• The ocean water has two characteristic properties: temperature and salinity. These two factors determine the ocean Currents and the flora and Fauna of the oceans

Ocean Temperature Characteristics

• The temperature of ocean water is a fundamental physicochemical factor influencing both sedimentation and marine life.
• The mean annual temperature of the oceans stands at approximately 17.4°C. Specifically, the Pacific Ocean maintains a mean temperature of 19.1°C, the Indian Ocean at 17.0°C, and the Atlantic Ocean at 16.9°C.
• Warm ocean currents play a role in transporting higher temperatures to colder latitudes, while cold currents contribute to cooling lower latitudes.
• Unlike fresh water, ocean water freezes at a lower temperature, around -1.872°C instead of 0°C. Additionally, water with higher salinity freezes at an even lower temperature.
• When ocean water freezes, the solutions are expelled, resulting in nearly fresh water ice. Oceanic ice generally does not exceed a thickness of 3 meters.
• The ice layer acts as insulation, preventing the water beneath it from reaching the colder air temperatures above. Consequently, the water beneath the ice remains cold but doesn't freeze. This sub-ice water also contains a high salt concentration, making it denser than surface water.
• A vertical circulation of ocean water occurs as dense water sinks to the ocean bottom, creating what is known as thermohaline circulation, incorporating both temperature (thermo) and salinity (haline). This circulation ensures that the deep ocean water maintains a temperature similar to the coldest surface water.
• In tropical regions, sunlight warms the ocean, with Equatorial surface temperatures reaching as high as 30°C. Conversely, near the poles, the average surface water temperature hovers around 0°C
• The highest sea surface temperatures (27°C to 30°C) are not near the Equator but a few degrees north of it. The lowest recorded temperature is -1.9°C near the poles.
•  Maximum and minimum annual temperatures of ocean water occur in August and February in the Northern hemisphere, and the pattern reverses in the Southern hemisphere.

Factors Influencing Ocean Temperature

Several factors impact the distribution of ocean water temperature:

• Latitude: Surface water temperature decreases from the Equator toward the poles due to the slanting rays of the Sun poleward.
• Prevailing Wind Direction: Wind direction influences ocean water temperature distribution. Offshore winds from land to sea elevate ocean temperatures, while winds from snow-covered regions in winter decrease surface temperature. In the trade wind belt, offshore winds induce upwelling of cooler water, and offshore winds accumulate warm water, raising temperatures.
• Unequal Distribution of Land and Water: Ocean water temperature variation between hemispheres results from the dominance of land in the Northern hemisphere and water in the Southern hemisphere. The Northern hemisphere ocean receives more heat due to its contact with land, as heat flows from continents to the ocean.
• Ocean Currents: Warm currents raise ocean temperatures, while cold currents decrease them. For example, the Gulf Stream (warm current) increases temperatures along the Eastern part of North America and the West coast of Europe, while the Labrador cold current reduces temperatures near the Northeastern coast of North America.
• Location and Shape of Area: Enclosed seas tend to have higher temperatures at the same latitude compared to open seas, which generally have lower temperatures.
• Additional minor factors such as submarine ridges, local weather conditions (storms, cyclones, hurricanes, fog, cloudiness), evaporation, and condensation also influence surface ocean temperature.

Temperature Ranges of Oceanic Water

Two temperature ranges of oceanic water are identified:

Daily Range of Temperature:

• The difference between the maximum and minimum temperature within a day (24 hours) is known as the daily range of temperature.
• The daily range of temperature for surface ocean water is almost negligible, around 1°C. Maximum and minimum temperatures are typically recorded at 2 PM and 5 AM, respectively. The daily range temperature is approximately 0.3°C in low latitudes and 0.2°C to 0.3°C in high latitudes.

Annual Range of Temperature:

• The maximum and minimum annual temperatures of ocean water are recorded in August and February, respectively, in the Northern hemisphere.
• The average annual range of temperature of ocean water is -12°C (10°F) but there is a lot of regional variation, which is due to regional variation in insolation, nature of seas, prevailing winds, location of seas, etc.

