supply to the Coastal Photic Zone

  • Primary productivity is higher in the coastal oceans than in the adjacent open-ocean waters primarily because there are more nutrients supplied to the photic zone.

Wind-Driven Coastal Upwelling

  • Coastal upwelling occurs where Ekman transport by the winds moves the surface layer offshore.
  • Coastal upwelling along the east coasts of the continents in subtropical latitudes does not supply nutrients to the surface layer because the upwelled water is brought up from the deep western boundary currents.
  • Coastal upwelling along the west coats of continents supplies nutrients to the surface layer by upwelling of water from below the relatively shallow thermocline.
  • The intensity of upwelling in coastal upwelling zones can be variable as winds also vary.  In many locations, upwelling is seasonal due to seasonal changes in prevailing wind direction.
  • Coastal upwelling often extends further offshore and is stronger at capes where the coastal current may interact with the topography to form eddies.

Biology of Coastal Upwelling Zones

  • In coastal upwelling zones, many organisms take advantage of the circulation that moves water onshore in the lower layer and offshore in the surface layer.
  • Primary production is low in newly upwelled water, possibly due to lack of certain micronutrients or dissolved organic compounds to reduce toxicity or enhance bioavailability of certain trace elements by forming complexes with them. However, these micronutrients or organic compounds are synthesized by some phytoplankton species and primary production, dominated by diatoms, rises quickly as the upwelled water moves offshore.
  • As the water moves farther offshore nutrients are depleted, diatoms are replaced by dinoflagellates, and productivity drops. Grazers and carnivores concentrate their populations in appropriate locations within the circulation. Grazing zooplankton concentrate near the maximum diatom concentration. Small carnivorous fishes concentrate farther offshore.
  • Some species produce eggs or larvae that sink so they are carried first inshore in the lower layer and then, after being upwelled, offshore in the surface water back to the adult feeding area.

Other Mechanisms of Nutrient Supply

  • Wind-driven upwelling and freshwater runoff are the dominant sources of nutrients in the coastal ocean, but other sources may be important in specific local regions.
  • Other sources include breaking of internal waves near the edge of the continental shelf, enhanced vertical mixing due to episodic large storms, fast tidal currents, turbulence, and eddies where currents flow over shallow topography or around islands.
  • In mid latitudes nutrients may be supplied seasonally when winter cooling and storms destroy the shallow summer thermocline and return nutrients from the water below this thermocline.

15.3 Seasonal Cycles

  • There are strong seasonal cycles of primary production in mid latitudes and high latitudes but little seasonality in tropical regions.

Polar and Subpolar Regions

  • In polar and subpolar regions, the water column remains well mixed year round except where haloclines form.
  • Where there are strong haloclines, productivity is nutrient-limited.
  • Where there is no halocline in summer, nutrients are plentiful and there is high primary production, but only during the short period of summer when light is not limiting.

Tropical Regions

  • In tropical regions, there is a strong persistent thermocline, nutrients are limiting, and primary production is low except in areas where upwelling supplies nutrients and in coral reef ecosystems where the symbiosis of zooxanthellae and corals provides for rapid recycling of nutrients.

Mid Latitudes

  • In mid latitudes, the water column is usually well mixed in winter but a shallow seasonal thermocline is formed in summer.
  • Primary production is light-limited in winter.
  • A phytoplankton bloom occurs in spring until nutrients become limiting and light availability increases. There is often a weaker fall bloom as cooling and storms mix some nutrients back into the photic zone.

Phytoplankton Species Succession

  • During a phytoplankton bloom, phytoplankton biomass is controlled by zooplankton that multiply quickly as phytoplankton production increases. Most phytoplankton are eaten by zooplankton. As a result the biomass (standing stock) of phytoplankton does not increase dramatically even during periods when phytoplankton productivity is very high.
  • There is a typical sequence of phytoplankton species dominance during spring and summer in many mid-latitude regions. Diatoms dominate until silica is depleted and are then replaced by flagellates.
  • The life cycles of many species of herbivores and carnivores are sequenced such that a particular life stage is reached at the time of year when a specific type of phytoplankton or zooplankton food is available.
  • Consequently, if the phytoplankton species succession during a given year does not follow its typical sequence (due to natural or anthropogenic factors) this can adversely or beneficially affect many other species in the food chain.

