Introductory Chapter: General Overview on Oceanography

1. Introduction

Spherical, gigantic, bright blue with large whites in areas of clouds, ice, and storm spirals. This is the Earth seen from space by an astronaut. Blue corresponds to the ocean that moderates the temperature, significantly influences the climate, and ensures the maintenance of life as we know it today. The human population has used the ocean since the beginning of the times, and the conquest of the seas is directly related to the evolution of human society, which establishes the movement of people relative to the sea, raw material, food, commodity/trade exchanges, energy, and as a waste disposal area, unfortunately currently contributing to the effects of climate change. About 6,24 billion people (78% of the world’s population) live within a radius of 200 km from the sea, making up the largest metropolises on the planet, almost all of which are directly connected to the ocean [1].

Seas and oceans are smaller segments of the only ocean that involves all the emerged land of the planet, divided only for purposes of convenience (social and political) and geolocation. During the Middle Ages, much of the maritime trade was carried out between the Mediterranean sea and other small seas in that region, becoming famous the “Seven Seas,” comprising the Adriatic, Arabian, Caspian, Mediterranean, Black and Red sea, and the Persian Gulf [2]. Currently, globalized human civilization, in constant exchange of information, responds with a much broader and more analytical look at the ocean. The new seven seas are made up of the North Pacific, South Pacific, North Atlantic, South Atlantic, Indian, Arctic, and Antarctic oceans. According to the International Hydrographic Organization, there are 61 seas on Earth, such as the Caribbean Sea (Central America), North Sea (Northern Europe), Gulf of Mexico (Mexico and USA), Bering Sea (between America and Asia), Persian Gulf (in the Middle East), and Hudson Bay (Canada and USA) [3].

Seas and oceans together cover 71% of the Earth’s surface, which corresponds to an area of 361,100,000 Km² and a volume of 1,338,000,000 Km³ (Table 1). Its deepest point is the Mariana Trench, 11,022 m deep in the Pacific, and the highest point is in the Hawaiian Marine Range (USA), a sea mountain 10.203 m high from the ocean floor (Figure 1). On average, the ocean has a layer 3796 m thick, with a temperature of 3.9°C and a salinity of 34.482 g of salt per liter of water, usually at a salinity of 35. In comparison, the surface land is only one thickness of 840 m and the Himalayan range (Nepal) with 8848 m. The deepest point, among all continents, is in Siberia (northern Russia), Lake Baikal with 1680 m deep, which dams 20% of the planet’s fresh melting water [4].

Portions of water on Earth	Volume (Km^3)	Percentage (%)
Salt water (Oceans and Seas)	1.338.000.000	97,0
Fresh water	35.000.000	3,0
Portions of water on Earth	Volume (Km^3)	Percentage (%)
Glaciers	24.000.000	68,7
Subterranean water	10.500.000	30,1
Permafrost	300.000	0,8
Surface water and atmosphere	135.000	0,4
Surface water and atmosphere	Volume (Km^3)	Percentage (%)
Lakes	91.000	67,4
Soil moisture	16.500	12,2
Atmosphere	13.000	9,5
Swamp water	11.500	8,5
Rivers	2.120	1,6

Table 1.

The proportions of water mass on the planet are distributed differently throughout the planet, whether in solid (ice), liquid (ocean, rivers, lakes, groundwater), or gaseous (atmospheric vapor) form.

The largest portion is known as ocean, being salt water.

Figure 1.

About 71% of the surface of the planet is covered by the ocean, and this volume corresponds to 97% of all the water in the Earth’s crust.

While the ocean may seem incredibly large, on a planetary scale, it is insignificant. In an image that portrays the surface of the Earth covering a paper globe of 12 cm in radius, the oceans would only represent the thin layer of blue ink that colors the paper, considering the 12 cm radius of the planet. The ocean accounts for about 0.02% of the planet’s mass. There is an immensely greater volume of water inside the planet than in the ocean, atmosphere, and rivers. The Northern Hemisphere has 60.7% of its surface area of sea and 39.3% of land, the largest portion of emerged land. In the Southern Hemisphere, its largest area is destined to the sea with 80.9%, and only 19.1% of land (Figure 1).

Oceanography is important because the oceans play a critical role in global climate, producing oxygen, regulating sea levels, and creating habitats for millions of marine species. In addition, the oceans are a vital source of food, energy, and mineral resources for humanity. For these reasons, oceanography is a fundamental science for understanding and sustainably managing the oceans and their resources [5].

2. First navigations

Historically, civilizations that used maritime transport (mobility or food) had greater development and greater territorial borders in relation to other cultures. The first written records of maritime trade date back to 2000 BC in the Mediterranean sea. The Cretans were the first people to establish maritime supremacy in the Mediterranean. After the fall of this empire in 1200 BC, the Phoenicians gained control and expanded the commercial zone beyond the Straits of Gibraltar. Greek culture began its dominance of the Atlantic Ocean in 900 BC. They were the first to observe a north-south current beyond Gibraltar, considering all this body of water as an immense river, called the “Okeanos.” However, these expeditions were closely associated with the coastal zone, very few ventured high sea. On the other side of the world, other peoples also took to the sea, such as the Chinese, who developed a complex waterway system that connected the various rivers to the Pacific Ocean. It is estimated that in 3000 BC. Polynesian peoples already moved easily between the islands of present-day Indonesia and South Asia, starting the colonization of islands in the central portion of the Pacific. These “sailors” were based simply on observing the Sun and stars during dawn and dusk [6].