Distribution of Temperature

Distribution of temperature are described below

Horizontal Distribution of Temperature

• The average temperature of surface water of the oceans is 26.7°C and the temperature gradually decreases from the Equator towards the poles.
• The rate of decrease of temperature with increasing latitudes is generally 0.5°C per latitude.
• The average temperature becomes 22°C at 20° North and South latitudes, 14°C at 40° North and South latitude, 10°C at 60°C latitude and 0°C near the poles.
• The oceans in the Northern hemisphere record relatively higher average temperature than in the Southern hemisphere.
• The average annual temperature of all the oceans is 17.2°C. The average annual temperatures for the Northern and Southern hemispheres are 19.4°C and 16.1°C respectively.
• The variation of temperatures in the Northern and Southern hemispheres is because of unequal distribution of land and water as the Northern hemisphere is made up of more land, while the Southern hemisphere is made up of more oceans.

Vertical Distribution of Temperature

• The maximum temperature of the oceans is always on the surface because it directly receives the insolation.
• The heat is transmitted to the lower sections of the oceans through the mechanism of conduction.
• Vertically the oceans are divided into three layers from the standpoint of thermal conditions of sea water, in the lower and middle latitudes.

Diagrammatic Representation of Vertical Distribution of Temperature

Three layers in the oceans from surface to the bottom are as follows

• First Layer The thermocline layer represents the top layer of warm oceanic water and is 500 m thick with temperature ranging between 20°C and 25°C. This layer is present within the tropics throughout the year but it develops in mid-latitudes only during summer.
• Second Layer The permanent thermocline layer represents the vertical zone of oceanic water below the first layer. It is characterized by a rapid rate of decrease in temperature with increasing depth.
•  Third Layer The psychrosphere layer is very cold and extends up to the deep ocean floor. The polar areas have only one layer of cold water from the surface (sea-level) to the deep ocean floor.

Salinity of Ocean 

• Salinity is the total amount of the dissolved salts in grams contained in one kilogram of seawater and is expressed as parts per thousand per cent.
• The salinity of water affects the waves, currents, temperature, fishes, organisms, planktons, etc.
• Isohaline is the imaginary line drawn on a map to show places with the same salinity. Salinometer is the instrument used to measure salinity.
• Salinity in Oceans: Impact and Distribution
• The average salinity of oceans stands at 35.2%, contributing to a decrease in the freezing rate, a reduction in evaporation, and an increase in density.
• Salinity induces water and ocean currents, with the latter moving from areas of lower salinity to higher salinity.
• An upper limit of 24.7% is considered for defining brackish water. Salinity at depth remains constant, leading to a marked contrast between surface and deep ocean zones. Lower saline water rests above denser, higher saline water, resulting in a distinct Halocline zone where salinity sharply increases.
• Increasing seawater salinity, while keeping other factors constant, raises its density, causing high saline water to sink below low saline water, resulting in salinity-based stratification.

Oceanic Salinity Quantity

• Sodium Chloride: 27.213
• Magnesium Chloride: 3.807
• Magnesium Sulphate: 1.658
• Calcium Sulphate: 1.260
• Potassium Sulphate: 0.863
• Calcium Carbonate: 0.123
• Magnesium Bromide: 0.076
• Total: 35

Factors Affecting Salinity:

• Depth: Salinity varies with depth, influenced by latitudinal differences.
• Currents: Cold and warm currents impact salinity decrease.
• Latitude: Salinity increases with depth in high latitudes, peaks at 35m in mid-latitudes, and decreases at the Equator.
• Location of Sea: The location of the sea affects how salinity changes with depth.
• Evaporation: Surface salinity increases due to evaporation, which is offset by fresh input from rivers or ice loss.

Importance of Salinity:

• Salinity influences the compressibility, density, insolation, absorption, evaporation, and humidity of seawater.
• The composition of water and the distribution of organisms and plankton are affected by salinity.
• Salinity impacts the freezing point of the ocean, with high saline oceans freezing more slowly, and evaporation slowing with increased salinity.