15.4 Algal Blooms

  • Nuisance blooms of dinoflagellates occur periodically off many coastlines and appear to be becoming more frequent and widespread. These blooms can be triggered by natural events or by anthropogenic inputs, particularly of nitrogen in treated sewage discharges.

Dinoflagellate and Other Phytoplankton Toxins

  • In a bloom, some dinoflagellates produce toxins that may not affect invertebrates, but can kill fishes and affect humans who consume the invertebrates.
  • Many areas of the coastal oceans support dense shellfish populations but many of these areas are closed to shellfishing, at least for parts of the year, as a precaution against dinoflagellate toxins that may be concentrated in the shellfish. In the United Sates and elsewhere, the extent of these closed areas is growing progressively larger.

Oxygen Depletion

  • Dinoflagellate blooms can cause oxygen depletion or anoxia when the blooms collapse and the organic matter increases the oxygen demand below a shallow summer thermocline or halocline.
  • Hypoxia and anoxia are becoming more frequent and widespread worldwide and may periodically cause massive kills of benthic invertebrates and sometimes fish.

15.5 Fisheries

  • Populations of fishes are extremely variable from year to year due to variations in natural conditions. Their population dynamics are chaotic.
  • Most of the world’s fisheries are being fished at, or above, their maximum sustainable yield.
  • Most of the world’s production of fish biomass occurs in a small percentage of the ocean area located in coastal regions and upwelling zones. This is primarily because food web trophic efficiency is higher in coastal regions and highest in upwelling regions.

15.6 Estuaries

  • Estuaries are the areas where fresh water from rivers and ocean water mix.

Geological Origin of Estuaries

  • Estuaries can be classified as coastal plain, bar-built, or tectonic estuaries, or fjords on the basis of their geological origin.
  • Coastal plain estuaries are river valleys drowned by rising sea level during the past 15,000 years.
  • Bar-built estuaries are created where longshore drift of sand creates an elongated spit that extends across the mouth of a coastal embayment.
  • Tectonic estuaries are formed where earthquakes raise or lower a section of coastline to create a valley.
  • Fjords are the steep sided glacier cut valleys that were drowned by rising sea level after the glaciers had melted.

Estuarine Circulation

  • Estuaries can also be classified as salt wedge, partially mixed, or well-mixed, fjords or inverse estuaries on the basis of their circulation characteristics.
  • The Coriolis effect deflects estuarine circulation. In the Northern Hemisphere, this deflects the landward flowing layer to the right side of the estuary (looking landward) and the seaward flowing layer to the left side. In salt wedge estuaries this inclines the halocline, and in partially and well-mixed estuaries tends to separate the flow so that water flows landward along the right side and seaward along the left side of the estuary (looking landward).
  • The volume flow of water in each direction in an estuary increases toward the seaward end of the estuary, where it is many times greater than the river freshwater flow rate.

Types of Estuarine Circulation

Salt Wedge Estuaries

  • In salt wedge estuaries, freshwater flows seaward over a layer of seawater that flows landward, with the layers separated by a strong, sharp, halocline. Lower layer water is slowly mixed upward into the freshwater layer along the length of the estuary, but little or no freshwater is mixed downward into the lower layer.

Partially Mixed Estuaries

  • Circulation in partially mixed estuaries is similar to that in salt wedge estuaries but there is more mixing between the layers and the halocline is more gradual with depth between the two layers.

Well-Mixed Estuaries

  • In well-mixed estuaries, the water column is well mixed vertically throughout the estuary and there are no distinct layers. However, salinity increases progressively seaward.

Fjord Estuaries

  • In fjord estuaries, estuarine circulation is restricted to the upper layer of the water column above the sill depth. Deep water in the fjord is stagnant and separated from the surface layer by a strong halocline.

Inverse Estuaries

  • In arid regions, salinity can be raised by evaporation so the seaward flowing water in an estuary is higher density than the ocean water outside. In these estuaries ocean water flows in as a surface layer over a seaward flowing lower layer of high salinity water formed within the estuary.