Trade and the conquest of new lands promoted travel increasingly ambitious, long, and far from the coast. It is undeniable that marine science had its beginnings linked to simple observations described by navigators. In 300 B.C., the largest library in the history of the ancient world was founded, with the largest collection of parchments, the Library of Alexandria (Egypt), considered the first university on the planet. Due to this source of information, marine sciences had a great leap forward in their applied studies. One of the most famous librarians who managed the library was the Greek Eratosthenes of Cyrena, the first to remarkably calculate the circumference of the Earth. Although Pythagoras had already reached the conclusion that the planet was round in 600 BC, it was Eratosthenes who estimated its size. The original figure published by the librarian in 230 B.C. differs by only 8% from the currently calculated real value (40.075 km) [7].

2.1 Scientific navigations

The first documented scientific expedition took place between 1768 and 1771, under the English flag of Captain James Cook, on the ship HMS Endeavor. Although the expedition had several goals, scientific observation was one of them. It was on this expedition that they “discovered” New Zealand and mapped the Great Australian Barrier Reef. The success of his first voyage resulted in two other expeditions: one (between 1772 and 1775) to the extreme south, being the first navigator to circumnavigate the world at high latitudes, which “discovered” Easter Island and reached latitude 71°S, although it did not find Antarctica. His last expedition was between 1776 and 1779, with the aim of exploring the high latitudes of the North (Canada, Alaska, and Siberia). On this voyage, Cook “discovered” Hawaii and charted the west coast of North America. He and scientists from the British Royal Academy collected samples of plants, animals, various marine organisms, and samples from the ocean floor. The detail in the description of your Pacific nautical charts is so accurate that it helped the Allies during World War II [8].

All of the above expeditions promoted great advances in marine science, even though none of them had academic research as their main objective. The first circumnavigation expedition with the exclusive main objective of marine sciences was the British ship HMS Challenger of 1872–1876. Another famous previous expedition, that of the ship HMS Beagle, was commanded by Captain Robert FitzRoy and naturalist Charles Darwin between 1831 and 1836. This resulted in remarkable discoveries for the theory of the evolution of life on the planet, but it was mainly focused on experiments and continental samplings, C. Darwin’s bestseller The Origin of Species (published in 1859), was one of the fruits of this splendid expedition, which inspired the future Challenger [9].

3. Physical oceanography

Physical oceanography is the part of oceanography that focuses on understanding the physical properties of the oceans, such as temperature, salinity, current, swell, and depth. These properties affect the circulation of ocean currents, the formation of islands, and continents, marine life, and the interactions of the oceans with the atmosphere and climate [10].

Physical oceanography focuses on understanding how temperature, salinity, and current affect the circulation of ocean currents. Ocean currents are important because they affect the global climate, help transport nutrients, and regulate sea levels. Physical oceanography also studies waves, including how they are generated, how they propagate, and how they affect marine life [11].

Physical oceanography also focuses on understanding the depth of the oceans and undersea topography. This includes the formation of seamounts, island chains, and the cycles of erosion and sediment accumulation. Undersea topography is important because it affects current circulation, carbon dioxide production, and marine life [12].

In addition, physical oceanography studies the interactions between the oceans and the atmosphere, including how the oceans affect global climate and how they are affected by climate. For example, oceans can affect global climate by transporting heat and water vapor, and climate can affect oceans through precipitation, evaporation, and ice formation [13].

In summary, physical oceanography is an important part of oceanography that focuses on understanding the physical properties of the oceans and their interaction with the atmosphere and climate. This is crucial to understanding how the oceans affect and are affected by the global climate and to ensure the sustainable management of marine resources [14].

3.1 Atmospheric circulation

The sun and atmosphere directly or indirectly control almost all dynamic processes within the ocean. The dominant external factors and energy sinks are sunlight, evaporation, infrared radiation emission from the ocean surface, and the ocean’s sensible heat from hot and cold winds. Winds control the surface circulation of the ocean down to about 1 km of depth. Wind and tides drive deep ocean currents [15].

The ocean, in turn, is dominated by a heat force that conducts atmospheric circulation differently from the equator to the poles. The uneven distribution of heat balance (loss and gain) across the ocean drives winds through the atmosphere. The sun heats the tropical ocean, which evaporates, transferring heat in the form of water vapor to the atmosphere. Heat is released when the steam condenses into rain. Winds and ocean currents transport heat toward the poles, where it is lost to the atmosphere [16].

Air heats up, expands, and rises at the equator; in the same way that it cools, contracts, and performs downward movement at the poles. However, instead of continuing from the equator to the poles continuously in each hemisphere, air rising at the equator is gradually deflected eastward as it moves poleward, that is. the air turns to the right in the Northern Hemisphere (NH) and to the left in the Southern Hemisphere (SH). This change in direction is caused by the Coriolis effect (a real effect that depends on the frame of reference), which, despite not causing the wind, influences the direction [17].

From the moment the air rises at the equator, a decrease in humidity by precipitation (rain) caused by the cooling and expansion. This drier air then becomes denser in the upper atmosphere as it begins to radiate heat into space and cools. After moving from the equator to about 30°N and 30°S latitude, the air becomes dense enough to descend to the Earth’s surface. A large portion of the descending air returns toward the equator when it reaches the surface. In the NH, the Coriolis effect influences the direction of surface air to the right. Despite being warmed by compression during its downward movement, air is normally cooler than the surface it flows through. As a result, the air warms up as it moves toward the equator but evaporates surface water and becomes humid. This moist, heated, and less dense air begins to rise as it approaches the equator, closing the cycle. This significant air loop is called the atmospheric circulation cell. There are two cells in the tropics (0° to 30°): Hadley cells. Two cells at mid-latitudes (between 30° and 50–60°): Ferrel cells and two cells at high latitudes (50–60° up to 90° - poles): polar cells. These three large atmospheric circulation cells described are also represented by the trade winds (northeast and southeast), west winds, and east winds, respectively [18].