Distribution of Salinity:

• Average oceanic salinity is 35%, with variations in each ocean, sea, lake, and bay.
• Horizontal distribution shows higher salinity near the Tropics of Cancer and Capricorn due to low rainfall and high evaporation.
• Equatorial regions have lower salinity due to high rainfall, maintaining an average salinity around 34-35%.
• On average, salinity decreases from the Equator towards the poles, with the highest salinity occurring in the subtropical high-pressure belt due to elevated temperatures, increased evaporation, and reduced rainfall. In the entire Northern hemisphere, the average salinity is around 34%, while it stands at 35% in the Southern hemisphere.

Marginal Sea Salinity:

• The Mediterranean Sea, Red Sea, and Persian Gulf exhibit relatively high salinity levels, ranging from 37% to 41%
• Despite its high latitude, the North Sea maintains high salinity due to an influx of saltwater from the North Atlantic.
• The Baltic Sea experiences low salinity due to substantial river water inflow.
• The Black Sea, South China Sea, Andaman Sea, Japan Sea, and Bering Sea have low salinity levels attributed to significant freshwater influx from rivers and low evaporation.

Inland Seas and Lakes:

• Inland seas, receiving regular salt supply from rivers, boost very high salinity as water evaporates, increasing salt content.
• Examples include the Great Salt Lake (Utah, USA), the Dead Sea, and Lake Van in Turkey, with salinities of 220, 240, and 330, respectively.
• Oceans and salt lakes become more saline over time due to increased salt input from rivers and freshwater loss through evaporation.

Vertical Distribution of Salinity:

• Salinity changes with depth, and its pattern varies based on the sea's location.
• At the Equator, salinity increases with depth but starts decreasing as one moves to deeper parts.
• In high-latitude regions, salinity increases with depth as water freezes.
• The Halocline zone, between 300m and 1000m, witnesses rapid salinity changes.

Oceanic Deposits:

• Ocean deposits, formed by continuous weathering, erosion, and remains of fauna and flora, are classified by Sir John Murray into four groups:

Terrigenous Deposits:

• Derived from the wear and tear of land, volcanic, and organic products.
• Mainly found on continental shelves and slopes.
• Classified into three classes based on particle size: mud, sand, and gravel.
• Mud is further categorized into blue, green, and red types based on color.
• These processes contribute to the diverse and dynamic nature of oceanic salinity and deposits.

Volcanic Deposits

• In volcanic regions, the continental shelf and slope accumulate primarily from volcanic activities. These materials undergo chemical and mechanical weathering and are transported to the ocean by the forces of running water and wind. What sets volcanic deposits apart from typical Terrigenous deposits is their composition, consisting of pyroclastic volcanic products and lava rather than quartz.

Organic Deposits

• These deposits are formed from the shells and skeletons of various marine plants and animals on the seafloor, transformed into mud and sand through chemical and mechanical processes. Distinguishing themselves from regular Terrigenous deposits, organic deposits are solely composed of calcium carbonate. They are further classified into Neretic Deposits, found on continental shelves and in shallow waters, and Pelagic Deposits, which are both organic and inorganic.
• Neretic Deposits are coarse-grained accumulations of remains from ocean organisms, including mollusks, skeletons, and calcareous vegetation. They are typically found in continental shelves and shallow waters near islands.
• Pelagic Deposits, covering about 75% of the seafloor, are formed from the remains of marine organisms and volcanic dust carried by the wind. They are classified into Calcareous Ooze, which contains calcium carbonate shells, and Siliceous Ooze, which is of organic origin and composed of silica.
• Calcareous Ooze includes Pteropod Ooze, formed from aragonite planktonic gastropods, and Globigerina Ooze, consisting of shells from planktonic marine protozoans. The latter forms a layer of soft mud at depths of 1000 to 3000 feet.
• Siliceous Ooze, of organic origin, is made up of silica and is largely present in the Pacific and Southern oceans. It can be further divided into Diatom Ooze, formed from microscopic unicellular algae, and Radiolarian Ooze, composed of siliceous radiolarian tests.