Particle and Contaminant Transport in Estuaries

  • Large particles carried by rivers are deposited primarily in the estuary where the currents usually decrease from those that flow down the rivers.
  • Small particles may be transported out of the estuary but tend to form clumps when they reach the salt water so small and medium sized particles tend to sink and be carried back landward in the lower layer of the estuary circulation.
  • As a result, estuarine circulation tends to trap particles and their associated contaminants in the estuary. Particles are often distributed in shallow wetlands adjacent to the main channel of the estuary, especially in the region near the landward end of the estuary.

Estuarine Biology    

  • Temperature, current speed, and salinity are all highly variable in estuaries, which makes them difficult places for organisms to live.
  • Changes in salinity are particularly difficult because they change the osmotic pressure. Therefore, fewer species live in estuaries than in the open ocean. However, estuaries generally have abundant nutrients and light so they typically sustain high biomass.
  • Benthic microalgae are abundant in some estuaries where light reaches the estuary floor, and rooted aquatic plants are found in very shallow areas and wetlands.
  • Estuaries are used as habitat by the juveniles of many species, as they provide abundant food and protection from ocean predators, particularly in wetlands.

Anadromous and Catadromous Species

  • Anadromous species live adult lives in the ocean, return to rivers to spawn, and then die. Catadromous species live in freshwater as adults, return to the ocean to spawn, and then die. In each case both adults and juveniles must transit the estuary, although in opposite directions.


Critical Concept Reminders:

CC.5 Transfer and Storage of Heat by Water (p. 413)

  • Water’s high heat capacity allows large amounts of heat to be stored in the oceans and released to the atmosphere without much change in the ocean water temperature. Water’s high latent heat of fusion allows ice to act as a heat buffer, which keeps the ocean surface water layer temperatures in high latitudes relatively uniform and near the freezing point. To read CC5 go to page 15CC.

CC.8 Residence Time (p. 412)

  • The residence time of seawater in a given segment of the oceans is the average length of time the water spends in that segment. The residence times of some coastal water masses are long and therefore some contaminants discharged to the coastal ocean can accumulate to higher levels in these regions than in areas with shorter residence times. To read CC8 go to page 19CC.

CC.11 Chaos (p. 430)

  • The nonlinear nature of many environmental interactions, including some of those that control annual fluctuations in fish stocks, mean that fish stocks change in sometimes unpredictable ways. To read CC11 go to page 28CC.

CC.12 The Coriolis Effect (p. 431)

  • Water masses move freely over the Earth and ocean surface, while objects on the Earth’s surface, including the solid Earth itself, are constrained to move with the Earth in its rotation. This causes moving water masses to be deflected as they flow. The apparent deflection is at its maximum at the poles, is reduced at lower latitudes, and becomes zero at the equator. In Northern Hemisphere estuaries, the Coriolis effect tends to concentrate the lower salinity water that flows down the estuary toward the left side of the estuary as viewed from the ocean, while the high salinity water flowing up the estuary tends to be concentrated toward the right side (In the Southern Hemisphere these directions of concentration are reversed). To read CC12 go to page 32CC.

CC.15 Food Chain Efficiency (pp. 417, 423)

  • All organisms use some of their food as an energy source in respiration and for reproduction. They also lose some of their food in excretions (including wastes). On average, at each level in a food chain, only about 10% of food consumed is converted to growth and biomass of the consumer species. To read CC15 go to page 49CC.

CC.16 Maximum Sustainable Yield (pp. 418, 423, 427)

  • The maximum sustainable yield is the maximum biomass of a fish species that can be depleted annually by fishing but that can still be replaced by reproduction. This yield changes unpredictably from year to year in response to the climate and other factors. The populations of many fish species worldwide have declined drastically when they have been overfished (beyond their maximum sustainable yield) in one or more years when that yield was lower than the average annual yield on which most fisheries management is based. To read CC16 go to page 51CC.


Reduce Text Size Increase Text Size Email Print Page

Chapter Features

Purchase the Introduction to Ocean Sciences, Second Edition ebook

Need technical support? Please visit our helpdesk.

Questions of comments about the site content? Contact the author.

Visit the author’s photo archive

Norton Gradebook

Instructors now have an easy way to collect students’ online quizzes with the Norton Gradebook without flooding their inboxes with e-mails.

Students can track their online quiz scores by setting up their own Student Gradebook.