This atmospheric circulation model described provides a very interesting understanding for physical oceanography. From understanding of these atmospheric dynamics, it is possible to extend the study to various phenomena or processes that occur in the earth-ocean-atmosphere system, such as monsoons, breezes (sea and land), storms, cyclones (tropical and extratropical), and even phenomena, such as “El Niño” and “La Niña” (also correlated with oceanic circulation) [19].

3.2 Oceanic circulation

As seen in the description of atmospheric circulation, there is an energy or heat balance between the equator and the poles through the atmosphere and oceans. This interface is extremely important. This energetic (or thermal) balance is essential for the dynamics of winds and ocean circulation. Energy transport across the oceans via ocean currents accounts for 10 to 20% of the heat distribution across the planet. Basically, sea water moves in currents, either shallow or deep. Surface currents affect only the shallowest tenth of the oceans, and their movement is influenced by heat balance and winds. In general, the movement of surface currents is horizontal, and they can also flow vertically according to the wind that blows near coastal regions or along the equatorial region. Surface currents flowing from the equator (low latitudes) transport heat to the poles (high latitudes), nutrients, and influence climate and weather. In addition, they are essential for navigation. Deep oceanic or thermohaline circulation is driven by density differences between water masses. Remembering that the density in the oceans is defined by the relationship between temperature, salinity, and pressure (due to the great depths). This circulation accounts for 90% of sea water below the surface layer [20].

In general, the Coriolis effect, the force of gravity and friction influence movement (direction, up and down, and intensity/velocity) of surface and deep ocean currents (thermohaline). The oceans are interconnected but do not perform significant water exchange between them, and this fact occurs because the water masses have different oceanographic characteristics (temperature, conductivity, salinity, heat balance), wave dynamics, tides, and currents that differ throughout the planet. Therefore, the oceans are divided into five large portions: Atlantic, Pacific, Indian, Arctic, and Antarctic [21].

4. Geological oceanography

Geological oceanography is an interdisciplinary area that combines knowledge from geology, oceanography, geophysics, geochemistry, geomorphology, and sedimentology. Its aim is to understand how the geological features of the oceans and seafloor emerged and how they evolved over time [22].

One of the main techniques used in geological oceanography is seismic, which involves the emission of sound waves that are reflected from different underground layers, allowing scientists to obtain a three-dimensional image of the marine subsoil. Other techniques include taking sediment samples, using sonar to map the seafloor, and conducting underwater drilling [23].

Geological oceanography is also important for understanding the evolution of undersea sedimentary basins, which are large areas of sediment deposition, and for studying undersea volcanic eruptions and undersea earthquakes. This information is critical for predicting potentially dangerous geological events, such as tsunamis, and for properly managing coastal zones and marine resources, such as oil and natural gas [24].

The coastline is one of the most dynamic natural features on the planet. Its position in space constantly changes on temporal scales of seconds (waves), hourly (high tides and low tides), daily (storms), seasonal (seasons of the year), annual (El Niño), decadal, secular, and millennial. The daily rise and fall of sea level and other bodies of water connected to the ocean (estuaries, lagoons, etc.) are caused by the interference of the Moon and the Sun on the Earth’s gravitational field. The amplitude of the tides (the difference in level between high and low tide) is a modeling element of the coastline, as a function of the current velocities associated with it. These tidal currents are significant in coastal sediment transport. Tidal currents have the capacity to modify the morphology of the coastline and the inner continental shelf [25].

Geological oceanography also plays an important role in understanding marine biodiversity and protecting marine ecosystems. Undersea geology can influence ocean currents, temperature, and the chemical composition of water, which has implications for marine life. In addition, geological oceanography can help identify and protect areas with high biodiversity and critical habitats for marine life [26].

In summary, geological oceanography is an important field that provides a fundamental understanding of the evolution of the Earth and oceans, as well as the interactions between human activities and the marine environment [14].

5. Chemical oceanography

Chemical oceanography is the branch of oceanography that focuses on the study of the chemical composition of the oceans and their relationship to the Earth and climate. The oceans are an important source of elements and chemical compounds that are essential for life on Earth, and chemical oceanography seeks to understand how these elements and compounds are distributed, transported, and transformed within the oceans [27].

Chemical oceanography focuses on issues such as the variation in the concentration of salts, the formation of chlorides, the concentration of carbon and other elements, as well as the interaction of these components with the atmosphere, continents, and marine life. In addition, chemical oceanography also investigates the chemistry of the deep oceans, including the chemical composition of the water, the concentration of metals, and the presence of organic compounds [28].

5.1 Water

Water is a molecule formed by chemical bonds between two hydrogen atoms and one oxygen. Because of the arrangement of hydrogen and oxygen atoms, water has angular geometry, making it polar. Thus, its positive part attracts negative particles, while the negative attracts positive particles. Water has intriguing characteristics, which make it a compound, despite being abundant, so special and unique [29]:

• High specific heat: Water needs an amount of high energy to raise the temperature of 1 gram of substance by one degree Celsius (°C). Therefore, water resists temperature changes and works as a climate moderator for the planet, transporting heat from low-latitude regions to high latitudes.

• High latent heat of fusion: It is a large amount of energy gained or lost for water in the solid phase to pass into the liquid phase, or vice versa, without having an increase in temperature.

• High latent heat of vaporization: Tt is large amount of energy acquired or lost so that the water in the liquid phase passes to the gaseous phase, or vice versa, without having an increase in temperature.

• Density: The density of pure water is 1000 g/cm³; already, the water sea has a density between 1.020 and 1.030 g/cm³. Water density is strongly related to salinity and temperature. Therefore, the density of sea water increases with increasing salinity and pressure and with decreasing temperature. Thus, the colder and saline water is, the denser it is, and the warmer and less saline it is, the less dense it is.