Inorganic Deposits

• These deposits are composed of microscopic particles of dirt, potentially causing inflammation, eye irritation, or redness.

Coral Reefs

• Regions of high biodiversity formed by the accumulation and solidification of skeletons of lime-secreting organisms known as coral polyps. Coral reefs are lime-dominated rocks, representing a symbiotic association between coral polyps and algae.
• Coral reefs are formed by colonies of various types of reef-building stony hard corals. Each coral colony consists of polyps that stay fixed in one place, providing a home to symbiotic algae.
• Each polyp slowly secretes a hard calcium carbonate skeleton, serving as the base or substrate for the colony. The living polyp attaches itself to the skeletal base, offering protection to both the polyps and algae from approaching predators.
• Calcium carbonate is continuously deposited by the corals in the living colony, contributing to the growth and structure of the reef. It is the gradual development of these hard skeletal structures that builds coral reefs over an extended period.
• Often referred to as the rainforests of the sea, coral reefs form some of the most diverse ecosystems on Earth. These fragile ecosystems are particularly sensitive to water temperature.
• Corals are distributed across the world's oceans, from the Aleutian Islands off the coast of Alaska to the warm tropical waters of the Caribbean Sea. The largest coral reefs are found in the clear, shallow waters of the tropics and subtropics.
• The Great Barrier Reef in Australia, the largest of these coral reef systems, stretches over 1,500 miles (2,400 km).

Conditions for Coral Formation:

• Temperature: Corals are generally found between 30° North and 30° South latitudes, requiring a temperature of 20°C for development. They thrive in depths of 45 to 55 meters where sunlight is abundant.
• Salinity: Corals develop in clean saline water and do not thrive in freshwater or highly saline conditions, avoiding deltaic regions.
• Oxygen and Plankton: Corals need oxygen and plankton for survival and growth, leading them to develop towards the continental slope.

Areas of Coral Reefs:

• Coral reefs cover approximately 284,300 sq km, making up just under 0.1% of the ocean's surface area. The Indo-Pacific region, including the Red Sea, Indian Ocean, Southeast Asia, and the Pacific, accounts for 91.9% of this total.
• South-East Asia and the Pacific, including Australia, contribute significantly to coral reef coverage, with 32.3% and 40.8%, respectively. The Atlantic and Caribbean coral reefs account for 7.6%.
• Coral reefs are rare along the American and African West coasts due to upwelling and strong cold coastal currents reducing water temperatures.
• In India, coral reefs are found in the Gulf of Kutch, Lakshadweep Islands, Gulf of Mannar, and Andaman and Nicobar Islands.
• The Great Barrier Reef, comprising over 2900 individual reefs and 900 islands, is the largest reef, stretching for over 2600 km off Queensland, Australia.

Types of Coral Reefs:

Fringing Reefs:

• These reefs grow directly from a shore, located very close to land, often forming a shallow lagoon between the beach and the main reef body.
• A fringing reef runs as a narrow belt (1-2 km wide), growing from the deep sea bottom with the seaward side sloping steeply into the deep sea. Coral polyps do not extend outward due to sudden depth increases.
• Fringing reefs are the most common type, found in various regions of coral reef development, including examples at the New Hebrides Society Islands off Australia and off the Southern coast of Florida.

Barrier Reefs:

• Extensive linear reef complexes that run parallel to a shore, separated by a lagoon. The largest in size among the three reef types, spanning hundreds of kilometers and several kilometers in width.
• It forms a broken, irregular ring around the coast or an island, almost parallel to it.
• Barrier reefs are less common than fringing reefs or atolls, with examples found in the tropical Atlantic and Pacific. The Great Barrier Reef off the North-Eastern coast of Australia, spanning 1200 miles, is the world's largest of this type.
• Despite its name, the Great Barrier Reef is not a single reef but a vast complex consisting of many reefs.