• Sound: The speed of sound waves in sea water is approximately 1.500 m per second, much higher than the speed of sound in air, which is why many marine animals use sound to orient themselves in the oceans.

• Light: The water layer can be divided into two denominations relating to lighting: photic zone and aphotic zone. The photic zone is the layer illuminated and heated by the Sun on the surface of the water, allowing all photosynthetic production. The waters located below this zone do not receive illumination, which is why it is called the aphotic zone.

• Effective solvent: The previously described electrical asymmetry of the water molecule explains its great solvent capacity, being able to attract negative or positive particles. Water is the substance capable of dissolving more substances than any other.

• Salinity: It is the total amount of inorganic solids dissolved in the water. The average salinity of the oceans is 3.5%, depending on evaporation, precipitation, and the amount of fresh water discharged from the continent. Sodium chloride and sulfate are the most abundant dissolved ions in sea water. The determination of salinity is done through the chlorinity of the water sample. Since the ratio of chlorinity and salinity are constant, the total mass of bromide, iodide, and chloride ions is measured.

• Composition of sea water: Sea water is added water solids and dissolved gases, which can be classified as conservative elements, trace elements, nutrients, and dissolved gases. Conservative elements are those that appear in high concentrations. Trace elements are those that, although found in very low concentrations, play an important role in water chemistry and biota. The nutrients are very important for the biota, mainly for the organisms at the bottom of the food chain, affecting all subsequent trophic levels. They generally have low values in surface waters due to the rapid consumption of photosynthetic agents, and in deeper waters, there is an accumulation of nutrients. The gases present in the air dissolve easily in water; thus, all gases are dissolved at the surface of sea water. Among the dissolved gases, the most representative are nitrogen, oxygen, and carbon dioxide.

• pH: In a volume of water, the more hydron (H⁺) and the less hydroxyls (OH⁻) the more acidic it will be; therefore, the reverse makes it more alkaline. The pH scale ranges from 1 to 14; the value 7 indicates the neutral point at which there is balance in the amount of H⁺ and OH⁻ ions, while the predominance of H⁺ ions indicates low pH values; the opposite indicates high values. Pure water is neutral, whereas sea water is slightly alkaline, ranging from 7.4 to 8.5. This is due to the large amount of CO₂ dissolved in sea water, which can produce H⁺, bicarbonate (HCO₃⁻), and carbonate (CO3₂⁻) ions, and which prevent pH change when acids or bases are added. Thus, sea water acts as a buffer solution.

Marine systems undergo changes in their chemical composition due to changes in the environment, usually associated with the entry of contaminants. It should be remembered that the environmental quality can be altered by the presence of these toxic agents. The main anthropogenic inputs found refer to the dumping of dredged material, urban and industrial effluents, leaching from rural areas, atmospheric inputs, and shrimp farming waste [30].

5.2 Chemical oceanography and the climate change

Chemical oceanography has important implications for understanding climate as the oceans are an important climate regulator, storing and transporting large amounts of heat and carbon dioxide. Chemical oceanography is also important for understanding biogeochemical cycles, including the carbon cycle and nutrient cycle, which are fundamental to marine life [31].

Chemical oceanographers use a variety of techniques to collect and analyze samples of water and marine sediments, including chemical analysis techniques, spectroscopy, and remote sensing. This information is important for the management of marine resources such as fisheries, tourism, and oil and natural gas exploration, as well as for understanding marine biodiversity and the health of marine ecosystems [32].

In summary, chemical oceanography is an important field that provides a fundamental understanding of ocean chemistry and its relationship to climate, marine life, and the wider environment.

6. Biological oceanography

Biological oceanography encompasses a wide range of disciplines from molecular biology to ecology to better understand the functioning of marine ecosystems and their inhabitants. Some key areas of study include [33]:

• Marine biodiversity: The study of the diversity of species and their distribution in the ocean.

• Marine ecology: The study of how marine organisms interact with each other and their environment.

• Marine phytoplankton: The study of microscopic plants that float in the ocean and form the base of the marine food chain.

• Marine zooplankton: The study of microscopic animals that float in the ocean and are an important food source for many larger marine animals.

• Marine primary productivity: The study of how marine plants produce organic matter through photosynthesis, which supports the entire marine food chain.

• Marine food webs: The study of the complex network of relationships between predators and their prey in the marine environment.

• Marine biogeochemical cycles: The study of the movement of important elements and compounds, such as carbon, nitrogen, and phosphorus, through the marine environment and the impact on marine life.

Biological oceanography also investigates the impact of human activities on the marine environment and its inhabitants, such as the effects of pollution, overfishing, and climate change [34].

The energy that most marine organisms need to survive comes directly or indirectly from the Sun. This produces enormous amounts of energy, which is captured by the chlorophyll present in organisms called primary producers. Solar energy is then transformed into chemical energy. From these reactions, energy is used to synthesize carbohydrates and other organic molecules that will be used by the producers themselves or will be ingested by other microorganisms present in the aquatic environment called consumers [35].

Photosynthesis is the main autotrophic process carried out by beings possessing chlorophyll, represented by plants, some protists, photosynthetic bacteria, and cyanobacteria. The rate of photosynthesis varies depending on the available light (intensity and quality), in addition to other factors, such as the amount of biomass, nutrients, etc. This process is considered dominant in the conversion of energy into carbohydrates, but energy production may also occur through inorganic molecules available in the medium instead of sunlight. We call this process chemosynthesis, and it is present in some marine forms, but it is small in relation to photosynthesis [36, 37].

Primary productivity, synthesis of organic matter from inorganic substances, is expressed in grams of carbon assimilated into organic matter per square meter of ocean surface per year (gC/m²/year). In this context, we call primary producers the autotrophic organisms capable of synthesizing food. The heterotrophic animals that consume these organisms are called secondary consumers, while the animals that feed on these organisms are called tertiary consumers, and so on up to the top consumers. It is important to note that as energy flows, much of it is lost as heat. The energetic interactions between producers and consumers are generally complex, which is why they are called food webs [38].

6.1 Pelagic organisms

Pelagic organisms live suspended in sea water, adrift, and interact in this place with members of very different sizes and characteristics. In this habitat, they share the need to maintain an upright position, in addition to obtaining food, among other basic needs. According to the form of life, we can divide the pelagic organisms into plankton and nekton [39].

Plankton is constituted by the group of beings that live suspended in the water, carried by the current, in aquatic environments of fresh, brackish, and marine water. Many of these organisms have their own movement but insufficient to overcome the force of the currents. Plankton organisms range from micrometers invisible to our eyes without the use of a microscope, to millimeters, with some exceptions of organisms that can reach from several centimeters to meters in length, such as the “Portuguese Man-of-War” (Physalia physalis) [40].

Plankton can be grouped depending on some characteristics:

• Basic cell structure: Prokaryotes or eukaryotes.

• Trophic grade: Autotrophic or heterotrophic.

• Nutrition: Photoautotrophic (light capture and carbon absorption inorganic), osmo-trophy (absorption of organic molecules), phagocytosis or phago-trophy (ingestion of particles)

• Horizontal distribution: Patches or aggregates.

• Planktonic microorganisms: Bacterioplankton (prokaryotes heterotrophs and autotrophs), phytoplankton (eukaryotic organisms, unicellular autotrophs), and proto-zooplankton (eukaryotic organisms, unicellular heterotrophs, Protozoa), zooplankton (heterotrophic eukaryotes organisms).

The phytoplankton community guarantees the ecological balance of the aquatic environment as it forms the basis of the trophic network and of all biological production in the seas. It is a group with a high degree of biodiversity, and new species are isolated and described every day. The microbial food chain of marine ecosystems extends throughout the entire photic zone of the oceans (where the presence of light reaches, up to 200 m deep on average), and it is also known as the microbial loop [41].

The trophic relationships of the loop group different microorganisms, which are mainly responsible for the processes of decomposition and remineralization of the compounds of the biogeochemical cycles [42].

Several other ecological actions can be performed by phytoplankton (adapted from [43]):

• Primary production in the oceans: This is directly related with maintenance of gas balance (CO₂-O₂). Phytoplankton, due to its ability to use CO₂ and release O₂, acts as an important factor in maintaining the balance in the concentration of these gases in water, even influencing physical-chemical characteristics, such as pH.

• Sedimentation of the material produced (biological pump): Through the sedimentation of organic matter to the bottom of the sea (through the death of microorganisms and the release of feces, mainly zooplankton), their removal from the pelagic environment for a long time occurs.

• Carbon removal is important as a climate regulator. In addition, the sedimentation of certain organisms gives rise to the formation of siliceous mud or diatomite (diatoms, radiolarians) and calcareous oozes (foraminifera). Silica from diatomaceous Earth is a chemically inert material and is used as an abrasive substance in filtration in sugar refineries, breweries, winemaking, base for dyes, and insulating material.

• Harmful phytoplankton blooms (HAB): They occur due to a large increase in cell concentration, with reduction of O₂ (at night, or in senescence), or by the production of toxins that are accumulated along the trophic chain.

Marine biodiversity is high in the pelagic environment as the plankton has a high species richness in addition to being in large numbers, usually greater than 30 coinhabitants in space and time unlike terrestrial ecosystems. This ability to inhabit the same ecological niche seems to be a paradox, but it can be explained by the great heterogeneity in the pelagic environment, providing the existence of micro-niches for different species [44].

The lack of knowledge about the life cycle of many planktonic microorganisms makes it difficult to recognize their real species. Ideally, organisms are identified based on morphological criteria (shape and size), ultrastructure, biochemical makeup (chemotaxonomy), and genetics. There is a consensus that the genetic constitution of organisms, details of ultrastructure, and the life cycle should, together, serve as a reference to determine phylogeny, and that phylogeny should determine taxonomy. Thus, the nomenclature should reflect the genome information. This ideal is still far away for protist and prokaryotic organisms and, in practice, the morphospecies are still of great importance in determining the diversity of phytoplankton and proto-zooplankton [45].

How many species are there? In addition to most microscopic species still being unknown, the exact number is unknown, as new species are described daily. Major advances were determined by the introduction of electron microscopy around 1970 and currently by molecular biology. There are at least eight major groups of phytoplanktonic organisms, the most important of which are diatoms (Bacillariophyta) and dinoflagellates (Miozoa). Other prominent groups are Coccolithophyceae and Cyanobacteria [46].

Primary production (PP) in the oceans can be divided into primary gross (the total organic matter produced, excluding the cellular respiration that occurred during the given period) and net primary (organic matter produced is accounted for discounting the “loss” by respiration). Some physical factors may interfere with primary production, such as the primary production rate approximately following the behavior of light with increasing depth of the environment (negative exponential curve). In the euphotic layer, the rate of photosynthesis is high, decreasing to the boundary of the euphotic layer, where the rate of photosynthesis approximately corresponds to the rate of respiration (net primary production = zero). Below the euphotic layer, respiration exceeds photosynthesis and therefore autotrophic cells do not grow [47].