Atoll:

• A roughly circular (annular) oceanic reef system surrounding a large, often deep central lagoon. The lagoon has a depth of 80-150 meters and may be connected to seawater through channels cutting across the reef.
• Atolls are located at great distances from deep-sea platforms, where submarine features like submerged islands or volcanic cones may aid in their formation.

Importance of Coral Reefs:

• Coral reefs serve as a source of food and shelter for various species, including fish, shellfish, fungi, sponges, sea anemones, and more. They are crucial support for human life, providing homes for species like crabs, shrimp, oysters, and clams.
• Coral reefs protect coastlines from ocean storms and floods and act as environmental indicators of water quality.
• Economically, coral reefs contribute significantly by attracting millions of tourists and serving as sources of new medicines.

Coral Bleaching:

• A negative phenomenon associated with the health of corals, where the loss of algae causes whitening and eventual death. 
• Coral bleaching is influenced by natural and human-induced factors such as sea temperature, sedimentation, freshwater dilution, and more.

Coral Reseeding project

• In 2018, scientists initiated the largest-ever attempt to regenerate coral on the Great Barrier Reef by harvesting millions of eggs and sperm during their annual spawning.

Marine Resources:

• The ocean is a valuable natural resource, providing food, jobs, goods, services, and economic importance for billions of people worldwide. Ocean resources include food, fuel, renewable energy, minerals, sand, gravel, and tourism.
• The ocean is mined for minerals and drilled for crude oil, playing a critical role in regulating Earth's climate by removing carbon from the atmosphere and providing oxygen.

Maritime Zone

• United Nations Convention on the Law Sen (UNCLOS) 1982 (Law of the Sea) of the is the only which stipulates a framework for state jurisdiction in maritime international convention spaces. It provides a different legal status to different maritime zones.
• It divides marine areas into five main zones namely Internal Waters, Territorial Sea, Contiguous Zone, Exclusive Economic Zone (EEZ) and the High Seas.

Classification of Marine Resources

Marine resources can be divided into the following categories 

Marine Biological Resources (MBR)

• The biotic and abiotic resources found in the oceanic water and bottoms are called Marine Biological Resources.
• They include marine water, inherent energy in the oceanic water (e.g., wave energy, tidal energy etc.), biotic life of marine water (plants and animals), marine deposits and abiotic elements (minerals, fossil fuels etc.), biotic and abiotic matter of ocean bottoms, benthic organisms etc.
• Marine Biological Resources (MBR) are divided into two parts i.e., food and non-food resources. 
• Food Resources On the basis of uses, marine food resources are divided into two types
  • (i) Protein rich food resources for the use as food for human beings (e.g. fishes).
  • (ii) Animal feed mainly for domesticated animals and the contribution of fish in the world annual income from marine resources of all categories stands second.
• Non-food Resources The important marine non-food resources include corals. They are animals in the groups of marine organisms. These are generally called rainforests of the oceans.

Marine Organisms (plants and animals)

• These are also divided into three categories on the basis of their habitats: Planktons These are floating and drifting micro plants and animals of photic zone. These are divided into phytoplankton (plant planktons) and zooplanktons (animal planktons).
• They include algae, strong and powerful floating and swimming marine animals, mainly fish. These marine animals move in all the zones of the oceanic environments. e.g., Pelagic fish, Demersal fish, etc.
• Benthos Benthic organisms include those plants (non-photosynthetic or non-phototrophs) and animals which live at the bottoms of the seas and oceans. e.g., epibenthic community, benthic organisms, inflora and infauna.

Prelims facts

• Oceanic salinity is low in the Equatorial Region because of

- Heavy rainfall, Cloudiness and Humidity [UPPSC (Pre) 2008]

•  Which sea has the highest average salinity ?

- Dead sea [UPPSC (Pre) 2012, BPSC (Pre) 2017)

• Major source of Oceanic salinity is 

- Rivers [BPSC (Pre) 2011]

• Which zone denotes water salinity gradient ?

- Halocline [UPPSC (Pre) 2016]

• Which one of the following salts contributes maximum to the salinity of seawater ?

- Sodium Chloride [UPPSC (Pre) 2000]

• Where is the Great salt lake located ?