In the marine environment, water column stability is important in phytoplankton and macroalgae (in coastal waters) ecology. The presence of a thermocline or halocline determines stability in the surface and illuminated layer, allowing phytoplankton cells to be exposed to light and providing high gross and net primary production. In conditions of vertical mixing in the water column (isotherms and isohalines), phytoplankton cells are displaced, remaining part of the time in the dysphotic (low light) or even aphotic layer, thus decreasing primary production. The relationship between light, primary production, and respiration in the water column is described by the Sverdrup model. In this model, the concept of critical depth (first defined by Sverdrup, 1953) stands out, that is. the depth at which the production of the entire water column is equal to the total respiration (Figure 2) [48].

Figure 2.

Light penetration (of different wavelengths) in the sea water column (coastal zone) and the vertical distribution of macroalgae.

Chemical factors also influence PP since phytoplankton actively incorporate inorganic nutrients by using enzymes associated with the cell membrane. The main nutrients or elements required for nutrition can be classified in different ways [49]:

• Macroelements: They are found in relatively low concentrations large, being rarely limiting in the environment: CO₂, Na⁺, K⁺, Mg²⁺, Ca²⁺, and SO₄²⁻.

• Micronutrients: They are found in lower concentrations, often limiting in the medium: NO₃, NO₂, NH₄, PO₄, and Si(OH)₄.

• Trace elements: They are present in minimal amounts, and may be limiting in some cases: Fe, Mn, Zn, Cu, and Co.

• Organic compounds (required by some organisms): Vitamins (B₁₂, thiamine, and biotin), organic binders, urea, and amino acids.

Another group of great ecological importance in plankton is the called zooplankton, in which heterotrophic organisms that feed on primary producers and other zooplankton organisms participate. Formed by animals and larvae of numerous species, the vast majority microscopic, have a certain ability to move in the oceans and seas. The locomotion capacity of zooplankton can be verified with the vertical migrations present in some organisms. It can be classified into two groups:

• Holoplankton: Organisms that spend their entire life cycle in the plankton, such as copepods (the most diverse class of crustaceans, and the largest group that make up the zooplankton), among other crustaceans (such as krill), filter feeder urochordates (salps), chaetognaths, and hydromedusas.

• Meroplankton: Organisms that go through only one stage of life in plankton, such as eggs, larvae, and animals in the juvenile stage, such as larvae of crustaceans, mollusks, and echinoderms. The larvae and eggs of fish are part of the meroplankton and are called ichthyoplankton.

Most of the organisms that make up the zooplankton feed on microalgae, although carnivores, omnivores, and detritivores are observed in addition to herbivores. On the other hand, they are food for many species of fish and other animals, such as the whale, which feeds almost exclusively on krill (pelagic arthropod), considered a key species in the Antarctic ecosystem. They are considered essential organisms for the maintenance of the aquatic ecosystem as are at the base of the food chain [50].

6.2 Nektonic organisms

Pelagic animals, which actively swim in the water column, are known as nektonic beings. Most are vertebrates (mainly fish), but some invertebrates are present in this classification, such as squid and some crustaceans. Turtles and mammals can be important species in certain areas, as can seabirds, especially as predators. The main groups of nektonic predators are:

• Cephalopods: They are the most evolved animals among mollusks, with many marine predators formed by squid, Nautilus, and Octopus. They are a source of great fisheries.

• Marine reptiles: Turtles, sea snakes, sea lizards (iguanas), and sea crocodile. The best known and most successful are the eight species of sea turtles, possibly due to the presence of their shell, which provides efficient defense. Although the only predator is man, there is attention to the spawning stages, in which there is a high mortality rate and the young when they are on their way to the sea.

• Seabirds: Of four groups of seabirds, the gull and the pelican are the best known as there are many species and these live close to the coast. However, the groups best adapted to the pelagic world are the albatrosses, petrels (Tubinares order, considered the most oceanic birds in the world), and penguins. Penguins, on the other hand, have lost the ability to fly; however, they use their wings to swim over long distances and with great dexterity of movement. Native to the southern hemisphere, they have adaptations to conserve heat, such as insulation with fabric adipose and fat feathers. Another important point to highlight is the presence of large colonies in areas of high productivity, such as the resurgence in Peru. Unfortunately, they are directly affected, with high mortalities, in pollution events (e.g., pesticides) and oil spills. They prey on squid, fish, and zooplankton.

• Fish: They are the group that dominates the nekton, characterized by great ability in swimming, through which they can move independent of ocean currents. There are more species of fish, and more individuals, than there are species and individuals of all other vertebrate groups combined. Some species have modifications that allow them to detect prey or avoid predators. They are divided into two large groups based on the material that makes up their skeletons: cartilaginous and bony. The class Chondrichthyes includes sharks, lampreys, rays, and and is characterized by cartilaginous tissue, jaws with teeth, pairs of fins, and an active lifestyle. Through vibrations in the water, detected by sensitive organs lined up below the surface of the skin, the tiger shark, and the hammerhead shark, among others, attract their prey. In addition, they have a very developed sense of smell, acting in hunting prey, which is composed of fish and marine mammals. In the class, Osteichthyes are approximately 99% of all fish today. In the Teleostei order are the organisms of greatest commercial interest: cod, tuna, and sole, among others. These animals feed on different types of prey depending on their size, location, and prey availability and may be planktivorous, piscivores, or both. They can perform migrations after prey. The vast majority are found in temperate waters, but species diversity is greatest in tropical and subtropical regions.

• Marine mammals: The Mammalia class is the most evolved group of vertebrates, including seals, whales, dolphins, and walruses, and constitutes a very diverse group of 128 species that depend on the oceans for their existence. This group does not correspond to a distinct biological group, but to a functional group that has in common dependence on the aquatic environment for food. This dependency manifests itself on several levels, not all being expressed by the totality of species. For example, dolphins and whales (cetaceans) are completely dependent on the marine environment at all stages of their lives, whereas seals feed in the ocean but breed on land. Marine mammals are divided into four groups: cetaceans, pinnipeds (seals, sea lions, and walruses), sirenians (manatees and dugongs), and fissipeds (carnivores with spread toes, such as the polar bear (Ursus maritimus) and two species of otter. They play a fundamental role in the maintenance and regulation of marine ecosystems, especially by regulating the populations of their prey species. These two factors, relevant global biomass and regulatory role, make them a fundamental component of the marine environment. This fact makes them if particularly important if we consider that, currently, about 23% of marine mammal species are threatened [51].