- USA [UKPSC (Pre) 2006]

• Where is the coral reef or fossil belt usually found ?

- Coastal areas between Tropic of Cancer and Capricorn [RAS (Pre) 2009]

•  World's largest coral reef is found near

- Australia [IAS (Pre) 2007]

•  The Great Barrier Reef is located in

- Pacific Ocean [MPPSC (Pre) 2015]

• The 'Great Barrier Reef' of corals is situated near the coast of

- Queensland [RAS (Pre) 1993]

Self Check

1. Which is the largest coral reef in the world?

(a) Kingman Reef

(b) The Great Barrier Reef

(c) Lansdowne Bank

(d) Lyra Reef

2. Where are coral reefs located?

(a) Mid-latitudes

(b) Subtropics

(c) Tropics

(d) High latitudes

3 . The zone of highest salinity is MPPSC (Pre) 2011

(a) the tropical zone of the sea

(b) the temperate zone of the sea

(c) the polar zone of the sea

(d) None of the above

4. Salinity in ocean water decreases when

(a) evaporation is more

(b) wind velocity is high

(c) humidity is high

(d) rainfall is high

5. Which one of the following statements about the growth of coral reefs is not correct?

(a) Coral can grow abundantly in freshwater

(b) It needs warm water between 23°C-25° C.

(c) It needs shallow saline water not deeper than 50 m.

(d) It needs to be supported in photosynthesis and

requires abundant sunlight.

6. Consider the following statements. UPPSC (Pre) 2018

1. Most of the world's coral reefs are found in the water of tropical seas.
2. More than one-third of the world's coral reefs are located in the territories of Australia, Indonesia and the Philippines.
3. Coral reefs support a large number of animal associations as compared to tropical rainforests.

Which of the statement(s) given above is/are correct?

(a) 1 and 2

(b) Only 3

(c) I and 3

(d) All of these

7. The acidification of oceans is increasing. Why is this phenomenon a cause of concern? UPPSC (Pre) 2012

1. The growth and survival of calcareous phytoplankton will be adversely affected.
2. The growth and survival of coral reefs will be adversely affected
3. The survival of some animals that have phytoplanktonic larvae will be adversely affected.
4. The cloud seeding and formation of clouds will be adversely affected

Codes

(a) 1, 2 and 3

(b) Only 2

(c) 1 and 3

(d) All of these

8. Which of the following conditions are necessary for the growth of coral reefs? UPPSC (Pre2016

1. Light conditions
2. Clear and sediment free water
3. 6% ocean salinity 
4. Tropical sea water with temperature ranging from 22 20°C to 21°C

Codes

(a) 1, 2 and 4

(b) 1 and 3

(c) 2 and 4

(d) All of these

9. Consider the following statements.

1. Most of the world's coral reefs are in tropical wate
2. More than one-third of the world's coral reefs are located in the territories of Australia, Indonesia, Philippines.
3. Coral reefs host far more number of animal phyla than those hosted by tropical rainforests

Codes 

(a) 1 and 2

(b) Only 3

(c) 1 and 3

(d) 1, 2 and 3

10. Where is the coral reef or fossil belt usually RAS/RTS (Pre) 2009 found?

(a) In a temperate climate zone about 18°C. 

(b) In coastal areas between Tropics of Cancer and Capricorn.

(c) On the East and West coasts of both continents and islands. ND DAL AN

(d) Cold sea coasts

11. The largest coral reef in the world is found near the coast of which one of the following countries

(a) Australia

(b) Cuba

(c) Ghana

(d) Philippines

12. 'Biorock technology' is talked about in which one of the following situations?

(a) Restoration of damaged coral reefs.

(b) Development of building materials using plant residues.

(c) Identification of areas for exploration/extraction of shale gas.

(d) Providing salt licks for wild animals in forests/protected areas.

Know Right answer

1 (b)

2 (c)

3 (a)

4 (d)

5 (a)

6 (d)

7 (a)

8 (a)

9 (d)

10 (b)

11 (b)

12 (a)