All marine mammals share four common characteristics: a hydrodynamic body, with appendages adapted for swimming; modified breathing system to retain large amounts of oxygen during long dives and displacements. Another important point is the ability to generate internal heat through high metabolic rates and conserve it through insulating layers of fat, and in some cases, even with the presence of hair. The fourth adaptation is related to the absence of need for fresh water due to the ability of their kidneys to excrete urine concentrated in salts, allowing the water necessary for their metabolism to derive from the oxidation of food [52].

The order Cetacea (cetaceans and sirenians) has the only mammals’ marine animals that spend their entire lives in the water. Unlike pinnipeds, which mainly use hair as thermal insulation, cetaceans have a thick layer of fat, the “blubber.” Hind limbs are absent, and propulsion is provided by horizontal caudal fins. The forelimbs do not have externally individualized fingers, having the shape of oars, and are used to maintain stability during swimming. They are divided into two suborders: Odontoceti and Mysticeti. Current true whales (raw whales) are characterized by their highly differentiated feeding apparatus due to the loss of teeth and the appearance of cornified epithelial tissue plates (fins) that are suspended from the roof of the mouth and serve to filter food from the water. This suborder Mysticeti includes right whales, gray whales, and pygmy right whales, while the suborder Odontoceti includes beaked whales, sperm whales, porpoises, Amazon River dolphins, killer whales, pilot whales, and belugas, as well as of dolphins [53].

6.3 Benthic organisms

They are those animals and plants that live associated with the sediment. Some can burrow (infauna), and others live on the surface of the sediment (epifauna). The benthic habitat can be shallow or deep, full of food, or somewhat barren; the fact is that the diversity of benthic habitats, and of organisms that live associated with them, is very large. Macroalgae forests, rocky intertidal zones, sandy beaches, marshes, and even coral reefs are part of this vast habitat [54].

The epifauna comprises the animals that live on or associated with rocks, stones, shells, vegetation, or on unconsolidated backgrounds. The infauna comprises all animals that live within the unconsolidated substrate layer, drilling into it, or simply living within it. There are still other ways to classify benthic organisms:

• Macrofauna: Comprises all animals that are retained in a 0.5 mm mesh sieve.

• Mesofauna: Animals that pass through a 0.5 mm mesh sieve but are retained in a 0.05 mm mesh.

• Microfauna- all other organisms (generally Protozoa): To survive in the benthic habitat, there are several functional adaptations, such as tolerance to different physicochemical aspects of different types of substrates, as well as structural adaptations, such as body shape and small and elongated bodies (vermiform) are common. The metabolism and activity can be modified by the substrate via nutritional aspects (ex: Filters are more frequent in sandy bottoms, where their filtration devices are not at risk of being clogged). In this way, the type of substrate, where the animal lives can modify the rate and forms of reproduction. The horizontal distribution on sandy bottoms is affected by the nature and size of the grains; the type, quantity, and form of organic matter associated with the substrate; the total area of the sandy substrate; and other environmental factors such as water movement, light, salinity, oxygen supply, and pressure.

7. Bioactive marine natural products

One of the fields with a successful history within Marine biotechnology is the bioprospecting of bioactive marine natural products. This subarea stands out as one of the most developed, with several substances being used commercially, and many others in preclinical and experimental testing phases. Until the 1950s, the marine environment went unnoticed by natural product scientists, mainly due to its difficult access. But it was from the 1970s onwards, with the advancement of diving techniques and equipment, that marine organisms became part of chemistry and pharmacology laboratories, starting their history [55].

The first efforts in the exploration of marine natural products were focused on easily available and collectible organisms, such as brown, green, and red algae, sponges, and soft corals, which quickly showed to produce a great variety of new molecules. With continued exploration, in partnership with the progress of oceanographic technology, other groups of organisms, more critical in terms of availability, were studied, and the arsenal of unique molecules from the marine environment grew [56].

In general, the exploration of marine natural products resulted and continues to result in the discovery of many substances, most designated for the pharmaceutical industry, but also occupied space in the areas of cosmetics, agriculture (pesticides), and shipbuilding (antifouling’s) [57].

7.1 Biopolimers

Products made from nonbiodegradable polymeric materials (such as plastic), which come from fossil sources, have become a problem due to the growing number of inappropriate discards, and the degradation time of these materials, which take many years in the environment. Researchers have been looking for alternatives, together with the industry, to minimize the environmental impacts caused by the inappropriate disposal of plastic products. Among the alternatives, in addition to reuse and recycling, the production and use of biopolymers, biodegradable polymers, and green polymers have been growing due to their technical and economic viability, presenting great potential for expansion. They can come from renewable sources, such as cellulose, potatoes, or be synthesized by bacteria from small molecules, or even be derived from animal sources, such as chitin or proteins [58].

Crustaceans, such as shrimp and crabs, produce chitin, the second most abundant polysaccharide in nature, after cellulose. Chitin is a very versatile substance for industrial application, in addition to being useful in other areas. It is used in the composition of agricultural fungicides. In medicine, it is used for the manufacture of hemodialysis membranes, in biodegradable surgical threads, as substitutes for artificial skin, healing for burns, and medicine capsules and insulin releasers. In cosmetics, chitin is used in the manufacture of shaving creams and moisturizing creams. Due to its ability to absorb fats, chitin is present in the composition of several dietary foods. It is also used in papermaking and the textile industry. The flocculant and coagulant actions of chitin are applied in the filtration of water in swimming pools, in water sanitation, and in the removal of heavy metals and oils [59].

Marine algae (seaweeds) represent one of the most abundant sources of relevant and widely used biopolymers. Agar, used in research as a raw material for gels and biological matrices; carrageenans, used as stabilizers and texturizers by the food industry and in cosmetic and hygiene product formulations; alginic acid, used as a biomaterial in medical sciences for skin grafts, dressings, and healing agents for serious cases, such as burns, as well as a vehicle for administering drugs or gene therapy and as a base in the preparation of dishes in gastronomy (Figure 3). Some mussels and barnacles have been explored for their adhesive properties for fixing on consolidated substrates. This type of “glue,” produced mainly by barnacles, has been used in surgical procedures, replacing the suture [60].

Figure 3.

Main seaweed polysaccharides: A – Alginic acid; b – Carrageenans; c – Agar.

Seaweeds are used in many countries for very different purposes: directly as food, extraction of phycocolloids, extraction of compounds with antiviral, antibacterial or antitumor activity, and as biofertilizers. Seaweed polysaccharides (phycocolloids), such as agar, alginates, and carrageenans, are produced on a large scale and have a wide range of applications in the food, pharmaceutical, and cosmetic industries. Many species of seaweed (macroalgae) are used as food and have also found use in traditional medicine because of their perceived health benefits. Seaweed is a rich source of sulfated polysaccharides, including some that have become valuable additives in the food industry because of their rheological properties as gelling and thickening agents (e.g., alginates, agar-agar, and carrageenan). The different phycocolloids used in the food industry as natural additives are (European phycocolloid codes): alginic acid, sodium alginate, potassium alginate, ammonium alginate, calcium alginate, propylene glycol alginate, agar, carrageenan, semi-refined carrageenan or processed Eucheuma species, and furcelleran [61, 62].

7.2 Biofuels

Fossil fuels are responsible for the emission of gases that intensify the greenhouse effect (heating of the earth’s atmosphere). The gravity of this fact could be minimized through the indirect use of solar energy to obtain fuels derived from photosynthetic organisms, which can be cultivated practically all over the world, in a renewable and nonpolluting way. Recent research indicates that the production of biodiesel from microalgae could radically change the fuel market. With a much higher oil production potential per area than traditional crops grown on land, microalgae have aroused worldwide interest in prospecting for biofuels [63].

The advantages arising from biodiesel from algae include their fast growth rates and their high yield per hectare, the fact that they do not contain sulfur, are nontoxic, and are highly biodegradable. Productivity is higher in controlled environments (photobioreactors), but other forms of production are also superior for open systems. Significant investment in research is still needed before high levels of productivity can be guaranteed on a commercial scale. In addition to producing oils, algae are also rich sources of vitamins, proteins, and carbohydrates. Several companies and universities are involved in algae biofuels, and in 2020, the development of a systemic approach to the sustainable commercialization of this biodiesel and its bioproducts was announced in USA [64].

In addition to biodiesel, bioethanol also paves the way within the marine biofuels, being produced from cellulose and alginate extracted from macroalgae or from tunicin, a component present in the tunic of ascidians. Sea squirts also contribute to methane production from their biomass [65].

7.3 Bioremediation

Ecological systems have a level of innate ability to break down contaminants or pollutants that adhere to them. The biological agents responsible for these automatic cleanings are often microorganisms from nature itself. The elimination or breakdown of environmental contaminants by living organisms is called bioremediation. Such microbial-mediated removal over time can take place completely without human intervention; however, the process can also be initiated by anthropogenic administration. The greatest bioremediation efforts in the marine environment are focused on oil spills or other petroleum product contamination, where Marine biotechnology can play a significant role in the final stages of total cleanup [66].

For this case, there are three bioremediation strategies used:

• Intrinsic bioremediation is the removal of oil naturally by biotic means from the environment itself over time and without human intervention.

• Bio-stimulation, which needs native microbial populations to degrade contamination. It is carried out by human intervention through the addition of nutrient fertilizer or other means that increase the rate of natural biodegradation.

• Bio-augmentation, which is the least common strategy and consists of adding an oil-degrading microbiota that complements the degradation capacity of native populations. It may be a rare lineage or one absent from the local community, or even genetically created to carry out the function of degradation. It is not widely practiced due to the environmental concerns of such an intervention [67].

One of the biggest concerns is the toxic polyaromatic hydrocarbons (PAHs) that make up the tar, found in the oil. Through DNA fingerprinting techniques, researchers have isolated marine bacteria that degrade PAHs. Currently, efforts are being made to understand how communities of natural bacteria can detoxify areas contaminated by hydrocarbons, as well as unravel microbial metabolism and growth in contaminated environments [68].

8. Conclusions

This chapter covers the main aspects of oceanography, the interdisciplinary science that studies the oceans, including their biology, geology, physics, and chemistry. It is an important area for understanding climate change, preserving marine resources, predicting natural events, such as tsunamis, and for safe navigation and fishing. Oceanography is also crucial to understanding Earth’s dynamics, and how the oceans affect the global climate. However, much remains to be discovered about the oceans, and oceanography remains an ever-evolving field with frequent discoveries and technological advances. In short, oceanography is a vital area for understanding and protecting our planet and its marine life.

Acknowledgments

Leonel Pereira thanks to the Fundação para a Ciência e Tecnologia, I. P (FCT), under the projects UIDB/04292/2020, UIDP/04292/2020, granted to MARE, and LA/P/0069/2020, granted to the Associate Laboratory ARNET.

Conflict of interest

The authors declare no conflict of interest.